I keep reading these threads where people say that the engine needs some back pressure on exhaust and others say it doesn't. My eyes are raw right now from all the reading, but no hard facts.

The biggest restriction to the exhaust seems to be the lead pipe. But if the engine needs back pressure, this would be counter-productive to improve upon.

Perhaps a science lesson? Any suggestions for a custom exhaust/race pipe if anyone were to think about it?

If this has been covered elsewhere, please point me in that direction. No flames...because I have searched and read alot already.

Thank you in advance!

I think this would be a very good topic to hear some hard facts on.

I've heard two types of opinions on the subject. One is that back pressure is good. In real life, this view seems to flow from the lips of "rice boiz." On these forums, several people have suggested back pressure is somehow correlated to torque (though I have no threads to point to).

On the other hand, I've heard you want to eliminate all back pressure. This view is commonly told to me by American Muscle fanatics from the days of old (e.g. my dad). This certainly seems the most logical to me, as adding bigger pipes yields more power, and race cars tend to get to keep the exhaust work to a minimum, occasionally running pipes from the headers directly out the hood.

But is there too much of a good thing? Is back pressure important?

An excellent explanation by my old friend Vaughn Nishimura, who was one of the first people to ever swap the KL motor in the the MX-3:

Actually, there is slight misconception in terminology that needs to be
cleared up. It is not that a certain amount of backpressure is needed for
optimum performance. A well designed exhaust system should not only
relieve backpressure, but go one step further and create a vacuum in the
system. When the next cylinder's exhaust valve opens, the vacuum in the
system pulls the exhaust out of the cylinder. This is what's termed
"scavenging". It just easier to call it as needing a little backpressure
than it is to explain a balanced airflow system to the average guy on the
street.

The physics of airflow dynamics dictate that the pipe should be only be big
enough to handle a given volume of air or in this case exhaust gases. Whether
it be an exhaust system or a A/C duct in your home first consideration is
the proper tube diameter. Many people think "Bigger is Better", but this is
not the case. The smallest diameter that will handle the airflow of a given
volume of air at a given air velocity should be used. This small diameter
will generate the velocity (air speed) needed to "Scavenge" or create a
vacuum in the pipe. In the case of a internal combustion engine, if the pipe
diameter is to small for the engine it will pull hard at low rpm's but at
some point in the higher rpm's the tube will not be able to flow as much air
as the engine is pumping out, and the engine will "clock out " early, not
reaching its potential peak rpm. In this situation it would require going
one size larger in tube diameter. This is why the general rule of thumb is
that the maximum exhaust pipe for a normally aspirated multi valve engine
operating under 8000rpm should only be .75 to 1 time the displacement of the
engine in litres.

This vacuum in the exhaust system also has a big effect on the intake side
as well. Because the vacuum is pulling the exhaust out of the cylinder this
creates a stronger vacuum pulse on the intake side that pulls a stronger
air/ fuel charge into the cylinder. If the pipe diameter is to big the Air (
Exhaust) become lazy and wants the intake side to push it out so in the case
of an engine the intake charge gets contaminated with exhaust gases. If the
pipe is to big for an application this can also cause restriction in airflow
because it is moving to slow and will actually start to back up in the
pipe.

This is an over simplified overview in airflow dynamics but you should get
the general idea.

A bit off topic, but for FI rotarys relieving backpressure can be big trouble - unless you upgrade all fuel and ecu requirements at the same time. Countless people have blown their 13BT's and 13B-REW's because they straight-piped the system and ran lean. This exists with many piston FI engines, but is especially prevelant in rotarys.

Originally posted by Red Devil
A bit off topic, but for FI rotarys relieving backpressure can be big trouble - unless you upgrade all fuel and ecu requirements at the same time. Countless people have blown their 13BT's and 13B-REW's because they straight-piped the system and ran lean. This exists with many piston FI engines, but is especially prevelant in rotarys.

But completely unrelated because the RX-8 is not a speed/density system like the previous 13b.
We are fortunate enough to have a MAF, so it will compensate all airflow increases up to its maximum rating.
In our case, that is about 20% more than what the car is capable of stock, or roughly 50 HP.

Thank you for the responces so far.

Here's some more questions to spawn off...

Just for shits and giggles, what would the affect be of removing the exhaust off entirely...I hear it shoots flames, but I am speaking perofrmance wise.

That was a good explanation about scavenging (creating a vacuum), but it doesn't describe a Rotary redlining at 9000 RPM. When it says exhaust pipe, does it mean from engine to the muffler or exit of the car. What volume would you use? 1.3, 2.6, or 3.9 Liters?

20%...Is that an arbitrary number? I'd surely love to put 40-50 Horses onto my RX-8 naturally aspirated.

and...where's rotary god and wakeech?

Originally posted by Ft Liquordale
20%...Is that an arbitrary number? I'd surely love to put 40-50 Horses onto my RX-8 naturally aspirated.

20% is what is left in voltage output above where the MAF reads at WOT the torque peak.

Yeah, why didn't the previous rotarys have MAF's? I'm sure there is an explanation, and maybe a very good one, but I never heard it?

The 13B-REW engine in the 3rd gen RX-7 used a speed density system - MAP sensor, no airflow meter. The 2nd gen RX-7 used a flapper door airflow meter - worked well, but was an intake restriction. The 1st gens (save the GSL-SE) were all carburated.

The speed density system in the 3rd gen has one weak spot - for speed density to work, the volumetric efficiency of the system has to be a constant. When you upgrade exhaust/intake, that changes the VE, and you run lean. Speed density basically does an equation to solve for air volume entering the engine - air temp, RPM, boost/vacuum, look it up in a table, there's your air volume. An airflow meter samples air volume directly.

The problem with an airflow meter is it's always a restriction. The newer hotwire airflow meters (like on the 8) are VERY low restriction. Speed density was cool at the time the 3rd gen was built, though :). I think part of it comes from using an EFI system that Mazda licensed from Nippondenso - it's basically a Bosch system, as was the 2nd gens.

Back to exhaust. When discussing exhaust, you have 3 factors - flow, velocity, and backpressure. Backpressure is always bad - that's a physical restriction. Velocity is good, and when you have the right velocity on a non-turbo engine you can "suck" the exhaust gasses out of the combustion chamber, creating great efficiency. If you have a lot of flow, however, you typically lose velocity - on a non-turbo engine, you need to find the balance between them. A small pipe will have great velocity, but will create backpressure at high RPM. A large pipe will have great flow, no backpressure, and no velocity.

Turbos are much easier - flow is good, more flow is even better.

In general, on a rotary you generally want pretty good-sized exhaust piping. Rotaries pump a lot of air for the size, and there's a ton of heat in the exhaust. But, you still have to factor in the whole system - a great cat-back system will only get you so far if you have a cat that's choking you down.

Good thing is Mazda did an excellent job on the stock exhaust system - it's light years better than the RX-7's various exhausts.

The other thing to remember with rotaries and exhaust is two things - noise and heat. You cannot believe the amount of noise a rotary can make - we're talking deafening. Heat is also a big factor - a non-turbo rotary pumps a LOT of heat out of the tailpipe. Cheap mufflers don't last long.

The good thing is all the cat-back systems built so far that I've seen have been of excellent quality. A lot of the above rules moreso apply if you're trying to make something yourself.

Dale

On the intake, there are different channels that open at different times (http://www.rx8club.com/showthread.php?s=&threadid=24649&highlight=intake). I'm pretty sure rotarygod has mentioned that a similar system would benefit on the headers. Would the intake "widening" have any effect on what the correct volume needed to be removed to have the most efficient exhaust? So, would it be more efficient to have more than one exhaust pipe (have a full secondary exhaust, in effect, that is only available after certain RPMs, like the extra intake channels)?

This is totally possible. At low RPM, you want high exhaust velocity, and at high RPM, you need a lot of flow to keep up with the massive amount of exhaust coming out. Most systems balance the two to have a good compromise.

I know there have been some manufacturers who've had variable mufflers - the Stealth had a variable muffler, if I recall. A "smart" exhaust system is feasible, but you're dealing with a LOT of exhaust heat - it's a harsh environment to put moving objects.

But, it is quite possible. Would take a helluva lot of R&D to develop a good system.

Dale

They managed something with turbos. Maybe a blowoff valve of sorts would work?

Now if only someone with money and time and engineers would read this thread... Borla? RB? It's worth a shot.

Nice to know everyone is thinking along the same lines as me. A blow-off valve may be kind of illegal and like I said before (but neither is removing the cat on a street application), but rotories are known to spit flames. Not good for things such as break lines. Theres a valve of this sort floating around this site somewhere that has a pull cord that effectively eliminates the exhaust. I wouldn't want to be the guinea pig.

I was thinking dual pipe with an internal valve that opens with air pressure. The problem as stated above is the heat. Does it matter where the valve would be?

Also, with these changes, would there need to be changes made to the valve timing on the intake?

Originally posted by Ft Liquordale
A blow-off valve may be kind of illegal ...

Huh? Blow-off valve? Do you mean a dump valve?
A BOV is for the pressurized side of a turbo system. The dump valve is like a remote wastegate, but for a normally aspirated vehicle.

Originally posted by Ft Liquordale
I was thinking dual pipe with an internal valve that opens with air pressure. The problem as stated above is the heat. Does it matter where the valve would be?

When I said "blow off valve of sorts" I was talking about the same thing. I didn't mean blow off into the air, but blow off into another pipe. Basically, have one pipe terminate into the main pipe with a pressure release mechanism between them that would open if a certain pressure was reached to allow the flow of air into the secondary pipe.

Then it wouldn't matter if there was fire. The only problem I could foresee is what the seal on the valve would be made of. It'd have to handle a great deal of heat.

I think starting it at the headers would be the best place. Otherwise, the headers are still the bottle neck. Basically, this system would have to be a FULL exhaust system replacement for it to be really functional. I don't think it would work right if it wasn't.

Re: MazdaManiac

Yeah... BOV... dump valve. Whatever. I was trying to give a name to "something that releases pressure by removing content" and BOV was the first thing that came to mind. I think you understand the point I was getting at.

Ft. Liquordale,

To answer your question, Racing Beat did a test on their in-house Renesis, and found a 5HP peak gain with no exhaust, and it was all top end. A nicely tuned cat-back system with acceptable sound levels will yeild between 2 and 4 HP, all peak.

Just making sure.

The better way would be a conical insert in the exhaust with a spring that could be calibrated to allow the exhaust to push the cone out of the way as exhaust pressure increased.

Tony, your input kind of proves what Mazda Maniac is saying. The higher frequency (= velocity) of keeping the exhaust on, helps the low end (not for back pressure). And well designed exhausts, even though not taking the exhaust entirely off, will create a vacuum at higher rpm. Thank you for the input.

Don't you love when things start to sink in?

How big are the exhaust ports from the engine/headers? And I don't think, from what I have learned so far on this board about the intake it matters so much if this 'valve' is on the headers or not. It is like an intrument. You should be able to tune it at any reasonable point. Right?

Originally posted by MazdaManiac
Just making sure.

The better way would be a conical insert in the exhaust with a spring that could be calibrated to allow the exhaust to push the cone out of the way as exhaust pressure increased.

Kind of what I am thinking, but the spring would have to be made of some crazy material to withstand the heat, otherwise, it'd become maluable after a long time of being exposed to high heat levels for extended periods of time. This is why it may be a better idea to move it further away from the engine.

Well, the EGTs in the RX-8 sit around 1600°F most of the time, but they can creep up to 1800°F. I've seen them go as high as 2000°F on mine.
That is too hot for aluminum, but a stainless steel spring would do just fine.

You would put it at the muffler end, anyway.

I'd imagine a two muffler (probably straight through) setup. If you ever seen the RX-8 store exhaust, this is the image in my head. With the valve on one side of the valve split. The sizing of the piping...no clue!

So who wants to be Bill Nye on this?

Originally posted by MazdaManiac
Just making sure.

The better way would be a conical insert in the exhaust with a spring that could be calibrated to allow the exhaust to push the cone out of the way as exhaust pressure increased.

I was under the impression that was how "blow off valves" worked. Just a spring that is set to a certain pressure... etc. Now that i think about it, we'd have to factor in how much flow the valve blocked... ew.

Anyway, as far as who should play Bill Nye, I wish Rotarygod would chime in. He was already pushing, from what I understand of his posts, for something very similar to this with the headers (which is why I even mentioned this in the first place).

I reckon the valve could be at the muffler, but we'd have to make the pipe all the way down hold the volume of air that would create the magic vacuum at the highest RPM, but have the part that the valve splits off of smaller so that it would create a bottle neck and only let the correct volume of air for lower RPMs through, if that would even work.

If it wouldn't (I don't know if it is volume in the tube, which would cause the muffler placement to fail, or volume released from the tube, which would allow the muffler placement to work, that make the magical vacuum happen), then we would have to split at the headers and have two pipes all the way down.

Many principles raised fail to show benefits on rotary exhaust systems for two major reasons. First, in order for scavenging to have any benefit there must be sufficient overlap between the intake cycle and exhaust cycle. The Renesis has what Mazda is calling "no overlap". Also, scavenging is best used in piston engines because the combustion chamber has available volume in which we may create a vacuum at certain rpm's. This sets up the intake cycle for a stronger pulse when the intake valve opens. Rotaries, on the other hand, have very little c.c. volume in which to create a vacuum. Some avatars have a graphic representation of the rotor's movement within the housing, so we can see that as the rotor orbits there is very little volume left as the rotor completes its' exhaust cycle. Second, rotaries don't experience the reversion effect that piston engines have because they don't have valves closing which cause a reverse pulse to travel back through the exhaust pipe. If they do have any reversion it pales in comparison to piston engines

Charles

Charles,

Was your post directed to Mazda Maniac or posts so far from everyone?

FTL

just wondering with all these discussions, what are your thoughts about exhaust modification available in the market? are they cost efficient or not? (wanted to get your opinion)

Originally posted by tRiX8
just wondering with all these discussions, what are your thoughts about exhaust modification available in the market? are they cost efficient or not? (wanted to get your opinion)

The best claim is 10 HP on an exhaust. Most people probably aren't seeing that much on their cars. I've decided to upgrade my suspension while the exhaust situation gets worked out. If it turns out there is no way to get more than 10 HP with a full exhaust, I'll decide how much a better sounding exhaust is worth to me.

The flywheel and ECU upgrades are the best bang for the buck as far as the '8 is concerned. I might get to those next.

Originally posted by Charles R. Hill
Many principles raised fail to show benefits on rotary exhaust systems for two major reasons. First, in order for scavenging to have any benefit there must be sufficient overlap between the intake cycle and exhaust cycle. The Renesis has what Mazda is calling "no overlap". Also, scavenging is best used in piston engines because the combustion chamber has available volume in which we may create a vacuum at certain rpm's. This sets up the intake cycle for a stronger pulse when the intake valve opens. Rotaries, on the other hand, have very little c.c. volume in which to create a vacuum. Some avatars have a graphic representation of the rotor's movement within the housing, so we can see that as the rotor orbits there is very little volume left as the rotor completes its' exhaust cycle. Second, rotaries don't experience the reversion effect that piston engines have because they don't have valves closing which cause a reverse pulse to travel back through the exhaust pipe. If they do have any reversion it pales in comparison to piston engines

Charles

well that would be scavenging inside the combustion chamber, which isn't to be confused with scavenging from resonant tuning.

anyway, the "smaller volume" arguement doesn't work, it's the same volume as at TDC (which on a piston engine is where the scavenging happens), and would have the same effect as a piston engine's head of the same compression ratio (in the 13BMSP's case, 10:1).

i'm not sure about the reversion issue, either.

Originally posted by robertdot
The best claim is 10 HP on an exhaust. Most people probably aren't seeing that much on their cars. I've decided to upgrade my suspension while the exhaust situation gets worked out. If it turns out there is no way to get more than 10 HP with a full exhaust, I'll decide how much a better sounding exhaust is worth to me.

no, on it's own i couldn't imagine a straight-through exhaust giving you 10bhp considering that at present factory tune the engine isn't even close to its mechanical potential.

but, with some significant amount of enhancement a lower restriction exhaust could offer a bigger benefit. on its won, yeah, it's just down to sound, and maybe flame-throwing capability.

Originally posted by wakeech
... it's just down to sound, and maybe flame-throwing capability.

My dad used to tell me stories about mounting spark plugs on the far end of the muffler, then having a switch to put power to them. Apparently there was enough un-burnt fuel to make some really cool flames. But this was back before the 70s... maybe even before the 60s...

I wonder how well this would work for the '8? Might be a fun project...

Most of those systems re-route the spark from the engine spark plugs to the rear plugs. So you rev the motor, flip the switch, and as the motor revs down, flame thrower! But that's just what someone told me, I'm not sure if that's true.

Originally posted by wakeech
considering that at present factory tune the engine isn't even close to its mechanical potential.

We get all these mixed signals from this board. First Mazda did such a great job tuning the 8 that it is really hard to anything extra to it. Then you go and say something like that! How do we bring out this 'mechanical petential?' Do we open up our hoods and give the engine a pep talk?

Originally posted by Ft Liquordale
We get all these mixed signals from this board. First Mazda did such a great job tuning the 8 that it is really hard to anything extra to it. Then you go and say something like that! How do we bring out this 'mechanical petential?' Do we open up our hoods and give the engine a pep talk?

They didn't do "such a great job" as to make it hard, they just made it very complex whcih makes it difficult.
The tuning is designed to be comfortable and get good emissions. It does this by use of highly convoluted logic and feedback systems.

Don't open your hood and give it a "pep talk". Give it a Greddy E-Manage.

Alright guys, I'll do my best to explain what I know about back pressure, but I could be wrong.

Overall, what I've heard, is that back pressure isn't really a problem on certain engines. In general, low revving torquey engines like a pushrod V-8 will actually loose some low end torque if too much back pressure is lost. The reason for this is that the back pressure is actually needed to help get in the incoming fuel and air mixture in the combustion chamber, as the valves overlap. The result of decreasing back pressure in some pushrods, is that some of the incoming air and fuel actually exists the combustion chamber before detonation, leaving it lean, and with less oxygen molecules to begin with.

The plus side to having less back pressure is when the revs are getting up there, the exhaust has a quicker exit. So higher revving cars would benefit from such. Variable valve timing, like V-TEC and such, would benefit the most probably, as there is less overlap at lower RPM's, then those that don't have variable valve timing. Overall, it depends on the engine.

As for rotaries, though I don't know much about backpressure with them, it seems as though having the least amount as possible would be best, as there doesn't seem to be any overlap between the intake and exhaust.

Feel free to correct me on something if you think differently. Hope this helped to some extent.

I guess I'll chime in on this one. MazdaManiac had a perfect quote on the first page of how everything works. There are so many people that still claim that backpressure is important for low end power. This couldn't be farther from the truth. As in MM's quote, the key is to not outflow your requirements. This has nothing to do with adding pressure. Show me a race car with backpressure and I'll show you a loser! If backpressure is important at one rpm, it has to be important at others as well. It is an all or nothing phenomenon. In a perfect world, we would have a vacuum in the exhaust.

I'm just going to throw out some random make believe numbers here for examples sake. If our exhaust only needs to flow 100 cfm of air at 3000 rpm, how is making a larger exhaust that flows more going to benefit at this rpm? It isn't but we need a higher flowing exhaust if we intend to make power above this point. When people say they need backpressure to get low end power, they are confusing this with making the system only flow so well. While this does meet the cfm needs of this rpm, it also adds pressure. They'd make far more power if the exhaust flowed the same amount with no backpressure. In a perfect world, our exhaust diameter would start out very small at low rpms and then progressively get largers in area as rpm's rise. This would keep constant pressure and velocity per rpm. Too bad we don't currently have the technology to do this.

Here's an idea for you guys that like to spend your evenings drawing new exhaust system designs on paper instead of sleeping. Run a dual pipe exhaust through dual mufflers. Both pipes split right after the header/exhaust manifold. Install a valve in one pipe. Trigger the valve to open at a dyno determined rpm. At lower rpm's, air only flows through one side. At the swithover point, the valve opens and adds the other sides area into it's own. Simple really. Doing it this way would minimize backpressure and keep velocity high. It is definitely much better than closing it off farther down the line with a larger area pipe feeding it. This is bad for velocity.

My header design does not change anything at any point. It is designed to trick the center port into resonating the same as the outer ports giving us the same acoustic phasing and tuning as the outer ports. This would actually make a worthwhile header that can actually be tuned to make a difference.

Scavenging deals with how well the exhaust velocity pulls the chamber clean which in turn pulls more air into the engine from the intake side. This can only happen with port overlap. The Renesis has 6 degrees of dwell (no overlap) so this can't happen. Our goal should be to get each rotor's gasses moving so efficiently down the pipes that they suck out gasses from the other rotor. This is exhaust scavenging. This can and is also affected by acoustic pressure waves. The less pressure we can leave in the combustion chamber itself when the port closes, the more intake air will get into it. So this basically rebuts what I just said and makes it possible to somewhat scavenge the incoming air even though there is no overlap. However the effect is not as great. If we could leave a vacuum in the chamber (we can't), when the intake port opens, the vacuum would suck air down the intake runners. The faster they start moving, the easier tey stay moving right up to the point where they are forcing themselves into the engine until the port is totally closed. This is getting a little too technical though so lets just stick on topic and say we need those gasses moving as fast as we can get them moving while at the same time keeping as little backpressure as we can. It is a balancing act.

Just to summarize this threads good points remember this. Backpressure is never important. We just don't want the pipe to be so large as to outflow our needs. This is difficult with a fixed pipe size which is why the wrong crowd thinks they are right. I should have just typed this to begin with so I could have saved 20 minutes of typing!

Are you sure this applies everywhere? Or does that all just apply to rotary engines. Back pressure is needed to some degree when you have a great deal of overlap with non-phasing cams I think. Please read my reply above, and tell me if it's right or not. I thought that was how it worked, but not sure.

no, seriously man.

backpressure, that's a pressure. pressure over an area (like, say, the diameter of your exhaust piping) is a certain force. force going back into the engine = bad.

i just wrote a thinger on this on performancescene.net the other day, read the first post (http://www.performancescene.net/forum/showthread.php?s=&threadid=134) by Mackenzie71, and then the second last one by me (http://www.performancescene.net/forum/showthread.php?s=&threadid=134&perpage=15&postid=25078).

backpressure is bad, what everyone who says backpressure is thinking is static pressure (which is the pressure of the gas in the pipe, which goes to determine its velocity, which determines the dynamic pressure... the lower the dynamic pressure the better, etc etc).

Originally posted by Ft Liquordale
We get all these mixed signals from this board. First Mazda did such a great job tuning the 8 that it is really hard to anything extra to it. Then you go and say something like that! How do we bring out this 'mechanical petential?' Do we open up our hoods and give the engine a pep talk?

yeah, i'll go on MM's tangent.

it isn't that they maximized the stock potential, really, but certainly the intake system and exhaust piping aren't the biggest restrictions on this car (it's well known that it's the tune of the engine, to save the cats). so "fix" that, and you're up some 20-40rwhp, just changing the fuel maps. so the engine isn't yet outstripping the exhaust's capabilities yet, so swapping it out for a different one isn't going to accomplish you very much.

and then, well, as soon as you get into serious modification (like, taking the lightest rotors ever made for a 13B block and make them lighter, give the intake side a perhipheral port, crank the redline up a few more thousand... well, you get the idea) then it's again an entirely other ballgame. and no, just about every racing body that allows the wankel to mingle with the piston hogties engine builders, 'cause of the nearly unbelievable power potential for the size of these motors.

Originally posted by rotarygod
Run a dual pipe exhaust through dual mufflers. Both pipes split right after the header/exhaust manifold. Install a valve in one pipe. Trigger the valve to open at a dyno determined rpm. At lower rpm's, air only flows through one side. At the swithover point, the valve opens and adds the other sides area into it's own. Simple really. Doing it this way would minimize backpressure and keep velocity high. It is definitely much better than closing it off farther down the line with a larger area pipe feeding it. This is bad for velocity.

yeah, i think we've all thought of this one before. the issue that i couldn't figure out was how the hell you valve it, and then (even worse) how to keep it sealed, nevermind how you'd design a method of contruction by which the welding wouldn't ruin your nifty little contraption.

Scavenging on a piston engine happens whenever the intake and exhaust valves are open at the same time. On a piston engine the lobe center angle combined with overall duration(camshaft specs) determines total overlap. Since Mazda designed the Renesis to be emissions-compliant, they have said the Renesis has no overlap. Until we do porting jobs on our rotaries the benefits are limited. Scavenging from resonant tuning is the same as considering reverson pulses, in concept. My impression on the c.c. argument is that the volume eventually reduces to near-zero because the apex sweeps past the exhaust port. If I am wrong, so be it. Either way, when my header is finally finished I guarantee it will look and perform like no other yet available. If not I will not release it.

Charles

I apologize if my post seems redundant. I failed to fully read all of the previous posts. I will say this; everyone on this thread is carrying the conversation to stay on-topic, which is cool. No bitchin' going on, either. Very cool. These attributes are specific to rotary owners aren't they? You guys rock.

Charles

i might only be 17 but i think MazdaManiac (http://www.rx8club.com/member.php?u=8162) has the right idea..
i've done alot of searching on this topic trying to figure out the best arguement for my friend who believes backpressure is need for a vehicle to operate. i dont believe backpresure is a good thing at all and if i had it my way it would be gone all together. one of the main reasons i believe this is if you've ever looked at an old skool hot rod alot of them have exhuast that run along the side with no muffler or cat. this leads me to believe that there is no backpressure in these cars at all seeming that the pipes are so short. not to mension, you have to believe if they've figured out a way to compress air and force it into an engine (turbo charger), they've found a way to suck the air back out through the exhuast pipe (as mazda maniac explained). I do believe on the other hand motor cycles and dirt bikes need back pressure cause they have a different kinda of system goin on in them (i think).
all the hard facts seem to point towards "you dont need back pressure", but thats just my opinion...

nice grave digging. and you're right, MM, BHR, RG, and others did have the right idea... 6+ years ago. this isnt a topic that is any longer debated or discussed. most people take this for granted as common knowledge

the principles applied to motorcycles are no different than any other internal combustion engine.

Backpressure is never a good thing. What people commonly think of as backpressure at work is really higher exhaust gas velocity. Once it gets too fast though power starts to rapidly fall off. "Backpressure" is really an inaccurate misused term anyways.

Just because you have a muffler does not mean there is going to be excessive exhaust pressure. You can also have a very quiet exhaust that doesn't functionally effect exhaust flow.

I am getting closer to the possible REAL answer on this forum. This is what I have found so far. Buku information direct from the people that designed and built the RENESIS engine. Follow the link. The references listed can be found by title name in pdf format thru Google.

http://papers.sae.org/2004-01-1790

It may not be the exact answer we are looking for, but I think I am headed in the right direction. Please consume a LOT of your favorite beverage. Some of this stuff is dry as hell.

Here is another link pertaining to a patent on intake and exhaust

http://www.patentstorm.us/patents/3981276/description.html

I do believe on the other hand motor cycles and dirt bikes need back pressure cause they have a different kinda of system goin on in them (i think).

Yes, you are correct. I own a 2006 Kawasaki ZX-6R 636 that has a mechanical servo damper in the under tail exhaust muffler. It is restrictive at lower RPM and open at higher RPM. Kawasaki states that it improves lower RPM torque and throttle response. Granted, you cannot compare a 14,000 RPM inline 4 cylinder piston engine to a 9,000 RPM rotary engine.

There is a common misconception that engines need backpressure in order to run properly, generate low end torque, etc. That is simply untrue. Backpressure is a bad thing. Always. Take a look at a top fuel dragster...how much backpressure do you think those zoomie headers make? Very little, and those engines produce 2000+ hp.

So, what is backpressure? Any fluid flowing through a pipe experiences drag on the walls of the pipe. This depends on a number of factors, including the diameter of the pipe, the smoothness of the inside of the pipe, the viscosity of the fluid, and the velocity of the fluid. This drag results in a pressure drop through the pipe. In order for the fluid to flow at all, the pressure on one end of the pipe must be higher than at the other. In an exhaust system, that pressure drop is what we refer to as backpressure. It's pretty obvious that the engine has to produce this pressure differential, so the less power it has to spend making pressure to push the exhaust out, the more power it can send to the wheels.

Given that exhaust pipes are pretty smooth, and that we can't change the viscosity (thickness) of the waste gas being forced through the pipes, we are left with basically 2 parameters we can have any control over: The pipe diameter and the gas velocity.

Unfortunately, the pipe diameter controls the gas velocity since the volume of gas is prescribed by the engine. So, we really only have one thing we can change. So, bigger pipes allow less pressure drop for a given volume of gas because the velocity is lower. The pressure drop (backpressure increase) is proportional the gas velocity squared, so if I double the gas velocity (by reducing the cross sectional area of the exhaust pipe by half) then I quadruple the pressure drop.

Well, there's an easy solution for that: Just make the exhaust pipe bigger. Bigger pipe, lower gas velocity, less pressure drop, so less backpressure. Wow, that was easy. After all, this is the way it's done for basically any type of commercial plumbing system. Need less pressure on a chilled water pipe or a natural gas line? Just make the pipe bigger.

But wait, there's a problem....Having a huge exhaust pipe has killed my low end torque!!! What's different? Oh, there's no backpressure!! Therefore backpressure makes torque!

Wrong.

An exhaust system is different than just about any other plumbing situation. How? Because the flow is pulsed, and this turns out to be a big deal. Every time a pulse of exhaust gas runs through the pipe, a strange thing happens as it passes, it has a little area of vacuum behind it.

Well, how big the vacuum behind each pules is depends on the gas velocity. The higher the velocity, the bigger the vacuum the pulse has behind it.

Now, this means that I can "draft" the next pulse, just like in NASCAR. In NASCAR, it's called drafting, in an exhaust system, it's called scavenging. You've probably seen this term used when talking about headers, but the same concept applies in the pipe.

I get the maximum scavenging effect if the gas velocity is high, so the pipe needs to be small. By maximizing the scavenging effect, I help to pull pulses out of the combustion chamber, which means the engine doesn't have to work as hard to do that.

This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished.

So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart).

So, the loss of torque has nothing to do with backpressure, and everything to do with gas velocity. So you need exhaust components that are not restricive (manifolds/headers, mufflers) and that are sized correctly for your application.

To further dispel the "backpressure is necessary" theory, try this if you want. If you have access to a vehicle with open headers, make a block off plate that will bolt to the collector. This plate should have only a 1" hole in it for the exhaust to flow through. That will give you PLENTY of backpressure, and zero scavenging. Then you can report back on how much low end power it has.

The one exception to sizing an exhaust is for turbo cars. Since the turbo is in the exaust stream, the gas flow spinning the impeller tends to come out of the turbo with the pulses greatly diminished. In this case, you can get away with running a larger pipe than on an equivalent HP N/A engine because you can't take as much advantage of the scavenging effect.

If I am totally off the mark on the following, constructive feedback is appreciated.

Decreasing back pressure will increase "flow". (I am using that loosely. More to follow.). The o2 sensors are telling the ECU that a lean environment is existing and the engine needs more fuel. Stock configurations can only supply so much fuel. Stock configuration will reach the tipping point when the oem injectors can only supply so much fuel. Once that point is reached, then lean means heat. Too lean means too hot.

Solution? That is up to the "tuners". A balance between increased fuel injection capability, increased intake air flow, and decreased back pressure will allow for more something, either HP, Torque, or wasted money outta your pocket. So my two cents worth is you cannot JUST decrease back pressure because you want to. There is a give and take depending on how you are going to run your RENESIS.

Hope I am not way off the mark. I aint no roket sientist... LOL

An exhaust system is different than just about any other plumbing situation. How? Because the flow is pulsed, and this turns out to be a big deal. Every time a pulse of exhaust gas runs through the pipe, a strange thing happens as it passes, it has a little area of vacuum behind it. Well, how big the vacuum behind each pules is depends on the gas velocity. The higher the velocity, the bigger the vacuum the pulse has behind it. Now, this means that I can "draft" the next pulse, just like in NASCAR. In NASCAR, it's called drafting, in an exhaust system, it's called scavenging. You've probably seen this term used when talking about headers, but the same concept applies in the pipe. I get the maximum scavenging effect if the gas velocity is high, so the pipe needs to be small. By maximizing the scavenging effect, I help to pull pulses out of the combustion chamber, which means the engine doesn't have to work as hard to do that. This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished.

So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart). To further dispel the "backpressure is necessary" theory, try this if you want. If you have access to a vehicle with open headers, make a block off plate that will bolt to the collector. This plate should have only a 1" hole in it for the exhaust to flow through. That will give you PLENTY of backpressure, and zero scavenging. Then you can report back on how much low end power it has.

I am a little bit familiar with what you stated above. There is an explanation of this on the following link. (I hate to keep linking pages. Saves me explaining all this again.)

http://forum.concours.org/index.php?topic=45090

But does this same theory apply to a rotary engine?

Totally different beast. Especially since there was major mods from the ported exhaust (Rx7) to the side exhaust (RX8)

Not trying to contradict you. I am a mechanic of sorts learning as much as I can about rotary engines.

We're getting deep into this.

I guess I can say that this applies to any internal combustion engine regardless of the design. As long as the engine has to displace exhaust gases.

I agree Easy, this is getting really deep.

For the masses, let me add this. Referencing the link from SAE, Mazda did an awsome job making the emissions California compliant. (Leave it up to Cali. Damn near everything causes cancer there for some reason.) A lot of research and design make the RX8 deliver the most hp and torque while still passing emissions. For those of us who want to get that extra juice, discussions like these open up questions that will eventually get answered.

My dad used to tell me stories about mounting spark plugs on the far end of the muffler, then having a switch to put power to them. Apparently there was enough un-burnt fuel to make some really cool flames. But this was back before the 70s... maybe even before the 60s...

I wonder how well this would work for the '8? Might be a fun project...


the reason that worked so well is because back then they were carbureted. most people back then would run a choke cable into the cab to make them run rich, which in turn would cause them to have large amounts of unburnt fuel. fallowed by the ignition from the spark plug. BLAM. flames

My dad used to tell me stories about mounting spark plugs on the far end of the muffler, then having a switch to put power to them. Apparently there was enough un-burnt fuel to make some really cool flames. But this was back before the 70s... maybe even before the 60s...

I wonder how well this would work for the '8? Might be a fun project...

Uh, who needs plugs, many of us already throw steady fire or fire balls...

rote8 makes a very good point :)

sure, if you ignore the zero overlap reality, pulling a vacuum against a closed small chamber and a flow path are two entirely different things

try porting the hole in your head to relieve the excessive cranial backpressure :lol:

Back pressure is never a good thing, but the myth comes from people that make the mistake of modifying their exhausts in a way that it would mess up with the resonance tuning of the OEM exhaust (in case it existed in the first place), which would cause overall performance to drop because the OEM system used resonance tuning to have the best scavenging effect with the REQUIRED muffler and cat's installed so that it made the best power under those conditions.

But there is no denying that a well designed and tunned free flowing exhaust is going to produce more power than a restricted system. There are a lot of factor that contribute to a good exhaust, like gas velocity, length and diameter of the headers, design of the collector, if its a 4-1 or 4-2-1 system etc... too much to explain for the sake of this argument but if you're is interested there's a book I read this past summer that explains all this in depth, its called Scientific Design of Exhaust & Intake Systems by Philip H. Smith and John C. Morrison. its a really interesting read if anyone is interested.

I want to make this clear that I'm re-posting this from a local automotive forum. I'm curious what the rotary community has to add to this.

Back pressure, Exhaust velocity and scavenging.
The myth: “engines need some backpressure.”

One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what its consequences are. I'm sure many of you have heard or read the phrase "engines need some backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

How the myth came about:

It is easy to see how this misunderstanding arises. Lets’ say that Max puts a 3-inch system on his normally aspirated car. He soon realizes that he has lost power right through the power band. The connection is made in his throbbing brain….

Put on 3" pipe = loss of backpressure = loss of power.

Max erroneously concludes that you need backpressure to retain performance. He has ignored the need for exhaust gas velocity to get that scavenge effect.

The other myth: “engines can get burned valves from not enough backpressure”


How this myth came about:

The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust for his.

Once these vehicles received performance mods that reduced backpressure, they tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

Some basic exhaust theory

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficiently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle; a 6 cylinder has 6 pulses and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

Backpressure and velocity.

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much, much, faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity.

Backpressure in its most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your power band is located 2-3000 RPM you'd want a narrower pipe than if your power band is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

So why is exhaust velocity so important?

The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).

Conclusion.

SO it turns out that engines don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.


Cited from various diffrent website.