Ft Liquordale

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!

robertdot

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?

MazdaManiac

iTrader: (3)

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:

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.

MazdaManiac

iTrader: (3)

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.

Ft Liquordale

Thank you for the responces so far.

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

Just for ***** 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?

MazdaManiac

iTrader: (3)

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

Red Devil

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?

dcfc3s

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

robertdot

On the intake, there are different channels that open at different times. 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)?

dcfc3s

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

robertdot

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.

Ft Liquordale

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?

MazdaManiac

iTrader: (3)

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.

robertdot

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.

Tony Orlando

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.

MazdaManiac

iTrader: (3)

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.

Ft Liquordale

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?

Ft Liquordale

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.

MazdaManiac

iTrader: (3)

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.

Ft Liquordale

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?

robertdot

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.

Charles R. Hill

iTrader: (7)

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

Ft Liquordale

Charles,

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

FTL

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)