Header theory:
09-20-2012, 10:14 AM
#326
Chapter 5 of "Street Rotary: How to Build Maximum Horsepower & Reliability Into Mazda's 12A, 13B, and Renesis Engines" by Mark Warner has a good simple explanation wrt some of this. You might be able to read it online with this search: Street Rotary: How to Build Maximum Horsepower & Reliability Into Mazda's ... - Mark Warner - Google Books
High gas velocity is helpful in encouraging inertial scavaging. Getting high velocity by reducing pipe diameter, increases overall resistance to flow (back pressure). If you put your thumb over most of the end of a garden hose, what happens? Velocity goes up, flow goes down. It is only if one can take advantage of the inertial pulse characteristics that smaller pipes are of any advantage.
Supercharging puts more mass into the combustion chamber to burn and exhaust. Without overlap, it's not blowing out any exhaust gas. Most of the mass of exhaust is *not* pushed out by the rotor (or piston). It's because P = nRT/V where the temperature has gone up substantially after burning. Open the exhaust port, and the hot gas starts flowing out without any help from rotor squeezing.
A chart from that book shows that a reduction of mean back pressure by a factor of 6 for a PP rotary increases power by a factor of 1.85; for a side-port that factor is 1.14 ! This makes perfect sense. Ignoring some temperature effects and possible effects of "bad" resonance, a 10:1 compression motor is at worst going to leave 10% of the exhaust in the chamber for the next fill, leaving 90% of the intake 'space' available for fresh air/fuel. If one could pull a good vacuum (zero pressure, absolute) on the exhaust stroke, then 100% would be available. 100/90 = 1.11 , ie, no surprise.
There's just not a lot to be gained.
High gas velocity is helpful in encouraging inertial scavaging. Getting high velocity by reducing pipe diameter, increases overall resistance to flow (back pressure). If you put your thumb over most of the end of a garden hose, what happens? Velocity goes up, flow goes down. It is only if one can take advantage of the inertial pulse characteristics that smaller pipes are of any advantage.
Supercharging puts more mass into the combustion chamber to burn and exhaust. Without overlap, it's not blowing out any exhaust gas. Most of the mass of exhaust is *not* pushed out by the rotor (or piston). It's because P = nRT/V where the temperature has gone up substantially after burning. Open the exhaust port, and the hot gas starts flowing out without any help from rotor squeezing.
A chart from that book shows that a reduction of mean back pressure by a factor of 6 for a PP rotary increases power by a factor of 1.85; for a side-port that factor is 1.14 ! This makes perfect sense. Ignoring some temperature effects and possible effects of "bad" resonance, a 10:1 compression motor is at worst going to leave 10% of the exhaust in the chamber for the next fill, leaving 90% of the intake 'space' available for fresh air/fuel. If one could pull a good vacuum (zero pressure, absolute) on the exhaust stroke, then 100% would be available. 100/90 = 1.11 , ie, no surprise.
There's just not a lot to be gained.
09-20-2012, 10:26 AM
#327
How do you reduce reversion? Is reversion having a negative affect on our engine? Again I am not sure because how do you determine exhaust reversion is occuring on this engine? Measure BSFE/VE versus torque? The normal signs and symptoms will not be there due to the no overlap.
09-20-2012, 03:46 PM
#329
ok i am following--but isnt exhaust gas actually pushed out by the higher pressure inside the combustion chamber caused in part by the higher temperatures, but also when the pressures between the chamber/port/header start to equalize then the rotor begins to push most of what is left out? Dont forget my sc also causes a pressure increase on its own.
No overlap is no overlap. There is nothing we can do about that without porting.
So the way I figure it, is to focus on backpressure reduction. Dont increase the pipes size to much and dont over cool the exhaust as both of these will help create back pressure by reducing velocity. It is obvious that you keep the system as a straight and smooth as possible. Big balancing act.
You have to remember that headers actually slow exhaust gas velocity. Their primary pipe cross section and collector area are larger than the oem manifolds/transitions and anytime there is an increase in pipe size the velocity slows and the gases cool. The header design also allows for increase cooling of the exhaust gases by having a larger surface area and being unshielded etc.
A longer tube header on our car may be beneficial by delaying the transition to the collector ?
Putting an exhaust system together for our engine is not a simple task.
I just think that exhaust wave tuning will not be of any use on this engine.
I still say "ZOOMIES"!!--lol--I wish. Wait --no I dont--couldnt drive the thing...
No overlap is no overlap. There is nothing we can do about that without porting.
So the way I figure it, is to focus on backpressure reduction. Dont increase the pipes size to much and dont over cool the exhaust as both of these will help create back pressure by reducing velocity. It is obvious that you keep the system as a straight and smooth as possible. Big balancing act.
You have to remember that headers actually slow exhaust gas velocity. Their primary pipe cross section and collector area are larger than the oem manifolds/transitions and anytime there is an increase in pipe size the velocity slows and the gases cool. The header design also allows for increase cooling of the exhaust gases by having a larger surface area and being unshielded etc.
A longer tube header on our car may be beneficial by delaying the transition to the collector ?
Putting an exhaust system together for our engine is not a simple task.
I just think that exhaust wave tuning will not be of any use on this engine.
I still say "ZOOMIES"!!--lol--I wish. Wait --no I dont--couldnt drive the thing...
09-20-2012, 04:54 PM
#331
Registered
iTrader: (2)
Join Date: Oct 2007
Location: Pacific Northwest
Posts: 16,684
Likes: 0
Received 240 Likes
on
110 Posts
OD, Something to think about:
You will always have a psi reading on the exhaust, because the exhaust will always be moving. Reducing the pressure of the exhaust will mean slowing it down. Unless you are looking for a way to slow down the exhaust while keeping it hot, I don't think cooling it is necessarily a bad thing if pressure drops? Sure, hotter means faster, but it also means more pressure.
It would be interesting to see what kind of power/pressure is possible from a ludicrously large diameter pipe, to see if the cooling you are trying to avoid is actually a bad thing.
You will always have a psi reading on the exhaust, because the exhaust will always be moving. Reducing the pressure of the exhaust will mean slowing it down. Unless you are looking for a way to slow down the exhaust while keeping it hot, I don't think cooling it is necessarily a bad thing if pressure drops? Sure, hotter means faster, but it also means more pressure.
It would be interesting to see what kind of power/pressure is possible from a ludicrously large diameter pipe, to see if the cooling you are trying to avoid is actually a bad thing.
09-20-2012, 09:15 PM
#332
i had one engine that was so low on compression I dont know if the exhaust was moving or not.......
But, I agree with you that there will always be some pressure in a street exhaust system.
I agree that velocity is important, I also agree that volume is also. That is where the experimentation needs to focus on concerning the exhaust improvements for this engine imho. Forget pressure waves and harmonics. Just get the gas exhaust OUT.
I am really thinking about the Smokey trick of trying to find a better low pressure spot in which to end the tailpipe. It may be a challenge to find a better one than oem.
you could always run a large oval shaped exhaust pipe if you wanted to get really big
But, I agree with you that there will always be some pressure in a street exhaust system.
I agree that velocity is important, I also agree that volume is also. That is where the experimentation needs to focus on concerning the exhaust improvements for this engine imho. Forget pressure waves and harmonics. Just get the gas exhaust OUT.
I am really thinking about the Smokey trick of trying to find a better low pressure spot in which to end the tailpipe. It may be a challenge to find a better one than oem.
you could always run a large oval shaped exhaust pipe if you wanted to get really big
09-21-2012, 12:11 PM
#337
For fooling around purposes, the only hope of sorting resinance things out is with pressure transducers in the exhaust and a recording scope triggering on ignition events. Mazda does provide a unique opportunity with out engines though to get pressure readings very close to the port openings --> the air injection ports.
A very slow transition in diameter from pipe to exhaust tip would aid in killing off reversion waves generated by the tip, though probably the more important ones come from the collector area. Never merging the center port into the other two would probably clean up intertial scavanging from the end ports.
If I could get a TeamRX8 style log for a reasonable price, I'd probably go in that direction.
YMMV
09-21-2012, 12:39 PM
#338
Registered
iTrader: (2)
Join Date: Oct 2007
Location: Pacific Northwest
Posts: 16,684
Likes: 0
Received 240 Likes
on
110 Posts
My comment was targeted toward OD's comment of putting his tips in a low pressure area created by vehicle movement through the air. If he wanted to he could probably create a very low pressure area somewhere near the back of the car to put the tips in, probably lower pressure than what currently exists.
Will it help? You are smarter than me there.
Will it help?
You are smarter than me there.
09-22-2012, 08:36 AM
#339
i am asking for a 12 degree merge with straight 3 inch transitional piece using v bands/OR a more traditional shaped flange ( so i can get good gaskets).
This is all in the works--just waiting for it to arrive so i can get the header/collector/clamps or flanges and midpipe modified/installed. I also will have to add the o2 bungs. I have decided to add 12 inches of length to each primary pipe.
I am still up in the air about what to do for a cat back. The best will be a single--but so far I dont like the sound of any single I have heard.
That siamese port sure causes some head scratching
. I, too, have wondered about the velocity from the centerport. I do think that a good log manifold may have some advantages in regard to the centerport. That being, a slower dumping port ( because of the diameter change) entering a faster flowing stream of gas from the front port would be "pulled" into the stream more? IDK for sure.
I do wonder what volume/mass difference there is between the center versus the others?
Cross sectional measures cannot really define what is actually flowing through that port.
I do wonder what this engine would sound like with a dedicated centerport exhaust pipe with only a small resonator on it? HMMMMM
This is all in the works--just waiting for it to arrive so i can get the header/collector/clamps or flanges and midpipe modified/installed. I also will have to add the o2 bungs. I have decided to add 12 inches of length to each primary pipe.
I am still up in the air about what to do for a cat back. The best will be a single--but so far I dont like the sound of any single I have heard.
That siamese port sure causes some head scratching
. I, too, have wondered about the velocity from the centerport. I do think that a good log manifold may have some advantages in regard to the centerport. That being, a slower dumping port ( because of the diameter change) entering a faster flowing stream of gas from the front port would be "pulled" into the stream more? IDK for sure. I do wonder what volume/mass difference there is between the center versus the others?
Cross sectional measures cannot really define what is actually flowing through that port.
I do wonder what this engine would sound like with a dedicated centerport exhaust pipe with only a small resonator on it? HMMMMM
09-22-2012, 12:33 PM
#340
Registered
09-22-2012, 08:13 PM
#341
My comment was targeted toward OD's comment of putting his tips in a low pressure area created by vehicle movement through the air. If he wanted to he could probably create a very low pressure area somewhere near the back of the car to put the tips in, probably lower pressure than what currently exists.
Will it help? You are smarter than me there.
Will it help?
You are smarter than me there.
but opinions will vary 
With my trusty rusty tape measure, I measure the frontal area of the car to be about 50 x 65 inches = 3250 square inches. With a Cd ~ 0.30 we'll say then it has an equivalant flat plate area of ~1100 square inches. A 2 psi difference across the area of the plate would result in 2200 lbs of force.
In other words, to get (at best) 15% drop of pressure for the exhaust gas to vent into, the car needs to be in a situation where it's encountering (at least) a ton of drag. (To estimate what the drag is at a given speed, you can work through it yourself like this Aerodynamic Drag - Craig's Website at Backfire.ca ).
With the various realities involved and the fact that the exhaust is on the back end of the car anyway, getting even a 2% lower pressure for the exhaust to vent to would be good, with an even more meaningless gain in hp resulting.
It'd be rather easy to measure pressure gradients at various potential outlet locations if one wished to. Or put it where it looks good; won't make much difference.
09-22-2012, 08:36 PM
#342
OD: I realize you do this for fun, and like to fiddle with stuff. I hope you realize that design-by-eyeball is seldom effective, or at least not in one pass without experiment, measurement, experiment, measurement and so on. You haven't characterized the setup you have now, so how will you know you're successful (unikely on first try) or be able to tell why you weren't after the first try?
Do you understand why exhaust velocity is important? If so, an you don't believe inertial scavanging is important, achieving high exhaust velocity will come at the expense of higher backpressure.
In the for fun category, treat the two end ports like a standard header (ie high velocity gas, smaller pipes), run the center port by itself with a "too-large" pipe for minimal back pressure (since resonant tuning won't work).
Do you understand why exhaust velocity is important? If so, an you don't believe inertial scavanging is important, achieving high exhaust velocity will come at the expense of higher backpressure.
In the for fun category, treat the two end ports like a standard header (ie high velocity gas, smaller pipes), run the center port by itself with a "too-large" pipe for minimal back pressure (since resonant tuning won't work).
09-23-2012, 11:01 AM
#343
Appreciate the analogy HiFlite. You are right I do this for fun. I get very curious about these things. I am in a fairly rotary isolated area. The closest in the know guy is Rick E at Mazmart and even Rick is not that involved with the street performance fi'ed msp engine. he is to busy with the other types.
Cam at Pettit is over 10 hrs away.
So this forum can be a big help.
I think I understand why velocity is an important aspect of exhaust flow--but i am not an engineer.
In order to get the most out on any exhaust system--testing is required.I totally agree with that. But, you do have to start somewhere. Zero overlap does actually make it a little easier in this regard. With overlap the cam etc can mask a poorly performing exhaust system. So that variable doesnt have to be dealt with in our engine.
Like all other peak performing systems --every component has to work together in a complementary way. if I had the proper equipment i would do a full analysis, but I dont and it is cost prohibited to purchase.
So I have to educate myself as best I can and begin somewhere. I am not getting any younger
I believe internal scavenging is not to be totally ignored, but it is not something to sacrifice velocity for--like a lot of headers do.
Perhaps you misunderstood me? Velocity is NOT more important than backpressure! It will end up as a balancing act between the two I think. I have 4psi now at wot from approx 4Krpm onward--which is not that bad, but it can be improved on without sacrificing any significant velocity.
Just like a good performance air intake, the details of the exhaust system can make a big difference. The wrong transitions, uneven matched pipes, to many bends, the wrong mufflers, welding slag in the system, exhaust port transition, curve too soon off the exhaust port etc--it all matters.
I may not be able to dial in the perfect exhaust system for my car--but i can make it better.
Cam at Pettit is over 10 hrs away.
So this forum can be a big help.
I think I understand why velocity is an important aspect of exhaust flow--but i am not an engineer.
In order to get the most out on any exhaust system--testing is required.I totally agree with that. But, you do have to start somewhere. Zero overlap does actually make it a little easier in this regard. With overlap the cam etc can mask a poorly performing exhaust system. So that variable doesnt have to be dealt with in our engine.
Like all other peak performing systems --every component has to work together in a complementary way. if I had the proper equipment i would do a full analysis, but I dont and it is cost prohibited to purchase.
So I have to educate myself as best I can and begin somewhere. I am not getting any younger
I believe internal scavenging is not to be totally ignored, but it is not something to sacrifice velocity for--like a lot of headers do.
Perhaps you misunderstood me? Velocity is NOT more important than backpressure! It will end up as a balancing act between the two I think. I have 4psi now at wot from approx 4Krpm onward--which is not that bad, but it can be improved on without sacrificing any significant velocity.
Just like a good performance air intake, the details of the exhaust system can make a big difference. The wrong transitions, uneven matched pipes, to many bends, the wrong mufflers, welding slag in the system, exhaust port transition, curve too soon off the exhaust port etc--it all matters.
I may not be able to dial in the perfect exhaust system for my car--but i can make it better.
09-29-2012, 10:41 AM
#346
stage one of my "exhaust " build complete. I have the header and extensions, it is hard to get a proper collector--companies are just not that responsive. The no overlap engine stalls them out to head scratching. Then I am put on the "back burner".
Perseverance time.
Perseverance time.
09-30-2012, 11:46 AM
#349
i understand team and I have decided not to even do a custom merge collector. I am trying to make sure that the formed collector I need, will have approx a 5 inch taper. I cant get a straight answer from them.
Its not easy to find a 3 to 1, 1.75 primary tube 3inch outlet, T304 ss, long taper, formed collector.
If anyone has a contact please throw it my way. Burns and spd are of no help so far.
Its not easy to find a 3 to 1, 1.75 primary tube 3inch outlet, T304 ss, long taper, formed collector.
If anyone has a contact please throw it my way. Burns and spd are of no help so far.
09-30-2012, 12:20 PM
#350
Is this what you're look for Denny?
http://www.mandrel-bends.com/catalog...04ss-1990.html
3-1 Merge Collector, Parallel, 1.75" Inlets, 3.00" Outlet, 304SS
Type: 3 to 1 Parallel Merge Collector
Inlet Diameters: 1.75" Inlets Slip Fit
Outlet Diameters: 3.00" Outlet
Material: 304 Stainless Steel
Length: 5"
Angle: 18 Degree
http://www.mandrel-bends.com/catalog...04ss-1990.html
3-1 Merge Collector, Parallel, 1.75" Inlets, 3.00" Outlet, 304SS
Type: 3 to 1 Parallel Merge Collector
Inlet Diameters: 1.75" Inlets Slip Fit
Outlet Diameters: 3.00" Outlet
Material: 304 Stainless Steel
Length: 5"
Angle: 18 Degree