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I follow and understand the principle that the NA motor needs back pressure to help with especially low end power. My M5 has SS headers and 3" pipes solely all the way back to the cans and I do not believe I'm forfieting power.
The car was run and dyno'ed before with the headers and the secondary cats in place and there was no actual difference to the present set up. It was quieter.
My current exhaust is not from any of the known aftermarket companies. It was built by ASR as a system sold with the NOS kits but it is not necessary.
It has been said that the factory headers are not restrictive and on another thread the opposite was said. So where is the confusion? When you look at the stock headers the tubes are pinched in so many places that it would make sense that they would produce restriction.
So what is the consenses on the subject?

Thank you,
M5Ranger...Juan
 

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I find it amusing as to all this talk about back pressure, free flowing etc. Exhaust systems are plenty and many shops talk about free flowing systems and "back pressure." Nobody likes to talk about EGV or Exhaust Gas Velocity, which is what makes power in an exhaust system. Does the M5 need 3" exhaust? No. The exhaust gas is at its hottest coming out of the engine. As the exhaust gas runs downstream towards the exit of the exhaust system it begins to cool. Expanding the diameter of the piping in the rear will cool the exhaust gas even more, making the gas dense and slowing the velocity down. This hurts performance more than help. In some of our GT systems we actually neck down the piping towards the rear of the car in order to maintain exhaust gas temperatures. The testing procedure is extensive, not to mention expensive but then again, someone can go to a muffler shop and have a few pipes welded together for a fraction of the cost compared to a well developed system. Your engine will only flow a certain cfm, given your engines efficiency. There are companies out there that would sell you an exhaust or header, etc. and when you lose power, they bring up "back pressure". In the end, the system should have been designed properly.
 

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I find it amusing as to all this talk about back pressure, free flowing etc. Exhaust systems are plenty and many shops talk about free flowing systems and "back pressure." Nobody likes to talk about EGV or Exhaust Gas Velocity, which is what makes power in an exhaust system. Does the M5 need 3" exhaust? No. The exhaust gas is at its hottest coming out of the engine. As the exhaust gas runs downstream towards the exit of the exhaust system it begins to cool. Expanding the diameter of the piping in the rear will cool the exhaust gas even more, making the gas dense and slowing the velocity down. This hurts performance more than help. In some of our GT systems we actually neck down the piping towards the rear of the car in order to maintain exhaust gas temperatures. The testing procedure is extensive, not to mention expensive but then again, someone can go to a muffler shop and have a few pipes welded together for a fraction of the cost compared to a well developed system. Your engine will only flow a certain cfm, given your engines efficiency. There are companies out there that would sell you an exhaust or header, etc. and when you lose power, they bring up "back pressure". In the end, the system should have been designed properly.
I guess it is the end of the day on the East Coast!!:hihi:

Thank you Eloy! I am not trying to act like an RPI lap dog, but do appreciate the info that is manageable for limited knowledge people. This concept can certainly be verified by other experts as to whether plausible/factual or not! :dunno:
cherrsagai
 

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RPI it would seem that I should go into the amusing business because I amuse alot of people these days.
The comments that you are denigrating have been said to me by more than one reputable tuner. Maybe there are different opinions or schools of thought.
Hell what do I know. I just spend the money.
My M5 runs very strong on motor. Somethings right.
:cheers: M5Ranger
 

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I follow and understand the principle that the NA motor needs back pressure to help with especially low end power. My M5 has SS headers and 3" pipes solely all the way back to the cans and I do not believe I'm forfieting power.
The car was run and dyno'ed before with the headers and the secondary cats in place and there was no actual difference to the present set up. It was quieter.
My current exhaust is not from any of the known aftermarket companies. It was built by ASR as a system sold with the NOS kits but it is not necessary.
It has been said that the factory headers are not restrictive and on another thread the opposite was said. So where is the confusion? When you look at the stock headers the tubes are pinched in so many places that it would make sense that they would produce restriction.
So what is the consenses on the subject?

Thank you,
M5Ranger...Juan
NA engines don't need backpressure. This is an often repeated fallacy. Unfortunately, many shops repeat this myth, making customers believe it. It's really sad how many shops still believe it in the 21st century. Let me state this again: NA engines don't need backpressure.

http://www.magnaflow.com/05news/magazine/05sportc.asp

BACKPRESSURE = TORQUE?
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.

Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.
http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm



All engines need flow volume. At the low end of the RPM range, they need velocity to create power. Smaller piping is a good way to get flow velocity to benefit lower end torque. Backpressure is a quick and dirty way to increase low end torque at the expense of higher end.

Some say that "an engine needs backpressure to work correctly." Is this true?

No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.
The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.
At low RPMs, you need exhaust VELOCITY. At high RPMs, you need exhaust VOLUME.

The best way to do this is to size your exhaust piping properly, and to use the lowest possible backpressure canisters. It's the velocity that helps scavaging: higher the velocity, greater the vacuum.

Backpressure is a shortcut to velocity, but not a great one, as it hampers power through most of the powerband.

Proper exhaust piping sizing + proper muffler design is the best way to maximize total power.


The most ideal setup:

1. Relatively small primary tubing to keep flow velocities high (SBC based musclecars winning NHRA and IHRA competitions are running 1.5" diameter primaries, and they have much more cylindar volume than we do).

2. Serpent based first bend:



3. True merge collector design with 15 degree angle:



4. Tri-Y design to broaden powerband (though ours would be weird, since one would be 3 into 1 and the other 2 into 1).

5. Megaphone with reverse cone after the merge

In terms of the primary length, longer tubes would give better top end, and shorter tubes would give better lower end.

This being said, a setup like this would be EXTREMELY expensive and not that practical considering our space constraints (it's tight under there).



With regard to the M5/M6's specific setup:

The stock headers have a lot of great benefits: equal length primaries, stainless steel, overall great design.

There are some compromises: There are areas where the piping is pressed down, but the overall piping area doesn't change that much. The relatively shorter piping trades some top end power for lower end torque. The attached main cat really robs power.

In terms of aftermarket headers, there are some advantages: mandrel bent with no pressed down sections, true merge collector on SS, stainless steel on SS. There are also some compromises: no longer equal length, bigger primaries slow down flow velocity, longer tubing robs some lower end torque.

In the real world, it's really a balancing act. We're trying to find the best compromise to reach our intended goals. The real question is if the areas where the stock header is pressed down restricts it more than the equal length helps it. From what we've seen, the real problem with the stock headers is that cat.

What we're trying to do is to reach the best compromise for our goals (good power throughout the powerband, keep low end strong since that's where this car needs it the most, stay with stainless steel instead of merely coated mild steel, give people a good inexpensive way to smog their car without spending loads of money, and stay within a reasonable price point). We'll be able to provide info soon on how well these goals were reached.

Anyhow, I've probably written too much and have bored 90% of the people here. So, for those who want to skip it all, at least take this away:

NA ENGINES DON'T NEED BACKPRESSURE!
 

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RPI it would seem that I should go into the amusing business because I amuse alot of people these days.
The comments that you are denigrating have been said to me by more than one reputable tuner. Maybe there are different opinions or schools of thought.
Hell what do I know. I just spend the money.
My M5 runs very strong on motor. Somethings right.
:cheers: M5Ranger
I think tone doesn't come across on the internet too well. Eloy's a bit of a jokester (and sadly, I'm often on the receiving end). Please don't take it the wrong way, he's not putting you down.

Truthfully though, in terms of exhaust flow design, tuners often explain only one part of the overall theory and neglect other, sometimes more important, parts. With only 1/2 of the information, it's easy to latch on to a less than optimal compromise.

Either way though, you do have a very stout car, and that's always a good thing. :M5thumbs:
 

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Discussion Starter · #27 ·
Thank you for the VERY informative post.. I stand corrected.. Exhaust Velocity is what is needed..

What tube sizing then do you replace the stock header cats with which you have found help the exhaust velocity and also help move the volume of air.. ?
 

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Yes, EGV. :)

Me a jokester? Yeah. Not as bad as M5baller though. There are a lot of information or myths that float around the industry and I've seen a lot of it over the past 10 years of being in business. What most companies will do is take or use the information floating around the internet and market products based on those assumptions. We arent about that. :)
 

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Gabe,

I am part of the 10% crowd. Good info!:applause:

As for Eloy being a jokester, he is not very good! ouich Believe me, I don't think he has ever made me laugh!:dunno:

That said, he is very, very honest and a artisan! I would rather hang out with a honest guy than a funny guy who is not honest, any day!lovelove

Now, please get the exhaust done!:hihi:
cherrsagai

Edit: I typed my response before I read Eloy's comments! See, his jokes suck!:1:
 

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Gabe,

I am part of the 10% crowd. Good info!:applause:

As for Eloy being a jokester, he is not very good! ouich Believe me, I don't think he has ever made me laugh!:dunno:

That said, he is very, very honest and a artisan! I would rather hang out with a honest guy than a funny guy who is not honest, any day!lovelove

Now, please get the exhaust done!:hihi:
cherrsagai

Edit: I typed my response before I read Eloy's comments! See, his jokes suck!:1:
Come on man. I have to have made you laugh at least ONCE! You should have known about EGV though. You got a lot of hot gasses coming out of you.
 

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Come on man. I have to have made you laugh at least ONCE! You should have known about EGV though. You got a lot of hot gasses coming out of you.
Why yes he does... He's got more gas than chevron
 

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Why yes he does... He's got more gas than chevron
Oh crap!:eek:oohhh:

The kid finally made me smile!:applause:

Congratulations!:applause: Now go thank Eloy!ouich
cherrsagai
 

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Discussion Starter · #33 ·
Im still gonna just hack my stock cats off the headers and have a straight tube which cones down to the factory size piping put in place..

Can this be done without removing the headers. If not then maybe I will take the RPI route..
 

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Im still gonna just hack my stock cats off the headers and have a straight tube which cones down to the factory size piping put in place..

Can this be done without removing the headers. If not then maybe I will take the RPI route..
The headers are buried in there. I can't possibly see how you're going
to cut the headers without removing them. Look at the fourth pic
on this page for reference:

http://www.evosport.com/product/proddetail.aspx?partnum=EVO.EXH.B60.H01

Also look at the fifth picture on this page:

http://www.m5board.com/vbulletin/showthread.php?t=96799&highlight=dixis+exhaust
 

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The headers are buried in there. I can't possibly see how you're going
to cut the headers without removing them. Look at the fourth pic
on this page for reference:

http://www.evosport.com/product/proddetail.aspx?partnum=EVO.EXH.B60.H01

Also look at the fifth picture on this page:

http://www.m5board.com/vbulletin/showthread.php?t=96799&highlight=dixis+exhaust
Marshall,

Good post!:applause:

The second thread reference with RPI installing a complete Dixis exhaust system says it all in terms of the thought process and follow through that RPI provides. If RPI provides this level of detail and research on a manufactured product for the car. Think for a moment and conclude what type of effort and time goes into a customized product. :M5thumbs:

What RPI is saying, is there is a thorough process and all contingencies are examined before a product is marketed. Some have taken a risky and maybe cavalier attitude that they can go to Joe or Juan's muffler shop and snip and tuck. Yes, via research of products like RPI and others that have been developed, you can replicate and maybe get lucky that it will last the test of time, however, RPI will (I hope) warrant their product and assist when things may need tweaking or re-engineering. That said, there are those mechanically minded, with experience in auto engineering that have competency and comfort in creating their own custom mods.

Just my two (whoring) cents worth.
:cheers:
 

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NA engines don't need backpressure. This is an often repeated fallacy. Unfortunately, many shops repeat this myth, making customers believe it. It's really sad how many shops still believe it in the 21st century. Let me state this again: NA engines don't need backpressure.

http://www.magnaflow.com/05news/magazine/05sportc.asp



http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm



All engines need flow volume. At the low end of the RPM range, they need velocity to create power. Smaller piping is a good way to get flow velocity to benefit lower end torque. Backpressure is a quick and dirty way to increase low end torque at the expense of higher end.



At low RPMs, you need exhaust VELOCITY. At high RPMs, you need exhaust VOLUME.

The best way to do this is to size your exhaust piping properly, and to use the lowest possible backpressure canisters. It's the velocity that helps scavaging: higher the velocity, greater the vacuum.

Backpressure is a shortcut to velocity, but not a great one, as it hampers power through most of the powerband.

Proper exhaust piping sizing + proper muffler design is the best way to maximize total power.


The most ideal setup:

1. Relatively small primary tubing to keep flow velocities high (SBC based musclecars winning NHRA and IHRA competitions are running 1.5" diameter primaries, and they have much more cylindar volume than we do).

2. Serpent based first bend:



3. True merge collector design with 15 degree angle:



4. Tri-Y design to broaden powerband (though ours would be weird, since one would be 3 into 1 and the other 2 into 1).

5. Megaphone with reverse cone after the merge

In terms of the primary length, longer tubes would give better top end, and shorter tubes would give better lower end.

This being said, a setup like this would be EXTREMELY expensive and not that practical considering our space constraints (it's tight under there).



With regard to the M5/M6's specific setup:

The stock headers have a lot of great benefits: equal length primaries, stainless steel, overall great design.

There are some compromises: There are areas where the piping is pressed down, but the overall piping area doesn't change that much. The relatively shorter piping trades some top end power for lower end torque. The attached main cat really robs power.

In terms of aftermarket headers, there are some advantages: mandrel bent with no pressed down sections, true merge collector on SS, stainless steel on SS. There are also some compromises: no longer equal length, bigger primaries slow down flow velocity, longer tubing robs some lower end torque.

In the real world, it's really a balancing act. We're trying to find the best compromise to reach our intended goals. The real question is if the areas where the stock header is pressed down restricts it more than the equal length helps it. From what we've seen, the real problem with the stock headers is that cat.

What we're trying to do is to reach the best compromise for our goals (good power throughout the powerband, keep low end strong since that's where this car needs it the most, stay with stainless steel instead of merely coated mild steel, give people a good inexpensive way to smog their car without spending loads of money, and stay within a reasonable price point). We'll be able to provide info soon on how well these goals were reached.

Anyhow, I've probably written too much and have bored 90% of the people here. So, for those who want to skip it all, at least take this away:

NA ENGINES DON'T NEED BACKPRESSURE!

^^^ Post of the year ^^^


Cheers ///
 

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Company Representative of RPI Power
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The headers are buried in there. I can't possibly see how you're going
to cut the headers without removing them. Look at the fourth pic
on this page for reference:

http://www.evosport.com/product/proddetail.aspx?partnum=EVO.EXH.B60.H01

Also look at the fifth picture on this page:

http://www.m5board.com/vbulletin/showthread.php?t=96799&highlight=dixis+exhaust

Correct. If it were THAT easy, we wouldnt be doing what we do right? lol. Good luck trying to get a clean cut with the headers still on. Even if they were able to cut it, how would they weld it?
 

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Wow, that was quite a bit of reading, and I skipped half of it!

Eloy did a good job of covering the bases. Many people talk about backpressure for torque, and like Eloy said, its BS. Most people dont understand the concepts of fluid dynamics, which deals with the EGV topic.

He did leave out some information though. Backpressure on NA cars does have a factual beginning. Carburetors arent sophisticated like EFI(obviously) and cant adjust for certain changes. Exhaust backpressure cause a reverse flow of air in the engine, and air would be double loaded with fuel, or became richer, and the engine would produce good torque levels. Now, when a free flowing exhaust was installed, this double loading effect was cancelled out, and the engine lost torque. This doesnt happen on modern engines, as the double loaded fuel would be detected, and the air/fuel ratio leaned out.

I just noticed Eloy posted the link to the UUC article with that information. Ive heard the concept talked about before, but UUC has the simplest and best explanation Ive seen.
 

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Great thread!
 
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Excellent thread. Although I am still confused whether to upgrade my headers to the Evosport or not? I love the sound and any extra performace would be welcome but whatr abouth the CEL issues? Can anyone enlighten me on this issue.
 
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