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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > NA Engine (non-turbo) / Drivetrain / Exhaust Modifications > Back pressure is never good- ever. (technical/dyno)



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      03-28-2014, 08:27 PM   #1
TheAxiom
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Back pressure is never good- ever. (technical/dyno)

http://my350z.com/forum/intake-exhau...for-n-a-2.html
Quote:
Originally Posted by Hydrazine
Back pressure (at least on our engine) is NEVER a good thing for NA applications.

It is a widely spread myth that some back pressure is good, but it is 100% FALSE.

There clearly are situations where smaller diameter pipes can outperform larger diameter pipes but this is not because of back pressure. This is because of scavenging. And when tuned properly, scavenging actually reduces back pressure as seen by the engine.

But be careful in the assumption that smaller automatically equals better performance. It is highly dependent on where the smaller diameter pipes are being used.

Small pipes near the engine can be good for scavenging and power, but as the pipes move farther and farther away from the engine the effect of scavenging rapidly diminishes. If scavenging cannot be taken advantage of, then small/restrictive pipes must be completely avoided.

Back pressure can only reduce power. Do not confuse this with scavenging.
Scavenging actually reduces back pressure on a tuned cyclical basis.

With each pulse released during the exhaust stroke of the engine, the pulse travels like a shotgun blast down the exhaust pipes. The high intensity blast creates a shockwave with a large positive pressure at the wave front. This wave front is traveling so fast that even when the piston reaches TDC and all the gas is expelled by the piston, the fast moving slug of exhaust gas doesn't stop and it rarefies the gas and creates a vacuum behind the shockwave.

It is the vacuum behind the shockwave that sucks out any remaining exhaust gas from the cylinder. This vacuum also pulls more fuel/air mixture through the intake valves during the intake/exhaust valve overlap period. And this is how more power is made. This is the scavenging effect. It vacuums exhaust gas out of your engine!:thumbup:

Adding back pressure can only kill off this vacuum that you want.

NOW HERE IS HOW THE MYTH STARTED.
IT WAS A MISINTERPRETATION OF TEST RESULTS.

Sombody a long time ago probably did the same series of dyno tests I did on varying pipe diameters. Like I did, they probably found that smaller diameter pipes can yeild higher HP and TQ. They probably mistook this for back pressure and put it out in the public. Smaller diameter pipes can provide higher performance when used properly. But small diameter pipes are only desirable when they are very close to the engine.

For example:
I did a series of dyno tests on various diameter test pipes ranging from 2.5", 2.25" and 2.0".

Before conducting the tests, my initial guess was that the larger diameter pipes would produce the highest HP with lowest TQ. And the smaller diameter pipes would produce the lowest HP and the highest TQ.

Well... I was 1/2 right...

As expected, the dyno testing showed the 2.5" diameter test pipes made the lowest TQ. And as expected, the dyno testing showed the 2.0" diameter test pipes made the highest TQ.

But here's the kicker. The 2.0" test pipes made 2 more HP than the 2.5" test pipes! ...It left me thinking "COOL.:thumbup: Smaller diameter test pipes make more TQ and more HP. That's a wining combination!"

So sombody a long time ago probably misinterpreted the smaller diameter as adding performance by being more restrictive. But this is not the case. It is because of increased scavenging. Smaller diameter pipes near the engine increase the velocity of the shockwave and thereby increasing the effect of scavenging. It was a misinterpretation of the results.

So I continued down this line of testing at the Y-pipe primaries. Using the 2.0" test pipes, I then tested various Y-pipe primary diameters. 2.0", 2.25" and 2.5".

The expectation was to see similar results... but not quite this time. At least not at the Y-pipe.

The 2.0" Y-pipe primaries did indeed provide the highest TQ, but it brought a good portion of the HP down. 2.25" primaries were better but could still be improved upon. The 2.5" Y-pipe primaries provided the best peak power and the best average power.

So dyno testing proved the best test pipe diameter is 2.0" diameter and the best Y-pipe primary diameter is 2.5".

I then continued further down this line of testing on the mid-pipe and made some more interesting observations. Testing mid-pipe diameters at 2.5", 3.0" and then a fully open Y-pipe.

What I did find was that there was no scavenging effect possible after the Y-pipe. There was nothing to gain from the smaller diameter what so ever. In fact, the only thing that had any effect was simple back pressure.

Using a open Y-pipe as the baseline I found that connecting a 3" single exhaust had no effect on TQ and with only a small 1.5 HP decrease.
The 2.5" midpipe slightly reduced TQ and was ~2.5HP down from than the 3" midpipe.

This series of tests established:
1) There was no scavenging possible after the Y-pipe.
2) A smaller diameter midpipe can only decrease HP&TQ
3) There will be rapidly diminishing returns beyond a 3" midpipe
4) With power to weight ratios taken into consideration a 3" midpipe can be considered optimum. 3" also allows more headroom for medium boost FI applications.

Going from 3" to a 3.5" midpipe may at best provide a 0.5HP increase. So from a weight point of view, going larger than 3.0" would be counter productive for NA applications.

I then conducted another series of tests at the end of the Y-pipe.
1) Attaching a 3" diameter butterfly valve with variable position restriction plate.
2) Attaching a 6" diameter parabolic diffuser to reduces pressure drop below that of a 3" open pipe.

The purpose of the butterfly valve restriction plate was to directly test the effect of raw back pressure on performance. And the results were very clear.
BACK PRESSURE RAPIDLY REDUCES PERFORMANCE.

I dyno tested the valve at various levels of flow restriction. From wide open to almost fully closed as back pressure was increased, performance rapidly decreased.


This set of dyno plots is proof positive that back pressure is the enemy of power and torque.

Let the myth of back pressure be permanently dispelled from the vocabulary of this forum!

After that series of tests I started another set of tests that decreased exhaust pressure beyond that of a simple open ended 3" pipe.
A 6" diameter parabolic diffuser was clamped onto the end of the Y-pipe. This was used to decrease flow resistance below that of a open pipe.

Dyno tests of the diffuser showed an instant 4-6HP increase over that of a open Y-pipe!:thumbup:

This picture below is a picture of a 5" linear diffuser. It doesn't perform quite as good as the 6" parabolic diffuser but the 6" diffuser is completely impractical for fitment and production reasons.
6" is too big for fitment under the Z and the parabolic shape also gives it a curvature that makes the production process MUCH more difficult.

This is the diffuser used on the MD ShockWave single exhaust system. It can also be attached directly to the Y-pipe for drag race applications.



So while there are rapidly diminishing returns with going to larger and larger diameter tubing after the Y-pipe, significant gains can still be made by use of diffusers.

The back to back dyno testing shown below was a simple open Y-pipe as the baseline and then with the diffuser attached.
I think this should be a sticky.
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      03-28-2014, 08:30 PM   #2
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And a follow up

Quote:



The 3.5" exhaust absolutely dominated the 2.5" exhaust that was on the car previously. This was the biggest find of the day. Since we were testing two new components at once (headers and new exhaust) we had to find a way to fit our old 2.5" exhaust that was on the car previously to really quanitfy the changes from the headers. We ended up mounting our dual exhaust upside down to make it fit with our y-pipe (don't ask how it worked, but it did!), and saw incredible torque losses with the dual exhaust compared to the single. A good x-pipe would have helped this dual exhaust, but I am still a believer that a single exhaust will always make the most power.

Whoever previously created this custom exhaust did no reasearch what so ever and the results showed it. Not only did the car sound horrible, the midrange power had losses of over 50 FT LBS OF TORQUE!!!! FIFTY!! It almost matched the top end power, lacking only a few hp, but it was the midrange where it was incredibly bad. Just wait until you see the dyno tommorow, it just goes to show how much science there really is in intake and exhaust tuning


[b]
Finally, a plot showing how inefficient the previous exhaust was. This is both with our headers, but the lower plot the dual 2.5" h-pipe exhaust and the other our 3.5" race exhaust

These tests were done on our Koni Challenge Race car, which is entirely stock engine wise with the exception of Nismo's most mild cam (R-Tune North American cams), and a Tilton clutch and flywheel (thats why it revs and sounds so wild). Intake is Injen with a BPI flowstack but will soon be re-done to 3.5" after 13" of tube length. The engine also has a motordyne spacer, that is it! Stock heads, block, compression, intake manifold & collector etc.
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      03-28-2014, 09:59 PM   #3
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Yo axiom break it down so prwtty much from the cats to res hes indicating that we use a 3.5 to 3.0 for the y-pipe and 2.0 for the muffler delete?
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      03-28-2014, 11:41 PM   #4
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Quote:
Originally Posted by dtea604
Yo axiom break it down so prwtty much from the cats to res hes indicating that we use a 3.5 to 3.0 for the y-pipe and 2.0 for the muffler delete?
I think it's other way
From manifold it should be 2.0 into a Y pipe that's 3.0 into the 5" linear end that I think is 3 to 5.0 ( I dont think 3 into 2 inch makes sense cuz of the back pressure )
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      03-29-2014, 03:18 PM   #5
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didnt read, but on older engines with worn valves backpressure can be a good thing. For example, some engines (i know first hand a honda F4i) will not run without an adequate amount of back pressure to form a better seal at the exhaust valves during the compression stroke. this may be just because its a bike engine and we tried to make it work with no exhaust at all, but the idea is still there that it may benefit a more efficient compression stroke with less exhaust gas escaping
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      03-29-2014, 11:52 PM   #6
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Also, with dual vanos (and other VVT technologies), you can tune how much overlap the intake and exhaust cams have throughout the rpm range, and change the scavenging effect on the engine. This really helps with maximizing your torque curve.
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      09-27-2014, 08:05 PM   #7
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Quote:
Originally Posted by mike-y View Post
Also, with dual vanos (and other VVT technologies), you can tune how much overlap the intake and exhaust cams have throughout the rpm range, and change the scavenging effect on the engine. This really helps with maximizing your torque curve.
The Revup 350Z has VVT on the intake and Exhaust.
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      09-28-2014, 03:27 PM   #8
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Not sure if the VQ motor being a V6 plays a roll in terms of back pressure, but a muffler delete even on the V8 M3 loses TQ. Making the M3 even slower from where it started.
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      09-28-2014, 04:00 PM   #9
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Quote:
Originally Posted by playground View Post
Not sure if the VQ motor being a V6 plays a roll in terms of back pressure, but a muffler delete even on the V8 M3 loses TQ. Making the M3 even slower from where it started.
Doubtful.
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      09-30-2014, 11:05 AM   #10
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I think it's a question of having the car tuned in consequence as well, if you just remove back-pressure, but keep the same programming, it might be problematic, all depending on the car I'm guessing, by problematic meaning that it would lose some tq
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      09-30-2014, 11:14 AM   #11
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Quote:
Originally Posted by mecbain View Post
I think it's a question of having the car tuned in consequence as well, if you just remove back-pressure, but keep the same programming, it might be problematic, all depending on the car I'm guessing, by problematic meaning that it would lose some tq
There is no power to be had by having "back pressure"
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      09-30-2014, 11:23 AM   #12
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Quote:
Originally Posted by TheAxiom View Post
There is no power to be had by having "back pressure"
All I know is that on all of the cars I had I replaced cats/exhaust with high flow ones, which helped performance a little, when removing cats/exhaust, I've lost performance in the low to mid range.... Got improved throtle response though, so some people could mistaken that for "better performance".....

Again I understand the theory, it's just that there might be other things into play here that we might not be aware about, simple example is that when removing cats you have to do something about the O2 sensors, if not the car goes in some sort of safe mod and you'll lose performance... Not giving this as an example to prove that backpressure is bad, as we're already aware of that... and there's a solution, but there might be other things that we're not aware about yet....

Just removing the resonator, some people noticed a drop in low end power here in the forum, if it would be 1 person I would say it's a coincidence, but read it quite often in some of the exhaust threads.

Last edited by mecbain; 09-30-2014 at 04:57 PM.
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      10-03-2014, 07:23 AM   #13
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I have done the resonator on my past e46 and after doing so I could immediately feel loss in low end torque

I did a muffler delete on the E90 and the shop used a smaller pipung from the resonator onwards and the power loss is not noticable compared to the E46 resonator delete
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      10-03-2014, 07:33 AM   #14
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Quote:
Originally Posted by mecbain View Post
All I know is that on all of the cars I had I replaced cats/exhaust with high flow ones, which helped performance a little, when removing cats/exhaust, I've lost performance in the low to mid range.... Got improved throtle response though, so some people could mistaken that for "better performance".....

Again I understand the theory, it's just that there might be other things into play here that we might not be aware about, simple example is that when removing cats you have to do something about the O2 sensors, if not the car goes in some sort of safe mod and you'll lose performance... Not giving this as an example to prove that backpressure is bad, as we're already aware of that... and there's a solution, but there might be other things that we're not aware about yet....

Just removing the resonator, some people noticed a drop in low end power here in the forum, if it would be 1 person I would say it's a coincidence, but read it quite often in some of the exhaust threads.
you're still confusing backpressure with velocity.

also, how many people with "muffler deletes" have dyno'd the results back to back? I'm guessing somewhere between zero to none.

there are so many factors at play in exhaust, who's to say backpressure changed with a resonator delete? how do you know it didn't *increase*?

finally, most of the cars we are talking about have cast iron manifolds or something really similar. they don't really take good advantage of the scavenging effect in the first place. tubular headers with tuned pulse lengths are expensive, and there's also the push for reduced emissions.
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      10-03-2014, 08:05 AM   #15
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Exhaust scavenging is a bigger deal at lower rpm's and that's usually why people lose torque when they delete cats. But they usually make more power at higher rpm's since the lower restriction helps when exhaust volume goes up. A resonator would make very little difference at all. The further back down the exhaust you go, the less difference it will make.

Very few people have done actual dyno's of the impact these changes make. Everyone goes off of "well it felt faster in the high end but a little laggy in the middle". That's not science, that's all perception. The difference that people are talking about is a few hp here or there depending on the car. That's pretty much not noticeable by a butt dyno.

About the only time you would actually want "backpressure" is if you had a Roots type supercharger. Those things compress the air in the intake manifold so often when people put headers on, they lose power since the air is getting pushed right through the combustion chamber.

Quote:
Originally Posted by mecbain View Post
All I know is that on all of the cars I had I replaced cats/exhaust with high flow ones, which helped performance a little, when removing cats/exhaust, I've lost performance in the low to mid range.... Got improved throtle response though, so some people could mistaken that for "better performance".....

Again I understand the theory, it's just that there might be other things into play here that we might not be aware about, simple example is that when removing cats you have to do something about the O2 sensors, if not the car goes in some sort of safe mod and you'll lose performance... Not giving this as an example to prove that backpressure is bad, as we're already aware of that... and there's a solution, but there might be other things that we're not aware about yet....

Just removing the resonator, some people noticed a drop in low end power here in the forum, if it would be 1 person I would say it's a coincidence, but read it quite often in some of the exhaust threads.
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      10-03-2014, 12:17 PM   #16
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never said there was no gain to be had in the higher rpms, I just mentioned that in my case and some of my friends when younger, removing the cats didn't give good results at the lower to mid rpm's...

now I'm not saying that it's because of back pressure or savaging effect or whatever... all I'm saying is that not theoretically, but practically that's my experience with removing cats and STOCK tune (excluding O2 sensor issue, if applicable), improved engine response, but loss of power down low...


As in regards to the comment about the removal of the resonator, personally I wouldn't thought it would made a difference (I mean PE doesn't have one...), however I saw the comment a few times here with people removing it and noticing that loss... Yeah I know that's not accurate and can't take buttdynos for real performance numbers, but when someone notices a loss rather than a gain, I'm tempted to believe them more than the other way around.
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      10-03-2014, 12:43 PM   #17
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based on dyno results, or "feel"?
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      10-03-2014, 01:58 PM   #18
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yeah dynos with my signature writen in blood as proof... yeah "feel", I can feel if my car is more responsive or not, I can also feel if my car is more sluggish at lower rpms... Even dyno's don't mean much for gain/loss of 5-10whp if not done on the same day/exact conditions and even then you have a few hp differences between runs... so let's not go crazy now....


in any case my point isn't to disprove what was posted by AXIOM, but to share my experience and raise a point where there are many things that come into play when removing cats/etc... and the end result might not always be as expected, meaning more power across ALL of the powerband...

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      10-03-2014, 02:33 PM   #19
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I think his point is that without a dyno you will never truly know what a mod does. Sure dyno's vary, but at least it's a reference point and somewhat repeatable. Perception and "feel" can be influenced by many things and a lot of times people swear stuff like an exhaust picked up huge power when really it's just louder and they gained nothing.

The original post by Axion is from Motordyne, those guys do some of the most extensive dyno testing of parts in the VQ world. I think the point was to dispel common myths about "backpressure". And posts that doing something makes the car "feel" faster or "laggier" are pretty much how myths begin.

Both of my cars feel different day to day even without changes. One day my 335 feels really responsive, the next it feels laggy even though conditions are similar. Same with my FX; so if I were to go by feel it wouldn't be accurate.

When you modify a car, there are a lot of different things interacting and so the only way to know if what you're doing has an impact is to measure it. Either take it to a dyno, take it to a track, take logs, etc. Otherwise, people are just guessing.
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      10-03-2014, 04:02 PM   #20
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Quote:
Originally Posted by shadow191 View Post
I think his point is that without a dyno you will never truly know what a mod does. Sure dyno's vary, but at least it's a reference point and somewhat repeatable. Perception and "feel" can be influenced by many things and a lot of times people swear stuff like an exhaust picked up huge power when really it's just louder and they gained nothing.

The original post by Axion is from Motordyne, those guys do some of the most extensive dyno testing of parts in the VQ world. I think the point was to dispel common myths about "backpressure". And posts that doing something makes the car "feel" faster or "laggier" are pretty much how myths begin.

Both of my cars feel different day to day even without changes. One day my 335 feels really responsive, the next it feels laggy even though conditions are similar. Same with my FX; so if I were to go by feel it wouldn't be accurate.

When you modify a car, there are a lot of different things interacting and so the only way to know if what you're doing has an impact is to measure it. Either take it to a dyno, take it to a track, take logs, etc. Otherwise, people are just guessing.
This -SO MUCH THIS.
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      10-03-2014, 04:09 PM   #21
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Quote:
Originally Posted by shadow191 View Post
Both of my cars feel different day to day even without changes. One day my 335 feels really responsive, the next it feels laggy even though conditions are similar. Same with my FX; so if I were to go by feel it wouldn't be accurate.

When you modify a car, there are a lot of different things interacting and so the only way to know if what you're doing has an impact is to measure it. Either take it to a dyno, take it to a track, take logs, etc. Otherwise, people are just guessing.
Quote:
Originally Posted by TheAxiom View Post
This -SO MUCH THIS.

k....:

- Your car probably feels different because it performs differently... if you just feel it different from day to day without any reason than I'm sorry but you have a problem...
- If your car performs differently because of X factors, why wouldn't that impact the dyno readings as well?

Some of you guys think you hold the absolute truth... get off your high horses, there's always a grey zone especially when you have so many variables...
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      10-03-2014, 05:03 PM   #22
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Are you actually saying that what you feel is going to be more accurate than a dyno? Have you ever put a car on the dyno? Would you buy a product based on people saying that they felt it made the car faster? Or would you ask for dyno claims?

I've been at the strip and there were times my car "felt" faster for whatever reason but the times were slower. Ie, I changed something and I really thought my car was faster, but it ran the same times or slower. So if I had never run my car, I would have sworn that the mod worked. But numbers don't lie. I've been disappointed plenty of times with stuff that I thought made the car faster, but the dyno proved that wrong.

If you're happy with a mod you do and you feel that the car is faster, that's great. We all mod our cars for ourselves. But basically telling people that the dyno doesn't matter does a disservice.

FWIW, I've deleted cats, used High flow cats, added resonators, changed mufflers, piping diamter, custom downpipes, basically on a previous car we built a turbo system from scratch along with exhaust. I was fortunate that a friend owned a shop with dyno so we could know what worked and didnt. I will say that what I felt and what the dyno said were often not in sync. The human mind is very unreliable.

Quote:
Originally Posted by mecbain View Post
k....:

- Your car probably feels different because it performs differently... if you just feel it different from day to day without any reason than I'm sorry but you have a problem...
- If your car performs differently because of X factors, why wouldn't that impact the dyno readings as well?

Some of you guys think you hold the absolute truth... get off your high horses, there's always a grey zone especially when you have so many variables...
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