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J. Dub J. Dub is offline
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Default Can too much exhaust flow (bigger pipe) be bad for performance? Calculations inside - 01-07-2016, 04:27 PM

Is there such a thing as too much exhaust flow?
I ask cause I recently installed an AE exhaust:
-true dual no H or X pipe
-No cats
-2.5" piping for most of the exhaust route

I then cut off the mufflers and installed pipe in their place.

So by my calculations: total CFM per pipe== 2.5" x2=1018 cfm. Est CFM per 2.5" pipe is 509. This is the maximum amount for this exhaust.

However,
according to http://www.summitracing.com/expertad...cfm-calculator the optimal CFM should be what you see below.

Maximum Engine RPM : 7000
Engine Size in Cubic Inches : 182
Street Carb CFM : 313.34
Racing Carb CFM : 405.5

So I wanted to ask you all. Does too big of exhaust piping have an ill affect on performance for our stock OEM turbo's?

I have recently read that a 7" FMIC can, at times, be too big and decrease performance for oem stock turbo's as well.

As I'm FBO and running a custom map with increased boost on the stock turbo's, I began to wonder about the exhaust CFM.
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rac rac is offline
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Default 01-07-2016, 06:45 PM

a big volume fmic will have more air to compress and therefore should theoretically reduce responsiveness. i run a 7" fmic on stock turbo's and didnt really notice this, however others have said the opposite.

a lot of people will say you cannot go too big post turbine on a turbocharged car, unlike n/a were optimal exhaust velocity is critical.

my personal experience would suggest that velocity still has some affect, i went from stock to ******* twin 2 1/2" exhaust, and then to ******* 3" single. I felt the 3" single was more responsive than the dual 2.5" and i didn't feel any loss up top. no dyno so maybe it was placebo, my 2.5" duals droned like h8ll.
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Default 01-07-2016, 06:47 PM

I've currently got a custom single 3.5" exhaust, with a ***, and no mufflers..

Whilst i haven't dyno'd the car yet for a HP number, the first thing i noticed was a loss in torque, and the entire torque band drastically changed.

I'm working with my tuner (trebila) to get some of this low/mid torque range back.
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rac rac is offline
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Default 01-07-2016, 08:19 PM

Quote:
Originally Posted by _ink
I've currently got a custom single 3.5" exhaust, with a ***, and no mufflers..

Whilst i haven't dyno'd the car yet for a HP number, the first thing i noticed was a loss in torque, and the entire torque band drastically changed.

I'm working with my tuner (trebila) to get some of this low/mid torque range back.
well there you go, probably not placebo for me then. it was a couple of years ago now. you could consider dropping it down to 3" if tuning doesn't help.
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Default 01-08-2016, 01:51 AM

In terms of power the best exhaust for a turbo car is no exhaust.


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Default 01-08-2016, 03:23 AM

Quote:
Originally Posted by rac
a big volume fmic will have more air to compress and therefore should theoretically reduce responsiveness. i run a 7" fmic on stock turbo's and didnt really notice this, however others have said the opposite.

a lot of people will say you cannot go too big post turbine on a turbocharged car, unlike n/a were optimal exhaust velocity is critical.

my personal experience would suggest that velocity still has some affect, i went from stock to ******* twin 2 1/2" exhaust, and then to ******* 3" single. I felt the 3" single was more responsive than the dual 2.5" and i didn't feel any loss up top. no dyno so maybe it was placebo, my 2.5" duals droned like h8ll.
Of course less boost but cooler air is something to factor on that one, rather than higher boost warmer air with a smaller IC.


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Default 01-08-2016, 04:55 AM

Quote:
Originally Posted by J. Dub
Can too much exhaust flow (bigger pipe) be bad for performance?
No. Never.
The ideal exhaust is the complete lack of exhaust. Period. Any exhaust is a restriction. Anyone that's familiar with pneumatics knows that you control the speed of a piston NOT by the air going into the cylinder, but by the air coming out of it. For the fastest, freest movement of the piston, there needs to be absolutely NOTHING restricting that exiting air.

In a car that must have an exhaust - like all street cars and the vast majority of race cars - headers are designed to recoup some losses by way of "scavenging" but you have to remember that in order to scavenge, there has to be scraps left over. Those scraps are waste - and it's always best to not have waste in the first place.
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Default 01-08-2016, 06:06 AM

Quote:
Originally Posted by Corrupted
No. Never.
The ideal exhaust is the complete lack of exhaust. Period. Any exhaust is a restriction. Anyone that's familiar with pneumatics knows that you control the speed of a piston NOT by the air going into the cylinder, but by the air coming out of it. For the fastest, freest movement of the piston, there needs to be absolutely NOTHING restricting that exiting air.

In a car that must have an exhaust - like all street cars and the vast majority of race cars - headers are designed to recoup some losses by way of "scavenging" but you have to remember that in order to scavenge, there has to be scraps left over. Those scraps are waste - and it's always best to not have waste in the first place.
Perhaps for for post turbo exhaust but for NA applications a proper header/collector configuration will make more power than straight ***** (zoomies) because of the exhaust pulse tuning.
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Default 01-08-2016, 09:07 AM

Quote:
Originally Posted by boost junkie
Perhaps for for post turbo exhaust but for NA applications a proper header/collector configuration will make more power than straight ***** (zoomies) because of the exhaust pulse tuning.
That must be why top fuel drag cars run zoomies...

The only exception to the rule of no exhaust = best exhaust is in the case of 2-strokes, but you still have to sacrifice large amounts of power across a large RPM range in order to utilize the tuning of those exhaust pulses. It also is a much different theory than scavenging on a 4-stroke that uses the resistance of fluids hitting each other, 2-strokes use actual sound waves and frequency tuning.
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Default 01-08-2016, 09:36 AM

Oh and by the way - the reason why people lose power when they put zoomies on their car when they had collectors - it's because their engine was built and tuned for the collectors. You can't just swap one for the other. It's like putting nitromethane in a gasoline engine then claiming nitro makes less power. Yeah, of course, but once you actually build the engine for the nitro, it makes more than the gas.
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Default 01-08-2016, 10:04 AM

Interesting read from everyone.
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Default 01-09-2016, 11:49 PM

This video did not discuss turbo engines, but it is still very informative for "most" applications.

https://www.youtube.com/watch?v=RWTARjxiqlo


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Default 01-11-2016, 03:37 PM

Quote:
Originally Posted by Bullitt1841
This video did not discuss turbo engines, but it is still very informative for "most" applications.

https://www.youtube.com/watch?v=RWTARjxiqlo
That's pretty good.
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ferocity02 ferocity02 is offline
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Default 01-11-2016, 04:30 PM

I've always assumed that some backpressure is good for NA engines, helping to maintain low end torque. For turbo engines, minimal backpressure is desired in order to maximize pressure drop across the turbine. I'm sure there's some velocity and scavenging variables in there somewhere.

I'm guessing that calculator is an oversimplification geared towards NA carbureted engines.


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Default 01-11-2016, 06:09 PM

Q-
Do you have a turbo?

A-
YES: NO EXHAUST
NO: TUNED LONG-TUBES

Funny cars don't count they run through so much volume of fuel and the fact a lot gets burnt in the exhaust changes they physics substantially.
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Default 01-11-2016, 09:43 PM

So what do you all make of this? http://www.passatworld.com/forums/vo...ust-stuff.html
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Default 01-12-2016, 03:23 PM

Sounds like a bunch of talk about small-engined cars (and in the DSM example, very low compression).

Like the others said above, you want maximal pressure drop across the turbine. The pre-turbo backpressure already exists due to the turbo, and you want to minimize post-turbo backpressure.

A 3" single is a power loss on this car once you start cresting 450-500whp.


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Default 01-12-2016, 03:36 PM

Quote:
Originally Posted by Corrupted
That must be why top fuel drag cars run zoomies...

The only exception to the rule of no exhaust = best exhaust is in the case of 2-strokes, but you still have to sacrifice large amounts of power across a large RPM range in order to utilize the tuning of those exhaust pulses. It also is a much different theory than scavenging on a 4-stroke that uses the resistance of fluids hitting each other, 2-strokes use actual sound waves and frequency tuning.
Top fuel are supercharged so yeah that changes the dynamics. Look at the quickest NA drag cars and they run collectors. Hrm, wonder why?
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Default 01-18-2016, 08:30 PM






I wonder if Cosworth or Porsche (respectively) were worried about having too little exhuast?


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Default 01-19-2016, 06:51 AM

Quote:
Originally Posted by Sered
Sounds like a bunch of talk about small-engined cars (and in the DSM example, very low compression).

Like the others said above, you want maximal pressure drop across the turbine. The pre-turbo backpressure already exists due to the turbo, and you want to minimize post-turbo backpressure.

A 3" single is a power loss on this car once you start cresting 450-500whp.
Couldn't be more accurate. As much as I hate "dump tubes" off of external wastegates they help a lot as when they are open the effectively make your exhaust larger when you need the extra flow.

There was a great article I read that going from a 3" to a 3.5" exhaust on a 500ish WHP Supra made an additional 40 whp or something silly like that.

One of the best tools I've ever purchased is a magnehelic gauge off Ebay. I've used it to quantify gains in intake system vacuum (restriction), exhaust pressure (restriction) as well as ducting and aero mods.
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Default 01-19-2016, 08:06 AM

Quote:
Originally Posted by BeatupVR4
Couldn't be more accurate. As much as I hate "dump tubes" off of external wastegates they help a lot as when they are open the effectively make your exhaust larger when you need the extra flow.

There was a great article I read that going from a 3" to a 3.5" exhaust on a 500ish WHP Supra made an additional 40 whp or something silly like that.

One of the best tools I've ever purchased is a magnehelic gauge off Ebay. I've used it to quantify gains in intake system vacuum (restriction), exhaust pressure (restriction) as well as ducting and aero mods.
I remember when everyone on supraforums back in the mid 2000s was switching to 3.5" exhausts and the single turbo guys were bumping up to 4" exhausts. It resulted in huge gains. Who doesn't want an effective 40-50whp gain from an exhaust? It's even easier on the motor and the turbos.


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Default 01-19-2016, 02:15 PM

I suppose 6 cylinder cars must be different that 4 cylinder cars. In the evos/dsm and s2000 turbo guys it been proven that a full 3 inch exhaust is good to about 650-700whp with no performance gain from going to bigger exhaust than a full 3 inch.


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Default 01-19-2016, 02:24 PM

Quote:
Originally Posted by Riceball777
I suppose 6 cylinder cars must be different that 4 cylinder cars. In the evos/dsm and s2000 turbo guys it been proven that a full 3 inch exhaust is good to about 650-700whp with no performance gain from going to bigger exhaust than a full 3 inch.
Not sure if I can believe 650-700whp. That's a LOT of power for a little 3" pipe. Horsepower (aside from how efficiently it is generated) is pretty consistent with how much air is moving out of the engine. Only thing I can think of is the headflow on the 4G63 and F20C5/F22C1 is much better than the JZ/7M family or the N54 making the pressure-drop post-turbo less important.

Definitely a given in the JZ world that bigger exhausts make more power.


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Default 01-19-2016, 02:40 PM

I believe Terry dynoed with his cut out set up and saw a 27 whp gain.
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Default 01-19-2016, 04:51 PM

Quote:
Originally Posted by Deen@StreetKingImports
I believe Terry dynoed with his cut out set up and saw a 27 whp gain.
That is on the 135i, which has a pretty bad exhaust. The 335i e90 that I have has no cats, just stock resonator and muffler. We tested that vs open dp and saw no gains at all with 800whp+


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