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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > 2.5" vs 3" exhaust



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      06-14-2007, 07:40 AM   #1
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2.5" vs 3" exhaust

I am most likely having a custom exhaust made soon for my 335 coupe and was wondering on this car is 3" exhaust worth it over the 2.5". On my previous turbo cars 3" was the way to go because they had larger turbo's that really benefited from the reduction in backpressure, but with the 335 I just dont feel much turbo lag.

Obviously the biggest gains would be seen with 3" downpipes and full 3" exhaust, but I dont plan on replacing downpipes, at least not yet.

I already have one 3" muffler I could use.

What do you guys think?



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      06-14-2007, 07:47 AM   #2
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nice "lil" muffler, LOL, but it's a loud one I guess. Anyway, I dont see too much benefit in 3" if the DPs remain stock, even if the guys has shown nice results replacing the stock exhausts with 2.5" catback... I think the best gain was in replacing the restrictive stock mufflers with direct flows.

The question is wether the added 3-5 HP (maybe) with 3" catback is worth of getting rid the clearance problems and the loud sound and drone on the highway...
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      06-14-2007, 08:06 AM   #3
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Quote:
Originally Posted by Sniz View Post
I am most likely having a custom exhaust made soon for my 335 coupe and was wondering on this car is 3" exhaust worth it over the 2.5". On my previous turbo cars 3" was the way to go because they had larger turbo's that really benefited from the reduction in backpressure, but with the 335 I just dont feel much turbo lag.

Obviously the biggest gains would be seen with 3" downpipes and full 3" exhaust, but I dont plan on replacing downpipes, at least not yet.
Had you two exhausts in your previous turbo cars? 2 x 2.5" is much more than 1 x 3".
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      06-14-2007, 08:33 AM   #4
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My previous turbo cars have been Volvo's so those cant have true dual exhausts as they are inline 5's.

Very true that a dual 2.5" is more volume than a single 3", but dual 3" would be that much more.

That muffler was planned on being a mid muffler on my 3" exhaust from my S60R. The car already had a larger Magnaflow at the rear but wanted a little more sound suppression. My R was pretty wildly tuned and was spitting fire on upshifts sometimes even with the Magnaflow in place. When I first bought it, the car only had 2 resonators and 0 mufflers w/ a 3" ceramic coated turbo back exhaust using a random technology cat.........sounded amazing but loud. Car was boosting over 20psi on a internally modified stock turbo housing.

I was thinking of doing the same setup but in dual exhaust for the 335 (4 total mufflers).

So there would be little mid mufflers like the one seen above as well as larger rear mufflers on each side. The Magnaflows really dont slow down the exhaust gases that much so I wouldnt expect to lose THAT much HP but it would provide for a nice deep tone and slightly lower db than some of the aftermarket kits out there.
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      06-14-2007, 09:18 AM   #5
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Go 3" if you can, its not going to hurt anything...
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      06-14-2007, 02:15 PM   #6
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Dual 3" exhaust is beyond overkill for a <400 whp vehicle. It will more than likely cause your low end to suffer (exhaust gases travel too slowly and cool in the pipes which slows them even more). Contrary to popular belief, a huge exhaust on a turbo car with small turbos is NOT the way to make power in areas where the turbos will shine.
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      06-14-2007, 09:56 PM   #7
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The best exhaust on a turbo car is no exhaust.

Go as big as you can, however keep in mind that a single 3in exhaust is plenty for 600+whp. There's no way you'll get anywhere close to that level with the stock turbos.

IMO, dual 2.5in is more than enough.
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      06-14-2007, 11:03 PM   #8
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Quote:
Originally Posted by A418t81 View Post
Dual 3" exhaust is beyond overkill for a <400 whp vehicle. It will more than likely cause your low end to suffer (exhaust gases travel too slowly and cool in the pipes which slows them even more). Contrary to popular belief, a huge exhaust on a turbo car with small turbos is NOT the way to make power in areas where the turbos will shine.
^^True
The reason no exhaust is great on turbo cars is that there is no way for the exhaust gas to cool on its travel to the rear of the car. If the piping is to big then the exhaust gas cools to rapidly and causes a turbulent flow. All this is bad for efficiency obviously.

If you have to have an exhaust system (99.9% of us) then you want to match the pipe sizing with you HP and space available. Dual 2.5 is great for this car. Remember, each path is coming from only 3 cylinders @ a total volume of 1.5 liters. Think about the old 1.6l hondas. They came stock with 1.5" exhaust. Even when turbo systems are added a 2"-2.25" exhaust is optimal.
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      06-15-2007, 12:27 AM   #9
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Quote:
Originally Posted by A418t81 View Post
Dual 3" exhaust is beyond overkill for a <400 whp vehicle. It will more than likely cause your low end to suffer (exhaust gases travel too slowly and cool in the pipes which slows them even more). Contrary to popular belief, a huge exhaust on a turbo car with small turbos is NOT the way to make power in areas where the turbos will shine.
+1

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Originally Posted by jam0321 View Post
^^True
The reason no exhaust is great on turbo cars is that there is no way for the exhaust gas to cool on its travel to the rear of the car. If the piping is to big then the exhaust gas cools to rapidly and causes a turbulent flow. All this is bad for efficiency obviously.

If you have to have an exhaust system (99.9% of us) then you want to match the pipe sizing with you HP and space available. Dual 2.5 is great for this car. Remember, each path is coming from only 3 cylinders @ a total volume of 1.5 liters. Think about the old 1.6l hondas. They came stock with 1.5" exhaust. Even when turbo systems are added a 2"-2.25" exhaust is optimal.
+1
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      06-15-2007, 07:30 AM   #10
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Is it that obvious, without measurement, that 3" is not as good (overall) as 2.5" on this car?
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      06-15-2007, 11:20 AM   #11
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I have installed exhaust systems on a few 335 and I would only tell people to install 2.5". 3" is not imposible but could be little tricky to install and I wouldn't see any way why it would be better.
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      06-16-2007, 11:43 AM   #12
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thanks guys, sounds reasonable to me that 2.5" is plenty........but if I can make it fit, 3" dual's might happen. We'll see.
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      06-16-2007, 11:53 AM   #13
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Quote:
Originally Posted by jam0321 View Post
^^True
The reason no exhaust is great on turbo cars is that there is no way for the exhaust gas to cool on its travel to the rear of the car. If the piping is to big then the exhaust gas cools to rapidly and causes a turbulent flow. All this is bad for efficiency obviously.

If you have to have an exhaust system (99.9% of us) then you want to match the pipe sizing with you HP and space available. Dual 2.5 is great for this car. Remember, each path is coming from only 3 cylinders @ a total volume of 1.5 liters. Think about the old 1.6l hondas. They came stock with 1.5" exhaust. Even when turbo systems are added a 2"-2.25" exhaust is optimal.
+2

Pipe diameter will affect velocity of the hot gas as it travels down the pipe- this is why people sometimes wrap the exhaust componants with "header wrap" as it keeps the gases hot and the flow up.

Exhaust diameter isn't a case of bigger-is-better, it's a case of optimizing the pipe size to the application to maximize flow. Pipes too large will result in a loss of torque on the lower end- the turbulence and loss of velocity aren't mitigated until a lot of air is being pushed through the pipe, so you lose out down low and gain up top
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      06-16-2007, 02:33 PM   #14
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Quote:
Originally Posted by JephryB View Post
+2

Pipe diameter will affect velocity of the hot gas as it travels down the pipe- this is why people sometimes wrap the exhaust componants with "header wrap" as it keeps the gases hot and the flow up.

Exhaust diameter isn't a case of bigger-is-better, it's a case of optimizing the pipe size to the application to maximize flow. Pipes too large will result in a loss of torque on the lower end- the turbulence and loss of velocity aren't mitigated until a lot of air is being pushed through the pipe, so you lose out down low and gain up top
That is completly incorrect.

With FI cars, the bigger the better, no exhaust is even better. Having dual 5in exhaust will have no negative effect on torque. To have an exhaust system that is big enough to cool the gases to the point where ANY power would be lost, more than likely wouldnt even fit under the car.

A good guideline to follow with FI cars is to keep the exhaust piping as large, or larger than the turbine wheel. In the case of the N54, dual 2.5in exhaust is more than enough with the stock turbos. If someone were so inclined going to dual 5in exhaust would not hurt the torque what so ever.
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      06-16-2007, 02:47 PM   #15
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If anyone is still confused about exhaust sizing, and the difference between NA and FI motors. Here is a good write up expaining why the theory of "too big of exhaust will hurt power" is wrong.

VE= volumetric efficiency

A smartly designed normally aspirated exhaust has a mechanism to convert exhaust gas energy into negative pressure. This is a collector. If the collector and primary tubes are designed properly (and many are not), they will actually enable each exhaust gas pulse to gently 'suck' on the other primary tubes as it passes through the collector and on out into the exhaust pipe. The greater the 'suck' action, the greater the positive effect on VE and the reduction in pumping losses. If the exhaust is enlarged enough to reduce exhaust gas velocity through the collector, the exhaust pulses do not have enough energy to make the collector work, and our power isn't there. This is often misinterpreted as not having enough 'backpressure', which is incorrect. Of course, if the exhaust is too small, this increases pressure in the system, which begins to cut into our power as well. Anything more than ~2 psi in a normally aspirated exhaust system is going to reduce power, so sizing normally aspirated systems is like designing/sizing a musical instrument to sound a specific musical note when we blow into it.

A turbo exhaust has no such mechanism to create something positive from exhaust energy. Once that exhaust pulse spins its way out of the turbine, it can do nothing but get in the way. If pressure is created on the backside of the turbine, it just reduces turbine efficiency and VE as well.

With a turbo car, a larger exhaust will make the most difference at peak power, where the engine is consuming the greatest rate of mass air flow. IF we find that a larger exhaust somehow reduced power, it's because it increased VE and leaned our fuel mixture enough to make a difference. If we re-tune, we will get our power back, plus some.
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