COBB 135i Performance Gain #'s (vs. 335i, 535i, z4i, etc.)

[jgoldsmith729]

There has been a ton of discussion on whether or not the midpipe or axleback provide sizeable gains to the n54 in the 135i. COBB shows the following for their expected 135i gains:

STG1: 21%HP / 28%TQ - STG2+: 46%HP / 50%TQ

335i, 535i, z4i:

STG1: 30%HP / 38%TQ - STG2: 51%HP / 55%TQ


COBB has stated that they have found the 135i's catback to be restrictive in their testing: http://www.1addicts.com/forums/showt...472814&page=52

I also spoke with their tech support and they stated that the majority of the restriction is in the axle back section.

Does anyone have any substantiated evidence on where the power gains come from (mid pipe, axle back, both), or the contrary?

Thanks.

[Ryan L]

Seems strange since many here claim the stock axle back is not really restrictive until around the 400 rwhp mark. Maybe those with dyno results can chime in.

[jgoldsmith729]

Just was doing a google site search and came up with my original thread. Apparently not a hot topic. Regardless......anyone have information?

[Vigilante375]

I've heard people say on here say that you need a restrictive exhaust because the turbo needs it.....right.....at least there is some actual testing by a trust worthy source to show the axle back IS restrictive. But like any car, the stock exhaust (headers on back) are very restrictive. That is why there are companies who make aftermarket exhaust systems to make the exhaust flow better and get the most out of the engine for power.

I'm sure with the N54 on the 135i the mid pipe and axle back need to be swapped out to get the same gains as the other 3 models did. But generally you only gain 5 to 10 HP at most from a tune, not noticeable on a car.

[Stohlen]

The exhaust system doesnt need to be restrictive to a point on any car. Otherwise you'll actually lose horsepower.

I do believe however that the N54 exhaust is significantly more restrictive than the N55 IIRC.

[Jaymax85]

for what its worth, i put a berk catback on my car with cobb stage 1 and it feels significantly faster in the butt dyno area, especially in the midrange and up top

[donsaxena]

I thought on a NA motor you need the back pressure, but on our cars less restriction is better.

[Stohlen]

All motors need backpressure to some extent. Like everything in cars, you have to find the perfect balance. A smaller pipe is going to have poor flow, but high velocity flow. A bigger pipe is going to have good flow, but low velocity flow. A motor needs velocity to help evacuate the cylinders of exhaust gasses, it creates a suction like effect that pulls the exhaust out of the engine. This works to a lesser extent on turbo cars as well, and while it doesnt have as much of an effect, it is still an important part of the engines operation. Take the exhaust off of your car at the turbo (or from the cylinders on an NA motor) and if you can stand the noise, you'll find that your car isn't as chipper as it once was.

I can go into all the details of exhaust pulses and the velocity effect if you like, but I find most people dont really like reading a novel and just skip over it. Essentially in short, you need to keep up as much exhaust velocity as posible to help evacuate the cylinders and pull exhaust through the turbo (rather than pushing it through) while not claughing the pipes and significantly lowering exhaust flow potentials.

[Vigilante375]

No matter if the engine is N/A or FI of some sort, the engine needs to BREATHE and let the exhaust out as quickly and efficiently as possible. So having a less restrictive exhaust will help but not going so big that you'll start losing power.

Stohlen explained it well and very in depth.

[donsaxena]

Originally Posted by Stohlen

All motors need backpressure to some extent. Like everything in cars, you have to find the perfect balance. A smaller pipe is going to have poor flow, but high velocity flow. A bigger pipe is going to have good flow, but low velocity flow. A motor needs velocity to help evacuate the cylinders of exhaust gasses, it creates a suction like effect that pulls the exhaust out of the engine. This works to a lesser extent on turbo cars as well, and while it doesnt have as much of an effect, it is still an important part of the engines operation. Take the exhaust off of your car at the turbo (or from the cylinders on an NA motor) and if you can stand the noise, you'll find that your car isn't as chipper as it once was.

I can go into all the details of exhaust pulses and the velocity effect if you like, but I find most people dont really like reading a novel and just skip over it. Essentially in short, you need to keep up as much exhaust velocity as posible to help evacuate the cylinders and pull exhaust through the turbo (rather than pushing it through) while not claughing the pipes and significantly lowering exhaust flow potentials.

Very good explanation thank you

[Cozy]

I guess this is why BMW felt necessary to build the PE.

[jgoldsmith729]

Originally Posted by Stohlen

All motors need backpressure to some extent. Like everything in cars, you have to find the perfect balance. A smaller pipe is going to have poor flow, but high velocity flow. A bigger pipe is going to have good flow, but low velocity flow. A motor needs velocity to help evacuate the cylinders of exhaust gasses, it creates a suction like effect that pulls the exhaust out of the engine. This works to a lesser extent on turbo cars as well, and while it doesnt have as much of an effect, it is still an important part of the engines operation. Take the exhaust off of your car at the turbo (or from the cylinders on an NA motor) and if you can stand the noise, you'll find that your car isn't as chipper as it once was.

I can go into all the details of exhaust pulses and the velocity effect if you like, but I find most people dont really like reading a novel and just skip over it. Essentially in short, you need to keep up as much exhaust velocity as posible to help evacuate the cylinders and pull exhaust through the turbo (rather than pushing it through) while not claughing the pipes and significantly lowering exhaust flow potentials.

Agreed, great explanation! A simplified and applicable version of my thermodynamics, fluids, and physics classes during eng school. Still curious about real world numbers......eg. dyno before/after midpipe and/or cat back modifications.

The reason I'm curious is because like others, I'd like to add some sound and more importantly HP to my 135i, but not at an exorbitant cost/hp. Hard #'s are the easiest way to justify it.