Exhaust Backpressure and Its Impact on LTFT Values

Question

This is a follow-up from my initial question which can be found here : LTFT and STFT values are off and don't make sense.

Car : BMW 2015 F10 M5 Modifications : Eisenmann Race Catback Exhaust, MSR Cold Air Intake. Everything else stock.

The summary of my problem is, LTFT values between Bank 1 and 2 are inconsistent where Bank 2 is 10% or more lower (-4% vs -14%) throughout the lower RPM band. At WOT, LTFTs line up and stay equal and behave normally.

Zaid has been incredibly helpful and we narrowed it down to the aftermarket exhaust I have.

Basically there is one big difference between the stock exhaust and the aftermarket one and that is the use of flaps. Each muffler (there are two) has 2 pipes (so 4 pipes total) where 1 pipe on each side has a flap connected to it that is operated by a valve. The valve is connected to a vacuum line. In the stock exhaust, at low RPMs, to reduce noise as well as reduce turbo lag and increase low and torque, the flap is closed increasing the backpressure. Over 3,500 rpm (or around that), the flaps open up. The aftermarket exhausts have no flaps (not just mine but a lot of others) and the standard installation procedure is that they zip tie the vacuum lines and plug them as well (mine has bolts plugged). So in an aftermarket scenario, car operates as if the flaps were always open.

Here is a picture of the eisenmann muffler right next to the stock muffler : https://eisenmannblog.files.wordpress.com/2012/11/8182708317_10988d68b2_h.jpg

Here is a picture of the eisenmann muffler on the car : http://farm9.staticflickr.com/8490/8206133927_6b3bec97bb_b.jpg

The theory we have at the moment is the lack of flaps is causing this rich condition I am seeing. What I don't understand is why this is happening on one side of the exhaust not the other. So far I have done:

• Check both exhausts and where they are clamped, clamps are extremely tight, and there is a good maybe 4-5 inches of stock exhaust inside the aftermarket pipe (after market slips on), so there is no way a leak can happen at the connection points.
• Check the vacuum lines on both sides, they are both zip tied and plugged with a bolt.
• Check the intake installation many times, no air leaks, all clamps are securely fastened.
• Check turbo inlets and charge pipes, no cracks or air leaks.

Returning the exhaust back to stock is not something I want to do. I spent several hours adjusting the tips of the aftermarket exhaust so that all 4 looked identical. Without knowing for sure, I dont want to touch it.

Any help is appreciated.

Thanks.

Answer 1

Possible Culprits

It is normal for fuel trims to be slightly different, but a 10% bank imbalance on a car that still smells like Munich seems odd.

There are two things which affect fuel trims:

• the physical system

  If you are confident that both intake and exhaust are air-tight then we will have to cross them off the list of potential causes.

• the integrity of the sensors/injectors

  This comment of yours about a slow response from Bank 2 has me wondering whether you have a lazy O2 sensor. It's the only thing I can think of right now, since that is the only bank-specific sensor which provides feedback for fuel trim correction.

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How to Test

The idea here is to confirm the slow response of Bank 2 O2 sensors compared to Bank 1.

You'll ideally need some graphing capability. A scope or an OBD-II app with graphing functionality. Pull up Bank 1 and Bank 2 STFT's

Introduce some propane gas into the intake of each bank to induce a negative STFT. Look for differences in response time and STFT magnitude. Based on your description, I'm expecting the STFT for Bank 2 to change more slowly and/or register a bigger % difference.

Answer 2

Gut reaction?

I went straight to fluid dynamics. It is likely that the aftermarket exhaust is made of larger diameter piping. Increasing the cross sectional area of a pipe will, all other things being equal, reduce the flow rate of a fluid through the pipe.

This exhaust is likely tuned for high RPM performance, when the engine needs to move a higher volume of exhaust gasses, and larger piping is needed. At low RPMs however, with a smaller volume of exhaust gas being expelled, the larger piping may have a detrimental effect, slowing the velocity of the gas.

It could feasibly slow enough that your O2 sensors read a higher concentration of gas and presume that the engine mixture is off. It would explain why things level out under WOT.