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  1. #1
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    Member No: 18884 trigondave is an unknown quantity at this point trigondave's Avatar
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    Exhaust Back Pressure ?

    I have heard that too much back pressure reduces engine performance, and that too little pressure has the same problem.

    My question. How does one determine the "right" amount after making engine modifications? Chipping. Shaving the Deck. Boring out.

    I road race a 320i (1.8L) with a 2.5" header that feeds into 3" exhaust. Am I robbing the car of power with this set up? Are there diagnostic measures I can use to optimize engine performance?

    Thanks everyone.

    Dave A.Dave A.


  2. #2
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    Re: Exhaust Back Pressure ?

    Engine displacement and compression and turbo use is a way to formulate the proper exhaust size for max power.

    Your current 3" configuration is just making a helluva lot of noise with the 1.8L. Now if you had a turbo on it the 3" setup would be decent if you were pushing over 1bar.

    You loose TQ when you go that big of a diameter. You might gain HP on the very top end if your car can push taht much exhaust volume. The risk that you are taking now with the 3" pipe is burning valves.

    I would recommend a 2.5" pipe after the collector and add a resonator piece in between teh collector and muffler. This will give you a bit of back psi enough to regain some TQ for the mid range. I like using the magnaflow round 2.5" stainless steel mufflers as resonators. They fit well in BMW's.

    I've played around with a lot of exhaust systems and dyno'd the results on various cars.

    Examples-
    2.5L Porsche engine with 11:0.1 CR and mods. I use a 3" high-flow cat with a 3" pipe. Results- Excellent throttle response and high rev HP, ok mid range. I cured the lack of TQ in the low-mid range by installing 4.13 gears. The car now chirps tires into 4th gear.
    I raised the power band for both TQ and HP with this setup.

    BMW V8 32V 10:0.1 CR, high-flow cats 2.25" inlet/outlet, y-pipe connecting them afterwards and dumping into a 3" system. There are a 'steps' in the exhaust in between the 3" pipe and the exhaust manifolds in order to retain the TQ for the street but yet after 4K rpm the engine pulls hard to the top end. Raised the TQ in the rpm range in order to not completely boil the tires up in the low revs and also now have a nice flat TQ curve up to 5252rpm. The top-end HP is very smooth from 4K up to 6k.

    Go with the 2.5" pipe and add a resontaor in between the pipe and exit point.

  3. #3
    Mel A
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    Re: Exhaust Back Pressure ?

    What about this setup on E30 M3?
    3" Y pipe from headers to front Borla race muffler (acting as resonator) instead of cat. with 3" pipe to Borla rear race muffler.

  4. #4
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    Re: Exhaust Back Pressure ?

    I bet its sounds good. Are you running the S14 engine stock or built? (bored/stroked/, high CR, cams, valves) or an E36 M3 swap? or V8? :-)



  5. #5
    Mel A
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    This setup on stock S14-same setup minus front

    Borla on 2.5 S14 race car.
    2.5 extremely loud,2.3 not too bad.

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