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Re: "Performance" Exhausts... A Must Read... (archive)
[ Follow Ups ] [ 6-series (E24) Message Board ] [ Msg. Board FAQ ]
Posted by Rod... on August 15, 2000 at 00:24:22:

In Reply to: "Performance" Exhausts... A Must Read... posted by Joe ///M on August 13, 2000 at 21:10:27:


According to SEVERAL people I have spoken to, including a man who built F1 motors in France, a free flow exhaust will HARM the performance of an e24, and could actually hurt the motor as well. The 3.5l motor was designed FOR backpressure, and a FREEFLOW system is NOT worth the money. The Euro M6 and US M6 should only be used WITH FACTORY MOTORSPORT EXHAUSTS!!! The Us 635csi is a bit more marginal, but you should STILL steer clear of the Free Flow!!!

Food for thought.

BTW, we dynoed my M635csi w/ w/out the free flow B&B, and I LOST 4HP from 2500-4500 RPM, and I lost over 10 horses at 5000!!! The torque only suffered a bit, 4-6 foot lbs. throughout the entire powerband. Needless to say, I sent my exhaust back to Billy... The "quieted" version of the B&B actually HAS backpressure, and is almost to BMW spec...

Joe ///M


Joe ///M

I'm sorry but this is absurd.
What you are experiencing is common and expected with many upgrades of the intake or exhaust systems.

The air flow volume through an engine is other than a linear function for a cylinder size at a given RPM. The early Motronic, with an AFM, use a map only for the fuel injector pulse width at or near full throttle or above a certain RPMs for setting the air/fuel ratio. Every intake and exhaust systems goes through intake and exhaust flow reverberations and resonance through the engines RPM range and the Motronic map is a look-up table to compensate for this non-linearity.

Therefore, if you change a component in the intake or exhaust system that affects the engine intake and/or exhaust flow reverberations and resonance at full throttle, the air/fuel ratio will be wrong for the stock BMW or the aftermarket chip tune.

That is why Dinan and JC will provide a chip that is to be used with certain intake and or exhaust components they also sell, i.e., free flow intake and exhaust, intake manifold, cams, cam sprocket, etc.

The fact isn't that the engine can't utilize and/or develop more torque with a free flow exhaust, it is that the Motronic chip is not mapped correctly for the component you installed. Modifications to intake and exhaust components can make an engine run lean or rich at a resonance or reverberation point so there isn't a cure-all for solving engine performance changes and loss of engine torque. Rarely will changing an intake or exhaust component increase an engines performance when its full throttle performance is map dependent. The best option is to select the components that will set the engine up the way you believe will make the most torque and then have a custom chip made if a tuner doesn't make one to fit your specific modifications.

That's life with the Motronic
I use to just rejet the carb after loading the engine up with all the goodies. Now it is a much more serious effort.
Rod…


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