Re: Wouldn't it make sense that. . . (archive)

the free-flow muffler in question would have to be matched to the correct performance chip in order to extract maximum performance? According to Dinan, the stock 635 factory muffler has excessive backpressure. They dyno test their products and claim +9 hp @ 5000 and +17 ft lbs torque @ 4000 by adding one of their free-flow mufflers (essentially a reworked factory unit) to a car with one of their chips. Not a lot, but still something. I know from my background working w/ old American muscle cars that just adding headers and/or a free-flow exhaust would often result in WORSE performance unless the carb(s) were adjusted to modify the air/fuel mixture. I would think that same principle would apply here as well, don't you?
Larry

I think Larry is damn close here.... Nevermind Joe ///M's response.

This is a long and not simple topic....

Simply bolting a "performance" exhaust upto a car is only going to give you so much HP increase (and can actually decrease HP/torque depending upon gas flow and engine head design). When those BMW engineers designed the intake/exhaust they were designed to work in together to produce the desired performance figures (ie torque/hp emissions/ fuel economy etc) they were looking for. Think of it as a "tuned" system.

If you change one aspect (ie exhaust) you also need to look at the intake side (and fuel requirements as well). The motronic fuel system is good at adapting up to a point, but must be reprogramed to change the ignition and fuel curves to "fine" tune any performance changes.

An automotive engine to produce more power (per liter displacement) must be made more efficent. Think of the engine as an air pump --the more air(and fuel) ingested and air (exhaust) expelled the more power it will produce. So anything you can do to get more air INTO and OUT of the engine will increase it's efficency (and power).

So that's why many makers either add more valves to an engine's head (ala BMW/Honda etc) or make valves in the head larger etc.

As mentioned, this topic can fill whole volumns. (with more always being written) If you haven't read a good book on engine efficency/head and gas flow design. Try Vizard's book on the BMC "A" series engine (or his books on modifying your V8 engine). Although this covers the old BMC engine and was orginally written in the 70's many (if not all) of the ideas/methods are used on most of todays internal combustion engines.

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