Pros & Cons of changing stock extractor or exhaust manif

[haffeel]

Guys,

Can the guru's here list the pros and cons in changing the stock exhaust manifold in a E30 325i car?

I for one can guess the first thing is, getting rid of the stock exhaust will make the engine more noisy. (IS THAT CORRECT)?

Secondly the fuel consumption can go throught he roof!
My buddy here disputes the fuel consumption thing as he claims, it is related to how heavy is your foot on the GAS and has nothing to do with the exhaust manifold.

Can any one shed some light?

I notice the stock exhaust manifold produces some back pressure, what will happen for the back pressure factor if we fix one of those fancy extractors ? Does all this contribute to fuel consumption thing.

thanks,

cheers

[haffeel]

This is the first time in this forum some one has no opinion on a subject. Come On guys .. I would like to hear some thing

[mrLEE30]

what the hell is an extractor??

as for manifold the stock item is pretty good as it is, the 325 uses a slightly bigger manifold then the 320 but you didnt mention what car you have.

as for back pressure less will allow the engine to flow easier, but only if you allow it to breath easier, so air filter change and perhaps a bigger throttle body, but then the next problem will be the injectors as they can only squirt a limted amount of fuel in, so they would need upgrading, and then of course so would the ignition managment etc etc etc

so to summise unless you are changing the entire exhaust system for something more special (by that i mean equal length headers and different cat and different silencer)then i would not bother

the amount of exhaust noise is dictated normally by the silencer, so different headres will have no noticable effect on noise

fuel consumption will not go through the roof if only the manifold is changed

mrlee

[Brianmoooore]

First point - The standard exhaust manifold and system on a 325 is pretty good.
Whether or not a replacement system makes more noise is entirely dependant on the design of the replacement system. They can make it as loud or as quiet as they like.
A N/A engine is an air pump. The more air it pumps, the more power you get. The air flowing down one branch of a manifold creates low pressure in another branch where it joins that branch, and this increases the air flow in that other branch, giving more power.
Problem is, because of the different speeds of the gasses at different revs, if this happens at one point in the rev range, then the opposite will happen at another point, and impede air flow.
BMW designed their engines to give a particular power and torque distribution curve. If you don't like this curve, then an aftermarket manifold can, if well designed, move this curve around, but gains in one place are likely to be accompanied by losses elsewhere.
You don't get anything for nothing, and BMW didn't make many mistakes.

[keggy]

wot about the m40 manifold? worth changin?

[mrLEE30]

glad brian said a similar thing to me, but then i just read the thread on the 18hp gain by changing the manifold maybe the standard manifold is not so good after all!!

but i DID mention that the length of the header pipes make most of the diference when designing the system so i stand by that

mrlee

[mrLEE30]

oh yeah i just noticed you said it was on a 325 too

[Brianmoooore]

glad brian said a similar thing to me, but then i just read the thread on the 18hp gain by changing the manifold maybe the standard manifold is not so good after all!!

but i DID mention that the length of the header pipes make most of the diference when designing the system so i stand by that

mrlee

If you look at the curves on that thread, you'll see it's swings and roundabouts..

[haffeel]

OK then I am going to keep the exhaust manifold stock. the header pipe how long should be the lenght?

Oh yes, my ride is a 325i with BBTB and a zone chip in.

Brian thanks again. You have saved my sanity once again dude

I will stick with the stock exhaust manifold.

TQ