How does the exhaust manifold design affect engine performance... ???

I'm always reading that engines need to breathe easier, so immediate logic would suggest that (in the case of a 4 cyl engine) 4 x short primaries each immediately exiting the bodywork into the world above would work best. I know this is not the case, but don't know why...

Anyone want to have a pop at summarizing good exhaust manifold design, or point me towards a reputable source?

Ta

Dont forget though that you need a certain amount of back pressure in the exhaust. A properly designed system will enhance exhaust scavenging leading to more power / torque.

Generally 4-2-1 systems give better mid-range torque than 4-1 systems.

It depends on lots of things but mainly reducing restrictions on the flow of gases.
Imagine that each exhaust port sends out a coke can size of gas, These have to get out.
So if they line up behind each other as the come out, they will flow, but if the crash into each other they will slow the flow down.
You have other effects like each pulse of gasses drawing the next one out by the negative pressure left behind it.
I remember on our imps it was beneficial to leace an oversized gasket as the intrusion in the flow forced a pulse back towards the head and helped the imp breathe.
They only true test is a rolling road and experimenting.
Also the useage of the engine is important to get the power where you need it.
Single primaries will work at extremely high revs (look at dragsters) it is the lower revs on road cars that benefit from the exhaust pulse drawing the next one out.

[Edited on 17/11/2009 by nitram38]

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Originally posted by Steve G
Dont forget though that you need a certain amount of back pressure in the exhaust.

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You need a small amount of negative pressure to help the exhaust gasses escape. Other than that it is just a case of trying to get the gasses out of the exhaust as quickly as possible with as little restriction as possible.

simply .... drop a stone into a bath. Watch the ripple - what happens when the ripple hits the edge, it bounces back.

Exhaust tuning is about turning the gas ripple to hit the exhaust port just around (i don't know if its before or after) it opens. It then bounces back and scavenges from the piston sucking more gas out. You can therefore tune the exhaust to improve efficiency in a certain area of the rev range.

Cheers fellas!

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Originally posted by MikeRJ

quote:

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Originally posted by Steve G
Dont forget though that you need a certain amount of back pressure in the exhaust.

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Back pressure is never a good thing to have for outright power.

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even the dragsters with just short pipes have them to tuned lengths, think music etc, there are resonant frequencies and stuff.
Also the ability to flow well is important. If a hot gas expands, it cools. So it should then reduce in pressure and flow easier. So larger primaries should help.....I think
and one more reason I suspect exhaust wrap is not a good idea for power.
All in all very complex stuff, and I suspect many OE manufactorers manifolds are very much better than many of the aftermarket ones these days. Just think of the budgets! I know exhausts are restrictive to make them quiet, but even some of those I expect are pretty good.

There are some good information on this page: http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=50

If the header is for a high revving (8000-9000 rpm) Duratec I would recommend the following design.

Fabricate a custom 4-1 header

Get a good quality 4-1 merge collector with 2” primaries, 2,5” throat, and a 3,0” exit.

Get some 1 ¾”, 1 7/8”, and 2” pipe.

The total length of the primaries should be around 65 cm, be sure that the length of all the pipes is within 3 cm.

Make the primaries with 3 equally long steps, start with 1 ¾” pipe, then 1 7/8” and then 2” pipe.

PWL

Cheers!