Bill Sherwood's Engine Page

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The Exhaust System

The exhaust side of things is a little easier, and follows much the same principles as the inlet side of things. The desired port shapes are the same, and in fact the only extra thing we want to do is to keep the exhaust gasses as hot as possible for as long as possible to reduce back pressure and increase gas velocity. Most people will opt for aftermarket extractor type exhausts, and by and large they are a good thing, but there are a few things to look at first -
For road & club use, you'll be best off with a 4-2-1 system on a four cylinder engine.
For high performance & race use, the only way to go is a 4-1 system.
The 4-2-1 system gives good all-round power, with no special power peaks or dips, while a 4-1 system give the best power at higher revs - At the expense of low end power.

Speaking of 4-2-1 extractors, this is the WRONG WAY to do it on a four cylinder engine! A 4-2-1 system must have cylinders #1 & #4 joined up, and #2 & #3 joined up, then further down those two pipes join into one. This is because four cylinder engines have a 180° crank, whereas virutally all V-8's have a 90° crank (the angle between the crankpins) and so the order in which the exhaust pulses come down the pipes is different. You can tell that the engine has these incorrent type of extractor on it, as they sound a lot like they have a dead cylinder and/or like a Subaru WRX.

No matter what system you decide on, all the pipes must be equal length to its' partners to make the extractor system work effectively, eg, #1 cylinder pipe must be the same length as #4's, and so on. This is normally very difficult to do, and so a few manufacturers aren't as careful as they should be in that area.
If you have a tubular extractor type system, then you will get a small gain on power by wrapping the system in asbestos bandage, or better still ceramic coating the pipes to retain heat internally.
The length of each of the pipes is also critical to where and how much power you'll get. Longer pipes will make more power at low revs, and vice-versa.
The size of the collector pipe - Where the exhaust pipes all converge - is also important. It doesn't matter how many cylinders the engine has, or how large it is; It all depends on airflow.
From the same magazine as I got the inlet length diagram, I also have one for the exhaust. It's used the same way, but you need to find out the number of degrees your cams have between when the exhaust opens to when the inlet opens, then read up to the revs, then to the numbers on the left hand side, which give you the length of the exhaust in feet. Another example, the same 4AGE as I mentioned before, you'd want the effective exhaust length (The distance from the middle of the exhaust valve to the first muffler) to be about 3 3/4' long.

With the length of the individual pipes coming from the head, a good rule-of-thumb for a 4-2-1 system is to make the 2-1 pipes (called the 'secondaries') join together at about half the distance that you've calculated in the above diagram. The 4-2 pipes (called the 'primaries) should be half that again.
With a 4-1 system, make all the primary pipes half the length in the diagram.
If your engine makes 200hp, then you'll need about a 2 1/4" ID pipe, and with more power you'll need a bigger ID pipe, of course. Turbo engines are a different beast, however - Bigger is (usually) better! With any turbo engine of reasonable power you'd have to think about a 3" pipe, perhaps larger ...
In any case, the tip of the exhaust should exit at a point where there is a low pressure region to minimise back pressure. It's a cheap form of weak turbocharging.

This is a picture I took of a hillclimb car in the UK when I was there in July 2001. This is about as nice a set of extractors as you'll find. Check out the welding, it's superb. Also look at the collector, as it's very good as well.
Perhaps the only complaint is that there are too many bend in some of the pipes, but in a racing car you rarely have the luxury of space ...

These are genuine Toyota TRD racing extractors, not the ones that TRD sell over the counter to the public.

There is also an optimum length for the pipes that come from the head, and this point is often right under the middle of a typical sedan type car's floor. Same rules as inlet systems though- Long pipes make more power at lower revs, and vice-versa. The easy way out of this is to put the first muffler at the optimum length, thus fooling the exhaust flow into thinking it's left the pipe.
The muffler should be a straight through type for minimum restriction and thus power loss, but since these types don't quieten down the engine very much then you may need a couple to keep everyone happy.

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Intro & Inlet system - Combustion chamber

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