Exhaust Header Design Ideas

By using a carefully designed exhaust header you can get a little bit more power out of your engine, by arranging for the primary pipe length to be such that the last bit of exhaust gas is sucked out of the cylinder just before the exhaust valve closes. The exhaust gas leaving a given exhaust port is accompanied by a high pressure pulse, which travels at the speed of sound away from the exhaust port. This pulse has a high pressure leading edge, and a low pressure tail. It is reflected back down the header pipe to arrive just in time to suck exhaust gas out of the cylinder. It also sucks some inlet charge in, which is handy, but some of it might just go out the exhaust, but there we are. The pipe length is therefore set so that the time the pulse takes to travel down and back matches the valve timing, and the engine speed. The speed of sound in the exhaust gas is probably not be the same as in atmospheric air because of the gas composition. The speed of the exhaust gas itself is more difficult to determine, but together with the engine capacity determines a sensible header pipe volume.

The graphs below have been generated from design equations (A. Graham Bell 1985, Performace tuning in theory and practise).

What you do with the exhaust gas after the header is of little importance, so long as the design is not constrictive, so there's no point investing in a decent header and putting a back box on with baffles in it, but on the other hand you probably won't get any benefit whatsoever from using a tailpipe the size of a dustbin.

The total header pipe length depends on your exhaust valve timing, which I now know is 56ºC BTDC.. I am assuming here that the basic design will conform to a 4:2:1 header, and the following dimensions will definitely put you in the right ball park.

The total header pipe length (from exhaust port to final collector) is given below:

The primary pipes on the header should be set to 15 inches, and so the secondary pipe lengths are given as follows:

The diameter of the header pipes is important because this determines the pipe volume for a given length. Obviously, it must be related to the engine capacity. I am assuming 1360cc.

The primary pipe diameter is given below:

... and the secondary pipe diameter.

Both secondary pipes now need to be put through a tapered collector to go to a single pipe leading to the first expansion chamber in the exhaust. Setting the taper to 8 degrees. the following graph is magically produced.

And then the bit of pipe which goes to the first expansion chamber is this long:

I forgot to add the diameter of this pipe... whoops. About 2" should do.

Disclaimer.

The author assumes no responsibilty whatsoever for any damage, intentional or otherwise, incurred on any motor vehicle, resulting from the material contained herein.

(If you break your car, it's your fault. Not mine.)