Quote:
Originally Posted by Madact
So, the trouble with primary diameter… the actual exhaust ports on the engine are oval 1.75” x 1.3125”, which is similar in area to a 1.5” ID tube - the above equations suggest anything over this is a waste of time and ideally smaller would be good, but we don’t want a ‘step’ in the wring direction, so 1.5” ID it is.
The primary length to the first set of Ys (our ‘P1’) is then the ‘P’ from the 5800 rpm calculation - 32”. Of course, many sources are quite insistent on the ’15 inch’ thing, and it would be nice to take advantage of anti-reversion effects, so we may as well put a step there, out to 1.625” ID, which is the next standard tube size.
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Why do you not want a step in the "wrong direction"? I think it is only going to be a problem if you have long duration race cams...
A 32" primary is excessive and will miss out on some wave effects which could give extra scavenging between pipes, I think that should be the entire length of the header - a lot of people who write up and explain these equations, including professionals, don't understand them!
15 inch is about right depending on the speed of the exhaust flow down the pipes, going a few inches either way will just change the rpm at which you get best effect.
The most important decision is the pipe diameter, this controls the speed at which the exhaust gasses flow down the pipe, the thinner the pipe the faster they go and the faster they go the more inertia they have and the bigger the vacuum they can then pull behind them when the cylinder empties which in turn means more exhaust gas gets pulled out of the cylinder leaving less work for the piston to do in pushing out the remaining exhaust gas. If the piston does work to remove the exhaust gasses then that uses fuel, if the exhaust gasses do it then it is done for free. Essentially you want the pipes as thin as possible without them being so thin that they block the flow, the target speed is normally half the speed of sound, taking into account that the speed of sound is a lot faster at exhaust gas temperature, the reason people try to keep the exhaust gasses hot is to maintain a consistent speed of sound, although most people don't realise that. Once you get passed the half the speed of sound the exhaust gasses start having difficulty flowing and that gets worse as you approach the sound barrier.
If you want it to work well at low throttle then you need small diameter pipes to keep the gas flow speed up, the pipe lengths should then match those of a race header as the timings of flow and wave reflections will all be the same. You just need to make sure that the pipes are big enough for the flow to remain not excessive at full throttle and full rpm.
At least that is how I see it, others understand it in other ways...