[Madact]

Many thanks Nigel, you've brought up a bunch of stuff I hadn't really considered, and spurred thoughts about a bunch more

Originally Posted by Nigel_S

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...

I was under the impression that a step down in diameter would reflect part of the initial positive pressure pulse back into the cylinder, as well as restrict flow more than a step in the other direction?

Originally Posted by Nigel_S

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!

I'd be right with you (as would the various formulae I've found) if I was designing for best scavenging at say 5800 rpm, but I'm looking for a torque boost closer to 2400rpm too . Though it looks like I may have to settle for less overall length (and torque boost at higher RPMd) due to space constraints.

Another nice thing about pulse resonance is that you get harmonics, so if there's resonance at frequency X from a long pipe (assuming closed pipe resonance, i.e. a restriction / reflector) you'll get a similar (albeit slightly weaker) effect at frequency 3X, so it's 'as if' you had a resonator 1/3 the length when you hit that frequency... not sure that will help of course

Originally Posted by Nigel_S

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.

Do you know whether the 15" thing is about pressure pulses or flow? I haven't been able to find any explanation of this figure, apart from a couple of old quantitative parametric studies done ages ago on 351ci V8's, and those didn't attempt to answer the 'why' of it .

Originally Posted by Nigel_S

... 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.

Not to mention that the speed of sound is also dependent on pressure, the size of the pipe (sound travels faster in a pipe than free air, and varies with diameter), and the fact that the speed of sound is measured relative to the medium, meaning sound travels faster going down the tailpipe than it does going back towards the engine . And of course pressure pulses travel at the speed of sound while mass flow is much slower, and positive pulses and negative pressure pulses react in different ways to steps in different directions, as does flow... Goodness knows I don't have my head around all of it yet...

Originally Posted by Nigel_S

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.

Well, if say, you're running at half the rpm that the race header is tuned for, then the engine will be half as far along opening and closing valves etc. compared to where the race header is intended to deliver the reflections at... I can't see that being a good thing.

Another smaller effect I just realised, it even that to get the equivalent (single-tube) reflection times at part throttle I believe you need longer pipes - for a single pipe t(reflection) = t(going down the pipe) + t(coming back) = d / (v + v1) + d / (v - v1), where d is pipe length, v is the speed of sound and v1 is the speed of flow. If your full-throttle flow v1 is 1/2 the speed of sound v, then for a quarter the flow rate the pipes need to be about 30% longer for the pulse to return in the same amount of time - that's quite significant. Looking at the graph, I was thinking that maybe an even lower speed might be useful to keep part-throttle tuning consistent, if that's desired - 1/3 the speed of sound would keep the resonant frequencies within 10%, for example. Of course that would only apply to part-throttle at high RPMs. Interesting implication there too, in theory of your header is tuned for say 5500rpm and you're running at 6500 for example, backing off on the throttle might actually bring the header back 'into tune' by slowing down the exhaust flow... the relative effect is larger the higher the flow velocity, and would have more impact on a header with a narrow power band effect, could be a useful insight.