exhaust things

[Steve Kerswell]

Been fiddling with the Sierra exhaust. I've always had a short pipe exiting under the sill in front of rear wheel. But it banged about a bit and made the wheel filthy very quick. Wanting the car a bit more civil, I bent up a full length pipe, fitted a silencer I had sitting around and a fancy tail silencer I had off the B+. It seems to me that the car now responds better to the throttle, definitely sounds better, but does seem to rev easier. I have done nothing else (yet). Interested in knowing if the longer 50mm pipe has worked miracles :-) Back pressure etc. I remember some of the stuff taught to me about pipe length but cant remember why ??

[Steve and Matt Pegrum]

You've now got two new silencers which certainly will affect the sound. Other than that pipe/silencer diameter will affect flow rate and therefore may improve engine breathing (depending how bad it was before) and therefore power.

Pipe length is less likely to be an issue I think. You are thinking perhaps of lengths tuned to engine revs but I think that's more of a 2 Stroke thing for bikes of yesteryear, where there aren't really such things as valves and cams

[James Doulton]

It might be partly a psychological effect. I know that when I have driven a car with a leaky exhaust then just driving slowly seems to be so laboured because as soon as you touch the accelerator then there is a lot more noise but very little action. It is a bit like that in my Rickman which has a side exhaust that was replaced and the new one is somewhat louder - it just seems like harder work, even though it actually goes quite well when pushed.

[James Doulton]

A colleague of mine had a Westfield with a 1760 crossflow and a wild cam. He spent ages getting his exhaust exactly 53" long (from the head to where the 4 branches joined) because this was exactly half the distance that sound travels in the time that the exhaust valve is open for at 7000rpm, or something. The idea is that the positive pressure wave travels to where the pipe opens into the common pipe and this causes a negative pressure wave to be reflected back up the pipe, just in time to suck a little more exhaust out of the cylinder.

I can think of a number of reasons why that is probably wrong - it doesn't take account of the gas speed, it assumes that the speed of sound is constant whereas temperature and pressure affect it, as does chemical composition. If it did happen to work out then it would only give a positive effect over a fairly small rev range.

As Steve says, it was quite important for two strokes.

[Big Vern]

I spent a lot of time in the 80's working on tuned and variable length intakes and exhaust.Tuned length intake and exhaust has very much fallen out of favour these days and rightly so. (mainly due to variable valve timing) The tuned lengths add a little in torque and power but only work in very narrow rpm windows, typically no more than 250rpm wide. once away from the target rpm's the performance drops off significantly as the positive and negative pressure waves that were helping will have reversed and now hinder performance. 'Spread betting' your intake and exhaust lengths help with providing a wider more favourable torque curve (greater area under the graph) which increases driveability thus reducing lap times and making the car more usable in traffic. The key to best performance is a wide smooth torque curve not a narrow bumpy one. I don't have much experience with two stroke but given how they work a negative pressure wave at the exhaust port would seem to make sense.

The was a very good article written many years ago by the late Bill Blydenstein which explained tuned lengths very well. I'll see if I can find it.

[Steve Kerswell]

Thanks for that Vern, all I know is the old pinto is definitely revving easier, maybe the old XR2i silencer was a tad restrictive? I've done nothing else to the car except changing silencer and going full length pipe. Intend taking it to a local car show over the weekend, so will get a good run out then.