standard exhaust performance

[Dave Mann]

JonE pid='17573' dateline='1539248138']I have always been intrigued by the Birkett statement in the green book suggesting long, narrow bore 7/8" ID tailpipes were best. This old forum 2014 thread is worth re-invigorating:

http://pub25.bravenet.com/forum/static/s...8&cmd=show

Ray White's comments on replacing the large bore rear pipe of his Swallow with an 1" bore (and finding it improved) contrasts with Sandy Croall's observation that when his LWB rear pipe fell off, performance improved (it became akin to the SWB system).

Any more recent experiences from those experimenting with standard manifolds and engines? Were Austin exhausts overspecced in diameter?

Prior to 2000 both our RNs had run with no exhaust tailpipe because those that were available were the incorrect shape passing over instead of under the rear axle. In mid 2000 David Martin informed me of the correct shape of the 3 speed RN tailpipe and I bought a LWB tailpipe, cut it in half and made 2 tailpipes for our cars and fitted them and thought no more of it. A couple of weeks later we set off one Friday evening for the first of Malcolm Parker’s Pennine Runs, first stop Matlock a run which takes about an hour on a quiet Sunday morning, not this time. To put it bluntly I could have pedalled faster, it wouldn't pull the skin off a rice pudding. After about 5 miles, I realised what the cause was, stopped & removed the tailpipe, the change in performance was dramatic. The rest of the weekend lived up to the Malcolm Parker tradition, but it didn't escape my notice that our slow RN wasn't as slow as some of the RPs on the run, in fact one owner commented on this and wanted to know what go faster tweaks I’d done to the engine. On returning home I looked into this and found that originally the 3 speed RN tailpipe was part of the silencer and not a separate pipe pushed into the silencer outlet, so I made a new tailpipe of larger diameter which fitted over the silencer outlet and found no reduction in performance. Remonstrations with the supplier brought no satisfactory response, passing it off by the fact that updraught carburettor engine needs a shorter larger diameter exhaust than the later side draught version. Just to prove that was a load of manure I fitted a 26Va side draught carburettor and tried both tailpipes, which produced similar results.
See photo, I know there are bolts missing, I'd just put it on for the photos.