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standard exhaust performance
#1
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:

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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?
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#2
[attachment=4122 Wrote: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:

[Only registered and activated users can see the links Click here to register]

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.


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#3
Even 1 inch pipe seems small. I need to make an exhaust for my special and asked at a local exhaust place what the smallest diameter pipe they could give me was. From memory it was 32mm and that was a special order! What do people use when making their own pipe these days?

Simon
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#4
Simon, my experience is that Alan's advice about 1" primary's is worth listening to, I would agree that the tail pipe should be larger however one also needs to consider state of engine tune.
Location: Auckland NZ
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#5
From what I under stood the tail pipe is the same diameter as the silencer's outlet pipe which required the fabrication of a coupler as seen in early pics:

.jpg   tail pipe joint .JPG (Size: 90.86 KB / Downloads: 628)

Stephen
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#6
(11-10-2018, 10:47 AM)jansens Wrote: [Only registered and activated users can see the links Click here to register]

you could also try motorcycle-custom shops instead of automobile exhaust specialist as these nowadays only seem to think in 'bigger bore is better bore' terms.
Motorcycle customshops also seem to fabricate their share of bespoke exhaust systems
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#7
I made the tailpipe for the Cheapskate Special from a builders wheelbarrow chassis. I think it is about 1 1/4" od. Anyway it was free! The silencer is made from 2 CO2 cylinders welded together. Also free.
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#8
Very interested in Dave Manns observations. Perhaps the exhaust explains the reported superior road tests of RN cars. Makes a bit of a puzzle of the published bhp (although may not be peak).
To summarise, I gather the improvement noted by Dave was due changing from a pipe which telescoped into the silencer to one which telescoped over the silencer, whereas the original would have been same as silencer.
Just to get it all straight can someone please clarify the id of RN original tailpipe, RP original?
Unlike cars with boots, provided not too many rust holes, with a short exhaust little chance of exhaust entering the cabin.
RP pipes can become crushed if the bump stops are defunct.
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#9
(11-10-2018, 07:34 PM)Dave Wortley Wrote: I made the tailpipe for the Cheapskate Special from a builders wheelbarrow chassis. I think it is about 1 1/4" od. Anyway it was free! The silencer is made from 2 CO2 cylinders welded together. Also free.

I thought my exhaust system was cheap, an old silencer for £5 and the tubing from some old Heras fencing. I like the Co2 cylinder idea Dave W.

Dave M
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#10
I am lucky to have a 7/8" pipe bender and made this tailpipe out of a steel tent pole. There was some discussion on the old forum at the time and a contributor who had flowtested a 7 engine was of the opinion that size was OK.  My wife had been complaining  the at the time that a side exiting exhaust was too noisy and smelly.  Quieter and no change in performance, standard maggy engine with a 1" SU.  cheers  Russell
 
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