Stuart accepts the Bertie Goodman Award during the 2010 National Australian Velocette Rally at Lennox Head, NSW from Anne Frampton (nee Goodman), Bertie's daughter and the Club Patron.
But in developing his Velocette record breaker, he has found time to be diverted into looking at the standard Velocette fishtail silencer used on the Venom/Viper/MSS and their derivatives...
Originally forming an article for the Australian Velocette OC magazine FishTailDownUnder (FTDU), which I've yet to use, Stuart was happy for me to pre-empt the article in FTDU with this post....
An Exhausting day on the Dyno
My rally Venom has been fitted with a virtually dead stock engine..... So before I refitted my special squish engine I thought it might be a good time to get to the bottom of its long term lack lustre patchy performance. I fitted a new piston and cast iron barrel and carefully checked everything but the performance was still only fair. A few days later it went dead flat and I staggered the 80 km home. After checking and changing everything to no avail and cursing somewhat, I aimed a disgusted kick at the silencer.....................and it rattled !!
Here at last was the problem...... I cut open the silencer and found the outer sleeve baffle had come away and jammed in the rear , choking the exhaust...... This baffle must have been cracked for some time and finally failed , and as the inner baffle on my bike is not attached to the pipe and had been fitted in various positions, this all conspired to give erratic exhaust effects, upsetting carburetion and performance. I fitted a plain pipe extension to scrambler specs and the bike went like a rocket. I was however curious (skeptical) about the efficiency of the silencer so I decided to do some testing on my dyno....... here are the results, which I am sure will cause some interest.
Most runs were between 3 and 5000 rpm and all power figures are given at 5000 rpm.... at this RPM the engine is not stressed and is well on the cam a bit past peak torque and it greatly simplifies comparison, all runs were with a 30mm Concentric with 270 main, needle no 2 groove, slide 3 ½ ....these were found to be “universal settings” .... A/F ratio fluctuated somewhat but was between 12.3 to 13 at full throttle so optimizing jets for exhausts was not bothered with but admittedly would have had some effect. Fuel was 98 octane, ignition adv 38 degrees, 17/8 cam, compression ratio is lowish at just a whisker under 8:1.
Velocette factory power figures were taken at the crankshaft with new well sorted and set up engines.... at 5000 rpm a reasonable 500 Venom engine should produce close to 30 HP at the crank .......The following figures are taken at the wheel ( where it counts ) so some power loss in the driveline ( chains, clutch, gearbox, rear wheel etc ) must be allowed for.
To establish a baseline the engine was set up on a straight through pipe 50 inches long.
Run 1.... 1 5/8” Straight through 50 “ ..... “scrambler” pipe...
no silencer................. 24.6 HP
Run 2 .... 1 5/8” Clubman pipe .... Fishtail .... baffle 10” protrusion in silencer ......... 19.5 HP
Run 3 ....1 5/8” Clubman pipe .... Fishtail... long baffle bottomed in outer baffle...... 20.2 HP
Run 4.... 1 5/8” Clubman pipe.... Fishtail ... NO inner baffle ....................................... 23.4 HP
Run 5 ....1 ¾ “ Clubman pipe .... special low resistance silencer ............................... 23.9 HP
Run 6 ....1 5/8” Clubman pipe ..... special low resistance silencer ............................ 24.7 HP
Run 7.... 1 ½” Clubman pipe ...... special low resistance silencer................................. 25.3 HP
Approximately 30 runs in total were made, often going back to earlier setups to check repeatability and accuracy. .... NB..... The “universal carb settings” should not be taken as gospel for any other bikes..... final carb settings ,particularly the slide and needle, should be arrived at for individual applications by careful road testing, although significantly different settings should treated caution.
A couple of points.....I did some runs from 2000 rpm up and as a generalization there was not a huge difference in any of the systems up to about 3000 to 3500 rpm, but from there on it became quite distinct.. I was surprised to see the small 1 ½ “ pipe give the most power..... although the difference between the performances of the three pipe sizes was less marked when fitted with the restrictive baffled Fishtail and the effect of the pipe sizes may have been different at higher rpm... No baffle resulted in reasonable top end but patchy mid range performance....Also surprising was the long bottomed out baffle being slightly superior to the shorter open baffle... The special low resistance silencer also gave considerably less fuel stand off from the carburettor. It is very apparent that when any baffle is fitted in these particular Fishtails the power is quite adversely affected. Varying the length of the 50” scrambles pipe was not tried, nor did I test megaphones or test at 6000 rpm, mostly I stuck to the 2500 to 5000 range......after all this is a two up rally touring engine and I was trying to optimize the setup for my own bike. Road testing has confirmed the dyno results.
Both Fishtails in question are a common current reproduction type I see fitted to many of the Velos in the club..... I do not know for certain if they are exact replicas of the original, but from what I have seen in drawings they appear pretty close internally .... I do however find it impossible to believe Velocette fitted silencers as restrictive as those I tested , in fact I doubt a Velo fitted with one of these silencers could ever perform very well.. If anyone has a guaranteed original genuine article, I would love to test it. There must be quite a few bikes out there strangled by their beloved pattern Fishtails.
Here are a couple of photos .... one of the various pipes and silencers trialled and one of the offending internal baffles .
Dennis.... Perhaps you might have a good diagram of the internal layout of a fishtail...... I have also recently done a lot of dyno work on carburation and ram tubes etc .... I trialled five different carburettors , Amals and Mikunis in one day ...and hopefully soon I will dyno some different cams so a couple of follow up articles may be of interest.