Vincent Related David Tompkins Vincent at Daytona

[vibrac]

As for comparing a Manx with a Comet I guess US Nortons are not like UK ones (Molnar etc) Now Ben on the Vincent twin won every race (except the one he dropped a valve in) last season and it was giving around 60BHp and 60ftlb torque flat lined from 2K to nearly 6K. and we are doing an engine upgrade this winter because we are just about equal pegging with the Manx contingent. remember we raced a series winning Comet for over a decade but never thought it would beat a 2000+ developed Manx (They aint what Bob Mac rode thats for sure, although they look as if they are)
Now barrels and big bore
I have a 90 bore with standard studs its off the Comet racer and I guess it goes back a long way, its in good condition but as i pictured a week or so ago its piston is cracked I have got another piston coming but the only head I have for a 90 bore has a welded insert in the head for the exhaust pipe stub,so although I am not a slave to standard I would like to use a standard head. However looking down the bore with the standard head in place, there is a 3mm (90mm-84mm/2) ring at the top where the standard head overlaps into the bore. As this is a road bike and I am looking for only about 8:1 CR, it would be nice if I could incorporate that as a .030" squish band and that together with a twinspark should give me a real tractor single and an easy life for the electric start.

 

[greg brillus]

The Vincent engine is a good strong design, but not an easy one to easily get big HP from........the basic issue with most all these old engines is the relatively primitive cam/valve train design. This coupled with the combustion chamber shape, and low port angles does not help........Running a good twin in even basic tune will be a winner, only due to the shear cubic inches alone........The downside is the massive weight.......An average twin even with many parts removed and lightened were possible is still nudging 90 odd Kg's just the engine unit........The big downside to this in racing is stopping it. The opening and closing of the valves is were it matters most........having massive duration big overlap cams seem great but the big downside is the huge loss in compression........I'm sure in my engine this is where i'm loosing out.........The cams and followers are very , very small, and so once you hop things up, the load on them goes through the roof.........This is probably the biggest weekness in a Vincent for getting bigger power gains........You need to snap the valve open quickly, hold it open for as long as possible, then snap it shut........with our valve trains, this is very difficult........A weak supply of oil delivery here will not help........ Symetrical race cam lobes, roller followers, and ratio rockers is the answer.......But then you need much stronger valve springs to cope.......lighter valves and other parts are a must........This is all known stuff.......The Horner brothers use this exact spec on all their engines........but it is mostly beyond all of us without very deep pockets or a full CNC machine shop like they do.........I am trying to do some but not all of these mods, in an effort to get some gain.........My ultimate gain would be to make the roller followers and symetrical cam set up........That would give the best gains I feel.......Making it all survive is another thing, so beefing up the entire valve train is the only way.

 

[bmetcalf]

Vibrac, with twinspark, would you have to go to the dread knitting needles to have a decompressor?

 

[Bill Thomas]

I think a lot of power can be made or lost with the exhaust ?,
When I was racing the pipes were getting shorter and shorter,
But now the rules say no Noise !, In UK.
I just ran a stock L/ning rep and could not be bothered at that time,
But my Special I raced before was much faster, It may have been the bigger pistons ,
But I think the the 2 into 1 with a huge out pipe did a lot to help.

 

[vibrac]

Vibrac, with twinspark, would you have to go to the dread knitting needles to have a decompressor?

There is I think little chance I would go back to knitting needles and I pondered long about loosing my greatly loved RE decompressor and then I experienced a week in Spain on the Columbres Rally with the Electric start Comet and realised I did not kick it over once, and every time I pressed the little red button I did not touch the valve lifter! so with a lower compression I dont think I need one. I could always pop the RE compressor in the tool kit with a ring pull just in case but it wouldn't be the first single I kicked without a decompressor

 

[Bill Thomas]

If stuck , Just roll it back on compression in gear,, Take it out of gear and kick,
I don't have anything on my old Comet, Since 1977 !.

 

[greg brillus]

A bit of a shame, I was hoping to spend some time playing with the Godet Grey Flash and egli racers of Luis Gallur's, but with the covid thing everything there has gone silent..........These are quite high spec engines compared to an original Vin single........ I think the obvious difference is the bigger bore shorter stroke, one piece crank with split con rod........This with a good oil pump allows the engines to run up to 8500 RPM with no issues, with over 9000 seen on the dyno. These extra revs give an instant boost to HP figures........The dyno figures I had seen last year showed about 52 to 53 on the Flash engine and the Egli was about 58........The Egli having seen some extra engine R & D carried out here in Australia gaining the extra power somehow........Of course with that, we were hoping to gain some assistance from Ken horner........But again to date this has not happened........ The roller cam/follower set up is the way to go, but it is costly and difficult........Amazing that Harley's had it all those years ago........It is something I will continue to chip away at. On my bike I am just changing the gearbox from the TTI alloy 6 speed to a magnesium 5 speed, and once my cylinder head comes back, then the fun of piston to valve clearance will be my next challenge. We are probably going to raise the inlet port and steepen the manifold/carby angle.........and then there's the Supercharger..........!!1@*&........?

 

[Bill Thomas]

If I build a 92 mm bore x 90 mm Stroke, Does that mean is over square ? ,
Surely that means it will be revvey ?, Not sure I want that,
Me being an old plodder now ?.

 

[greg brillus]

Bill yes over square........same stroke as stock at 90 mm, no not revvy as you might think, just way more torque. You can gear them up similar to a twin so the motor does not rev so hard for the given speed. I find a descent 600 Comet is a blast to ride........If the factory could have made them this way, the sales of twins might have been far less. I think from memory a 600 Comet with a 36 mm carb and Mk 2 type cam puts out about 35 Hp..........Good fun to ride. The 600 Norvin single road bike I built a few years back was my inspiration to build my racer. Cheers.

 

[Chris Launders]

There is I think little chance I would go back to knitting needles and I pondered long about loosing my greatly loved RE decompressor and then I experienced a week in Spain on the Columbres Rally with the Electric start Comet and realised I did not kick it over once, and every time I pressed the little red button I did not touch the valve lifter! so with a lower compression I dont think I need one. I could always pop the RE compressor in the tool kit with a ring pull just in case but it wouldn't be the first single I kicked without a decompressor

Is there room to fit a chainsaw one, they are only 10mm,Just push them down and they snap shut when the engine fires.