E: Engine Squish

[Monkeypants]

Well my last performance road bike was a KTM RC8 and with several upgrades and powercommander on the fuel system it was a monster of a V twin engine (150 HP) The later ones had 175 hp........So by comparison our Vincent engines are very primitive and docile, even when we try to hot them up.........Your 11:1 pistons probably give about less than 7:1 with the late timing of those cams, and this drops the dynamic compression very much..........The old school cams/followers and valve train in these engines is what limits the available horse power, the bottom end is capable of much more.......like most all engines really.

The bottom end is what fell apart first on Picador testing
IIRC the first twin crank let go at a sustained 5400 rpm and 48 bhp. The mainshaft broke. As they continued testing on other engines they broke more mainshafts and also had the big pin connection give way. Turns out the Vincent crank is not "as strong as the Forth Bridge", as the journalists loved to write.
It's all detailed in MPH, can't recall the issue.
The problem was solved with the Picador crank.

Glen

 

[Monkeypants]

Hi Greg not sure what dynamic compression is? are those hp figs real or just ones taken of the crank with no generator etc. as most of them are. How quick is it and what does it do a quarter in?
My old bike did a timed 150 mph with just a top fairing and 166 mph the following year with a Super Nero copy fairing at the Classic Bike Speed trials on petrol and in race trim nearly 20 years ago. Not exactly Docile!
Cheers Roy

I think Greg was referring to my 1360 as docile, even though I tried to hot it up!

Glen

 

[Chris Launders]

I visited Mark Cook who runs CCM Britain ltd a couple of years ago and he said is personal bike (BSA B50 based) is putting out 63 bhp, he makes all his own internals etc but they are still 500s
The cranks are ground from hardened billet as he said hardening them after machining distorted them more than was acceptable to him, even then they are scrap after a season.
I suppose off road racing puts different strains on an engine to road racing though.

 

[Bill Thomas]

There are a lot of Road going 600 cc or 600 plus Comets around,
But I have never seen numbers ?, Power or speeds , Rev's etc,
People just say they are good ?, And can keep up with Twins ?,
Would love to know more ?.
Cheers Bill.

 

[vibrac]

That figure of 40 thou from Terry Prince is interesting certainly as a Comet road bike I am not investigating the higher rev range as we do on the twin racer. In reality the initial measurements on the protruding piston and the top of the liner and thickness of the barrel packers I can juggle with will give me either 30 or 54 thou squish however there are questions of gaskets and squashing of components under 32ftlb of 4 head nuts so next job will be my trusty "play dough" exercise* which not only gives me clearances but rolled in a ball and dropped in my measuring jug of water gives me CR as well (look Ma! CR found and no rings fitted yet!)

*https://www.vincentownersclub.co.uk/threads/compression-ratios-in-the-toy-shop.8213/

 

[greg brillus]

My mate Dave's 600 Comet goes very well........Mk 2 cam and I think 36 mm Dellorto, Grosset twin spark ignition running about 26 degrees on 95 octane fuel puts out about 35 to 36 Hp.......He upped the rear gearing to keep the revs down........Fun to ride, way more torque than a stock 500. Funny it takes little in the way of changes to make a Vin go better........well up to a point anyway.

 

[greg brillus]

Sorry forgot to mention this........With the 600 Comet, the clutch is a Conways Honda type.......With the tune of this engine, the clutch is right on the verge of slipping........We found ATF to be the best for this, any other modern slippery oil in there and you wont stop it slipping..........All good fun.

 

[Bill Thomas]

I have a 45 rear sprocket on, But I am a Dreamer !,
I am sure we go too big on Carb's, Unless going for Flat out speed,
I think a 32 mm Mk2 Amal will be fine for me, I do like them, But Dellorto if I was Rich !,
In UK we have 99 octane Petrol , But not sure what is in it ,
Does not smell the same to me ?.

 

[Monkeypants]

That figure of 40 thou from Terry Prince is interesting certainly as a Comet road bike I am not investigating the higher rev range as we do on the twin racer. In reality the initial measurements on the protruding piston and the top of the liner and thickness of the barrel packers I can juggle with will give me either 30 or 54 thou squish however there are questions of gaskets and squashing of components under 32ftlb of 4 head nuts so next job will be my trusty "play dough" exercise* which not only gives me clearances but rolled in a ball and dropped in my measuring jug of water gives me CR as well (look Ma! CR found and no rings fitted yet!)

*https://www.vincentownersclub.co.uk/threads/compression-ratios-in-the-toy-shop.8213/

Terry's figure is 50 thou, but that takes into account a standard Vincent twin bottom end.
I set mine at 40 thinking the extra rigid crank would allow a tighter squish. No problem, so far.
So those figures are for twins. The Comet might have less crank flex?
It seems the Comet crank withstands high rpm use better than the twin, judging by the success of the Grey Flash and and Gray Flash replica racers.



Glen

 

[timetraveller]

Tim, your posting #25; I measure the internal volume of a standard post war cylinder head several years ago and made it 103.5ccs. This was done with a thin perspex cover, to avoid the meniscus effect, with a slight bleed hole and a pipette.