E: Engine Squish
- Thread starter vibrac
- Start date
Hi Tim, Yes that's what I have set mine to on my 1272cc motor. I used the " crushing of solder" method and a micrometer. Remember I have Terry Prince "bathtub" heads with a huge 20 degree squish angle and flat top forged Omega pistons machined to match so things may be different. I started at 0.040" and then went down top 0.030" and stopped there. It may have come from "Tuning for Speed". Phil knew a " Thing or Two"
Cheers Roy
Cheers Roy
And the effect is remarkable. I remember helping Roy at a race meeting at Brands Hatch years ago. I had ridden my bike to the meeting so was well used to handling Vincents. At some stage Roy needed his bike moved further up the padock and I offered to ride it up the hill for him. I made a total prat of myself by stalling the bike, three times, trying to pull away at 2,000 rpm as I would have done on my road bike. The cams in the bike at that stage produced very little power at 2,000 rpm. Later Roy fitted the Terry Prince heads and the bike ticked over like a traction engine and pulled away at low revs. There were other changes as well but my impression was that the squish heads made a huge difference to the efficiency of the combustion. What a pity that Terry's cylinder heads and barrels seem to be no longer available. Is anyone else thinking of continuing their manufacture?
Last edited by a moderator:
Yust looked Terry s notes up: between
1.2 - 1.4 mmmmmmmmmm
1.2 - 1.4 mmmmmmmmmm
50 to 55 thou ?,
Hi again, As I understand it large squish clearances reduce the effect considerably. So the closer you can get without touching the better. Ideally the whole squish area should remain clear of combustion products (Carbon Build-up) I am not sure but I think my 1272 piston/cylinder head setup is quite different to Terry's 1200. What I did is documented in the series "Once More into the Breach" that I wrote for MPH over quite some years. Basically get the squish right and you can considerably retard the ignition and decrease the main-jet size with a substantial power increase. My compression ratio ended up at 11.3 to 1. and I use 50/50 Avgas and Esso Super.
Note this is on a Full race bike which gave 104.6 bhp at 6543 rpm at the crank and 93.7 bhp at the rear wheel. Torque was 90.9ft lbs at 4350 rpm and changing the 40mm Smooth bore carbs to 37mm reduced the power to 91.6 bhp but increased the torque to 92.4 ft lbs at 4366 rpm
I only wish I had taken photos as I went as David Dunfey did for his SUPER book (that I have just started to read.)
All the Best
Roy
Note this is on a Full race bike which gave 104.6 bhp at 6543 rpm at the crank and 93.7 bhp at the rear wheel. Torque was 90.9ft lbs at 4350 rpm and changing the 40mm Smooth bore carbs to 37mm reduced the power to 91.6 bhp but increased the torque to 92.4 ft lbs at 4366 rpm
I only wish I had taken photos as I went as David Dunfey did for his SUPER book (that I have just started to read.)
All the Best
Roy
I think .030 will work well. The minimum is about .010" Tighter than that and you risk the pistons hitting the head. I think that anything less than .060 will give a squish effect. My Vincent only has 8:1 pistons so there is no squish at all and no pinging ever. I think that the Rapide engine relies on the swirl from the intake port, to keep the flame speed up to avoid pinging. I have had some Triumph engines with 10.8: 1 compression ratio and those engines relied on squish prevent pinging.
My race single runs 0.030" with a 12:1 piston on methanol........Its output is about half what Roy's engine does........I'm not sure if that is good or not........The valve train and cam/followers is the big limiter on these.......Trying to do something about it, but not an easy job. I'm interested to know what type of cams Roy uses and the lift, plus opening/closing points.........It's a work in progress.
For once my interest is not from a racing point of view. I am building new top end on My electric start Comet (Road), and I am loath to open out a standard head to 90mm so I simply want to use the 3mm land formed by the 84mm head to 90mm bore discrepancy and the top of the piston to give me a bit of pep to my relatively low (still to be measured ) CR. I am also firing twin plugs on the BTH (I have them so why not use them) so finding a good ignition timing will be fun. Fortunately my son Ben is installing a rolling road intended for classic bike tuning so we should be able to get it right.
Last edited by a moderator:
Tim, Hate to sound negative, but the twin spark BTH will probably cause you more grief than you might know........The issue is that the advance curve is the same as per the single spark version, only the output wires are simply doubled up to drive 2 output coils. What this means is that if you run say 26 to 28 degrees full advance, the fully retarded possition will fire way after TDC. If your engine has electric start you might get away with it, but on the kickstart it will be difficult to start......Good luck with it all.