In another thread that I don't want to hijack Greg Brillus wrote:
"If any of you are running the squish band top end kits with twin plugs you need to be absolutely sure of what ignition system you are running, if it is not right you can be sure of a piston failure. There is a "lot more" involved with some of these bolt on improvements than one might at first think."
Since I am using one of these big bore kits this concerns me of course. I have already learned that I should use piston skirt clearances of 5-6 thou rather than the 3.5 recommended. Even though I live in Maine, the new climate we seem to be experiencing means a few weeks every summer when we see temps above 33C.
I will be using a Power Arc programmable ignition we are developing that allows me to design my own advance curve. Bob Larmour built the housing. I have been running an alpha prototype successfully on my B Rapide for a year, but we needed to get a few changes made, and are in the beta stage development now.
As delivered from PA and modified for Vincent engines it has four curves available. and is a multi spark system with three sparks and adjustable dwell and rev limit
The top curve is the most advanced and is used when the two sensor wires are grounded. the bottom curve is the least advanced and results from ungrounding the two sensor wires. the two middle curves are intermediate and will allow selecting an advance that fits a standard single plug road going Comet or twin. I intend to use a Voes so under load I could automatically drop down the advance to something like the last curve here.
And here is a "playing in the sandbox" curve that is at the extreme low end for a Comet dual plug squish engine. Completely unscientific and the result of 5 minutes playing around with the curve. I know some dyno time may be warranted, and we are planning on it, but this shows what is possible.
As you can see, the curve is programmable all the way up to the rev limit. Davidd has found that a curve as low as 19 or the low 20's may be useful, so we will see. All still in the developmental stage now.
Finally, other recommendations from the top end fitting instructions:
Ring Gaps
TOP CHROME 0.015”-0.020”
SECOND 0.010”- 0.020”
Oil hole is optional with forged CP pistons
Before finally fitting of cylinder muff and piston the squish
clearance must be checked, it should be .050”to .055”
Any thoughts on whether these figures are OK? Especially ring gaps. I know Greg recommends using the oil hole, though others have said it is not really necessary
Thanks