Just used one of those online calculators and worked out my comp would be 12.34:1 with alloy heads. Has anyone pushed the limits of this Shell 100 stuff yet. Would be able to get away with retarding it back.

I have a second set of heads with bigger chambers, that worked out to be 10.58:1 compression, would I best best using those with more advance or the others (12:1) with the timing retarded (if possible)

you may think of getting an octane booster....
NOS makes a really high boost one (i think about 10 points??)

otherwise mabye look at a thicker gasket, although im not sure how much difference that would make...

as to retarding the timing, im not sure. you could do it although i would assume you would loose some bottom end power, and gain in the top. simular to haveing a smaller stroke. not sure though...

it depends if you are trying to keep the stock ECU happy or not.

running really tight squish (piston to head clearnce) will help reduce the likelyhood of knock.don't go less then 30thou
basicly pay very careful attenion to the whole build, get very chamber the same cc, make sure every piston has the same deck hight, and any dishes or domes in the pistons are exactly the same.
you could even go as far as coating the chamber and valves to stop hot spots causing pre-ignition


with both sets of heads, do they have the same ports but one set with smaller chambers? or are they completly different?

go 10-1 comp. ratio if pistons stock,other wise forged for strength, on 12-1 comp.ratio,100 shell be ok adjust timing to suit cam spec. - will 100 shell supply be consistant or vary -new line?

Thanx for the advice guys. I ended having someone with a 5.0L buy the higher comp heads off me for a reasonable price, so that was the decision maker for me. Should get him 10.5 to 11:1 comp on his ball tearer of a Windsor. Perfect for 98 fuel.

The lower comp heads will give me less ifs and buts, plus happily run on 98, which is more widely available anyway. Plus leaves the options open for a bit of nitrous of low boost blower at a later stage.