Pete 55Tbird (1/6/2012)
I have been following this discussion on deck height and I have a couple of questions. The original poster said his motor was going to be a street driven, mild cam and have 9.5 compression( static).
The questions are
1 At what point approaching zero deck (piston top to block top) would a .009 difference make any measurable difference?
While Jepito doesn’t mention in this thread what bore and stroke he’s working with, a run of the mill bored 292 with a 0.009” variance between piston heights accounts to about 0.16:1 variance in compression ratio assuming all the combustion chamber volumes are the same. That’s not much in the grand scheme of things but it is a variance that’s worth reducing if concerned about overall performance or the long term bearing wear.2 Would balancing the crank etc be of more benefit for the same cost?
Pete. Balancing is in a separate category from engine blueprinting and is equally important. It’s not one of those things that can be substituted for an engine build area that’s not up to par.3 With a 9.4 static compression ratio and a mild cam wont the ignition timing have to be retarded so much that performance is compromised
No. But 9½:1 is a safe limit for 91-93 octane fuel with the smaller cam grinds before ignition timing constraints come into play. As the intake durations get longer, then the compression ratio can also get larger than this and still optimize the ignition timing. The EMC Y engine was still running at 38° total timing although the static compression ratio was 10.8:1. This worked because the dynamic compression ratio was 8.5:1 and that’s where the camshaft that was being used was a critical part of the low octane fuel combination. There were zero detonation issues with this combination on a fuel that was 86 octane (MON method).4 Even with a true zero deck you still will have to use a cylinder head gasket of some thickness so how does that figure into your assumptions.
As long as a composition head gasket is being used, then zero deck simply reduces the quench area which as a general rule should be no less than 0.035”. That means you could potentially run the piston 0.005” out of the hole with a 0.040” thick gasket and get away with it. Reduced quench allows for higher compression ratios while also reducing the propensity for detonation. As a general rule, quench areas up to 0.065” are tolerable before detonation comes to the forefront with all else being equal.