Piston-to-head clearance affects...
Piston-to-head clearance affects "quench distance" in an engine. It's a fine line to walk when finding the minimum clearance available for your type of engine.
So Many Questions
Q: I build some restricted 500 4412 carb engines for some local racers, and I have some questions. First, how tight can you run the piston-to-head clearance on an engine like this? I have run it as tight as .030. Is there a point that it gets too tight and adversely affects burn rate or hurts power in some way? We are restricted to a solid cam, but we can run any exhaust we want. We can also run alcohol. We must run a production head with no porting. Would you put oversize valves in a Vortec head? As far as cams go, would a single pattern benefit me?
A: Consider that alcohol tends to burn slower than gasoline. Because its latent heat of combustion is roughly half that of gasoline, an increase in volume is necessary to approach equal or higher power levels. You already know this, but it sets a stage for comments that follow.
While the intake-to-exhaust flow relationship for the Vortec heads is pretty good for gasoline and rules limitations, you might want to consider two possible changes to the intake side. One is to increase valve size and the other is to use a dual-pattern cam that favors a later intake valve closing point. This can allow an increase in volumetric efficiency and help compensate for the alcohol's lower heat of combustion (compared to gasoline) by the potential for increased fuel volume. Larger intake-valve size may help net airflow at higher lift values, but it can lead to reduced flow velocity in the under-4,000rpm range if that's important to you. The camshaft might be an easier, more effective and less expensive area to investigate.
Eddie Dickerson Chassis Specialist...
The head-to-piston clearance issue may be a little less defined. Since you didn't reference compressed gasket thickness, I'll assume you're not talking about deck height but total quench distance. Generally, you'd like sufficient "quench" to keep mixtures active (for improved combustion activity or "mixture motion") but not so tight that it reduces net burn. Opinions will vary on this subject, but we've seen clearances in the 0.024 to 0.028-inch range run successfully. Combustion chamber shape can influence this minimum especially with alcohol.
You may also discover that quench height reduction can have an effect on optimum ignition spark timing. For example, if you're "over-quenching" the burn, then you may find an additional one to two degrees of spark timing is required (short of detonation) as compared to optimum spark if the burn is progressing normally. While you may consider this nit-picking the issue, it's simply to suggest there's an explorable relationship between flame rate and spark timing. Rather than look for all the reasons, this is only to suggest that changes in quench height should be accompanied by spark timing experimentation in small increments.
Hopefully, this will stimulate some thoughts.Jim Mcfarland
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