Thread: Domed pistons and quench?

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Originally Posted by FAYLUR

I've never used domed pistons so I've never had a need to know this but can someone explain if you should be concerned as much about quench when using domes? If you go with .035"-.045" then the head combustion chamber will have to be the proper or 'matching' CC's? Or do domed require more quench? And,,,,if the pistons are designed to provide say,,,12.5:1 with 64cc then using a 76cc would give too much quench? Is this right?

You probably wouldn't be concerned about quench with domed pistons because you'd probably be using a fuel that would be of sufficient octane to prevent detonation in the first place. The quench takes place between the flat crown part of the piston and the flat part of the cylinder head, opposite the chamber. With a dome in the way, the mixture couldn't "jet" across the chamber the way it can with a flat or dished piston.

" if the pistons are designed to provide say,,,12.5:1 with 64cc then using a 76cc would give too much quench? Is this right?"

No, you're confusing static compression ratio with quench. Quench is the clearance between the piston crown and the cylinder head at top dead center and is set at between 0.035" and 0.040" if using steel rods. The idea is to "jet" the mixture across the chamber to mix it up for complete combustion and also to eliminate any "dead" spaces within the cylinder/chamber.

If you had a piston that was designed to give 12.5:1 static compression ratio with a 64 cc chamber, then changing heads to use a 76 cc chamber would lower the static compression ratio to 10.6:1. The quench would remain the same, as long as you used the same piston and the same deck height.