Mailbag: What is Engine Quench?

Q: What is engine quench?

A: Quench is the distance between the top of the piston at TDC and the cylinder head surface. It is also called “squish” or “piston-to-head clearance.”

(Image/Summit Racing)

How is engine quench measured?

Quench is a calculated dimension. It uses the following formula:

Quench Deck Clearance + Compressed Head Gasket Thickness

How does engine quench affect performance?

The right amount of quench can promote fast, complete burn of the air/fuel mixture. It creates turbulence, which forces the air/fuel mixture toward the spark plug. This reduces the possibility of detonation. It also leads to cleaner emissions.

What is the right amount of engine quench?

Engine quench should be as tight as possible, without the piston contacting the head. Recommended quench depends on your max rpm and the type of connecting rods you use.

Max RPMConnecting Rod TypeQuench Range
6,000 or lessSteel0.035 - 0.045 in.
6,000+Steel0.038 - 0.043 in
AnyAluminum0.050 - 0.065 in.

Tech Notes

  • Aluminum rods get longer as they get hotter. Increased quench will prevent the piston from contacting the head.
  • Don’t run more than 0.060 in. quench trying to lower compression ratio. This will slow the combustion process and could cause detonation.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.

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