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Quench Quest

Squish, Quench, and Piston-to-Head Clearance

Jeff Smith Apr 1, 2002

Step By Step

This illustrates the basic layout of a piston, deck height, head gasket, and combustion chamber. Piston- to-head clearance is the combination of the deck height clearance of the piston and the compressed (installed) thickness of the head gasket.

Quench in a wedge-shaped combustion chamber is the area between the top of the piston and the flat portion of the combustion chamber. A flattop piston with a 0.005-inch negative deck height (below deck) along with a 0.41-inch gasket (compressed thickness) represents a piston-to-head clearance of 0.046 inch, which is tight.

This photo shows the stock cast piston used in the Goodwrench service replacement engine. This is an older-design piston that uses a full dish (concave design) that eliminates most of the quench effect regardless of the piston-to- head clearance.

These three pistons represent three different head configurations. The flattop piston with valve reliefs (left) is the simplest. The middle piston is a half-dished piston that utilizes a dish to reduce compression while maintaining the flat portion of the piston to create a quench effect. The domed piston (right) increases compression but still retains a flat portion for the quench effect.

Hi-Tech offers the Jim McFarland-designed Swirl/Quench hypereutectic piston designed to increase swirl and turbulence, and intended for use with the older, bathtub-shaped small-block Chevy combustion chambers. Independent testing has found as much as 5 percent improvements in torque and horsepower.

Taking the Swirl/Quench concept one step further, JE now offers a different configuration as a dished piston that still uses a mild ramp and dimples to create turbulence in the chamber and to direct combustion by-products toward the exhaust side of the chamber to increased cylinder scavenging. Here too, you can see a dished piston with a flat portion to increase the effects of squish.

7 As the piston reaches TDC, the air/fuel mixture is squeezed out from the quench area and creates turbulence in the chamber.

Chamber design can have a dramatic effect on combustion efficiency and power. Compare the old D-shaped small-block chamber...

...to the much more aggressive kidney-shaped chamber that requires less ignition timing to make more power, often with less fuel.

The more we learn about engines, the more it becomes apparent that every detail is important when building a high-performance engine. Sure, the big components like heads, cams, intakes, carbs, and exhaust are crucial to building power.

But there are also those subtle details that often go unnoticed. If you’ve ever wondered how a professional engine builder squeezes 450 hp out of a small-block while your identical twin barely makes 390 hp, the difference is probably in the details.

All the tuning and games we play with camshafts, cylinder heads, and headers are aimed at helping the combustion process. That’s when the mixture starts to burn and the air and fuel turn into cylinder pressure that pushes the piston down. But there are plenty of subtle little things that can influence the combustion process that you may not have heard of before. That’s what we’re going to look into in this story.

For decades, sharp engine builders have known about “quench” or “squish.” These terms refer to the area in a wedge-shaped chamber designed to create turbulence in the chamber as the piston approaches top dead center (TDC). This squish effect can also occur in other types of chambers as well. We will limit our discussion in this story to small-block Chevys, but the basic facts relate to all wedge-shaped combustion chambers.

The quench area is the tight area between the flat portion of the piston and the flat portion of the combustion chamber in a typical wedge-style chamber. As the piston reaches TDC and the mixture begins to burn, the air and fuel located between the piston and the head is squeezed or squished out into the dished portion of the combustion chamber. Think of the turbulence that occurs when you smash a tomato with a large mallet and you get the idea. With a flat-top piston, this squish area can be very tight. This is also the tightest clearance between the piston and the cylinder head. Since mechanical contact between the piston and the head is not advisable, most production engines rely on a piston-to-head clearance of 0.060-inch or more in this area.

Unfortunately, this is not an ideal piston-to-head clearance for optimal squish. But because of production tolerances, factory engines usually fall on the larger side of the clearance for obvious reasons. But when it comes to optimizing a performance engine for more torque and horsepower, this is an area where a knowledgeable engine builder can squeeze out a little more power.

The Squish Effect

Since wedge-style combustion chambers rely on the squish or quench area to create turbulence in the combustion chamber, an intriguing effect occurs in the combustion process. To better understand this process, imagine that the intake valve opens and a rush of air mixed with fuel enters the combustion chamber area. The piston comes screaming up toward TDC at 5,000 rpm (almost 3,000 feet per minute) as the intake valve closes. As the piston reaches TDC, a virtual hurricane of fuel and air is squished out into the chamber from this tight area between the piston and the head. While this turbulence sounds bad, the opposite is true. This turbulence has the effect of more thoroughly mixing the air and fuel into a much more homogenous mixture that tends to burn much more quickly and efficiently.

One way to produce maximum power from an engine is to use the least amount of fuel necessary to create maximum power while attempting to burn all of it. Given this, if you can evenly mix the air and fuel into a homogenous mixture with an extremely fine mist of fuel, you will make outstanding power.

Unfortunately, the opposite is also true—varying pockets of lean and rich mixtures within the cylinder when the spark plug fires will cost power and the combustion process will not be as smooth. Excessively lean or rich pockets within the chamber directly affect the rate of combustion and the amount of pressure applied to the piston. Rich mixtures tend to burn slower, while lean mixtures generally burn at a faster rate than a “proper” air-fuel mixture. If modifications to the chamber or piston affect these rates, the ignition timing will also need to be changed to optimize power.

What is the proper air/fuel mixture? In the last few years, the answer has been changing as the area between the combustion chamber and the top of the piston becomes more efficient. For example, the classic air/fuel ratio has always been 12.5:1, meaning 12.5 parts of air for every one part of fuel. But many race and properly designed street engines can make best power with air/fuel ratios now approaching 12.8 to 13:1.

So now let’s introduce a tighter quench space into this equation. All of the respected engine builders who we’ve talked to are firm believers in minimizing the quench clearance. According to Ken Duttweiler, the tightest quench he recommends is around 0.050-inch. He has built engines with far tighter clearances than this, but much of this depends on the piston-to-wall clearance. All pistons tend to rock slightly as they transition through TDC and this rocking motion reduces the piston-to-head clearance. Smaller-diameter pistons with tight piston-to-wall clearances don’t rock nearly as much in the cylinder bore compared to larger-bore pistons with wider piston-to-wall clearances.

Since piston clearance plays such a big part in piston-to-head clearance, it is possible to run a piston-to-head clearance tighter than 0.040-inch if you feel brave. Noted horsepower hero John Lingenfelter says that clearances of 0.037 to 0.040 inch are possible, but you must know what you’re doing. The late Smokey Yunick also recommended a quench clearance of 0.040 inch as a safe but critical clearance.


So what are the benefits of all this squishing and quenching? The benefits are small, but >> often important. Pump-gas engines that run on the ragged edge of detonation, for example, can greatly benefit from a tighter piston-to-head clearance to reduce rattle. That sounds contradictory since increasing compression should lead to increased detonation. All the engine builders we spoke to mentioned that tightening the quench (reducing the piston-to-head clearance) to get it under 0.050 inch will increase the static-compression ratio, but this tighter clearance also creates a more powerful squish effect. This additional turbulence creates a more homogenous “soup” in the chamber, reducing the harmful effects of lean air/fuel ratio pockets. With all other variables being equal, this contributes to creating an engine that is less prone to detonation.

We tried this on a recent dyno flog of a 383ci small-block. To keep the compression at around 9.5:1, we used a set of 0.050-inch head gaskets that created a wide piston-to-head clearance of around 0.060 inch. CHP engine flogger Ed Taylor swapped in a set of 0.040-inch Fel-Pro head gaskets and then tested the engine again. We witnessed only a marginal gain of around 2 to 3 hp (less than 1 percent), but it’s doubtful that the marginal increase in compression was responsible. Clearly, tightening quench with a thinner gasket had something to do with the increase in power. Tightening the quench area often results in the reduction of ignition timing requirements. This can then lead to a reduction in negative work (the cylinder pressure rising while the piston is still approaching TDC). This often is evidenced by a gain in low- and mid-range torque.

There is plenty of discussion about the net effect of squish and quench. While it’s doubtful that this will ever amount to more than a few horsepower in any street application, it does offer some distinct advantages when it comes to increased engine efficiency, better fuel mileage, and driveability. If you’ve ever wondered why certain engines run better than others, this could be one reason why.


JE Pistons
Huntington Beach, CA 92649
Hi-Tech Engine Components
Salt Lake City, UT 84104

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