The first question asked about this engine by its new owner was: "what is a tight squish motor?"

I realized that I had not explained that term when I put up the gallery, nor had I explained it in correspondence with her as the Project progressed. I will endeavor to explain the term, and the approach.

*****
UPDATE: August, 2007. I have been working on improving my 'explanation' here for the last several days. I'm not finished yet. So if some things don't seem quite in order, patience is requested.

I'll get it done as time permits.

*****

"Squish" is a term that is used to describe what goes on when certain things take place inside the motor; and has three main components that work together to create "squish"

1. Squish Action
2. Squish Wave
3. Squish clearances; Squish dimensions [areas; sizes]

"Tight Squish" is an Approach to building a motor that optimizes #3 in order to maximize #1 and #2.

[there is a synonymous term..."quench"...that is often used to describe some of what "squish" is and does. I use "squish" and "Tight Squish" rather than "quench" because I believe that those terms are descriptively more accurate than 'quench' regarding what is occurring, and much more complete in covering and accounting for what is occurring than does the very narrow definition of the term 'quench'.]

1. Squish Action:

...is what occurs to the air/fuel mixture in the cylinder, and in the combustion chamber, as the piston comes up to TDC on the compression stroke. As the piston nears TDC, the piston crown comes close to the cylinder head, and compresses the air/fuel mixture.

...in the SOHC redblock, the combustion chamber areas of the head have some surfaces areas that are flat; and those flat surfaces are where portions of the piston crown will come into close proximity [at TDC].

...where those surface areas of the head and piston come into close proximity, the compressing of the air/fuel mixture will be the greatest. And that extra compressing...or squeezing...or 'squishing' doesn't actually compress the air/fuel mixture more:

....it squeezes the air/fuel mixture out from between the head and piston crown close spots, out to the areas in the head that are more open: the main combustion chamber.

Squish Action is this squeezing...this squishing...of the air/fuel mixture out from between the places where the piston comes closest to the cylinder head.

2. Squish Wave:

...The squish action generates pressure waves..."Squish Waves"...that greatly increases the turbulence of the air/fuel mixture in the combustion chamber as the piston approaches TDC to finish its compressing of the air/fuel mixture.

[Squish "waves", because there are two places in our SOHC redblock combustion chamber that has squish surfaces (areas).]

Increasing the turbulence of the air/fuel mixture is a very GOOD thing to have happen.
...because that turbulence increases the homogenation of the air/fuel mixture...making it more uniform, and thus easier to burn, AND burn more quickly.

AND: [this is a very BIG, and important "AND"...]

this increased turbulence ALSO spreads the flamefront around a lot faster and more completely...

...BECAUSE the squish waves are occurring while the air/fuel mixture has already started to burn [the spark plug fires, and commences ignition, while the squish waves are moving].

3a. Squish clearances:

In order for squish action, and the resulting squish waves, to actually occur, the piston crown has to come within a certain distance of the cylinder head...a certain 'close proximity'.

So....how close does the piston have to come to the head?

...to within at least 40 thou [0.040in]

...and closer [tighter] than that if possible.


WHY so close?

...to generate the strongest squish waves possible, AND:

...to take advantage of, and use, the boundary layer cooling effect.

'Boundary layer cooling effect', also known as the 'law of the wall', is what happens to the air/fuel mix where it is in close proximity to the metal surfaces in the combustion chamber. Right up next to the metal, a layer forms that is about 20 thou [0.020in] thick. This boundary layer is cooled by its proximity to the metal surface: it is cooler than the rest of the air further away from the metal. The coolness of this layer makes it less capable of igniting and burning.

This 'layer' of cooler air, or air/fuel mix, DOES NOT WANT TO BURN. One could look at the boundary layer as being inert: it does not want to burn; and it actually acts like an insulator because it does not want to burn.

In actual fact, it is the boundary layer's cooling effect and barrier effect that enables a piston made out of aluminum to be exposed to combustion temperatures that are more than twice the melting point of aluminum...and NOT melt.

...for something SO thin, to be able to keep temperatures approaching 2500F [or more] from melting the aluminum that is ONLY 20 THOU AWAY from that heat, you would think that that boundary layer was more like a layer of asbestos that was 1/4 of an inch thick.

That boundary layer IS pretty tough.

[it is so tough, that the only thing that can actually penetrate it is the pressure waves that occur when detonation [very rapid uncontrolled combustion...as in 'explosion'] occurs. When detonation occurs, the boundary layer protecting the head and the piston can be ruptured; which is why you can see particles of aluminum on spark plugs, or on the pistons, after detonation occurs. Those detonation pressure waves blast through the boundary layer, and melt some of the aluminum, and splash it around.]

3b. Squish dimensions [areas and sizes]:

How much surface area of the head and piston crown that is actually squish surfaces is also a factor in how much squish action and squish wave occurs when the piston gets into the squish zone.

Many motors do not have very large squish areas because of the design and layout of the combustion chamber and the piston crown beneath it.

Fortunately, the SOHC redblocks do have a good amount of squish area to work with and use. If the squish areas' percentage of total bore area is in the 20% range, that is very good.

The most commonly used pistons in the SOHC redblocks have dished piston crowns, which reduces the total squish areas a bit. Using pistons with flattop crowns will maximize the squish areas' size and percentage of total bore area.


Now we can talk about "Tight Squish" and how it works.

With Tight squish, we optimize the squish clearances to take the most advantage of the boundary layer cooling effect, so that we can maximize the squish action and the squish waves.

To put the boundary layer cooling effect to work, the way to do that is to have the piston crown come close enough to the cylinder head so as to overlap the boundary layers:

...the boundary layer on the bottom of the cylinder head is 20 thou thick
...the boundary layer on the piston crown is 20 thou thick

...bring the piston crown up to where there is LESS than 40 thou clearance between it and the cylinder head, and you have overlapped the boundary layers:

...and have achieved Tight Squish.

[and because you have overlapped two 'inert' layers, there isn't much room between the piston crown and cylinder head...in those 'squish areas'...for any ignition or combustion to occur. You have 'quenched' the potential for ignition in the tight squish areas.

To achieve good squish action...the squish wave...the increased [hyper] turbulence...the clearance between the piston crown and the cylinder head needs to be set up to be less than 40 thou [0.040in]. Clearances greater than 40 thou are considered out of the squish range. Clearances smaller --tighter-- than 40 thou are considered tight squish.

[Getting the squish clearance close to 40 thou...such as 44 thou...will give some squish action and wave; but it will not be as strong as it could be with a tighter squish clearance. And if more than 40 thou, the boundary layers do not overlap, so those benefits are also not realized.]

I can see at least three questions that need to be [asked and] answered:

1. What is a good squish clearance to shoot for, generally speaking?
2. How tight can you go? [What is the limit?]
3. How tight is too tight? [or is there a too tight?]

1. What is a good squish clearance to shoot for, generally speaking?

...35 thou

2. How tight can you go? [What is the limit?]

...the limit is when the piston hits the head. So obviously you do not set things up so that the pistons hit the head right there on the engine stand, or when the motor is idling.

What you do have to do is to take the rule of thumb on piston to head clearances into account. And the rule of thumb is based on the amount of rod stretch that will occur as the RPMs rise.

the rule of thumb for piston to head clearances:

...provide 4 thou of clearance for every 1000 RPM, and add 4 thou as a cushion.

example: for an engine RPM redline of 7000 RPM, that would be 4 thou X 7 = 28 thou, plus 4 thou = 32 thou.

This IS a general rule of thumb, intended for street motors using steel rods.

3. How tight is too tight? [or is there a too tight?]

Yes, there is a 'too' tight. If the pistons hit the head before the RPM redline is reached....yup. too tight.

...as was described to me: "set the squish clearance so that the pistons will not hit the head until you are ~200 RPM above the redline."

That was not said with any intent of sarcasm. Rather, it was intended to be illustrative of how tight one can go.

Looking at it pragmatically, the answer to whether there is a 'too tight' or not has to be 'Yes'.

Why is that? Because there has been a fair amount of experimentation done by a number of people building competition motors of various types, and some of those people have tried different squish clearances. Some motors liked really tight squish; others did not.

Asking whether or not there is a too tight squish clearance for the SOHC redblocks, the answer would also have to be qualified 'yes'...

...because of the physical limit stated in 2. above,

...and because I do not know for sure just how tight one can go. As I build more motors, I am working towards finding out just how tight I can go.

[I have a block on the stand that I plan on setting the squish clearance at ~24 thou...with a redline of 6000. How it runs and performs will further my research.]






Why worry about squish clearance? With good squish action, the turbulence is better. This increased turbulence helps the air and fuel to burn faster. If the air and fuel burn faster, there is less chance of detonation occurring; and peak metal temperatures of the piston crown and cylinder head are reduced. With the faster burn, there is a better chance of deriving as much power as possible from the burning air and fuel to force the piston down on the power stroke. Fast burn is a good thing. Tight squish helps make fast burn possible.

Fast Burn...is a term that describes the goal of setting up a tight squish motor; and is also used to describe a similar Approach as Tight Squish. Fast Burn [the Approach] uses tight squish, but it also incorporates a redesign of the combustion chamber in the cylinder head [the roof and walls], and of the piston crown [the floor] that makes the combustion chamber as small as possible, to maximize the squish surfaces, the squish wave, and the squish action, but to do so in a manner that does not adversely affect flow past the valves. Fast Burn is a refinement of the old Open Chamber vs Closed Chamber design Approach to combustion chambers.

The SP motor has been set up to have a piston crown to cylinder head clearance of 0.037in. That is tighter than 0.040in; that makes it a tight squish motor. But since the combustion chambers in the cylinder head were not redesigned and modified as Fast Burn CCs, I cannot really call it a Fast Burn motor. But, because I improved the floor of the combustion chamber by way of shallower dish B230ET pistons, I did make it a faster burn motor.


...that was the very abbreviated explanation. A more complete explanation would involve further discussions of 'boundary layer cooling: the law of the wall'; as well as subjects such as endgas reduction; quench areas; proximity heat transfer; the factors affecting just how tight the squish can be on a particular engine; the "Ignition Chamber" vs the "Combustion Chamber": the differences and similarities of the two, and why it is so beneficial to optimize the use of the "Ignition Chamber"; and a few others that I am sure that I am forgetting...

...and take up several hundred pages...

To summarize:

Squish is good. Tight Squish is better. Fastburn is the goal.