"Squish " Education

[iknowashortcut]

Hello. I was wondering if you guys could educate me on the squish. What it is, what it does, what it means, etc. and how to improve on it. Thanks so much. I learn so much from this site.

 

[Brian VT]

Basically, it's the space above the piston at TDC. Less space=more compression.
It can be "improved" sometimes by using a thinner, or no, cylinder base gasket. Too much compression will result in the fuel detonating before it should. A higher-octane fuel may prevent this. (octane kinda makes the fuel less volatile)
You need to do a squish test and also determine where your piston is, in relation to the ports, at BDC before performing any mods. or you may do more harm than good.

 

[iknowashortcut]

Thank you very much for the information. So my next question is: how do you perform a squish test?

 

[RTK]

I test it with a couple of pieces of solder inserted through the plug hole then turn the engine over and squish the solder, pull out and use a micrometer

 

[iknowashortcut]

One more question: it was mentioned that you had to check where the piston was at BDC in relation to the ports. What is the importance of this?

 

[Brian VT]

If you use a thinner base gasket, or none, then you are lowering the cylinder and it's ports.

Also, here is a link to detail on measuring squish. This exact method isn't practical for a chainsaw engine because most of them don't have removable heads but it will give you an idea of the process and tell you what type of solder to use (typ. squish on a stock chainsaw is .020"-.050").
General

 

[CTYank]

Hello. I was wondering if you guys could educate me on the squish. What it is, what it does, what it means, etc. and how to improve on it. Thanks so much. I learn so much from this site.

Back in the '30s Harry Ricardo, who basically developed the modern Otto-cycle engine, discovered that a squish-band was very helpful in creating turbulence in the combustion chamber. This had the effect of reducing likelihood of detonation, other factors being equal.

Directed turbulence from squish-band around most of piston crown is common in diesels, to promote mixing of fuel and air. For completeness of combustion.

 

[RES]

Back in the '30s Harry Ricardo, who basically developed the modern Otto-cycle engine, discovered that a squish-band was very helpful in creating turbulence in the combustion chamber. This had the effect of reducing likelihood of detonation, other factors being equal.

Directed turbulence from squish-band around most of piston crown is common in diesels, to promote mixing of fuel and air. For completeness of combustion.

I know I am getting off track here a little. The early VW diesels had a seperate combustion chamber along side the main chamber. The diesel fuel was injected into this chamber. It was referred to as the "Ricardo" chamber. I always wondered what this meant. On the newer TDI engines this was eliminated. Thanks for the information.

 

[iknowashortcut]

Brian VT thanks much for the link to measuring squish. Very informative.