Squish test.
One thing you should do is a squish test. It is just as important to know this or more as anything else on your engine.
Take a couple piece's of solder around 1 to 1.5mm thick and around 1 1/4"long and put your piston about a 1/4" below top dead center.
Put a very little dab of grease on the piston above the wrist pin on both sides of the piston. Lay your solder Tight to the Bore across
the top of the piston on your grease, right above the wrist pin. Now turn the engine over with a Wrench and squish the solder. Do this at least 3 times until you are happy with a repeatable measurment. Squish is measured tight to the bore. If you have us do anything to your engine we will need to know this. If you ship us your engine do not worry about it.
Compression- Psi- Ratio- Cylinder head design-Octane
A two stroke has 2 compression ratios.
Corrected compression ratio. This is figured when the piston closes off the export.
Uncorrected ratio. This is the swept volume ratio.
Corrected compression ratio
A lot of engines advertise the corrected compression ratio number. This number really doesn't tell you anything if you have not measured other specs in the engine.
Power valves can also change this number . Higher ratios in the closed position and lower ratios in the open position. This is the reason for a power valve. A power valved engine has 3 compression ratios.
Closed, Open and Uncorrected.
Psi
We hear about it all the time. Why does this engine have this much compression and this other brand engine X does not? That depends on a lot of different engine specs.
Does it have a power valve?
What kind of port arrangement does it have?
It really does not matter what the psi on the gauge tells you. It would be nice if that were true. But we can not determine anything from psi,even on two of the same engines,deck height squish can all be different.
If someone says you need higher octane at 210psi that could be false or true. Psi is only a reading on a gauge and that is all. You can not determine octane from psi. I have built engines that need high octane at very low psi. Psi is a good indicator how the piston and rings are if you know the psi when the engine was fresh. Port arrangement plays a big part in how much psi you will have as does a power valve and some other things. If your engine has lower psi compared to someone else, but the engine starts easy and runs great I would not worry about it.
Uncorrected comp ratio.
This number at least tells you something. But then again we still can not choose a octane or performance from this
number alone. Cylinder head chamber,comp ratio,,squish, squish velocity ,pipe, ignition timing, play a big role in the fuel octane and performance you are after. Squish velocity requires a good deal of information to be correct ,bore,stroke,rod length,port arrangement,max rpm,piston used,squish and some other information must be calculated to have a correct velocity. What is the correct velocity? Every engine is different on the cylinder head design,compression,squish velocity required. I would probably never just cut .010 or .020, off a cylinder head, that seems to be pretty common for decades, for people doing something themselves. There trying to tighten up the squish and raise the comp ratio,It is also a good way to get in trouble. If a shop recommends this. I would think a little before you let them do it. Ask them some specs and see what they say. Sometimes, in some cases, it is necessary change the chamber/head design/comp ratio/velocity so much that the stock head can not even be used. But then again it all depends on everything else.
