This has been argued about on the RC Universe engine forum.  What I would do is run the piston up to TDC and drip rislone or similar in from a finely graduated syringe.  Fill it up to the bottom of the glow plug hole.  Figure the volume of oil and add that to the bore X stroke volume and divide same into the volume of oil.  I think this would give you what is called mechanical compression ratio.  Some say you should measure the cylinder volume from the top of the exhaust port. I have never done this, so it is all theoretical.

I borrowed a compression gauge from Tom Martin (It came out of GMA's estate) but wasn't sure it would really give me the info I was after.. Trying to compair heads and head shims on a couple different engines to come up with meaningful numbers.

Lets say for example I have a stock ST 46 that runs great on 10% nitro and I have a hemi ST 46 that also has a plasma P&L. I would like to add or deduct enough head shims on the hemi engine so the compression is the same and see if it will run on 10% nitro as well as the stock engine..

I suspect the compression gauge will give me a different reading between the two engines even if the compression ratio was the same due to loss of compression past the ring.

Is this making any sense or am I out in left field someplace..

Thanks Dick & Jim.. This is a new ball game for me and am trying to understand it better than just... Oh guess I'll try 2 head shims and see what happens 

This has been argued about on the RC Universe engine forum.  What I would do is run the piston up to TDC and drip rislone or similar in from a finely graduated syringe.  Fill it up to the bottom of the glow plug hole.  Figure the volume of oil and add that to the bore X stroke volume and divide same into the volume of oil.  I think this would give you what is called mechanical compression ratio.  Some say you should measure the cylinder volume from the top of the exhaust port. I have never done this, so it is all theoretical.

I did this with an engine but a little differently. 

1. Move piston up 'til exhaust just closes. 
2. Fill cylinder with air tool oil.
3. Pour oil into measuring container. (I used a graduated test tube)
4. Record amount.
5. Position the Piston to TDC.
6. Pour oil from container back into engine.
7. Record amount left in container.
8. Find the difference between both measurements.
9. The ratio can be determined from the results of both measurements.
10. Now clean up the mess.

Of course all of this is meaningless unless you know the compression ratio of the engine is supposed to be in the first place.

How 'bout piping in here Randy? 

Hi Randy and thanks again...

Among other things that have to do with different heads for Stalker 61's...

I have an unknown hemi head for a ST 46, two engines with ceramic p&l's, and a couple stock engines. Just trying to see if I can make some sense and learn a little out of what I have discovered trying to run these engines on various levels of nitro.

You have given me the one main piece of info I was trying to come up with(.003-.005). With the other info in this thread I think I have a handle on how it needs to be done..

Thanks gang..