Topic: Calculating compression ratios

[triple]

G,day fella,s. How do I go about calculating compression ratios on my motor.
It is currently closed up. Do I need to remove the head or can I work through the plug hole. ??


thanks ..

[SOHC Digger]

You can work through the plug hole. You will need something to precisely measure liquid. Set the engine to top dead center. Fill the cylinder with engine oil to the plug hole, measuring exactly how much you put in. Call this measurement "A". Then turn it over to bottom dead center, and measure exactly how much additional oil it takes to reach the plug hole again. Call this measurement "B". Measurement "B" is also your displacement per cylinder. For example, the CB750's are actually 736cc four cylinder engines; 184cc per cylinder. So measurement "B" for a CB750 SHOULD be 184cc (or 184mL).

Compression Ratio = (A+B)/A : 1

So for example, on my 78 CB750F, if my measurement for A was 23mL, and my measurement for B was 184mL, then:

CR = (23+184)/23 : 1
CR = 207/23 : 1
CR = 9:1

Once you're done, draw as much oil as possible back out of the cylinder and expect some smoke until it burns the remainder off.

[SOHC Digger]

This is where math is awesome. Now that you know how to find measurement "B", you can figure out if the guy who claims the bike he's trying to sell you is bored out to 915cc is on the level or full of #$%* before you buy it.

[triple]

Thanks digger. Do I assume that dividing by 23 is a constant ? used on all cr calculations or just in this instance ?

cheers

[SOHC Digger]

No, just for my example.

[NickO]

The figure "23" (i.e. 23cc's) is the unswept volume of the combustion chamber on THIS particular engine triple. It is NOT a constant.

[mec]

Then turn it over to bottom dead center, and measure exactly how much additional oil it takes to reach the plug hole again.

how do you do that? in BDC either in-valve or ex-valve is open.

mec

[SOHC Digger]

Then turn it over to bottom dead center, and measure exactly how much additional oil it takes to reach the plug hole again.

how do you do that? in BDC either in-valve or ex-valve is open.

mec

Good point. You're going to have to back the valve lash adjuster off all the way so the valve isn't open.

[SOHC Digger]

And actually, looking at the 750, with the cylinders canted forward, the spark plug hole is below the top of the combustion chamber. You would need to tilt the engine so the plug is pointing up. Might not be practical in all situations. Otherwise you won't get a true reading on measurement "A" and will only be able to calculate displacement.

[mec]

Good point. You're going to have to back the valve lash adjuster off all the way so the valve isn't open.

sorry, will not work:
not sure, but i believe you will gain ~4mm, at BDC valves are 5-6mm open.

mec

[SOHC Digger]

Good point. You're going to have to back the valve lash adjuster off all the way so the valve isn't open.

sorry, will not work:
not sure, but i believe you will gain ~4mm, at BDC valves are 5-6mm open.

mec

If that's the case, then I guess the cam needs to be removed at least.

[PeWe]

Why remove cam? It must be enough to remove the rocker arms from the cylinder to check, right?
I changed cams 2 weeks ago. I should have known this procedure that seems to be an easy way to find out the real CR. I'm very curious about my engine's CR.
I have to remember coming winter when I'll upgrade, before taking it apart. Later compare with the chambers volume so I'll know cyl block and pistons share of it.
Replace Spark plug with something with a smaller hole, a clear pipe that can have a mark for oil level. Sparkplug hole must point up so the chamber will be filled completely, right?
-Which oil? 

[SOHC Digger]

You are right, PeWe. You only have to remove the rockers.

Yes, the plug hole must point up to completely fill the combustion chamber.

It's just my opinion, but I would use whatever oil you use in your engine in the first place. That way anything that leaks past the rings will not contaminate the engine oil. You should be able to get most of it back out and the residual amount left will burn off.

[triple]

Good point. You're going to have to back the valve lash adjuster off all the way so the valve isn't open.

sorry, will not work:
not sure, but i believe you will gain ~4mm, at BDC valves are 5-6mm open.

mec

how about filling the chamber with oil on the power stroke ?? all valves will be closed until the piston reaches BDC

this should work....

[johno]

Nah, as merc says the exhaust valve opens x degrees before bottom dc gotta stop valve operation  as digger suggested.

re oil, using engine oil would be a very gooey slow process getting it all in then all out , try something lighter, I use auto coolant , no rust issues, got some colour making it easy to see in a burette , nice and quick, blows out the plug hole nice and easy with a few kicks etc

[PeWe]

Auto coolant, the model for alu engines, right?
My car must have a G30 (VAG G12). Its a red one... Others agressive to the aluminum.

I can use oil, flush it clean with white spirit or similar. Use a big syringe (100-150ml) to fill oil, suck it out and flush.