verifying compression ratio of an already built engine?

[jmz]

Is there an accurate way to check compression ratio of an assembled engine?

How accurate would it be and could someone here offer a step by step process?

Thanks in advance,

[cgarr]

In the car or on a stand?

[jmz]

Originally Posted by cgarr

In the car or on a stand?

Either and/or both. I've got one 2.8 litre engine on a stand that I'd like to verify and another in a car.

[Mark Henry]

Tilt it up fill with measured fluid. Oil will work but attacks rubber used in syringes.
When done tilt the engine upside down to drain the oil.

Then do the math putting the cc measurement where it says deck height.

don't know about in car.

[NICE 69 S]

Tilt the engine until the spark plug is pointing straight up. Put the cylinder at top dead center. Fill the cylinder with a burette until the fluid is at the bottom of the spark plug hole. Calculate displacement of one cylinder and add the volume of the combustion chamber that you just measured. Divide this total by the same combustion chamber number and you will have the compression ratio.
Bob B

[Walt Fricke]

I made a fitting topped by a clear plastic hose which screwed into the spark plug hole, and filled through that. That way I could fill to a mark I could clearly see, and not guess when I was just at the bottom of the plug hole. And I could rotate the engine a bit to persuade myself that I hadn't trapped any air. I had measured what it took to fill the fitting up to the mark, so that got subtracted.

There is also a nifty instrument which will give accurate results, even with an engine in the car. It is called the Whistler, and it works by blowing air into the spark plug hole at TDC firing (both valves closed). It reads the audio frequency produced (remember pipe organs from high school physics). That gives you the same result as the oil method for what the fully compressed volume is. The instrument is adjusted to what the displacement is, so it reads out as a compression ratio. Over $1,000, if still available. A fair number of shops have this, as it works on 400 cubic inch V8s as well as on much smaller cylinders.

Maybe Ernie J has one? Or knows someone who does up there?

[aircooledfool]

divide compression reading by 1 atmosphere or 14.7 psi. should put you in the ballpark