65 396

I have my 402 big block on an engine stand, with the heads attached.
I need to know the existing compression ratio. I am hoping it is 10 -1.
I have a burrett and I know how to use it. Is there a way to fill the cylinder with fluid (marvel mystery oil) and determine, based on how many cc's the cylinder holds, what my compression ratio is? How many cc's of oil equals a 10-1 compression ratio?

AZDoug

swept volume of cylinder + CCs of TDC volume/CCs of TDC volume,

Should get you there. Assuming you know the bore is stock.

Doug

1COOL60

Static compression ratio (as you are trying to determine) is a ratio of the total volume of the cylinder/combustion chamber with the piston at BDC vs. TDC. In theory, you can fill the cylinder/combustion chamber with the piston at TDC, take your reading, then move the piston to BDC and fill again (with both valves closed, of course). Divide the larger number by the samller number to get the CR. The amount of oil needed is determined by such variable as piston configuration, combustion chamber volume, head gasket compressed thickness and such.
160

DansYellow66

Will this really work on an assembled engine? I would think in the time it took to fill the cylinder that some oil (MMO is fairly thin) would have started to leak through the ring end gaps. Maybe not enough to matter?

GTOguy

You need to do the math on the cylinder/piston and then use the oil or fluid to cc the combustion chamber in the heads. You also need to factor in head gasket thickness. Filling the cylinders with oil is not the way to go about it.

63split63

Years ago when I was involved in stock car racing we had a compression ratio rule . The tech inspector had a machine we called a whistler . It was put in the spark plug hole and the engine was cranked over to determine the comp ratio .
I don't know if they are available now .

Here it is ! Bill

http://store.katechengines.com/whist...ster-p174.aspx

tuxnharley

This is a deep memory from a long time ago, but I seem to remember a simple mechanical device like an air pressure gauge that you could just screw into the spark plug opening, and then crank the engine over. It would record the maximum and minimum pressures (two indicator needles), and from that you could calculate the CR. I'm talking about a looong time ago, like back in the 70s. Does that sound familiar to anyone else?

Easy Rhino

I am not familiar with such a device, but it makes sense from a universal gas law sense; PV=nRT. Factor down to PV/T = PV/T, ignore temperature change, and factor down to delta P, delta V.

Probably like the racing test device mentioned above.

However, all this could be done by pulling a head and taking a small number of measurements.

Faslane

I use this on my turbo car, it seems to be spot on for tuning purposes. there is also the squish, that is important on the turbo car, but it may be no affect on N/A engine.

http://www.csgnetwork.com/compcalc.html

DansYellow66

The last engine I had built was an iron head Ford engine the buider said he wanted to hold to 9.0 compression. From his build sheets I was able to find enough information to calculate my compression ratio with the exception I think I had to assume a compressed head gasket thickness. I had the head chamber volumes after his head work, the machined bore and stroke dimensions, and the piston dish volume. Amazingly, it calculated out to 8.99 compression ratio.

My point being that if you have engine blueprinting and build sheets from a builder you may be able to caluclate it with a fair degree of accuracy.

Pop Chevy

Filling the engines chamber with oil is the most accurate way to do it. BUT , the rings need to be sealed off. I smear grease or vaseline around the top of the piston then put the head back on with a used head gasket. You must also tilt the engine so the plug hole is at the top. Fill the chamber to the bottom of the threads and you've got it ! I used to race NASCAR short track and we had a compression rule. I stayed out of trouble using this method and went fast.

65 396

I've cc'd many engines without smearing grease around the top of the piston. If a cylinder has been leak tested at 1 - 2 % leak, it doesn't need any help sealing the rings. I was hoping there was a formula that could be used when knowing the bore size (4.125) and determining exactly how much oil it takes to fill the cylinder to the top or bottom of the spark plug hole with the piston at TDC.

DansYellow66

That makes sense - if you are willing to take the top off of the engine to do so.

SWCDuke

The attached article may be useful, and here's the link to the CR calculator I use: http://www.csgnetwork.com/compcalc.html

Duke