CRX: Compression ratio formula

[Eddie John Cruz Jr.]

quote:  Thanks... I had a feeling that it was a 12 valve engine but wasn't sure. I also wasn't sure if it was the A3 or A2. As for the compression, I was actually looking for the ratios; 9.1:1, 9.2:1, etc... :^) Thanks again for the info...   Robert K. Kuhn CRX Owners Group President (http://www.crx.org/southcal) I read somewhere where the stock compression ratio is approximately 9.0to1. This is a stock head and head gasket combination. Any milling or change in head gasket thickness will affect the compression ratio. Here is  how you find the compression ratio for your engine:                                         COMPRESSION RATIO FORMULA   I took this off of a site so I take absolutely no credit for this handy tool. You know how sites are;  here today gone tomorrow...... 1. Determine the displacement of your engine. Displacement formula is:                            BORE X BORE X STROKE X .0031416 = DISPLACEMENT                            Example: 92 x 92 x 82 x .0031416 = 2180cc 2. Determine the swept volume of one cylinder:                            Example: 2180cc divided by 4 = 545cc 3. Determine the deck volume of each cylinder. The deck volume is the distance from the top center of the piston to the top of the cylinder when the piston is at top dead center. Measurement is made in thousandths.                            Example: on cylinder #1 you measure and find you have .020" deck height.                            BORE X BORE X DECK HEIGHT X .01996 = CC"s                            Example: 92 x 92 x .020" x .01996 = 3.378cc's                   4. Measure the volume in each cylinder head:                            To do this, use a piece of  plexiglass cut to fit in the cylinder head to cylinder                            mating area. Drill a 1/4"  hole in the center of the plexiglass. Lightly grease                            the edge of the plexiglass  and install in the head. (Spark plug and valves                            have to be installed) with a  syringe graduated in cc's fill the cylinder head                            chamber up with a light weight oil. Record the measurements. Repeat for                            other three chambers. Average cc volume of a new head chamber is                            between 47 and 51 cc's. 5. You now have all the measurements to determine your compression ratio.                           one cylinder swept volume + deck cc + head cc                           deck cc + head cc                           Example: 545 + 3.378 + 48 = 596.378                           3.378 + 48 = 51.378 = 11.6:1 In this example, 11.6:1 is the actual unadjusted compression ratio for one cylinder. Add cylinder shim thickness which will increase the deck cc volume and refigure compression ratio. This must be done for all cylinders. Average deck height shim for use on a 2180cc engine will be between .150" and .185". We use a .177" shim for AVGAS and a .216" or .256" shim for unleaded premium auto fuel. Lower is better for increased engine life and fewer valve related problems. I hope that this helps.   Ed "Sharky" Cruz 1986 CRX Si