Computing Compression

[Phred]

In reply to an inquiry on the compression ratio of an L28 with dished pistons, N-42, .060 surface, etc. I was troubled with not being able to reply with any accuracy. I work daily, machining, adjusting, and setting up compression in racing engines, and this was my response. Hopefully, it might shed some light on a problem encountered at some time by most Z rebuilders.

Ed,

You have too many unknowns in your engine to nail down a C.R. within half a point. For starters, how many cc's are contained within the dish of the piston? More important is the combustion chamber. If it was brand new, you could go with factory cc info, but a used head may have had a valve job or two, slightly sinking the valves (adding cc's). Or it may have already been surfaced, (removing cc's) if so how much? Which head gasket will be used? Different gaskets contain different amounts of area (cc's).

The following will explain how to compute your true compression ratio. To do this everything needs to measured in cc's, (cubic centimeters). Once you know what cc's your working with, a simple math formula will give you the bottom line, without question. There are a few special items you will need to beg, borrow, or buy. The first is a cc burette. This is a glass or plastic tube graduated in cc's. They are commonly found with a 50 or 100 cc capacity. (100 prefered). The next is a cover plate. These can be bought (Moroso) or made yourself. The plate is commonly a 6"x6" square piece of plexiglass 1/2" min. thickness with a 1/4" chamfered hole in the middle. Last, a pocket calculator.

The object of this adventure is to determin these unknowns:

1) Actual displacement of one cylinder.

2) Displacement of the cyl. gasket

3) Displacement of area above piston @ TDC

(dome is negative, dish is positive number)

4) displacement of combustion chamber

5) total displacement in cc's above the piston @ TDC

(found by adding and or subtracting the above figures)

The following example is a 240 engine I built for SCCA ITS

Stroke: 2.902

Displacement: 2454.5cc's

(formula: bore x bore x 16.387 x .7854 x stroke x # of cyl.)

(16.387=cc's per cubic inch)

(.7854= area of a 1" circle)

Displacement of head gasket: 6.9cc's

(3.375x3.375x16.387x.7854x.047 - same formula as above)

(3.375= inside dia. of head gasket)

(.047 = compressed thickness of gasket)

Displacement of area above piston @ TDC: .52 cc

`Displacement of combustion chamber: 41.4 cc's

(determined by use of cc burette and plate,

valves and spark plug sealed with light grease)

Total disp. in cc's above piston @ TDC: 48.82

basically- 6.9cc head gasket

.52cc block volume

41.4cc combustion chamber

------------

48.82 cc total volume

Now that simple formula using total volume and displacement:

409.08 (displacement of one cyl.)

----------

48.82 (Tot. volume)

+ 1 = compression ratio

basically: 409.08 divided by 48.82 = 8.38:1 + 1 = 9.38:1 C.R.

Thats it! If I didn't screw up my math. Kind of looks complicated, but once you do it a few times its a snap. This is worth more than 2 cents, so I'll call it my nickels worth.

Phred

Gresham, Or.