Alfa Romeo Repair: alfa engine compression, venolia pistons, compression gauge


Question
I have modified my 2000 litre engine and now get 200-215 lbs per square inch per cylinder..per my commression gauge. How do I convert that into compression ratio such as 9.5:1 or 10:1...Thanks  

Answer
Chris,

I'm afraid you can't do it that way.  You have to know the volume of the combustion chamber and the volume of any piston dome, plus the bore and stroke to tell.  I've seen formulas to convert to approximate C.R's but they flatly do not work.  

What did you do to the motor? Did you change pistons?  The Motronic Spiders, used 10.1:1 pistons and the earlier ones were nearly a point lower.  

Our Spider race car uses the Borgo 10.4 pistons and we have the head milled to the minimum total height or even a hair less.  Milling doesn't raise it that much but I'd say we are in the 10.7 or 8 range.  We run 4 gallons of 93 pump gas and 1 gallon of 110 racing fuel.  Our compression is in the range of yours.  
We also race a full house 1300 in a Giulietta and it has Venolia pistons at around 12.5:1 and the compression gauge on it, won't hit 200lb/sq.in.

Compression gauges are a good reference tool, but there are so many variables and ways to do it that results can be deceiving.  Squirt a few cc's of motor oil in the spark plug hole before you try it, and see how high it goes.  Spin on it for a couple dozen revolutions with the intake wide open and see how high it goes.
It's really important to use it the same way everytime.  Write it down along with all the measurements and over a few thousand miles or several races, you will be able to see wear and possible failure using it.

Please don't hesitate to write back.  You will find the most you can learn is by picking other people's brains.  Even if mine is sometimes empty. hahaha

dave