That depends on the rules, If they are measuring the "static" compression or the "cranking" compression which is measuring cylinder pressure. Cranking compression or pressure will fluctuate based on cam timing and engine temp would also play a factor. Compression tests are usually based either hot or cold. To get a true static compression would usually require tearing down the engine and measuring piston to deck clearance, combustion chamber volume cc, compressed head gasket thickness, bore volume. Trying to tech compression without tear down would require some kind of range based on cranking cylinder pressure.

We got checked for compression this year. They used the katech whistler. It was before the races so the car had not even been started. It was with a tenth of our calculated compression.

http://www.katechengines.com/motor-sports/services/whistler/

Not sure how it exactly works but I think its measures the volume of a cylinder at BDC and again at TDC by using some sort of gas. not 100% sure though

once the machine gets that info it can figure out the ratio.

I am pretty sure (not positive) it is cylinder temp.  I think it comes with a thermometer and everything.  I certainly appreciated pulling a spark plug vs pulling a head.  You still have the option to pull everything apart and measure if it doesn't give you the number you expect (says you're illegal).

VV had that issue a few years back.  One car measured high.  He pulled the head off to prove he was legal.

My numbers always matched my calcs though.

The whisler is accurate but only if you know the ci of the engine being tested, That was the issue with me at VV. It is measuring a known volume of air and as the piston gets closer and closer to TDC the air blowing into th e machine is measured when its at its smallest amount it converts it to comp ratio. My comp was mathamaticly done before asembly and its is 10.58 to 1. The wisler showed it to be 10.6 because it goes to the nearest 10th. On the other hand the ci pump showed me 385+ ci. There is no way the ci pump  can tell you how big your engine is unless they are all turned over at the same RPM thats why I was tore down and measured.

Mark you can call Or talk to Brad about how he will check

2 different tools. Compression/Cubic Inches. Got it Vlasic?