Banzai Racing made a good thread pointing out some of the problems you run into when you check compression without the Mazda tool. https://www.rx7club.com/3rd-generati...sting-1098715/ One of the biggest concerns is the effect of cranking speed, but altitude affects the reading as well.

In that thread I suggested people use the compensation curves found in the service manuals. I checked a 2nd gen Rx-7, 3rd gen Rx-7, and Rx-8 manual and the curve is the same. But it's not entirely obvious how to actually do that accurately. So I went ahead and made a Microsoft Excel tool to assist. The tool is available here on Google Drive:

https://drive.google.com/file/d/0B_j...ew?usp=sharing

It uses the cranking speed (from some source besides the tachometer in the instrument panel), compression reading in psi, and altitude that can be easily looked up online (whatismyelevation.com). The compensation formula came from determining the linear slope of the curves in the service manual diagrams. This isn't as good as using a real Mazda compression tester but it's a step towards a more accurate understanding of engine health.

 

Very nice! I am planning to try this out and do some experiments .

Dale

 

I can foresee this helping in a few scenarios:

Low cranking speed due to discharged battery/no battery charger, underrepresents compression (your compression is better than you think)
High cranking speed due to battery charger & strong battery, overrepresents compressions (your engine is weaker than you think)

higher altitude guys (not necessarily mountains) without battery chargers hooked up think their engine is weak but actually it's probably fine.