shinchrono

The previous owner of my FD says the compression test came out
9.6, 9.6, and 9.6 in front, and 9.4, 9.5, and 9.5 in the rear. as i was reading through the searches, i kept reading numbers in the 6's and 7's! i was wondering...does this mean the car is running a very high boost? if so i'm concerned about the engine running lean...

or is this a conversion from one compression unit to another? (?->psi, for example). i know the psi should be somewhere around 10, so is this a reasonable assumption? please help me with your wisdom

thanks!

bee

lol,

Boost is in no way related to compression. If you have a boost gauge, you should be running a max of 10 psi of boost, assuming your car is stock. As for the compression, those are excellent numbers. So good that I would probably not believe them unless some documentation was provided by the person or place who performed the test.

bee

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turbojeff

iTrader: (30)

Actually many stock cars run 11-12psi on the first turbo, dropping to ~7psi by redline.

Otherwise I agree with bee, the boost and compression are two different things. Numbers in the 6s are really too low but within Mazda specs. Numbers in the 7s are acceptable, 8s are great and I don't think I've ever seen an engine with numbers all in the mid 9s. Compression depends on cranking speed and engine temp. Nominal cranking speed is 250rpm, if yours was cranked faster the compression would be higher.


Jeff

Nathan Kwok

In the past some dealers have converted the compression numbers over to a scale of 10, so 10 is perfect (normally 8.5 kg/cm^2 is considered perfect). Why someone would do this is beyond me I suppose it makes it easier to understand. The bottom line is, if there are no units listed and/or you cannot verify the test was done hot, then I wouldn't put much stock into the test. Either way though as long as the numbers are consistent with each other you should be fine.