Looked around the forums and couldn't really find a straight enough answer for this. For an S4 13BT, what ~should~ the compression be at when it's new, when it's still good enough to modify a bit, when it's on it's way out, and when it's completely pooched?

Thanks in advance (Y)

 

New is right @ 118-120psi (broken in)

Modifiable is 95+ (preferably at least 100psi)

On it's way out is anything 90psi and below.

Blown could be anywhere from 80psi on all faces, to 0psi on 2 faces, etc. Depending on the actual damage.

 

It's very RPM dependent. Check the FSM for RPM compensation.

 

Well, those figures are assuming you have a fully charged battery and a good starter. Also, it will read low if it has been sitting for a while (several months+)

 

I'm looking at the FSM, and it's telling me 85psi @ 250rpm, +/- 15psi depending on engine speed.

This is a pretty far cry from the figures I'm getting from you guys?

 

Thats pretty damn low....
Now that I think about it, why would they have PSI figures? Shouldn't they have figures for using the rotary compression tester?

 

Yeah, that's 85psi with their compression tester, but the only reason they mention to use their compression tester and not a typical piston one is because the piston one would only show the highest reading. Only reason I could see for this is that a piston engine's pressure wouldn't vary with each stroke, while on a rotary we're testing 3 different chambers/ faces. So, solution? Hold the pressure release valve open and watch the needle jump. I'd assume this would work just as well only because I can't see a reason that it wouldn't.

Maybe I'm just too narrow minded since I'm so used to piston engines, but I really can't see any other reason as to not use a standard compression tester.

 

No, the Mazda compression tester reads on a scale of 1-10, it has no actual units for compression. Also, when using a piston tester you hold down the pressure release button to get the compression on each face.

 

You can also get inflated figures if you have super cranking RPM.
85psi is the base minimum, you don't want this. The numbers the others gave are a good indicator if your rpm is 250 approx.

The mazda numbers are real numbers. They are in Kg/cm^2 i think. You can compute the conversion factors easily. And yes it can read over 10, I have seen it.

 

Right out of the FSM: