Low compression experiences
 

Interested in hearing peoples experiences running low compression (particularly from an old/tired motor, rather than a popped engine, but happy to hear all)


So I bought a compression tester recently, where a hot test gave:
Front, 4.95, 5.33, 5.11 Bar
Rear, 4.88, 4.80, 5.10 Bar

5bar = roughly 72psi

Which is generally considered pretty low, I'm not too worried at the moment, after talking it through on a rx7 whatsapp group with some people.
The car starts hot+cold, has been sat, runs pretty rich (plugs came out black when doing the test), not losing coolant and its fairly even compression across the board. It's an old street ported rebuild (10 years back now and a few owners previous)
So I've decided to drive it properly for a tank of fuel or two, and get my new ecu in (and clean up the map), and test again to see where I stand. Hoping there is some carbon deposits etc affecting compression which may clear.


Interested to see how far people have pushed things and general experiences with low compression and rebuilding.
I plan to rebuild it myself when the time comes, so trying to figure out the right balance between getting some more miles out of her, and "ease" of rebuild. For example I wouldn't want to have to replace housings + rotors to gain a few hundred miles before popping.
Obviously its a how long is a piece of string scenario as you can't really predict these types of things

My last car had compression in the 90-100psi range. It ran great, just burned more oil.

First off, I'm guessing you used a rotary compression tester? Which tester did you use? Does it correct for RPM?

Also, when you tested was the throttle fully open? Numbers will be lower with the throttle closed.

If it does start, run, and drive, and start good when warm, you're OK for now but I'd start saving money. It could also be worth a de-carbon. I would look at rebuild before things get really bad - right now you probably have all good hard parts, if an apex seal blows that will trash a rotor and rotor housing. They may still be usable at this point.

Dale

also, you should compensate compression for altitude.

Foxed.ca - Rotary Compression Calculator

use the calculator. Compression numbers need to be converted to a standard speed and altitude.

Or just live at sea level like I do :)

Dale

Yeah those numbers are corrected for altitude and to 250rpm.
I'm using a rotary specific tester ( https://www.digitalcompressiontesters.com/ ) very similar to the aftermarket ones you guys use in the US

It does seem like I'm chancing things a bit but it's running well, before doing the test I wasn't expecting great number (as its 10 years since last rebuilds) but wasn't expecting so low either

A street port can drop the numbers a little bit too, some of the intake charge can go out the exhaust, dropping the amount of air in the chamber.

Does it ever struggle to start when hot?

Dale

No struggling at all, that's what keeps me in two-minds what to do.
Did a coolant change a little while ago with the lisle funnel, no champagne bubbles etc either

Just can't decide what to do, definitely going to drive a bit more, but think I will end up pulling it pre-emptively

if it hot starts and runs fine, whats the problem?

i ran mine a little too hot on track one time, and it lost compression (hot start issue), kept driving it and compression actually came back over time. starts easy now