Hi been a while I just need some info on the following. My car has developed a hot start issue, first thing to do is compression check please see results below
front rotor cold 92. 103. 99
Front rotor hot. 80. 87. 83

Rear rotor cold 103. 105. 98
Rear rotor hot. 94. 95. 90


All values are psi at 250rpm and altitude corrected. Does engine need a rebuild my runner is saying some numbers low but it's ok.. I'm not sure

 

You are still good there. I wouldn't rebuild yet.

May want to upgrade to the RX-8 starter, Banzai Racing has bolt-in starters. Will crank the engine faster and make hot starts easier.

Also worth going through the ignition system, ancient plugs and wires or bad coils will give you problems.

Dale

 

Dale I already have the rx8 starter. So maybe I have an ignition issue..

 

Compression numbers look fine. I agree, focus on ignition - start with fresh plugs and new wires if they've been in there a long time.

 

Low 80s warm will cause hot start issues. Especially in mid summer when heat soak sets in like crazy.

 

if it was mine i would take it on a longish drive where the engine can get up to temp and run well and then retest it.
your results are even, which is good, but low, which might indicate that the tester is just low, or maybe the engine wasn't happy or something.

story time, car number 1 is an Rx8, and tech pulls it into the bay running on one rotor, compression tests it, and one rotor is 8.5's, and the other is 5's, so its bad right? tech, Mike #2, calls Mazda expecting an engine, but they want him to replace the ignition system (this is a 2004 Rx8 and would have been 2006-2007, so still warranty), we all shrug, but its their dime, so we replace the ignition system. Mike #2 (we had 35 people, and 8 Mikes!) fires it up lets it warm up in the stall. retests it, and both rotors are 8.5's now, car is fixed..... moral of the story if the engine isn't running right, it will effect the compression test.

car 2 was some piston engine thing, maybe an Mx3? and the guys just decided to all compression it with their testers. results were all over the place, same engine, same day, different tester = different results. so the regular compression testers are pretty worthless, you're looking more for even numbers with those because that is about all its good for.

Mazda's specs are minimums and they want 100psi at 250rpm with a warm engine, so i'd let the engine run somewhere where it can run happily, and then retest

 

This is good info for us, thanks J9. I have the RCT and have tested probably 6 or 8 engines with it over the past few years. 3 being new rebuilds and the rest were mid mileage 50-60k. None of them tested over 100 psi which was concerning to me on the new rebuilds, but I concluded after some time it was the tester.
I need a compression tester tester. Haha

 

so you need a box with a spark plug thread and a pressure gauge?

pressure gauges in general suck, we have 2-3 in the race car trailer, and they vary. since tire pressure is important we just pick one and only use that one
i like these, but i've got the basic one cause i'm cheap, and mine just do street cars, https://www.longacreracing.com/produ...%9d-Tire-Gauge
the shipping is kind of crazy with these, it comes in a big box with like 3 pounds of paper catalogs....

 

Bad coils often have issues when they get hot, but still work fine when they are cold.

 

I use a sparkplug with a welded in 1/8npt boss, that a 150psi pressure sensors screws into, read the results with an oscilloscope, I reckon its a pretty accurate way to go. Kind of a faff though.

 

that’s pretty much what they all are other than the transducer being built directly in the plug housing.