When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Unexpectedly High Compression Results at 94k km – Realistic or "Unicorn"?
Hi everyone, looking for some insight on my recent compression test results.
I got my rx8 car just over a year ago and I have changed the new upgraded starter, new coils, leads and spark plugs alloy radiator, Cooling fan control kit to keep it cool.
The results I just got are significantly higher than the photos shared by the previous owner.
Current Results (Raw @ 225 RPM):
Front Rotor: 105, 106, 105 PSI
Rear Rotor: 110, 110, 108 PSI
The Question:The previous owner’s results were notably lower than these. My new results are sitting at ~95% of factory-new specs, which seems incredibly high for a 94k km car.Is it realistic for an engine to "gain" compression like this? I’ve been premixing 200ml per tank and the car is now running a lower-temp thermostat/fan kit (averaging 82°C). Could a carbon cleanup or the change in testing method
account for such a jump?
if the engine is cold or hot......you are supposed to do a hot test to get a more true "running" number which will be quite a bit less. If I was to bet it would be this.
if the engine is full of oil or fluid that may help seal
Starter speed, make sure to use the corrected value. However try to avoid really slow data if possible
Overtightening or undertightening the sensor can affect the result. Depending on your comp tester you may use the leading or trailing hole too so keep that in mind.
I think if the engine sits for any period of time, the housing to apex seal conformity/sealing is affected by surface rust, carbon, sticky apex seals, etc. Once you put some miles on it, it often "comes around" a bit. When I got my car, it was a used engine from Japan with about 60k miles on it, and then the previous owner had it running like crap on and off for about a year. I got it fixed and drove it in a variety of scenarios (including hard mountain runs), and it went from high 80s on the front rotor to high 90s after about 2,000 miles.
If you got the upgraded starter, why is the test rpm 225? The 2kw starter makes 290 or so starting speed. Those comp values but at 290rpm would be more in line with your average 94k km engine.... but having great compression isn't unheard of. Right choice of oil, right drive cycle, right build from the factory, sometimes the stars align
That said, I've had compression "rise" too, my 2015 comp test (actually with the same kind of tester as in the pictures) showed lower than any other test since (with a different tester). You could try to get a second opinion with another tester... or you could enjoy the car and not sweat it if it's not having issues.
because most street jockeys still challenge or are possibly not aware of my hand built and blueprinted professional Renesis race engine experience …
because I specifically included the race engine part of the comment in that reply. The difference being between simply buying and using parts per the factory generic numbering/alphabet system vs. clearancing and hand fitting seals to very specific dimensions for that purpose.
You do understand that ideally a 10:1 compression ratio should yield an ~145 psig result around sea level, right?
I know of factory REW RX7TT engines with 9:1 compression making 124-130 psig compression results.
.
You specifically stated; “…anything below 120 psig @ 250 rpm is less than ideal.” I understand what you’re trying to say, but to some of us lowly “street jockeys” (whatever that’s supposed to mean), it may come off as misleading. If you do a test at home and your readings are ~118, that’s not “less than ideal”, that’s virtually perfect for a stock engine per Mazda’s specifications.
And maybe I missed it, but I’m fairly certain he never mentioned his engine was a race engine. 🤷♂️
Last edited by FasterNLouder; Feb 26, 2026 at 07:56 AM.
You specifically stated; “…anything below 120 psig @ 250 rpm is less than ideal.” I understand what you’re trying to say, but to some of us lowly “street jockeys” (whatever that’s supposed to mean), it may come off as misleading. If you do a test at home and your readings are ~118, that’s not “less than ideal”, that’s virtually perfect for a stock engine per Mazda’s specifications.
And maybe I missed it, but I’m fairly certain he never mentioned his engine was a race engine. 🤷♂️
Ignore his comments, and just look at what he says. ***** and strikes...
Well, compression=power to a great extent, but not perfectly. What he stated is true, not misleading. Not a fanboi of his, to be sure, but what he said was right. These engines leak massively compared to piston engines, and whatever you can do to eliminate that leakage goes a long way for both power and longevity.
If you get 118psi, drive it and enjoy it. That means it's making whatever power the factory spec can produce.
Last edited by kevink0000; Feb 26, 2026 at 09:20 AM.
Ignore his comments, and just look at what he says. ***** and strikes...
Well, compression=power to a great extent, but not perfectly. What he stated is true, not misleading. Not a fanboi of his, to be sure, but what he said was right. These engines leak massively compared to piston engines, and whatever you can do to eliminate that leakage goes a long way for both power and longevity.
If you get 118psi, drive it and enjoy it. That means it's making whatever power the factory spec can produce.
I’m just trying to maintain the original context of this post—which is someone trying to understand whether their compression is good or not. A statement simialar to “under 120 psi isn’t good” is misleading in the context of a stock engine being within OEM specs or not. Rotary engines are almost never an apples to apples comparison with piston engines, so why start now?
I’m just trying to maintain the original context of this post—which is someone trying to understand whether their compression is good or not. A statement simialar to “under 120 psi isn’t good” is misleading in the context of a stock engine being within OEM specs or not. Rotary engines are almost never an apples to apples comparison with piston engines, so why start now?
Hmm. Well it all goes to the original context of the OP post in my view. Why not have more information, rather than less?
"Less than ideal" is definitely not the same as "isn't good".
Maybe slow down and digest what has been said. It's clear enough in my opinion.
