Question on Engine condition - compression numbers S2
08-10-2019, 10:59 PM
#1
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S2 compression numbers appear low - but runs and starts well
I am interested in a car with 80K+ miles. 2009 - 2011 S2.
Engine compression was just done 'hot.' It was pulled into a driveway for the test directly from driving cross town and a compression test done without cool down.
Engine RPM: 290
Altitude: 300 ft
Rotor 1 PSI: 95 91 93 Avg. 93 (About 6.5 kg/cm2)
Rotor 2 PSI: 90 92 94 Avg. 92 (About 6.5 kg/cm2)
From what I can tell, this puts the numbers five or six blocks into the red on the chart, if I am reading it properly. Can someone confirm? I also tried normalizing it at foxed.ca.
However, the car is reported to drive well, and start-up fine when both hot and cold.
The private tech who was very generous with his time discussing the test said the numbers may be artificially low because it was hot vs warm. He suggested maybe getting 7 PSI more. Even with an additional 7 PSI, the numbers are in the red.
I'd like to get some views on interpreting these compression numbers. real world. The car is clearly well cared for in all respects.
Could this car have a lot of life left? Why would it start hot and cold and run well, if its the equivalent of what appears to be 6.2 normalized to 250 RPM? Does it make sense to take a leap of faith? I'll put less than 4K miles a year on it, but I'd like to do one driver education weekend a year.
The compression tester was not a factory tester but a relatively new Twisted Rotors TR-01 v2. Is there a reason to question the numbers? Would a second test on another unit be called for, perhaps from a Mazda dealer? What conditions might create inaccurate results whereby the engine actually has less wear than indicated?
Also reported there was only one flood, back in 2017. It was started and moved 10 feet. Owner had it towed to Mazda Dealer and new plugs and wires were installed. This was 15k miles ago. No incident since.
Welcome all thoughts.
Engine compression was just done 'hot.' It was pulled into a driveway for the test directly from driving cross town and a compression test done without cool down.
Engine RPM: 290
Altitude: 300 ft
Rotor 1 PSI: 95 91 93 Avg. 93 (About 6.5 kg/cm2)
Rotor 2 PSI: 90 92 94 Avg. 92 (About 6.5 kg/cm2)
From what I can tell, this puts the numbers five or six blocks into the red on the chart, if I am reading it properly. Can someone confirm? I also tried normalizing it at foxed.ca.
However, the car is reported to drive well, and start-up fine when both hot and cold.
The private tech who was very generous with his time discussing the test said the numbers may be artificially low because it was hot vs warm. He suggested maybe getting 7 PSI more. Even with an additional 7 PSI, the numbers are in the red.
I'd like to get some views on interpreting these compression numbers. real world. The car is clearly well cared for in all respects.
Could this car have a lot of life left? Why would it start hot and cold and run well, if its the equivalent of what appears to be 6.2 normalized to 250 RPM? Does it make sense to take a leap of faith? I'll put less than 4K miles a year on it, but I'd like to do one driver education weekend a year.
The compression tester was not a factory tester but a relatively new Twisted Rotors TR-01 v2. Is there a reason to question the numbers? Would a second test on another unit be called for, perhaps from a Mazda dealer? What conditions might create inaccurate results whereby the engine actually has less wear than indicated?
Also reported there was only one flood, back in 2017. It was started and moved 10 feet. Owner had it towed to Mazda Dealer and new plugs and wires were installed. This was 15k miles ago. No incident since.
Welcome all thoughts.
Last edited by stevekat; 08-12-2019 at 06:32 AM.
08-11-2019, 02:18 AM
#2
If the test is accurate, she's dead, Jim.
It might still drive well, but how many other RX8's have you driven? Would you notice if 20 horses were missing?
You coild get 10000 miles out of it or 10 miles before issues show up.
That said, my own car once tested at 85-88 psi @ 250rpm with an aftermarket tester like that one. That was 5 years ago. Every test before or since then has shown passing numbers, so somehow that tester lied. If you really want the car, you're welcome to have it retested at Mazda. Just make sure the Mazda dealer's machine is working, they seem to have many broken ones.
It might still drive well, but how many other RX8's have you driven? Would you notice if 20 horses were missing?
You coild get 10000 miles out of it or 10 miles before issues show up.
That said, my own car once tested at 85-88 psi @ 250rpm with an aftermarket tester like that one. That was 5 years ago. Every test before or since then has shown passing numbers, so somehow that tester lied. If you really want the car, you're welcome to have it retested at Mazda. Just make sure the Mazda dealer's machine is working, they seem to have many broken ones.
08-11-2019, 03:12 AM
#3
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I have a 2009 R3. This would be a car to keep at my family's place when I visit. I am not sure if I would notice a missing 20hp, but a complication is that I would be buying from a distance. I have not driven the car, yet. I thought it might be curious that the two rotors numbers are so close, and that the faces on each rotor are fairly close, but at the same time low failing numbers.
Last edited by stevekat; 08-13-2019 at 04:31 AM.
08-12-2019, 07:11 AM
#4
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The numbers being so close could mean a number of things that are not mutually exclusive.
It could mean that...
The car may start and run fine, hot and cold, and that's good. That said, I've read stories of engines with better compression having problems and engines with worse compression being asymptomatic.
When I had my engine rebuilt, it was asymptomatic before hand. A year later (with very similar compression test results as when I sent it off) it had occasional hot start problems. ¯\_(ツ)_/¯
If you buy this car, set aside an extra $4000 for when the car becomes undrivable ($3k for the motor from Mazda, $1k for freight, taxes, fluids, incidentals). Like Loki said, that could be in 10 miles or it could be in 10k+ miles.
It could mean that...
- The motor is just generally tired and worn fairly evenly.
- There is flaking chrome on both front and rear housings affecting all faces of both rotors evenly.
- Any number of other things I can't think of right now because the caffeine hasn't hit yet.
The car may start and run fine, hot and cold, and that's good. That said, I've read stories of engines with better compression having problems and engines with worse compression being asymptomatic.
When I had my engine rebuilt, it was asymptomatic before hand. A year later (with very similar compression test results as when I sent it off) it had occasional hot start problems. ¯\_(ツ)_/¯
If you buy this car, set aside an extra $4000 for when the car becomes undrivable ($3k for the motor from Mazda, $1k for freight, taxes, fluids, incidentals). Like Loki said, that could be in 10 miles or it could be in 10k+ miles.
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