Compression numbers
03-01-2016, 05:33 PM
#3
40th anniversary Edition
I got my compression test. What do these numbers mean? (re-write from RIWWP Moderator))
You should have gotten numbers in a format something like this:
Rotor 1: 7.5, 7.6, 7.5
Rotor 2: 7.2, 7.3, 7.3
250 RPM
- The 6 numbers, 3 for each rotor, are your compression scores, one for each face of each rotor in the engine.
- If you don't have all 6 rotor face scores and at least 1 RPM number, your compression scores are going to be vague and hard to interpret.
- You may also have your compression scores in a different scale, such as PSI.
- The RPM is the speed at which the test was done.
- The RPM is critical to interpreting the numbers, as rotary compression changes with engine speed, especially at low RPM.
- Since the engine is spun by your starter, this is ALSO a good indication of the health of your starter!
The general guideline for how good, or bad, the scores are, WHEN NORMALIZED TO 250 RPM AT SEA LEVEL!
- 8.5 and up: Congratulations, you have a stellar engine! Compression scores this high are rare. If you think that this may be too high, there may have been excessive oil in the housing.
- 8.0-8.4: This is a very good engine! You should have very good power and as long as you stay on top of the rest of the failure points possible, it should last a long time
- 7.5-7.9: This is an acceptable engine. Most engines from Mazda seem to be in this range after the break-in period. Stay on top of the other failure points possible, and you should get at least 40-60k more out of this engine, if not more.
- 7.0-7.4: The engine has some life left in it, but start looking for replacement options. Compression loss is going to start speeding up from blow-by combustion gasses eating away at the seals.
- 6.5-6.9: Officially failing. The engine doesn't have all that long to 'live'. Compression loss is accelerating due to blow-by.
- 6.0-6.4: Failing significantly. Very prone to flooding even with new starter, battery, and ignition. It will have trouble starting when hot, power loss especially down low, and noticeable difficulty idling.
- 5.5-5.9: Failing badly. Extremely prone to flooding. Will be nearly impossible to keep it idling when hot. Significant power loss.
- 5.0-5.4: This engine is probably only able to start with a pull start, daily use is nearly impossible.
- Under 5.0: How is this engine even running!?!
Caveats:
- If the test was done incorrectly, this can skew the numbers up or down.
- If the engine has excessive oil inside the housings, the compression numbers will be reported as higher than they actually are when the engine is running
- If the test was not done at 250rpm and at sea level (neither of which is likely), the numbers you have will need to be normalized to 250rpm at sea level. Use Mazda's calculator here: Foxed.ca - Rotary Compression Calculator
- This is largely my opinion, based on seeing hundreds of compression scores on the site over the years. I am not using anything scientific to back it up. As always, your mileage may vary.
The black line is the failing line. Any 1 rotor face at a 6.9 or lower is failing.
You should have gotten numbers in a format something like this:
Rotor 1: 7.5, 7.6, 7.5
Rotor 2: 7.2, 7.3, 7.3
250 RPM
- The 6 numbers, 3 for each rotor, are your compression scores, one for each face of each rotor in the engine.
- If you don't have all 6 rotor face scores and at least 1 RPM number, your compression scores are going to be vague and hard to interpret.
- You may also have your compression scores in a different scale, such as PSI.
- The RPM is the speed at which the test was done.
- The RPM is critical to interpreting the numbers, as rotary compression changes with engine speed, especially at low RPM.
- Since the engine is spun by your starter, this is ALSO a good indication of the health of your starter!
The general guideline for how good, or bad, the scores are, WHEN NORMALIZED TO 250 RPM AT SEA LEVEL!
- 8.5 and up: Congratulations, you have a stellar engine! Compression scores this high are rare. If you think that this may be too high, there may have been excessive oil in the housing.
- 8.0-8.4: This is a very good engine! You should have very good power and as long as you stay on top of the rest of the failure points possible, it should last a long time
- 7.5-7.9: This is an acceptable engine. Most engines from Mazda seem to be in this range after the break-in period. Stay on top of the other failure points possible, and you should get at least 40-60k more out of this engine, if not more.
- 7.0-7.4: The engine has some life left in it, but start looking for replacement options. Compression loss is going to start speeding up from blow-by combustion gasses eating away at the seals.
- 6.5-6.9: Officially failing. The engine doesn't have all that long to 'live'. Compression loss is accelerating due to blow-by.
- 6.0-6.4: Failing significantly. Very prone to flooding even with new starter, battery, and ignition. It will have trouble starting when hot, power loss especially down low, and noticeable difficulty idling.
- 5.5-5.9: Failing badly. Extremely prone to flooding. Will be nearly impossible to keep it idling when hot. Significant power loss.
- 5.0-5.4: This engine is probably only able to start with a pull start, daily use is nearly impossible.
- Under 5.0: How is this engine even running!?!
Caveats:
- If the test was done incorrectly, this can skew the numbers up or down.
- If the engine has excessive oil inside the housings, the compression numbers will be reported as higher than they actually are when the engine is running
- If the test was not done at 250rpm and at sea level (neither of which is likely), the numbers you have will need to be normalized to 250rpm at sea level. Use Mazda's calculator here: Foxed.ca - Rotary Compression Calculator
- This is largely my opinion, based on seeing hundreds of compression scores on the site over the years. I am not using anything scientific to back it up. As always, your mileage may vary.
The black line is the failing line. Any 1 rotor face at a 6.9 or lower is failing.
The following 2 users liked this post by gwilliams6:
johnnyg21 (02-02-2022),
triangleman404 (04-27-2021)
03-02-2016, 08:56 AM
#5
The dealer said they're passing numbers but on the results document they gave me it showed that rotor one is at minimum for two faces and below minimum on the third.
I contacted Mazda USA and they agree with me that although the car runs, it's on the low side. They said they will be calling me in the next day or two to "assist me in getting a new engine core."
I also mentioned that my starter was working great prior to taking to the dealer and now it sounds super weak and barley starts.
They said their compression tester "wasn't working" and that they had to borrow one from another dealer.
I can only assume that they cranked it a dozen times on one machine and then another dozen times on the second.
I contacted Mazda USA and they agree with me that although the car runs, it's on the low side. They said they will be calling me in the next day or two to "assist me in getting a new engine core."
I also mentioned that my starter was working great prior to taking to the dealer and now it sounds super weak and barley starts.
They said their compression tester "wasn't working" and that they had to borrow one from another dealer.
I can only assume that they cranked it a dozen times on one machine and then another dozen times on the second.
03-02-2016, 10:33 AM
#6
I don't get it. when converted to kgf/cm2 and normalized to 250 rpm, all your rotor faces are above 7.0 kgf/cm2. That's passing. I also have the workshop manual numbers that say the minimum at 240 rpm is 660 kPa. So it's passing but not great. If Mazda gives you some "assistance" with the engine core, then props to them, but I'd be surprised.
Last edited by Jastreb; 03-02-2016 at 11:05 AM.
03-02-2016, 10:44 AM
#7
SARX Legend
iTrader: (46)
Well it depends how they gave him the readings, if it is directly from the Mazda tester, hooked up to the computer then it is already normalized.
How much time/mileage do you have left under the warranty? They might hook you up, they did for me, I got a reman installed at 96,000 miles. It only lasted 24,000 miles or so but still,
How much time/mileage do you have left under the warranty? They might hook you up, they did for me, I got a reman installed at 96,000 miles. It only lasted 24,000 miles or so but still,
03-03-2016, 06:37 AM
#8
40th anniversary Edition
Coming up this May on the 8 year mark with my 2008 40th Anniversary edition,which was bought new, with me the sole owner. So I will be getting a compression test done in April. Car runs great, engine starts and pulls great, but it is a must to do the compression test before engine warranty runs out. Will post the results.
08-26-2016, 08:40 AM
#9
The general guideline for how good, or bad, the scores are, WHEN NORMALIZED TO 250 RPM AT SEA LEVEL!
- 8.5 and up: Congratulations, you have a stellar engine! Compression scores this high are rare. If you think that this may be too high, there may have been excessive oil in the housing.
- 8.0-8.4: This is a very good engine! You should have very good power and as long as you stay on top of the rest of the failure points possible, it should last a long time
- 7.5-7.9: This is an acceptable engine. Most engines from Mazda seem to be in this range after the break-in period. Stay on top of the other failure points possible, and you should get at least 40-60k more out of this engine, if not more.
- 7.0-7.4: The engine has some life left in it, but start looking for replacement options. Compression loss is going to start speeding up from blow-by combustion gasses eating away at the seals.
- 6.5-6.9: Officially failing. The engine doesn't have all that long to 'live'. Compression loss is accelerating due to blow-by.
- 6.0-6.4: Failing significantly. Very prone to flooding even with new starter, battery, and ignition. It will have trouble starting when hot, power loss especially down low, and noticeable difficulty idling.
- 5.5-5.9: Failing badly. Extremely prone to flooding. Will be nearly impossible to keep it idling when hot. Significant power loss.
- 5.0-5.4: This engine is probably only able to start with a pull start, daily use is nearly impossible.
- Under 5.0: How is this engine even running!?!
May I ask where you have this summary from, because I think it's extremly exaggerated? I currently have a daily! driver with <6.3 front and >5.4 rear rotor compression. No cold start/idling problems at all and neglectable hot start problems. I just have major power loss at highway speeds or hill climbs when the engine is hot...
Harry
BTW: it's also a 40th anniversary model with 74.5k miles on it
08-27-2016, 08:44 AM
#10
40th anniversary Edition
My post and the chart are (as stated) from a forum post by RX8club moderator RIWWP. If your compression numbers are as you state 6.3 and 5.4 (depending on your starter rpms and your altitude relevant to sea level) then your engine has low compression. Rotary engines with low compression can still run for months and maybe even years with poorer than normal performance, but will eventually need replacement or rebuilding. Any hot start issues or loss of power are often signals of low compression. Time to save for a rebuild.
If you had read through the new owners thread, you would have seen this here:
https://www.rx8club.com/new-member-f...-202454/page2/
If you had read through the new owners thread, you would have seen this here:
https://www.rx8club.com/new-member-f...-202454/page2/
Last edited by gwilliams6; 08-27-2016 at 03:51 PM.
08-27-2017, 11:14 PM
#12
40th anniversary Edition
RX8 rotary Compression test chart
Look for your numbers here on our chart. They look like psi numbers.
If there were normalized to sea level and around 250rpm, then yes they are very low. Tell us more about the car and its driving condition and who did your rotary compression testing ?
If there were normalized to sea level and around 250rpm, then yes they are very low. Tell us more about the car and its driving condition and who did your rotary compression testing ?
Last edited by gwilliams6; 08-27-2017 at 11:19 PM.
08-28-2017, 03:14 PM
#14
Rotard420
It's not my car, but one I was looking to purchase. Thanks. Guy seemed to think they were good numbers lol still on the hunt for a good one, have a couple prospects. An 05 gt with 69k miles, always premixed, oil changed every 1800 miles, has coil pack upgrade. Doesn't say whether it's a bhr kit or just let conversion. Other is a 09 r3 with 90k at a dealership for 8,000, the 05 is 2,800 USD 10,000 & 3,500 CAD
Last edited by blainezoomzoomrx8; 08-28-2017 at 03:23 PM.
08-28-2017, 04:21 PM
#16
40th anniversary Edition
yes you did the smart thing, getting those compression numbers before any purchase. Be sure you get them for any RX8 you are seriously thinking of buying. Not sure what all the three price numbers are after the 05, but see which car speaks to you and which is in the best condition .
08-28-2017, 06:47 PM
#17
Rotard420
yes you did the smart thing, getting those compression numbers before any purchase. Be sure you get them for any RX8 you are seriously thinking of buying. Not sure what all the three price numbers are after the 05, but see which car speaks to you and which is in the best condition .
The 09 r3 is $8,000 USD which is $10,000 CAD.
I'm located in Canada, but I thought it convert it, so others reading could get a better idea of what they cost.
Sorry for any confusion
08-28-2017, 09:27 PM
#18
40th anniversary Edition
Just needed a coma after the US Price. I understand now.
Depending if the 09 R3 was first sold later than September 09, you would still have a small bit of time under the engine warranty.
On one hand, there were definite improvements to series 2 models that could be worth the extra money.
https://www.rx8club.com/series-ii-te...s-i-ii-161665/
On the other hand, The 05 is cheap enough that you would have money left for any upgrades or needed maintenance items, especially if it was well-maintained and the compression numbers are good.
I personally could never fit in the R3s recaro seats, and I like my moonroof, even when hot-footing it. Again, you have to sit and drive in them both, and see which one truly speaks to you and your personality, if they are both mechanically sound.
You won't mind paying more if the R3 is the right fit for you, and conversely you will be thrilled to have gotten a sweet bargain if the GT fits the bill at less than half the price.
Depending if the 09 R3 was first sold later than September 09, you would still have a small bit of time under the engine warranty.
On one hand, there were definite improvements to series 2 models that could be worth the extra money.
https://www.rx8club.com/series-ii-te...s-i-ii-161665/
On the other hand, The 05 is cheap enough that you would have money left for any upgrades or needed maintenance items, especially if it was well-maintained and the compression numbers are good.
I personally could never fit in the R3s recaro seats, and I like my moonroof, even when hot-footing it. Again, you have to sit and drive in them both, and see which one truly speaks to you and your personality, if they are both mechanically sound.
You won't mind paying more if the R3 is the right fit for you, and conversely you will be thrilled to have gotten a sweet bargain if the GT fits the bill at less than half the price.
Last edited by gwilliams6; 08-28-2017 at 09:37 PM.
08-28-2017, 09:37 PM
#19
Rotard420
Depending if the 09 R3 was first sold later than September 09, you would still have a small bit of time under the engine warranty.
On one hand, there were definite improvements to series 2 models that could be worth the extra money.
On the other hand, The 05 is cheap enough that you would have money left for any upgrades or needed maintenance items, especially if it was well-maintained and the compression numbers are good.
I personally could never fit in the R3s recaro seats, and I like my moonroof, even when hot-footing it. Again, you have to sit and drive in them both, and see which one truly speaks to you and your personality, if they are both mechanically sound.
You won't mind paying more if the R3 is the right fit for you, and you will be thrilled to have gotten a sweet bargain if the GT fits the bill at less than half the price.
On one hand, there were definite improvements to series 2 models that could be worth the extra money.
On the other hand, The 05 is cheap enough that you would have money left for any upgrades or needed maintenance items, especially if it was well-maintained and the compression numbers are good.
I personally could never fit in the R3s recaro seats, and I like my moonroof, even when hot-footing it. Again, you have to sit and drive in them both, and see which one truly speaks to you and your personality, if they are both mechanically sound.
You won't mind paying more if the R3 is the right fit for you, and you will be thrilled to have gotten a sweet bargain if the GT fits the bill at less than half the price.
Really the biggest advantage of the r3 for me is the dealer it's at has a 6 month warranty, and I know that the trans in the s2 is way stronger. The extra oil injector isn't really nessesary being that Ill be running premix ( most likely with the sohn adapter)
And I like the first gen front end, as well as the aftermarket support.
08-29-2017, 12:29 AM
#21
Rotard420
Not pretending to be informed. I know the differences, I've driven both gens , I've owned a rotary, and I've taken them apart. I know how they work. No need to be an *******, based on an assumtion, so **** off
08-30-2017, 04:03 PM
#25
Super Moderator
