compression rating
09-08-05, 06:10 PM
#2
Go Hard....or Go Home
The only proper way is with a Mazda compression tester. Lots of people claim they can do it, however you need to measure all 3 pulses of each rotor as it turns, and the Mazda compression tester does that.
09-09-05, 02:22 AM
#3
If you have a regular compression tester you can get a rough idea of how even your three pulses are. This should be enough to tell you if you have a blown apex or side seal on it's way out.
All you have to do is remove the valve at the end that holds the air in so that the needle bounces with the compression instead of staying at the highest rating. Then have someone crank the engine and watch for the bouncing of the needle. If it consistantly bounces to the same level with every compression stroke, you haven't blown anything.
The only way to get accurate pressure readings though is with the Mazda tester (either the new digital one or the old one with the paper ticker thingy). That'll give you a max and min pressure reading of every chamber individually and you can interpret that to find out how strong your compression is.
Compression tests AT Mazda will cost you. Around here there are no rotary shops with a tester though, so it's the only choice.... I've seen the testers on ebay, but the 700USD+ price tag has scared me off.
Jon
All you have to do is remove the valve at the end that holds the air in so that the needle bounces with the compression instead of staying at the highest rating. Then have someone crank the engine and watch for the bouncing of the needle. If it consistantly bounces to the same level with every compression stroke, you haven't blown anything.
The only way to get accurate pressure readings though is with the Mazda tester (either the new digital one or the old one with the paper ticker thingy). That'll give you a max and min pressure reading of every chamber individually and you can interpret that to find out how strong your compression is.
Compression tests AT Mazda will cost you. Around here there are no rotary shops with a tester though, so it's the only choice.... I've seen the testers on ebay, but the 700USD+ price tag has scared me off.
Jon
09-09-05, 08:46 AM
#4
thanks guys...a guy wanted to buy my fb so he took it to some shop on the Queensway i was there ,those guys took both the trailing plugs at the same time and crank it over it readabout 65 is that good or bad...... car works mint no smoke no hard start he said it should be 100 can anyone give advice got 124k on car and does the 2 plugs have to be out at the same time
09-09-05, 01:43 PM
#5
65psi?
Page 1-12 of the '85 FSM gives two charts for looking at compression. One shows how pressure drops as altitude increases, the other shows compression in relation to cranking speed.
http://www.fallingrain.com/world/CA/...mond_Hill.html
So you're not even 1,000 ft above sea level, so the altitude chart doesn't matter.
Assuming your battery, battery cables and starter are good, you should be cranking at roughly 250rpm and showing roughly 85psi.
If he's expecting 100, he's crazy. Even at sea level your engine would have to be turning over at almost 400rpm to get numbers like that (the norm is 250rpm).
So yeah. Your compression IS low, but expectedly so for a 20+yr old car. If the engine has never been rebuilt, then it doesn't surprise me. Is it at "omfg the engine's gonna blow" levels? No. Just make sure the OMP is functional and that engine should give you many more miles. Mine shows about the same compression right now.
If the dude is looking for 100psi, or even 85psi (what a NEW engine shows) then he's got unrealistic expectations.
Go here:
http://www.wankel.net/%7Ekrwright/ca...85_manual.html
Print out section #1 : Engine
Look at page 1-12 and give it to the dude so that he knows what he should be looking for.
Oh, and about the "both plugs out at the same time" thing, they say to only do that for the -SE, but that's because of the gas vapor that gets shot out of the 12a. With the -SE you can just unplug the injectors.
And whythefuck are they testing the TRAILING? I just noticed that. They should be taking one LEADING plug out at a time and testing there. That would affect your compression reading alot. Get him to go back and do it right!
Jon
Page 1-12 of the '85 FSM gives two charts for looking at compression. One shows how pressure drops as altitude increases, the other shows compression in relation to cranking speed.
http://www.fallingrain.com/world/CA/...mond_Hill.html
So you're not even 1,000 ft above sea level, so the altitude chart doesn't matter.
Assuming your battery, battery cables and starter are good, you should be cranking at roughly 250rpm and showing roughly 85psi.
If he's expecting 100, he's crazy. Even at sea level your engine would have to be turning over at almost 400rpm to get numbers like that (the norm is 250rpm).
So yeah. Your compression IS low, but expectedly so for a 20+yr old car. If the engine has never been rebuilt, then it doesn't surprise me. Is it at "omfg the engine's gonna blow" levels? No. Just make sure the OMP is functional and that engine should give you many more miles. Mine shows about the same compression right now.
If the dude is looking for 100psi, or even 85psi (what a NEW engine shows) then he's got unrealistic expectations.
Go here:
http://www.wankel.net/%7Ekrwright/ca...85_manual.html
Print out section #1 : Engine
Look at page 1-12 and give it to the dude so that he knows what he should be looking for.
Oh, and about the "both plugs out at the same time" thing, they say to only do that for the -SE, but that's because of the gas vapor that gets shot out of the 12a. With the -SE you can just unplug the injectors.
And whythefuck are they testing the TRAILING? I just noticed that. They should be taking one LEADING plug out at a time and testing there. That would affect your compression reading alot. Get him to go back and do it right!
Jon
09-09-05, 01:46 PM
#6
Just a side note. With a pressure rating in PSI (as they gave you) shows me that they're not using a rotary compression tester.
65psi = the highest pressure of all three compression strokes. Without a test on a rotary tester, you could be reading that with a blown apex seal (because the compression between the other two seals is still alright).
Not that I'm saying you have a blown apex, but the potential buyer doesn't seem to know enough to do a compression test properly, so he shouldn't have any expectations from it.
Jon
65psi = the highest pressure of all three compression strokes. Without a test on a rotary tester, you could be reading that with a blown apex seal (because the compression between the other two seals is still alright).
Not that I'm saying you have a blown apex, but the potential buyer doesn't seem to know enough to do a compression test properly, so he shouldn't have any expectations from it.
Jon
09-10-05, 11:26 PM
#7
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most of the inaccuracies with compression testing in my experience comes with the starter. either the battery is too weak to maintain a steady testable RPM, or the starter itself is worn. you have to take this into account as mentioned.
i think a better test for engine condition is how it runs and what type of fuel mileage you are seeing on the highway. if it's close to stock then the engine is running fine, but of course after all this time is a little worn...
also note that to obtain the max compression rating, the engine must be warmed to operating temp first, and there must not be ANY flooding of fuel...
i think a better test for engine condition is how it runs and what type of fuel mileage you are seeing on the highway. if it's close to stock then the engine is running fine, but of course after all this time is a little worn...
also note that to obtain the max compression rating, the engine must be warmed to operating temp first, and there must not be ANY flooding of fuel...
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09-11-05, 10:55 PM
#8
also... they checked with the TRAILING!
have them check again with the LEADING plugs out and the trailings in, put the tester in the bottom hole and don't use any of those stupid extender things....
make sure the engine is warm when the test is done, and (less important but still) that the engine is at WOT.
You'd want both plugs out at the same time, yes.
but checking compression in the trailing holes is not going to give anywhere NEAR as high a number as when you check the leading hole.
I'd say anything over ~80PSI on a 12A is probably a pretty solid motor, when you tested with the LEADINGS out and the engine warm!
have them check again with the LEADING plugs out and the trailings in, put the tester in the bottom hole and don't use any of those stupid extender things....
make sure the engine is warm when the test is done, and (less important but still) that the engine is at WOT.
You'd want both plugs out at the same time, yes.
but checking compression in the trailing holes is not going to give anywhere NEAR as high a number as when you check the leading hole.
I'd say anything over ~80PSI on a 12A is probably a pretty solid motor, when you tested with the LEADINGS out and the engine warm!
09-12-05, 01:16 AM
#9
I'd say anything over 75 is still strong if it's the original motor.
85psi is "this engine just came from the factory" perfect
75psi is "even after 20 years, I can still bury the needle"
65psi is "this engine will need a rebuild, but I can still do 170 on the queensway if I push it. With a good OMP this engine has a few years left" (mine)
below 65 is worrysome.
Those numbers are *my personal opinion* based on what I've read in the FSM. Don't take that as law, but it makes sense.
Jon
85psi is "this engine just came from the factory" perfect
75psi is "even after 20 years, I can still bury the needle"
65psi is "this engine will need a rebuild, but I can still do 170 on the queensway if I push it. With a good OMP this engine has a few years left" (mine)
below 65 is worrysome.
Those numbers are *my personal opinion* based on what I've read in the FSM. Don't take that as law, but it makes sense.
Jon
09-12-05, 09:56 AM
#10
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As long as you have 3 consistent pulses and at least 80psi, yes in theory the motor is running, however at 80 its not performing well anymore. New motors put out over 100psi.
We tested my 12A this summer, i got 115 psi on the front and 110 psi at the rear. Anything less than 100 i would be considering rebuild as i wouldnt want a tired motor in my car. It is believed by many that less than 80 means blow anytime. And we found this out this summer.
So if your at 65psi, id be thinking about a rebuild.
We tested my 12A this summer, i got 115 psi on the front and 110 psi at the rear. Anything less than 100 i would be considering rebuild as i wouldnt want a tired motor in my car. It is believed by many that less than 80 means blow anytime. And we found this out this summer.
So if your at 65psi, id be thinking about a rebuild.
09-12-05, 01:21 PM
#11
Hey Taylor,
I'm thinking about a rebuild, but I haven't actually tested mine yet. I've got the compression tester, but I haven't actually checked it yet.
All I know is that with a rebuilt carb and a Mallory Comp 70/Holley 1-4psi FPR I can only do 170 on the queensway no matter how long I hold it.
I don't know how you got over 100psi, since Mazda's *factory shop manual* for an '85 says that you should be getting 85psi on a new mazda engine.
The rebuild will wait though. I want to do the body work first. As ricer as that sounds, I want to do it before the current paint gets too bad, and before I develop any rust. From what Will's told me about the cost of repainting his SA, it'll be quite pricey to get a good quality job done on my FB. The rebuild will wait until after that.
Tell you guys what. If I have time before work today I'll comp test my engine and see what I actually get.
Jon
I'm thinking about a rebuild, but I haven't actually tested mine yet. I've got the compression tester, but I haven't actually checked it yet.
All I know is that with a rebuilt carb and a Mallory Comp 70/Holley 1-4psi FPR I can only do 170 on the queensway no matter how long I hold it.
I don't know how you got over 100psi, since Mazda's *factory shop manual* for an '85 says that you should be getting 85psi on a new mazda engine.
The rebuild will wait though. I want to do the body work first. As ricer as that sounds, I want to do it before the current paint gets too bad, and before I develop any rust. From what Will's told me about the cost of repainting his SA, it'll be quite pricey to get a good quality job done on my FB. The rebuild will wait until after that.
Tell you guys what. If I have time before work today I'll comp test my engine and see what I actually get.
Jon
09-12-05, 11:27 PM
#12
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well, we must be reading different info. Ive always believed, heard and read that a new Mazda motor would be in the 120-125psi range. 85 would definetly be low. Maybe 85 is before its gone through its break in stage.
My car isnt fast by any means, but with the 425 cfm Yaw carb, Road race header and straight pipping and a few other items, 170km/h comes and goes fairly quickly. Although i have only 20k on the motor. Nik and I comp tested mine in the spring and it had the #'s i stated above. A free flowing exhaust and rebuild im sure would do wonders for your car.
Getting back on topic. Nigel if you have 65psi, say a quick thank you everytime you turn the key and the car actually starts.
My car isnt fast by any means, but with the 425 cfm Yaw carb, Road race header and straight pipping and a few other items, 170km/h comes and goes fairly quickly. Although i have only 20k on the motor. Nik and I comp tested mine in the spring and it had the #'s i stated above. A free flowing exhaust and rebuild im sure would do wonders for your car.
Getting back on topic. Nigel if you have 65psi, say a quick thank you everytime you turn the key and the car actually starts.
09-13-05, 02:16 AM
#13
Taylor,
He got 65psi off Trailing.. that number means nothing
[hijack]
Well.. I didn't get a chance to test mine today, maybe tomorrow.
I've got a Sterling carb, which should also flow between 425-465cfm (I got one of the earlier builds, before they got it to flow the same as the Holley 465). I aim to get a nice exhaust and a rebuild ASAP after my paint is done. I know my car can do a lot better.
I also plan to get tuning it... I just need to pick up some jets, a reliable fuel pressure gauge, and find a way to re-mark my pulley for timing marks so I can time it.
I'm getting one of those "transistor trick" boxes to mount a 2nd Gen coil for my leadings. I figure if I set my timing and rejet the carb, I should squeeze a bit more out of this tired motor, which will hold me over until I get an exhaust and rebuild it.
[/hijack]
He got 65psi off Trailing.. that number means nothing
[hijack]
Well.. I didn't get a chance to test mine today, maybe tomorrow.
I've got a Sterling carb, which should also flow between 425-465cfm (I got one of the earlier builds, before they got it to flow the same as the Holley 465). I aim to get a nice exhaust and a rebuild ASAP after my paint is done. I know my car can do a lot better.
I also plan to get tuning it... I just need to pick up some jets, a reliable fuel pressure gauge, and find a way to re-mark my pulley for timing marks so I can time it.
I'm getting one of those "transistor trick" boxes to mount a 2nd Gen coil for my leadings. I figure if I set my timing and rejet the carb, I should squeeze a bit more out of this tired motor, which will hold me over until I get an exhaust and rebuild it.
[/hijack]
09-13-05, 09:43 AM
#14
Ok, before more people get confused:
-588kPa or 85 psi @ 250 rpm is within Mazda spec for compression.
-147kPa or 21 psi @ 250 rpm is the differential limit between rotors.
85 psi corresponds to 6 kg/cm2 on a Mazda factory compression tester.
Yes, that is acceptable according to Mazda spec, but, new and strong engines generally will show 8+ kg/cm2 on the mazda tester. Just like strong engines will show 100 psi+ on a piston tester.
Also, the factory test is performed through the trailing plug hole.
-588kPa or 85 psi @ 250 rpm is within Mazda spec for compression.
-147kPa or 21 psi @ 250 rpm is the differential limit between rotors.
85 psi corresponds to 6 kg/cm2 on a Mazda factory compression tester.
Yes, that is acceptable according to Mazda spec, but, new and strong engines generally will show 8+ kg/cm2 on the mazda tester. Just like strong engines will show 100 psi+ on a piston tester.
Also, the factory test is performed through the trailing plug hole.
09-13-05, 01:10 PM
#15
My Bad
Page 1-10 and 1-11, step 4 on both pages "Remove the Trailing side plugs"
Okay, so he tested in the right place... But I still see charts in the FSM saying that the reading *should* be 6kg/cm2(85psi).
Mazdatrix' site is down right now but they list the rotor compression ratings. IIRC the FB has 9.4:1 while the N/A FC has 9.7:1
Which engines are you thinking of that show 8+kg/cm2?
John, maybe I'm missing something. Pull up page 1-12 of the '85 manual I posted above and tell me what I'm reading wrong!
Jon
Page 1-10 and 1-11, step 4 on both pages "Remove the Trailing side plugs"
Okay, so he tested in the right place... But I still see charts in the FSM saying that the reading *should* be 6kg/cm2(85psi).
Mazdatrix' site is down right now but they list the rotor compression ratings. IIRC the FB has 9.4:1 while the N/A FC has 9.7:1
Which engines are you thinking of that show 8+kg/cm2?
John, maybe I'm missing something. Pull up page 1-12 of the '85 manual I posted above and tell me what I'm reading wrong!
Jon
09-13-05, 03:45 PM
#16
You are reading nothing wrong. The error is in your interpretation.
85 psi, 588 kPa, 6 kg/cm2 is the minimium acceptable compression. In other words, as long as you have that, with not more than 21 psi difference between rotors, your engine is still ok according to Mazda.
Just like minimum apex seal height is 6.5mm to be within spec. Less is bad, but more is better.
Nowhere does it say that 85 psi (6 kg/cm2) means the "engine just came from the factory perfect". Cause it is not.
My engine with brand new housings tested at over 115 psi a few years ago.
Couple of people doing over 8 on the Mazda Tester : https://www.rx7club.com/showthread.p...ssion+readings
85 psi, 588 kPa, 6 kg/cm2 is the minimium acceptable compression. In other words, as long as you have that, with not more than 21 psi difference between rotors, your engine is still ok according to Mazda.
Just like minimum apex seal height is 6.5mm to be within spec. Less is bad, but more is better.
Nowhere does it say that 85 psi (6 kg/cm2) means the "engine just came from the factory perfect". Cause it is not.
My engine with brand new housings tested at over 115 psi a few years ago.
Couple of people doing over 8 on the Mazda Tester : https://www.rx7club.com/showthread.p...ssion+readings
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