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Old 11-16-2009, 04:07 PM   #1
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Default Compression numbers with low compression pistons

I have done some searching and couldn't find anything. What should the compression numbers be for our engines warm with low compression (8.8 or 9) pistons? I am goig to do a compression test this week and am just curious.
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Old 11-16-2009, 09:26 PM   #2
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Did the compression test on my buddies car and it is 135-135-135-150. I know NA they are around 180, but would lower compression pistons make the compression this low? Also, why would just one of the cylinders be higher?
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Old 11-16-2009, 09:37 PM   #3
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Don't know about low comp piston compression, but from what I've read/heard good compression on a regular miata is supposed to be 185-200. 165-185 is OK and anything less is bad. Also a <10% variation between cylinders is very important. Anything more is bad.
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Old 11-16-2009, 09:41 PM   #4
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That's what I have always known but was unsure of these numbers. They were with the car warm, so that rules out cold engine. I just don't understand how 3 could be so far off of where they should be but be so close to each other. I am going to do a leakdown test tomorrow.
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Old 11-17-2009, 01:18 AM   #5
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um if i saw numbers in the 30's it would tell me **** is not good, i would hope you could atleast hit 150. Considering the 9-1 people run up to like 190 or so.
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Old 11-17-2009, 01:24 AM   #6
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135's isn't good, no matter the compression. How does it run? Smoke?

Do it again wet and see what it does.
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Old 11-17-2009, 02:04 AM   #7
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I was wondering the same thing, does the compression ratio have a direct impact on compression numbers?? I don't think so...

Its the shape of the dome of the piston (generally speaking) that determines the compression ratio, its the rings that determine the compression sealing inside the cylinder, right?

My brother compression tested his built turbo b16 motor EM1 last week, and could only muster 125 across the board..
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Old 11-17-2009, 02:08 AM   #8
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It runs fine and doesn't smoke. The only thing that is weird about it is how the first three are exactly the same. If they were different, then I would just say we need to replace the rings and hone the block. I just wanted to make sure before we did that. I am going to do a leakdown test tomorrow and see what comes up.
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Old 11-17-2009, 02:11 AM   #9
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Quote:
Originally Posted by NoMiEzMX-5 View Post
I was wondering the same thing, does the compression ratio have a direct impact on compression numbers?? I don't think so...

Its the shape of the dome of the piston (generally speaking) that determines the compression ratio, its the rings that determine the compression sealing inside the cylinder, right?

My brother compression tested his built turbo b16 motor EM1 last week, and could only muster 125 across the board..
Yeah that is low enough to where i would be looking at a rebuild imo. Unless the car was cold and the TB was still closed.
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Old 11-17-2009, 01:31 PM   #10
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OK did a leakdown test and it is showing it as only having about 5% leaking. I am really at a loss here. If it has low compression, it should be leaking right?
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Old 11-17-2009, 01:47 PM   #11
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compression was done when warm right?
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Old 11-17-2009, 01:56 PM   #12
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Yup. Engine was warm and throttle open. By warm I mean I let it idle for about 10 minutes prior. Do I need to do more than that? I always assumed that would get it warm enough
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Old 11-17-2009, 03:44 PM   #13
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I was gunna say bad valves... but leak down wasnt pathetic.
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Old 11-17-2009, 04:16 PM   #14
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Bad compression tester?

I've had someone tell me their compression numbers are crap. I threw my gauge on it and it was a perfectly healthy motor.
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Old 11-17-2009, 04:47 PM   #15
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Quote:
Originally Posted by crashnscar View Post
Bad compression tester?

I've had someone tell me their compression numbers are crap. I threw my gauge on it and it was a perfectly healthy motor.
I would expect this as well. I've had cheap testers read all over the board, some way high and some way low.
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Old 11-17-2009, 05:11 PM   #16
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If you have big lumpy cams and/or mis-timed stock cams, then that will give you low compression numbers even with non-low-compression pistons.
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Old 11-17-2009, 09:53 PM   #17
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OK we took the engine out tonight to take a look at it. The mechanical timing was correct, but the tension pulley on timing belt wasn't putting a whole lot of tension on it. It was slide pretty much all the way out and locked in place. I am taking the head off right now to get a look at the cylinder walls. I will post up pictures to see what you guys think.

I also borrowed another compression tester from a friend and that one gave us about the same results. It had about 140 in the first three and 150 in the number 4.
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Old 11-17-2009, 10:09 PM   #18
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Also, we did put a cap full of oil and it raised the compression to around 185. I know this points to the rings, but the leakdown test didn't show anything. I expected to hear air coming out the dipstick and a decent leak percentage.
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Old 11-17-2009, 10:13 PM   #19
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Does movement of the piston rings across the cylinder walls affect the compression? Maybe while the engine was cranking the compression was allowed to escape through the pistons stroke, but on a leak down test the piston stops and holds compression better? I'm not a compression diagnosis expert obviously. But yeah, rings sound like the problem.
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Old 11-17-2009, 10:30 PM   #20
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Common sense would lead me to believe that too. Head is off and the walls have some signs of up and down wear, but when I rub my finger across them, I don't really feel anything. I am about to upload some pictures.
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