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Old 09-17-2009, 12:14 AM   #61
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That looks like a high compression piston to me. It is domed instead of being dished. Looks like a 10-10.5:1 piston....
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Old 09-17-2009, 04:42 AM   #62
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That looks like a high compression piston to me. It is domed instead of being dished. Looks like a 10-10.5:1 piston....
Supertech 1:8.6

His:
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Old 09-17-2009, 04:55 AM   #63
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84mm 8.7:1 supertech. Who bought parts for your motor?
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Old 09-17-2009, 08:24 AM   #64
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Old 09-17-2009, 08:39 AM   #65
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1.6
8.x:1
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Old 09-17-2009, 09:03 AM   #66
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damn that's a shame, suppose that would solve the mystery
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Old 09-17-2009, 09:22 AM   #67
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I hate to be right sometimes.
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Old 09-17-2009, 09:40 AM   #68
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That looks like a high compression piston to me. It is domed instead of being dished. Looks like a 10-10.5:1 piston....

how are you so godlike?
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Old 09-17-2009, 10:59 AM   #69
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Old 09-17-2009, 11:02 AM   #70
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Holy ******* ****... I bought these myself straight from Belfab! I ASKED for the 9:1 pistons, even had a conversation with the owner about it. I wonder if they sent me the wrong ones? 10:1+ compression and 19 psi on pump gas FTL.

My compression test numbers were coming in at 190-195. I can search up on it, but offhand, does anyone know what the compression test should be for a 99 head with 9:1 pistons?

EDIT: OK here is the deal. I called Supertech, not Belfab, but Supertech. In the 83.5mm bore (what I have) they only have a 8.6:1 and a 9.5:1 piston; The 8.6:1 is a flat top piston. The 9.5:1 is a small dome top. Based on that, I would say these are at least 9.5:1 pistons.

The Supertech 9.0:1 only comes in a 84mm bore, and has an 'almost flat' top.

Last edited by ZX-Tex; 09-17-2009 at 01:30 PM.
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Old 09-17-2009, 08:09 PM   #71
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Holy ******* ****... I bought these myself straight from Belfab! I ASKED for the 9:1 pistons, even had a conversation with the owner about it.
Wouldn't surprise me, he really fucked me over on my order a while back. Sorry to hear that man. :(
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Old 09-17-2009, 08:54 PM   #72
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Did he do anything to resolve it?

OK according to the 1999 Shop Manual, the 'standard' compression test pressure is 209 psi. This is with the stock 9.5:1 pistons of course. So, assuming that my compression tester gauge is reasonably accurate, and using the ratio of the absolute pressures (stock versus my test), then my compression ratio calculates out at 8.9 psi, which is close enough to 9.0.

So, despite the appearance of the piston tops, I am not sure that compression ratio is the issue here.

Last edited by ZX-Tex; 09-17-2009 at 09:11 PM.
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Old 09-17-2009, 08:56 PM   #73
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Your pistons look nearly identical to my 84mm 10:1 Wisecos. Definitely, absolutely, no way they're low compression.
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Old 09-17-2009, 09:33 PM   #74
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Originally Posted by ZX-Tex View Post
Did he do anything to resolve it?

OK according to the 1999 Shop Manual, the 'standard' compression test pressure is 209 psi. This is with the stock 9.5:1 pistons of course. So, assuming that my compression tester gauge is reasonably accurate, and using the ratio of the absolute pressures (stock versus my test), then my compression ratio calculates out at 8.9 psi, which is close enough to 9.0.

So, despite the appearance of the piston tops, I am not sure that compression ratio is the issue here.

The idea that you can calculate compression ratio from a compression gauge is laughable. If you want, bolt your head back on and CC a cylinder - turn the motor to TDC, fill the cylinder with oil, and measure (with a burette, to the tenth of a CC) how much oil you put in.

(Swept volume + CC measurement)/CC measurement = compression ratio
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Old 09-17-2009, 09:44 PM   #75
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I understand what the compression ratio is. I am not calculating compression ratio straight from the psi reading. I know that does not work. What I am doing is comparing the difference between two readings, with the only difference being the volume difference in the chambers due to the pistons.

Look, you are preaching to the choir so to speak. But there are several issues here to be considered:

1. Before I contact Belfab, particularly given their dodgy nature, I need to be sure I am on the side of the righteous here. I want to be positive that these are pistons with a CR higher than 9.0:1 before I complain they are not. This is potentially a big deal.

2. I just tested my compression tester against another known good gauge and it seems to be accurate within 1 psi at 100 psi (as high as my shop compressor goes). So, assuming my compression tester gauge is reasonably accurate, and the stock pistons are 9.5:1, why did the compression test not test higher than stock, but lower? No matter what, if the Supertech CR is higher than the stock 9.5:1 CR, then the compression tester reading would HAVE to be higher than what the manual calls out.

I know how to do a compression test BTW. Fully charged battery, WOT, crank while watching the gauge until it stops rising.

3. It is possible, even plausible if you look at the pictures, that though these pistons clearly have more material at the center of the crown, they may have less (lower than a flat top) at the edges of the crown. Look around the valve relief pockets on the pictures of the Supertech pistons shown above. Thanks by the way of posting those guys, that was great The ones on the low compression pockets look deeper, meaning (possibly) more material at the edges (outer circumference) of the crown.

One thing I can do here, though it will not be for a few days, is run the compression test again when I reinstall the new motor. Same gauge, same starter, same head, same valves, same everything, except the new pistons (FM Wiseco 9.0:1). If the reading is the same, then the CRs have to be the same.
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Old 09-17-2009, 09:46 PM   #76
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Get pics of known 9:1 cr pistons, and this will all be ended.
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Old 09-17-2009, 10:53 PM   #77
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Agreed. Not sure where to get that from. Googled it with no luck. All the Supertech vendors, and Supertech themselves, have generic pictures. Belfab is not exactly as customer oriented as FM so I doubt I will get them to send me or post up a picture like FM did.
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Old 09-17-2009, 11:06 PM   #78
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Quote:
Originally Posted by ZX-Tex View Post
I know how to do a compression test BTW. Fully charged battery, WOT, crank while watching the gauge until it stops rising.
I never said you didn't, but even the most accurate test with the most accurate tester won't (IMO) provide accurate compression ratio results. Compression tests test the ability of the motor to hold compression; when the gauge maxes out, you're losing pressure through the rings or valve seats or (god forbid) head gasket. You can take a fucked-up high comp motor and test it, and it'll test lower than a 7:1 turbo screamer.

You haz pm.
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Old 09-17-2009, 11:41 PM   #79
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I never said you didn't,
I know, no worries. I just wanted to remove that as a source of error before anyone brought it up. I have read posts about people doing compression tests and wondering why their numbers were so low, only to find out when asked more questions that they were doing something stupid like not opening the throttle at all, or only cranking for a second or two

I agree about the other variables you mentioned above. I do not think they apply here though. Except for the #1 piston damage, the rest of the motor was fine. I am going off of the highest compression test number from the other three cylinders, which were 190, 195, and 192 psi. Leakdown on the other three was good too, and consistent.

Also, I forget if I mentioned this before, but just in case I sent the injectors off to Injector Rehab for cleaning and flow testing. Before cleaning they were all within 1% of each other. So the detonation was not caused by a lean charge in cyl #1, at least not from the injector. We will not get into the age old argument about the adequacy or inadequacy of the stock injector rail
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Old 09-18-2009, 11:01 AM   #80
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those have got to be the 9.5:1 pistons. compare the domes on the stock miata pistons for 9.4:1 and 10:1. the dome is in between.

the 8.6 pistons definitely have a SLIGHT dish.

8.6:1 83.5mm:


stock 94-97 8.8 and 9.0 pistons:


99 9.5:1 piston:


94, 99, and 01 pistons posted above:


Your piston...


the dome is wider than the stock 9.5, but it's also lower (flatter) than the 2001 10:1 so it can't be higher than that. the supertech catalog only lists an 8.6, 9.0, 9.5, and 10:1.
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