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
http://lh4.ggpht.com/_Oml-XV4A08o/SM...8%28001%29.jpg
His:
http://lh3.ggpht.com/_jh2HiMm4nKQ/Sr...0/P9160195.JPG

Attachment 203690

84mm 8.7:1 supertech. Who bought parts for your motor?

1.6
8.x:1

damn that's a shame, suppose that would solve the mystery

I hate to be right sometimes.

how are you so godlike?

Holy fucking shit... 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.

Wouldn't surprise me, he really fucked me over on my order a while back. Sorry to hear that man. :(

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.

Your pistons look nearly identical to my 84mm 10:1 Wisecos. Definitely, absolutely, no way they're low compression.

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

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 :bigtu: 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.

Get pics of known 9:1 cr pistons, and this will all be ended.

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.

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.

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 :facepalm:

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 :)

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:
http://photos-c-8.ak.fbcdn.net/hphot..._2652097_n.jpg

stock 94-97 8.8 and 9.0 pistons:
http://fixjunk.com/solomiata/solomia...toncompare.jpg

99 9.5:1 piston:
http://fixjunk.com/solomiata/solomiata2/99piston.jpg

94, 99, and 01 pistons posted above:
http://fixjunk.com/solomiata/solomia...toncompare.jpg

Your piston...
http://lh3.ggpht.com/_jh2HiMm4nKQ/Sr...0/P9160195.JPG

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.

Just for comparison's sake. Wiseco 84mm "10.0-10.3:1".
They are more different than I remembered. Looks like there is more meat in the area that fractured on yours and the dome has smaller circumference but is taller. They're actually pretty radically different than everything I see here.

Keith, if you read this and get a moment, could you post pictures of the FM Wisecos?

I just got confirmation from Supertech based on some pictures I sent them of my piston. These are 9.5:1 pistons...
Belfab... :facepalm:
Now I need to call Belfab and tell them they messed up. Anyone care to lay odds on getting anything other than a BS excuse or an empty apology?

dont u think its a little late?

my pistons came with an invoice with the partnumber and description

on the piston packaging there was a sticker with the part number.

You see, that is the thing... I would have checked that as soon as I opened the box. I suspect they were mislabeled. He claimed when I talked to him that they sort the pistons by weight to match them up. Could have happened then. Either that or I had a major brainfart and missed it.

At any rate, I am not sure 1/2 a point of CR is enough to get seriously pissed off about. Now if they were 11:1 or something stupid...

Ever take pictures of the goodies as UPS dropped them off just to post up?

Might be your only proof, a zoomed in pic of an incorrect part number.




So does this mean you're still planning on port WI since that's no longer the conclusive failure theory?

Jim Belfiore is a piece of work. He'll do nothing.

You might try the vendor for direct replacement. I doubt they are sold with any warranty at all, but it couldn't hurt I guess.

I have to go back and look at my photos to see if I took a picture of the boxes the pistons came in. I am not sure that I did. The boxes are long since disposed of. It is always the stuff you throw away you end up needing later...

I am still definitely doing the port WI. I have all of the parts, and already drilled and tapped the manifold. I even found a new Aquamist HSV that someone was selling cheap so I'll have valve controlled proportional WI. It should be a nice system.

Keith sent me this picture earlier...

The difference in all of these pistons is radical, I wonder which Wiseco's are in my car?

Why is there a little bump or dimple on the piston?

I believe it is an indicator on which way to install the piston. One should be able to tell by looking at the valve pockets but it is there anyway. The wiseco and the supertech both have it.

You are 100% correct. Even the stockers have it.

Yep. The dot points towards the front of the engine. Guess they figured something simpler than gauging the relative size of the valve pockets would be handy.



Hmm. Where have I seen that exact piston before? :rolleyes:

Yeah it is not a bad idea. It makes it more obvious to the error prone which way to install them.

I don't think my pistons had that dot - I had to be somewhat careful since they printed the "Supertech 84mm" marking upside down on one of them.

facebook

ZX-Tex - if the WI wasn't distributing evenly, #1 piston should show more carbon than the others. The pllugs show it. Is this the case with the pistons?

ZX-Tex ??

I'll have to look. No idea on the plugs they are long since mixed up.

Compare to a factory piston which has a cutout:

Side by side

SUPERTECH:
https://www.miataturbo.net/forum/att...0-piston-1.jpg

FACTORY: Notice the cutout
https://www.miataturbo.net/forum/att...ton-cutout.jpg

I just bought a set of JE 84mm 9:1 pistons for my '02. What do you guys think? am I going to be in trouble? they just went with the block to the machine shop. The head did not go with them as I am doing a mild pocket & relieve job here in my shop. the '02 has stock VVT cams. Does anyone run this setup?

so is there any conclusions here? are bel-fab and supertech the same pistons? should i be scared of supertech pistons? stick with je or weiscos?

Belfab SELLS Supertech pistons. IIRC there was never any question about this.

I still suspect that the cause of my #1 failure was uneven distribution of WI. But, that thin web area on the piston certainly does not help as it could create a hotspot that would lead to local detonation and more heat, leading to more detonation, etc. The rest of the piston looked fine, just that one spot.

So my motor has been reassembled for some time now with the FM Wiseco pistons and is waiting to go back in the car. If I could do it over again I would probably research it to be sure it is OK, then have that web around the intake pocket removed a-la the pictures JasonC shows above. They would have to be rebalanced but this is all no big deal for a good engine shop.

I hate that this happened to you, but thanks for documenting the situation. If the same situation happens to someone else, hopefully supertech will due the right thing and offer parts/labor compensation.

I guess the other question is how much boost do you plan to run, i have the 9.5:1 supertechs & so far they are fine, i wanted to run stock compression so i had good low down torque, i dont think i will ever see more then 18 psi though

I have this concern now as well. I was planning to have diyautotune tune my car to the limit of the MS MAP sensor ~ 22psi. I wonder how much this weak point can withstand with no detonation problems?

I've had 2 machinists tell me it won't be a problem in the absence of detonation.
I'd asked them if they thought it would be a good idea to mod the pistons to look like the factory ones.
I didn't bother then - however, I don't plan to run ludicrous boost.

Keep in mind I was running up to 18 psi with a GT2871 with an aggressive tune so I was well over 300 HP based on other data here. If you do not plan on pushing your engine as hard it may not be as big a deal.

without having to go back & read your thread over again, how aggressive was your tune? did you have it dyno'd? how much timing did you pull out of it?

What are you fueling with? Are you low comp? With even stock comp, I wouldn't think you could run 22 psi on that turbo on street gas. Maybe I'm wrong, but watch for det carefully.

Any updates?

I finally got the engine back in the car. I should have it running again pretty soon.

It's about time! I've been real interested in your build.