Darwantae951

I always thought the 10.6:1 pistons looked like chinamen from the Kung Fu movies...ok, technical information may now continue...

whalebird

That's good info about the 9.7 pistons. I have a couple of conflicting sources on the matter. PET does not differentiate between 9.5/9.7 either. Curiously, PET does not differentiate between Mahle and KS, and Porsche literature claims they are interchangeble IIRC; they are different in a couple of ways though.
Any idea what the "+" and "-" on the crown of Mahle pistons denotes?

M758

Who said the US NA does only 125?


Also the US 944 has a taller 5th gear it accelerates slower. I have had my 944 spec to 132 mph at Cal speedway. This was 6000 rpm in 5th in draft of a Honda. Speed comes from the calcuated gear ratios (.829 short 5th) and tire diameter using 225/50 R15.

M758

That is because they are not different. THERE ARE NO 9.7:1 pistons. PERIOD. Funny how you don't trust PET...

As for the reason Porsche does not differential between Mahle and KS pistons. Simple.... Porsche considers them to be the same even if there are detail supplier differences. Porsche considers them the same for form, fit, and fuction. Not sure what the + and - on the crown denote. I do know that the numbers on the pistons are the tolerance group. 0, 1,2 0 and 1 are most common 2 is quite rare.

Cochezz

M758, What is the size difference between tolerance group 0,1 and 2?

From the factory would a motor be all 1 or 2, or is it mixed?

Thanks

MAGK944

Tolerance groups were used by the factory to match engine casings.

From the tech spec manual:
Group 0 = 99.98mm
Group 1 = 99.99mm
Group 2 = 100mm
Group 1 0 = 100.48
Group 1 1 = 100.49
Group 1 2 = 100.5

Cochezz

Ok so .0004" is the difference.

Do you find the motor are all one tolleracne group or is it matched?

Thanks

Cochezz

Oh yah and what is a "tech spec" manual???

M758

I have never found a mix block. All are bore to the same group.

MAGK944

There are a bunch of them here:
http://www.ebay.co.uk/sch/oharazmanualz/m.html?_nkw=944&_sacat=0&_odkw=&_osacat=0&_trksid=p3911.c0.m270.l1313

Search eBay for your car model/year and you might find one. They were supplements to the factory service manuals.

Like M758 I haven't found a mix of groups in the same block. I can only assume that during production they used different casting/boring equipment and the groups were created to meet the variations in the machines.

Cochezz

Ok sounds good

Thanks

North Coast Cab

I have aftermarket JE flat top pistons in my motor. We figured they were 10.8:1 when we built it before messing with the head. I am pulling the motor right now to sell so I can install a 3.0. Let me know if interested. Oh yeah, it's a 2.6 dry sleeved block with lots of other tricks.
Also, on another thread Ed Baus is parting a 2.5 flat top JE piston motor.

M758

My thought is they were hitting the tolerance limit on the OD for the pistons. As such they created 3 tolerance groups and put the pistons in those groups. Then the machined the blocks to one of the tolerane grousp. I believe may be easier to hold a tight tolerance on the ID bore of cylinder than on OD of the piston.

whalebird

I think you miss understand me. The appearent absence of 9.7 is good info. I've never heard reference to the mistake in the factory literature and am still not convinced without factory supplements that it was a misprint, but it is plausable. I really don't care what Porsche considers to be acceptable replacements in these motors, because they don't care. There are slight variations between these pistons that I would not feel comfotable mixing in the same motor. The wrist pins for example, are piston (Mahle/KS) specific. So I question the availability of OEM pins from the aftermarket.
Trust PET? I do typically, but tons of old parts that are NLA or are superceded are no longer listed. Only the superceded (replacement) part and it's number are published. If there were a 9.7 piston, and it no longer exists, PET may be of little help. Maybe I will break out my microfische and see what Porsche offered when these cars were made.
I'm still curious what the engine builders prefer regardless of compression ratio. JE have a sorry track record in Porsche motors(air cooled especially)and is a budget choice given they cannot iron coat their pistons legally. I wouldn't build a motor with them, but thats me. I suspect KS and Mahle motorsport may be able to supply a replacement for our motors at a resonable cost, but that remains to be seen. I also am curious if Mahle pistons iron coat their pistons the same that KS does. KS pistons look and wear differently.
Just a few thoughts I have...my intentions are good.

M758

Look my day job is an engineer. I work for a large company that makes complex machines. One of the jobs it did was customer support and one aspect of the job. Was parts lists and manuals. So it have modified documents like PET for Customers before. I know how these documents are created and under what circumstances you change part numbers. So PET may not be perfect, but it is better that most sources we havoc access too.

There are no such thing as 9.7:1 pistons. Never existed ever. I keep trying to say that and have been saying that for at least 10 years. The 9.7:1 number is myth created 15 year ago when some one created a FAQ on the net. It was wrong then and is wrong now. I haw taken apart a lot of us spec motors and. Never seen anything like a 9.7:1 piston. Also I have never heard if another 944 racer ever having such a piston ever. There is one simple reason. They don't exist and never have.