The parting guy is "A Part Above" - seems reputable. If these really are identical to OEM, it would be a great find, but we'd have to see pictures, get weights, etc. to verify that.

FWIW, the 3 cars I've known to make 139 RWHP have low-comp. pistons & shaved heads. My prevoius, fresh high comp. motor made 137 ( no head shaving).

Well,
Aftermarket pistons are not strictly allowed in 944 spec.

However if they the equivalent replacements of the same weight then you are just replacing like for like. That would be ok.

Now with respect to oversize and tolerance groups.


There are 3 tolerance groups
0
1
2

0 and 1 are the most common. These are not overbores, but the stock bores falling the stock "normal" tolerance. What is done is to ensure a good stug fit blocks and pistons are put in 3 groups so that when you build them you can keep a close tolerance between the piston and bore, but be able to manufacture them to a looser tolerance. Pistons and blocks will be stamped with a 0,1 or 2 to denote the tolernace group.

Overbores
There are also two standard overbores. 0.5mm and 1mm. These are not allowed since they require the blockes to bored out and then you must get replacement pistons. This expenensive becase you need to get the block bored using the special Sunnen process to deal with alusil bore material and then need to get new pistons since used overbore pistons are not common. As such it will cost ALOT to rebore and re piston 944 motor. You can probably get complete race cars for less than the cost of a rebuilt oversize 944 motor. Since the series is trying to limit costs we said NO to overbores. The major reason is that it is very common technique used in other racing classes to try to gain 2-3 hp. I don't think it to be cost effective, but at the same time having new guys THINK the need to overbore a perfectly good motor because the "fast guys" do is not what we are about.


The good news for use however is that most 944 blocks last a long long time. Borewear is not that critical on these motors. As such reboring is not really neeed 95% of the time. If the time comes that we cannot get used blocks we can revist the overbore rule to save motors. We will however cross that bridge when we get there.


So Rick if those pistons are direct replacements equivalent in weight and shape they in the motor they will provide no benefit over the stock factory piston. As such I would not have issue with using them. If they are lighter or have some improvements that might make them better then they are not legal.

Careful Joe.

As I understand it the OE piston is aluminum with a metal skirt to protect it from the “hard bits” in the block’s alloy. I don’t think there is an OE equivalent made the same way. They now use “tough skirt” coatings, which my have other benefits.

It does seem like the legal “fast ticket” is to update to the higher compression “late” OE pistons. That means you need to find a late motor or short block, or try to source expensive new OE pistons from outside the US.

At some point in the future does it make sense to spec a piston and allow sleeving?

Aftermarket piston sets with rings can be had for quite a bit less than the prices I’ve seen for OE.

If you don’t go that route can you please rule out the later OE pistons so we don’t have to buy entire motors or short blocks to find pistons. (My choice as someone new to the class.)

p.s. You should be able to hone the 944 blocks in any hone. You might only need different stones. It’s on my list of things to figure out. I’ll let you know.

Karl,
I am not sure what you are talking about on the pistons. They are a solid aluminum.

As for replacement pistons they need to be equivlanent to the original pistons. If not they are not legal. If you buy replacement pistons you need to PROVE they are equivalent. If not they are not legal.

As for the use of high comp pistons. These things came in every 88 944 or 924S. The had a slightly smaller dish in the top of the piston that cause an increase in compression from 9.5:1 to 10.2:1. Our class limit is 10.5:1 and was set for two purposes. Firstly to allow room to shave the head incases of warpage and then allow a slightly higher compression that results. Secondly to allow cars 9.5:1 pistons to equal that same 10.2:1 compression as the 88 cars.

In 6 years of practice the 88 pistons have never proven to be clear winner or must have parts. In fact low compression motors tend to run very well with higher compression motors. I advise nearly everyone when doing a rebuild have the head shaved for flatness NOT compression. Just make sure it seals well and is flat and go racing. The inpractice power differences are impossible to determine.

As such while the 88 pistons on paper seem like the hot ticket there has never been a run on them since in practice they don't make any difference. If you can get one great... but I would never pay a premium for one.

With respect to sleeving or overbores
For now there no need to sleeve or bore the blocks. There is still an ample supply of useable hardware out there for cheap. That may not always be the case, but for now it is not going to happen. If at some point it becomes impossible to maintain these cars due to old parts we may need to adjust the rules to allow for reasonable replacments. Even now that kind of thing is allowed. One example is the front spoiler which can be fiberglass since orginal plastic ones tend to be very old and trashed in junkyards and a new one costs 4x a fiberglass one. They still must appear stock however to limit any advantage.

So no need to do it now as it will just add cost to motor builds.

I guess it's time to take the motor apart.

Thanks for the information...again.

Based on Porsche's Engine Repair Manual:

www.9ss1.dk/porsche944/garage/vol1/vol1.html

It appears that both Mahle and Kolbenschmidt were OEM suppliers of pistons and rings for the 944. Only difference I see in the specification are the tolerences for the #2 ring.

Bruce