Will the real RS Piston please stand up

[Allen Henderson]

RS Engine geeks and others,

Recently, I was told that a very original RS turned out to have different pistons than we normally associate with an RS which has engine type 911.83. The pistons, which are believed to be original, have a kind of flat on the bottom. A similar piston is pictured here, the one with the purple background (sorry the illustration isn't better but it is the most similar to what was seen-no camera at the time!). The other two photos are of what I always thought of as a "RS Piston," One is from an engine I just took apart, you can just make out the Mahle number, 90P27, inside the piston. The other shot is the top and side of a brand new one. Can anyone shed any light on this recent "finding." Thanks.

-Allen-

[orange911]

Looks like an attempt to gain a little CR. If not stock, then it may have been done by builder if the car was raced.

[Allen Henderson]

The purple background image is just to show kind-of what the piston looked like. The mechanic said he 1. hadn't seen them before and 2. thought they were OEM and original to the RS. Too bad he didn't take photos or get the numbers but he didn't.

-Allen-

[72targa]

If the picture with the purple background represents what you saw...and you are noting the flat surfaces next to the valve head cut-outs on the piston surface, those flat surfaces (indicating shaving down the piston surface) would tend to LOWER the compression ratio (i.e. there would greater volume in the cylinder chamber at the height of the compression stroke). This is assuming that a "stock" piston was machined down. Otherwise, these are not the Mahle's we know and love. And if Jerry Woods had never seen them...

[tom1394racing]

Here's pics of the real 8.5 CR Mahle 2.7 RS pistons.

The Mahle P/N under the dome should read 90P27

[RSTarga]

That one next to the mahle box looks more like a 2.2 s piston with 9.5 compression so those might be 2.7 higher compression pistons

[Allen Henderson]

90P27. Now that you mentione it the new one I posted doesn't look right. Let me double check it. Still, the question remains if the RS had two pistons, early and late? Thanks.

-Allen-

[tom1394racing]

There was only one 2.7 RS spec factory motor. It had the 8.5 CR pistons. There were (are) other piston manufacturers who made (make) higher CR pistons for the 2.7. You can buy JE's at 9.5 or so CR that will make a better running motor than the original 8.5 CR RS.

[Allen Henderson]

Other than custom, one-off, etc pistons (and my unforutunatetly) misleading photo(s)) is there any disagreement: from what has been seen, ONLY ONE RS 911.83 OEM piston? Thanks.

-Allen-

[andrew15]

That is correct - OEM Mahle RS pistons were only 8.5:1 compression.
Andial in conjunction with Mahle did make sets in 10.3:1 compression as well - they looked like your new piston photo on the right in your first post.

Regards,
Andrew M