My puppies have arrived

[blue72s]

[Shuie]

so you are doing a 2.6 or a 2.8?

[blue72s]

2.6, of course.

[Shuie]

great! Finally there is hope for an actual measured compression ratio on a 2.6

[blue72s]

I'm afraid I'm not going to be that quick in building an engine because I don't yet have all the engine parts.

My piston dome looks shallower and friendlier than I thought. It looks just like a 2.2S piston. The piston shown in Wayne's book on pg 91, figure 4-22 looks nothing like mine so the one in the book must be an aftermarket.
If you're desperate for CR measurement, why don't you ask Henry as he has built a 2.6.

[Scott Clarke]

Blue72S-
Are those RSR pistons, or the more recently developed pistons intended for non-RSR combustion chambers?
-Scott

[blue72s]

Scott,
Did Mahle ever make 92mm sets for non-RSR cumbustion chambers?

This is what Andial told me before I bought it:

The original 2.8 RSR piston and cylinder has not been available from Porsche for over 20 years. Mahle changed the design of the piston in only one aspect. The oil control ring was changed from a 4mm to a 3mm for better efficiency. Other than that the piston is no different than the original.

[Scott Clarke]

Blue-
About 3 years ago Mahle and/or Andial (I don't remember which) ran ads introducing a series of "Sport" pistons in , I believe, 9.5:1 and 10.5 or so CRs (@ 70.4mm stroke) in at least 90mm, and I think 92 and 93mm. these were intended for use with normal cylinder heads and carbs or MFI. I looked into these at the time (I called Andial and discussed CRs, availabillity, etc.), but decided on JEs instead as they were a lot cheaper and easier to get. I did physically see a set that would produce about 9.8:1 for a short stroke motor (intended for 2.7 twin plug street motor). The pistons domes looked much like those in your photos. Perhaps someone has better memory than mine.

[blue72s]

Originally posted by Scott Clarke
I did physically see a set that would produce about 9.8:1 for a short stroke motor

at what deck height?

[Scott Clarke]

At a "normal" deck height. I.E., they were intended so that you wouldn't have to do anything funky to get that CR.

[rswannabe]

Blue,

Could you post what the writing is on top of the pistons? Might help in identifying exactly what they are.

Brooke

[blue72s]

[rswannabe]

Great. I'll comapre them against the set I have and get back to you. They look to have the same dome size and valve pocket depth.

Brooke

[Steve@Rennsport]

Just another note on those 92's,....

The piston domes are made for the 72cc 2.8 RSR heads. Using these in any 2.2, 2.4, or 2.7 that have the 66cc-67cc chambers will give an approximate 11.2:1-11.3:1 CR at .040 of deck height. Caveat Emptor on using these in a street engine. Its possible to get the CR down to 10.5:1 but it takes a lot of work.

These newer cylinders differ from the original RSR ones by the deletion of the CE ring groove. These do not use any head gasket. That eliminated the cracking problem at the tops of the bores.

[Shuie]

Steve, any chance you know how much the compresion drops using them with the same deck height and a 66mm crank & rods to make a 2.6? Does it get to an acceptable range for twin plugs & street gas or will blended race gas be necessary?

TIA

[blue72s]

As I don't have any valves with me right now, I made paper circle cut outs in the same sizes as RSR valves - 49mm and 41.5mm to see what I've got. Guess what? They fit in the pockets exactly without any clearances. Shouldn't there be any clearances, in case pistons move sideways a little?

[Tim Walsh]

Th tolerances are tight enough that I don't think it's possible for it to move a litte sideways.

[Steve@Rennsport]

Originally posted by Shuie
Steve, any chance you know how much the compresion drops using them with the same deck height and a 66mm crank & rods to make a 2.6? Does it get to an acceptable range for twin plugs & street gas or will blended race gas be necessary?

TIA

Hi Sherman:

The 92mm RSR pistons with the early 66mm crank gives an approximate 10.25:1 CR. Of course, you MUST measure your chambers and do the math to see what the CR will be in YOUR engine as that always varies.

This setup works fine with twin-ignition and 92 or better pump premium.

[blue72s]

Thanks!

Just one more question (I think): Should the pockets be re-angled to match the 2.7 valve angles?

As you may know, on RSR head, valve angle was reduced to an included angle of 55°45' with an exhaust angle of 30°15' from vertical and an intake angle of 25°30' from vertical.
I do not yet know the 2.7 valve angles, as I can’t find any references in any books.

[Shuie]

Thanks Steve!