Mahle P&C set for 2.5l type 911/70 engine with 10.3:1 CR?

[matteo68]

My 2.5 SR engine is being built as an accurate type 911/70 long stroke (2,492 cc) and we are about to complete the bottom end with a 70.4 mm std/std crank and as a next step need to source the correct 86.7 mm diameter P&C set with 10.3:1 compression ratio as stated in the table in ‘Information…for Sports Purposes’ 2 Ed:



Whilst I would very much like to use a Mahle P&C set, I note their 2022 Porsche Application Guide states that the PS86-002 set has a CR of only 9.3:1 which would be too low for my engine, whereas their short-stroke 2.5l PS82-002N set does have a 10.3:1 CR in order to achieve the 275 bhp this type 911/73 engine achieves.

The Porsche Spare Parts list of Nov'71 for the 911 S MY72 (competition type) specifies the pistons & cylinders for the racing engine with a part number of 911.103.925.71 and a diameter of 86.8mm (which I think must be a typo as 86.7 mm diameter is correct and stated in the Sports Purposes document) - see attached extract:



I have searched all over the internet but cannot find any reference to this part number anywhere apart from in the Spares List.

Can anyone (gled49, longballa, letsrollbabe, boyer73S, erniew?) advise what I should do in the circumstances to achieve the correct CR please? Do I need to get Mahle to make a custom set and if so, where would I get all the data they require when I don’t even know what the deck clearance should be to achieve 10.3:1!

In the meantime I am searching through all the various ESR ST threads at the moment and have found this really useful post from Gib - anyone have a spare NOS set of original Mahle 86.7s by any chance?!! (not expecting any replies to that tbh!)

[gled49]

The one I did used real factory twin plug heads at 67cc, with a 1.1mm deck it was 10:1. It used original 906 cams and HBF injection. Factory HP rating was not achieved. The cylinders are bore in and use 908 base gaskets. G

[matteo68]

Thanks Gordon.

We've just found these which would appear to be exactly what we're looking for: https://msroadrace.com/index.php?rou...product_id=435

Matt

[gled49]

When I worked with Jerry Woods, he suggested comp ratios were figured with “0” deck.

[matteo68]

That’s really helpful too Gordon and makes sense: the guy at Mahle Motorsports said: “For this part (PS86-002) to be 10.3 to 1 maybe 70.5cc isn't the right chamber volume, maybe they're running tighter than 1mm deck clearance, or a combination of both? These have a dome volume of 25.9cc if that helps any.”

[68S_SK2]

Hi Matt,
You could ask Woessner in Germany. They are very well known maybe even better than Mahle.
I think your dimension is not listed, but I'm sure they can machine them.

https://www.woessner-kolben.de/produkte/auto/

BR
Claudius

[gled49]

At 70.5 & 26 dome it will be tough to get there. We were 67 & 27 dome.

[matteo68]

Thanks Claudius, a good suggestion but hopefully we’ll be able to get the ones from MSRoadRace shipped to the UK without too much difficulty. We just need to find out if the cylinders are Nikasil coated like the originals apparently were.

Cheers
Matt

[matteo68]

Just had this confirmed by Jason Griffin at Mahle:



Very happy now!

[Ernie W]

A photo of the type P/C…