compression ratio for a short stroke 2.6

[Shuie]

Has anyone ever measured the actual compression ratio of a short stroke 2.6 (92mm RSR P&Cs, 2.2 crank and rods)? Any chance its under 11:1? there was a thread recently where someone made a good guess at the CR, but I never saw anyone mention an actual measured value.

Thanks!

[aj748]

The motor in my '71S was built as short stroke 2.6 (92mm RSR P&Cs, 2.2 crank and rods, single plug heads & MFI, '67S distributor) around 1973. I have dyno sheets from back then showing it put out 238 hp at the flywheel, as well as notes from the owner at the time that the compression ratio was approximately 12:1! Unfortunately I don't have measured data to back that up.

That motor lasted approximately 15 years and 15K miles before it melted a piston. It was rebuilt with another set of 92mm RSR P&Cs, but this time the pistons were machined for 9.8:1 CR. With no other obvious changes the motor dynoed at 232 hp (different dyno too). With the quick advance distributor from a '67S, I use Sunoco 93-94 octane without detonation problems.

Andy

'71 S
'88 M3
'99 748

[blue72s]

the compression ratio was approximately 12:1!

How can it be? It isn't physically possible.

[Tim Walsh]

Blue,
the 2.8 RSR pistons are designed to be used on a larger domed head. I don't remember the actually numbers, but used with regular 2.7 heads the CR is outragous. Something like 13:1 or more depending on deck height.

[Shuie]

Tim's right. This is mentioned in Bruce Anderson's book as the caveat to using these pistons without using the correct RSR heads.

I have read here that there is a compression drop from what would be the ~12:1 with same heads when you use a 66mm crank and 2.2 rods to make a 2.6 instead of the 2.

Anybody have a measured compression ratio and deck height numbers for a 2.6 (92mmx66mm)?

[blue72s]

If memory serves, BA's book states that :

RSR p+c's x RSR heads x 70.4 stroke = 10.3:1

RSR p+c's x 2.7 heads x 70.4 stroke = 11.3:1 to 11.75:1

Using a 66 stroke for the latter will bring compression down by about 1.0 or 1.5

[blue72s]

My guess is:

Deck Height_______CR
0.030____________10.8
0.040____________10.3
0.050____________10.0
0.060_____________9.8
0.070_____________9.6

BA's book says that that deck height in 911 engines is usually in the 0.030 to 0.040 inch range.
However Wayne's book recommends between 0.050 to 0.060 for stock engines, or for high performance a thinner deck as tight as 0.040 if trying to achieve high compression.

[rswannabe]

What did you assume for the piston dome volume and head volume?