Measuring compression ratio

[Garnet]

http://www.cafebeemer.com.au/blog/Compr ... ratio.html

In case you just have to know.

[mattcfish]

Yeah, I've been following that thread. BMW stock 9.5 R100's may not be 9.5:1. I wonder if that goes for aftermarket pistons too, gulp.

[Garnet]

I haven't followed the thread over at Adv but I wonder if the pistons match the heads. There are so many R100 pistons and so many R100 heads that saying that "these" pistons are 9.5 to 1 is realy not possible.

In your case your pistons where designed to match pre 76 heads, so you be confidant that you are very close to advertized.

The proof is in the butt dyno.

[Roy Gavin]

Maybe he actually has 8.2 to one pistons - the top looks similar to the 8.2s in my GS

But I share his suspicion - when I compare my 9.5 B44 to a 7.2 C15 there is no mistaking the difference, there is a snap and urgency to the B44 that just isnt there on the low comp C15, or on any of my supposedly 9.5 to 1 Beemers either.

[mattcfish]

I haven't followed the thread over at Adv but I wonder if the pistons match the heads. There are so many R100 pistons and so many R100 heads that saying that "these" pistons are 9.5 to 1 is realy not possible.

In your case your pistons where designed to match pre 76 heads, so you be confidant that you are very close to advertized.

The proof is in the butt dyno.

Yes, I was only wondering if BMW (as unlikely as it seems) didn't get the compression right, then if Siebenrock had been following BMW's specs and not actually creating there own......
The butt dyno does seem to confirm the Siebenrock pre 76 pistons are 9.5:1. Although my dual plugging, light clutch, and 336 cam could be confusing matters some. I can't make it ping like the old 9:1 R90.

Didn't R100 heads have numbers on them to identify one from the other? Is there a list out there of head cc's on airheads?