I'd have thought that any difference in power will be directly related to the degree to which the engine can contain and compress the combustible mixture, with the 'psi at cranking' figure being a fairly good indicator of that.

It's not just barrels and pistons which are involved, so I'm always amazed that folk will take an engine apart and put it together again without giving the valves a bit of a fettling.
There are only 4 of them, after all..

As mentioned elsewhere, an 8.5:1 engine should show 150 psi and a 9.0:1 engine 175psi and iirc, some race engines I've worked on have shown over 200 psi.
( Bear in mind that the higher the CR, the more critical it becomes that the ignition timing is spot on, especially if 95 octane fuel is used. The 'peg in hole' setup should only be a starting point.)

Tickover smoothness is something else, I find that a heavy flywheel is what's required for 'steam engine' type smoothness...

ken.

8.5 vs 9. Well look at the gear box ratio in 2CV6, Ami 8 and van boxes etc + 125/15 vs 135/15 tyres. A bit of a jungle to work out what compression ratio to run on which gear box. 9.0 on 2CV6 box running 125/15 tyres compared to 8.5 is next to nothing when we talk top speed. You gain some torque but doubt you can feel it.

a well fettled 8.5:1 will feel better than a tired 9:1, have 9:1 in the blue car and it was fast, however its tired now and will be getting a quick re build for Salbris and as always its more than just the CR that has a bearing on the behaviour, carefull setting up of all the components will produce a nice tight sprightly engine

looking at drive shaft alignment and other things at the moment

Thanks, all - interesting

Tell you what, if you can't decide, then fit an 8.5:1 one side and a 9:1 the other, and see which side pulls the car along faster.

9:1 on the L/H [passenger] side 8.5:1 on the R/H side .....Yer 2CV will go 'round roundabouts a lot quicker ..... Honest ...

T.

Yeah we did that on the oval car my dad had a few years back.

ahem
One of the Scotroen "Sharks" had an engine like that-and it was far from slow

As one head always seems to get hotter than the other, maybe by using an 8.5:1 piston in that side, the ensuing lower compression/heat may sort that problem out?

Perhaps drilling a small hole in the 9:1 piston to let the extra compression out would equal it out?