PTwo
Hello,
I'm in the process of setting squish and sorting the compression ratio out.
I've measured the combustion chambers and the piston dish volume. I say 'dish' because, even though my pistons are domed, due to the deep valve reliefs they increase the cylinder volume by 4.5cc.
My measured combustion chamber shows 34cc. I haven't measured the other one yet.
Doing the calculations, I end up with 10.3:1. This is with my current 1.35mm squish, 0.3 base and 1.2mm head gaskets.
To improve the squish, I will remove the bottom gasket (0.3mm) and use 1.14 head gasket from Cometic, which gives me ~1mm squish.
With the above change, my compression ratio goes up 0.5 to 10.8:1.
If I skim my heads 1mm, I don't know what I end up at as I haven't seen an 851 head volume after skimming. The closest I've seen is Marvin measured a 748 head unshrouded at 29cc, which is said to be similar. Has anyone measured the volume on a 1992 head with 1mm skim?
With 29cc heads, and some machining of the outermost piston dome to clear the head, creating a (~5mm) squish band in the area, I would end up at around 11.8:1.
Now, the real question... what is my target CR? Brad mentioned 13:1 at some point and that is what has been in my head. Griff rightly pointed out the fuel requirements with that high compression, and also the stress on the starter. This is primarily a road bike so those are things I must consider. The above solution to get ~11.8:1 sounds to me like a good solution/compromise. The only unknown seems to be my combustion chamber volume after skimming.
Thanks in advance,
Peter
