[edit] About a minute after posting this I realised that I had my brain on backwards.
But I will leave the post here so people can see what my (early Sunday morning) thinking was anyway.
[/edit]

In Andy's first post above it states that the compression ratio is 7.9 : 1
And item (4) in the same post states that the combustion volume should be 11.6cc +/- 0.1cc
ASSUMPTION: this volume is to the top of the spark plug hole? (TSPH)

Surely, if the combustion volume is 11.6cc (TSPH) on a 125cc cylinder, then
If we take the spark plug head filling volume to be 2.0cc, which is a totally rough calculation made by myself, then the REAL combustion volume will be pretty close to 9.6cc. I think I can remember being told once that spark plug volume for NSR250 is 2.4cc but it doesn't really matter too much for the purpose of this post.

So,.. Compression ratio = (124.8cc + 9.6cc) / 9.6cc = 14.0 : 1 (NOT 7.9 : 1 as stated by HRC ??)

Maybe I am missing something, please let me know if I am.
But from where I am standing, it seems as though HRC do NOT know what they are talking about. (tongue in cheek, of course)

For an MC18 in standard trim, I have measured the squish to be 1.1mm and the combustion volume to be 13.3cc (TSPH).
This gives a compression ratio of 12.2 : 1

I have modified my MC18 to have 0.8mm of squish and 12.8cc of combustion volume (TSPH)
This gives a compression ratio of 12.65 : 1
I am hoping that this will be OK for pump gas. I intend to use 95 octane or higher ULP.

I have no real data for the MC21, which is what is being discussed in this thread of course. However, it is going to be very similar I ASSUME.


Last edited by TuckerBag on Sat Feb 25, 2012 11:04 pm; edited 1 time in total