Compression ratios

[mtiberio]

I see that the Brozz manual states 9.2:1 for 150 and 200cc motors but 8.7 for 229cc motors. I see the TT250 advertised as having both 8.7 and 9.2:1 ratios. Anyone know? Also I see some folks here have milled their heads. While I could do a clay job and determine how much can be removed via first principles, can anyone tell me a safe value that can be milled from a 8.7:1 motor?

Thanks
Mike

[JerryHawk250]

On my Hawk which was 9:1 I remove 1mm which bumped it to 10:1. You could go a little more as there is a good bit of clearance for the valves.

On my X22R instead of milling the head i'm installing a 11:1 piston and possible install a thin head gasket later on.

[franque]

While I could do a clay job and determine how much can be removed via first principles, can anyone tell me a safe value that can be milled from a 8.7:1 motor?

Based on the motors that you're referencing, you have a Hondoid motor? Realistically, due to production variations because of different manufacturers and tolerances, since you know what a clay job is, you already know what you should probably do. Safe is relative, I'd suggest you measure and figure out yourself what you think is ideal.

[Megadan]

Keep in mind that there are two places you can remove material from these engines as well. Taking 1mm off the deck of the cylinder head is the cheap and easy way to bump the compression, but far from ideal. If you really want to get into proper performance machine work then you would want to mill the cylinder itself to reduce the quench area to .035" or less. The downside is that on most of these machines it requires taking at least a couple of mm off the cylinder in order to achieve this, which if no other changes are made will easily put the compression ratio well into the 11:1 range.

The best thing you can do is to do as Franque suggested and take the time to measure everything out.

If there was a "Safe" figure across the board for the CG motors however, no more than 1mm would be my suggestion. Without doing a CC if your head, and measuring the other variables of your specific engine though, nobody could say for sure what your end result would be, That said, given the bore and stroke of the CG250 being equal, ignoring the small variables, that will still net you around 10:1. But with a margin of error of a couple tenths for said small variables.

A good example of this is the head I used to port and mill had a shallower quench bevel cut into the chamber than the stock Hawk head, so as I cut the deck down, my chamber volume decreased a lot more than if I were to have milled my Hawk head, which actually caused my final calculated compression to be higher than if I had used said Hawk head.