Thread: Increasing compression ratio?

While head is off 4 cylinder Series 2 motor, am considering having engine reconditioner shave some metal off to increase compression ratio. Am interested in comments from those who have done this or changed from a head with 7:1 compression ratio to 8:1. Presumably with the latter, should be a small increase in power and improvement in fuel economy. Also, if shaving the head, how far is it worth going increasing compression before benefits are outweighed by problems created including knocking? Keeping in mind that want to use the common 91 octane petrol.

The early 7:1 heads do not have enough metal to safely shave a significant amount off. Later ones can, but check whether it already has been done or was 8:1 to start with.

The distinguishing feature is that the later type head has a square boss about 15mm across cast into the top of the head, outside the rocker cover, and opposite the carburettor. This should (but not always does) have an "8" if it is an 8:1 head. 7:1 heads either have a "7" or are unmarked. The numbers are stamped and may not be very clear. I have seen references claiming that the newer heads can be safely skimmed to 9:1, but I think you would need to run on higher octane fuel for that to work. For 8:1 you may need to run on 95 or 96 octane to avoid having the ignition set so retarded that you lose a lot of the benefit.

A modest increase in compression from 7:1 to 8:1 will give a noticeable improvement in power or a little better fuel economy if you don't use the extra power, provided the ignition timing is adjusted to suit both the compression and the fuel used. There will be a slight reduction in tractability at very low engine speeds.