More info found on the The Mirage Turbo/Colt GT forum. 43cc chambers will take the stock 1g 4g63 compression from 7.8:1 to 8.2:1. If you do a search for 4g67 over there it will bring up alot of threads. 4g67 is 1.8. Some 1.6 heads have big ports, but AFAIK all 1.8 heads are big ports.