JohnC Posted July 18, 2020 Report Share Posted July 18, 2020 Hi brains trust, About 20yrs after rebuilding my CP-series engine, I'm going to undo a stupidity, and I'd like some advice on compression ratios. The stupidity was a cock-up when I got the cam reground but ended up with a profile better suited to a Cosworth 16V. Anyway, I'm putting in a CP-series cam this time. At the same time I'm having the head gas-flowed, and skimmed to get the CR optimal. Martin Jay has also worked his magic on the dizzy. The PI has been faultless for 50k, and I have a wide-band AFR sensor installed, so I can tweak fuel delivery to my heart's content. So, the question is, what CR to aim for? Here in Oz we have plentiful supplies of 98 RON with no ethanol, and that's what I plan to use. What CR is a good target? The manufacturer spec was 9.5:1, but I suspect I could go a little higher. I'm thinking 9.8:1, but maybe 10:1? Opinions welcome, particularly if based on hard-won experience! Cheers, John Quote Link to post Share on other sites
Nigel Triumph Posted July 18, 2020 Report Share Posted July 18, 2020 My CP series engine is around 10:1 compression ratio. It runs beautifully on 99 RON fuel, but will pink slightly on 97 super unleaded. Standard 95 octane doesn't suit it at all, the ignition has to be retarded stop pinking, making performance very subdued. Nigel Quote Link to post Share on other sites
DRD Posted July 18, 2020 Report Share Posted July 18, 2020 Mine runs at 9.75 and I need to use super unleaded plus an octane booster to get 99. Anything less and it pinks. Here Shell V Power is 99 and it likes that as well. Quote Link to post Share on other sites
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