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I checked search and didn't find any posts about a recommended compression ratio for 4.2 engines. My 4.2 needs an overhaul and I'm wondering what the ideal compression ratio would be for 87 octane pump gas. I'll probably use a Crower Mileage/Torque cam, Idle to 3500 rpm, redline: 4000 plus. Is 9.0 or 9.5 going to cause pinging or should I stick to 8.5?

 

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Can't speak for the 4.2, but just rebuilt my Buick 350 for my '70 Wagoneer and it knocked pretty bad on 87 octane. Compression was 9.2:1. Was supposed to be 8.8:1, but the figures for the pistons in the Federal Mogul catalog were incorrect. Initial timming was at 15 degrees I think. Kept retarding to get rid of pinging, but the power kept dropping off each time. The engine is apart again and will get new pistons that should yield 8.5:1 give or take. Hopefully that'll solve the problem. If I were you, I'd stay under 9.0:1 if you want to run regular gas.

 
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