Posted by Matt Wilson [108.162.221.169] on Saturday, February 25, 2017 at 11:02:52 :
In Reply to: Compression Ratio posted by Matt Wilson [162.158.69.102] on Friday, February 24, 2017 at 16:02:59 :
Thanks for the responses, guys. Like some of you have said, I also tend to have a hard time believing that I have to keep the CR below 7:1. Almost 25 years ago, I cc'd the head on my 230 and came up with 6.7:1, which was stock for the year of my truck (1949), and I had it milled down 0.080" and it then measured 8:1, or very close, and I never had any pinging issues. The rest of the engine was stock, except the cylinders were 0.040 over. I lived in El Paso at the time, which is at 4000 feet and gets HOT, and I bought the cheapest gas, which would have been either 86 or 87 octane.
Fast-forward about a decade, and living in the Fort Worth, TX area at just under 1000 feet, I rebuilt that engine, which entailed milling another 0.010 - 0.020" off (the machinist couldn't remember exactly how much), and boring out the cylinders to the next oversize, which was 0.060", so I'm certain the CR was at least 8:1, and probably more than that. I also used the VPW cam, but the rest of the engine was stock. Still the cheapest gas was used. No pinging.
The engine core I'm about to get is likely to be at 7:1 or more already, before I even touch it. I was only going to be aiming for about 7.5 or 7.75 anyway, and it may be difficult to stay below that, depending on what prior rebuilds have done, but if I can, I probably will, just to be on the safe side. My plan for this engine is to use a mild cam again, perhaps from VPW, perhaps from another source, and more carburetion, such as a double barrel or a dual-carb setup. I can see how the added carburetion might push the engine closer to pinging, because more air and fuel will get crammed into the cylinders, but I still have a bit of a hard time seeing how it's going to be a problem. In the end, I guess we'll see, and if I get pinging, then I'll adjust the timing, and/or use higher octane gas, and if that is not good enough, the I'll try to find an earlier head with lower compression.
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