Posted by Matt Wilson [162.158.69.102] on Friday, February 24, 2017 at 16:02:59 :
What are you guys running for compression ratios in your flatheads these days, with today's fuels? What issues have you had with pinging/detonation/spark-knock. Also at what altitudes are you running at, and what fuels do you use (octane and ethanol content), and fuel additives? What are your outside summertime temperatures? How hard do you push your engines in terms of amount of throttle (not necessarily rpm)? Also, what's your carburetion setup - stock carb, two-barrel, dual carbs, etc?
I'm interested in what you have to say for any of the flatheads - 218's, 230's, 236's, 251's, 265's and whatever others you may have.
Reason I ask is because I've spoken with a gentleman who I believe is very familiar and experienced with our flatheads, and he says that with today's fuels (specifically E10 87 or even 93 octane), at my altitude (less than 1000 feet here in the DFW, Texas area) and my summertime temperatures (100 deg F or more), I run a great risk of pinging/detonation if I run more than about 7:1 CR unless I go to 100 octane gas.
We happened to be talking mostly about the 25" long engines (251, 265), but I got the impression that he thought it would be very similar with a 230. And yet I've seen a number of posts by various people over the years (many of them not that long ago), who say they've operated with 7.5:1, 8:1, 9:1, sometimes more, so either this gentleman is mistaken (but he's very reputable, it seems), or the folks who posted these CR's are using high-grade fuel, additives, or they are operating in even lower altitudes (can't be much lower) and/or lower temperatures or compensating in some other way...or maybe they posted what they THOUGHT were their compression ratios, but they were mistaken.
I was rather blown away when he told me to limit my CR so much. He strongly encouraged me to use 100 octane fuel to avoid these problems.
I'd like your input, as it will guide me in an upcoming rebuild.
Thanks.
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