Corvette Forum Tech Talk: High Compression Engines
From the deepest archives of Corvette Forum, we bring you a golden-oldie topic, circa 2002.
One number that you rarely hear associated with a 454 big block Corvette is “12.5:1.” As far as compression ratios go, that’s the equivalent of stuffing 10 pounds of potatoes in a five-pound bag. Well, that was the case for someone who posted on our forums over 16 years ago (yeah, that post is old enough to drive), asking how to lower the compression of a 454 in order to turn it from essentially a race engine, into a street engine.
Needless to say, the basics of engine tech haven’t changed, so we decided to peek at just what it takes to get an engine like that to work in a car that’s not a dedicated racer.
Compression ratios above, say, 10.5:1 are entering the territory of running on fuel that you simply can’t get at every gas station. Attempting 12.5:1 on pump gas, even 93 octane, would pretty much self destruct.
Throwing fuel out as a variable, and only going with pump gas (we’ll assume 93), we’re left with only a few options. Using fuel injection, rather than a carb might have some advantages due to the customization of timing and sensors to detect knock. Or, as stated by forum member Ken Markusa, a change of cams could be done to “bleed off” some of the cylinder pressure, but it would have to be an enormous cam to pull that off.
In the end, lower compression would be the only solution, and that could be done with new heads or the best and easiest option, new pistons. With new heads, you can change the chamber shape. But again, you could change the quench area, and end up getting into a whole lot of technical details that could spawn off a number of other articles.
Pistons, however, can be shaped to the already existing cylinder heads, and perhaps be dished in order to lower compression, but still retain a flat top area to keep the quench area the same. Many custom shops can make a piston to exactly fit your application, too, rather than just buying a set and hoping for the best.
With so many variables at play, we have to wonder, what combo you would throw together to run a high compression engine?
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