In your case, the standard bore of a 390 is 4.05 inches and your 0.030-over engine is 4.080, so the formula points to a gap of 0.017 inch. That's right in the range you've measured, which is actually pretty good for a set of standard, non-file-fit rings. The 0.035-inch average your builder quoted is a bit large; most of the experts we spoke with felt that ring gaps on a basic engine should be kept below 0.030. It's better to err on the big side, since if you go too small and the rings butt you'll quite possibly be spending some more money.
Another consideration for future projects is the position of the top ring groove. Some modern piston applications place the top ring groove higher on the piston than conventional designs, often as a means of reducing crevice volume that can contribute to emissions. An exceptionally high top ring requires additional endgap as it is closer to the chamber and therefore, exposed to more heat. In those cases, confer with the piston manufacturer to get input on recommended specs.
David Freiburger: Also, thinking has changed a bit as far as gaps for the top and second rings. Rather than setting both to the same spec, builders have commonly gapped the top ring a little wider than the second ring, thinking that the top ring is exposed to more heat. More modern thinking puts a slightly larger gap on the second ring to allow for gas expansion between the two rings to prevent ring flutter that unseats the ring and loses seal.
More Info
Speed-Pro/Federal Mogul; Southfield, MI;
800/560-1400; federalmogul.com
Numbers Game
John Maltby, Burlington, VT: I've just completed four years of restoration on a '67 El Camino. The drivetrain consists of a 396 with almost everything else stock. The only upgrades are a 600 Holley, TH400, and a 3.73:1 Posi rear. I'm running 235/70 rubber all the way around. I run idiot lights, so I need to compute the best rpm to cruise at without putting in a tach. I want to base this only on the speedo, which is dyno-tested and is dead-on. Is there a formula for figuring rpm off this info? Math is a heck of a lot easier than writing this!
Jeff Smith: You're way too easy on us, John. There are several formulas based on what you're looking to find. It sounds like you want to know what your cruise rpm would be at a given vehicle speed. That formula looks like this:
RPM = (mph x gear ratio x 336) / tire dia.
We're assuming your "235/70" is using a 15-inch-diameter wheel, which measures a tall 28 inches. Plugging this diameter into the equation at 65 mph:
RPM = 65 x 3.73 x 336 / 28RPM = 81463.2 / 28RPM = 2,909
Your theoretical cruise rpm at 65 mph would be 2,909 rpm. That's accurate only if you are running a lockup converter. But we'd guess you've got a stock or near-stock converter that slips roughly 200 rpm at cruise. So add this to the 2,909 rpm figure, round it off, and you're looking at a cruise rpm of 3,100 at 65 mph. You can do the math to figure out the rpm for other speeds.
Here are some other formulas worth writing down:MPH = rpm x tire diameter / gear ratio x 336Gear Ratio = rpm x tire diameter / mph x 336Tire Diameter = mph x gear ratio x 336 / rpm
Hydramatic Magic?
Brian Fagan, Little Rock, AR: I've been looking to do a tranny swap in my '69 Pontiac LeMans that currently runs a 400 with a TH350 trans, and after looking at some of the late-model four-speed automatics, I came across some info on the GM Hydramatic four-speed from the '50s and '60s. This was a four-speed offered by GM in B.O.P. cars that was evidently fairly bulletproof, especially if treated to the B&M Hydro-Stick program. These transmissions were referred to as dual-range, so I'm guessing the overdrive in Fourth gear could be activated in other gears with valvebody alterations, which may have been included in the B&M version. I'm thinking a Hydro-Stick for the LeMans, giving me brutal strength, manual-shift capability, and overdrive. Why aren't these more popular today? Am I missing something?