You’ve got questions, we’ve got answers. We work with the Summit Racing tech department to help you tackle your auto-related conundrums. In this week’s Mailbag, we’re rebuilding a big block Chevrolet engine to work with a nitrous oxide system.
Q: I have a 1973 El Camino with a factory 454 big block. I have updated it with a COMP Cams roller cam (.623/.651 inches of lift) with matching lifters and valve springs, roller rocker arms, a Weiand Team G intake with a 750 cfm Holley carb, MSD Pro-Billet distributor and 6AL ignition, and Hooker headers with three-inch dual exhaust and Flowmaster mufflers. The transmission has a trans-brake and a 4,500 rpm stall converter.
I want to rebuild the engine for use with nitrous oxide. Will I need to change the pistons and rings? Will the stock connecting rods be OK? I was thinking about getting a new set of cylinder heads and perhaps a gear drive. Finally, what rear axle gear ratio should I use with this new engine combination? The car will be used at the track and as a Sunday driver.
A: Here’s what we’d recommend for your big block:
If your engine will operate below 6,800 rpm and make less than 570 horsepower, use Edelbrock Performer oval port heads. For an engine making over 570 horsepower and running above 6,600 rpm, use the Performer rectangular port heads. Beyond 7,200 rpm, consider using World Products Merlin cylinder heads.
With the oval port heads, use a Speed Pro forged piston with Sealed Power file-fit rings. The piston has a .225-inch dome and will yield around 10.5:1 compression. If you use the rectangular port heads, go with a TRW-L2372AF forged piston with SLP-R-9334 file-fit rings. If you take about .180 to .200 inches off the domes of these pistons, you will get 10.5:1 compression.
The factory forged rods will be fine up to 7,000 rpm when properly prepped, including new bolts. If the engine will be operated at 6,000 rpm or less, use 7/16-inch bolts. Beyond seven grand, use an aftermarket rod like a Manley or a Lunati.
4. Gear Drive
Unless you have a high-dollar, lightweight valvetrain, you don’t really need a gear drive. We would use a good timing chain set like a Cloyes true roller.
5. Rear Axle Gear Ratio
Your car can’t be used as a highway cruiser due to the high-stall torque converter, so you can go bigger on the gear. We’d recommend a gear ranging from 3.90 to 4.56 depending on tire height.