It's All About Communication
Q: In your Oct. '08 issue, you said to machine down the center divider 0.500-inch on an Edelbrock Performer RPM for cylinder communication from all four barrels. Why wouldn't you machine the whole divider down for full flow from the carb?
A: The trimming of the center divider is an old-school trick to increase communication between the upper and lower planes of a dual-plane manifold. We're sure it was tried before this, but it was widely used on the early big-block aluminum manifolds back in the '60s. This communication effectively raises the powerband (rpm) of the engine, and makes the engine think that the carburetor is larger than it really is. It's basically a poor man's single plane intake.
Cutting the center divider down 0.500-inch on the Performer RPM inlet manifold will give you a few extra ponies at higher engine speeds without killing too much slow-speed performance. The 0.500-inch point was a rocking point of gains. Anything more than this didn't give you anything upstairs, and would hurt slow-speed torque below and at torque peak. We mapped this out on the engine dyno on a 450-plus-horsepower, pump-gas 350 small-block. You can achieve similar gains by adding a 1/2-inch open spacer under your square flange carburetor. We cut down the divider because we didn't want to run a spacer. Having the engine dyno gave us the tools to map out the proper amount to cut without ruining the intake.
Q: I have a '70 Camaro SS with a 383 stroker engine, built by Smeding Performance. It's a mild stroker with 371 hp at 5,000 rpm and 441 lb-ft at 3,800 rpm (dyno tested). The camshaft is a hydraulic roller tappet, 0.480/0.480-inch max lift, 212/222 duration at 0.050-inch tappet lift, on a 112 lobe center. The cylinder heads are aluminum 2.02/1.60-inch stainless steel valves with 64cc combustion chambers, 175cc intake port volume, a Performer EPS dual-plane, and 9:1 compression ratio.
The car has a Tremec TKO 500 manual transmission with 0.68 Overdrive in Fifth gear. The rearend is a 12-bolt with 3.42 gears. Will I need a higher numeric rearend, because I really have no use for the Fifth gear unless I drive it at a very high speed at the motorway (as we call it here in Norway), far over the speed limit. I'll guess 3.73, 3.90, or maybe 4.10 will be my options.
The Camaro is street only. What would you recommend? Thanks.
Svein Erik Josefsen
A: Your thought process is right until you run into your 3.27 First gear. Going lower with the rearend gear will make First more useless than it all ready is. The gear multiplication you have with the 3.27 and the 3.42 rear gear give you a combined First gear ratio of 11.18:1. With your torquey 383, you'll blow through First by 36 mph at your horsepower peak of 5,000 rpm. This is assuming that you have a 27-inch tall tire on your Camaro. If you wish to go lower with your rear gear, we wouldn't recommend going any lower than 3.73 gears.
Yes, you have an 0.68 Overdrive Fifth gear. This will just mean that your 383 will live longer. Trying to achieve peak vehicle speed in Overdrive shouldn't be your worry. Get all the speed you wish in Fourth and let your engine loaf along the motorway in Fifth. Enjoy your Camaro over there!