Q. My 406 small-block has 10.25:1 compression, 15/8-inch headers, AFR aluminum 200cc heads, a 650 double-pumper carb, an Edelbrock Air-Gap intake (dual-plane), and a cam with 250/256 duration at 0.050-inch tappet lift, 0.519/0.523-inch max lift, ground on a 110 lobe separation. The car is mostly street driven. It weighs 3,600 pounds and has run high 7s in the eighth-mile. I’m running a Turbo 350 trans with 3,500 stall (only get about 2,900 out of it), 4.33:1 rear gear, and a 31-inch-tall tire. The engine will turn 6,800 rpm at most. I run Flowmasters with 3-inch pipe to rearend with turndowns. Since the last 4 feet of pipe past the mufflers is 3-inch pipe, would it hurt top end performance in the eighth to go to 21/2-inch pipe the last 4 feet? What about low-end torque and midrange with my heavy car? I’m sure the car would be a little quieter too!
A. It appears that you have a couple of pieces of your combination pulling different directions. Most 400 small-blocks like 15/8-inch primary pipe headers from our experience. The 250/256 duration at 0.050-inch tappet lift is going to build some top end performance matched with your 200cc inlet runners. The 650-cfm carburetor on a dual-plane intake is going to limit the engine’s rpm range. Going with a larger carburetor, like a 750-cfm, should help the upper-rpm power without much of a torque penalty. You didn’t mention if the camshaft is a hydraulic or mechanical lifter. With your current combination, you could easily be all done by 6,800 rpm.
Let’s get back to your exhaust. You mentioned 4 feet of muffler tubing after your Flowmaster mufflers. This must mean you’ve installed the mufflers right at the collector of the headers. If this is the case, you’re killing the tuning and scavenging of the mufflers. You want Flowmaster mufflers placed as far back (mid-ship) in front of the rear axle. You also want to utilize a balance pipe between the collectors, as close to the header as possible. Best case, you’d want to run a 3-inch balance pipe and at least a 21/2-inch. These changes will help the slow-speed torque of your engine, and make an appreciable gain in horsepower. You definitely don’t want to go with 21/2-inch tube on your 406. Your engine has the potential of making 500 hp with the components listed. To support that power you will need 3-inch tubing. Rework your exhaust and enjoy the benefits. CHP
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