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1963 Buick Riviera Replacement Distributor - Ask Marlan

One study that takes changing cylinder volume into account is P.C. Vorum's 1976 ASME paper "Short Pipe Manifold Design for Four-Stroke Engines." Essentially, the paper recommends combining optimum primary and secondary pipe velocities with a Heimholtz-like resonating chamber as found on some exhaust muffler designs. It presents a mathematical model for taking the various factors into account, although not every iteration is fully explained or documented; apparently, the author held something back so he could prosper doing contract work for OEM manufacturers-but it's a good starting point if you're math and engine-savvy! Copies of the out-of-print paper (reference ASME 76-WA/DGP-4) can be purchased from the Linda Hall Library, an independent research library of science, engineering, and technology.

Linda Hall Library Document Services
Dept. CC
5109 Cherry St
Kansas City, MO 64110-2498

Through Thick and Thin
I have a '95 Camaro Z28 with an automatic transmission and a set of 2.73:1 gears in the rearend. I was thinking of upping the gearing to 3.73:1 to improve off-the-line performance, using a thicker gearset so I wouldn't have the expense of upgrading to a three-series carrier. My mechanic advised against it because it might be noisy. The car is driven about 2,000 miles a year. It has an RK Sports high-flow converter, a Flowmaster muffler, MAC weld-on subframe connectors, and a K&N air filter. The exhaust is already loud, and I would be able to live with a little more noise. What are your thoughts on this?

Jason Burks
Lockport, IL

There's nothing inherently wrong with thick-flange ring gears. In fact, Ford has always varied production ring-gear thickness rather than change differential case flange heights. Assuming proper installation technique, any inherent noise problems are not a design problem, but instead indicate a gear-manufacturing quality control deficiency-namely, poor lapping technique. U.S. Gear has a good reputation in this area, and recently introduced thick 3.73:1 gears for the GM 7.5/7.625-inch 10-bolt rearend (PN 01-875-373X).

U.S. Gear
Dept. CC
9420 S. Stony Island Ave.
Chicago, IL 60617-3695

Performance with a Price
I drive a '79 Volvo 262 Bertone with a '90 Mustang H.O. 302, AOD trans and all computer functions intact. I'm running cast-iron manifolds to 2-inch pipes to a single three-way cat. From there back, it's all Volvo 1 7/8-inch exhaust, and ends at a muffler from a six-cylinder Camaro. I drive the car a lot and really appreciate the quietness. What do you feel is the minimum-size exhaust that I should use, and will a freer-flowing exhaust really help my gas mileage?

Bob Price
Drexel Hill, PA

Up to a point, reducing exhaust restriction generally helps gas mileage. But going too large can overscavenge the engine and result in a mileage reduction. Proper exhaust size is a function of both engine output and displacement. Ford used a full 2 1/4-inch dual exhaust system on the Mustang, complete with dual cats and mufflers. Assuming the engine is stock, in theory, 2-inch dual pipes should be adequate, as indicated by the accompanying exhaust system recommendation chart courtesy of Flowmaster Mufflers. Specifically, it's the 1 7/8-inch single-pipe and weak little single six-cylinder muffler that's holding you back. If there's insufficient room for dual pipes and mufflers after the converter, use a single 2 1/2-inch pipe and corresponding low-restriction aftermarket muffler. The converter inlet and outlet should likewise correspond to the inlet pipe sizes.

Flowmaster Inc
Dept. CC
2975 Dutton Ave.
Santa Rosa, CA 95407-7800

Suggested Exhaust Pipe Sizes for Street Performance
Pipe Diameter (In) Horsepower Engine Size
Single Dual
2.0 2.0 100 150-200 ci
2.25 2.0 150
2.5 2.0 200
2.25 2.0 150 200-250 ci
2.5 2.0 200
2.5 2.25 250
2.5 2.0 200 250-300 ci
2.5 2.25 250
3.0 2.5 300
3.0 2.25 250 300-350 ci
3.0 2.5 300
3.0 2.5 300 350-450 ci
3.5 2.5 350-400

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