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Q&A with Kit Foster: March 19, 2009

This week, Kit fields questions on an Edelbrock CA6 manifold and how to determine rpm's. Also, continuing discussion on possible condenser problems with a 1956 Lincoln Premier; and the search is on for spark plug wire configuration for a 1934 Ford flathead V-8.

Q. I bought this manifold in the 1960s for $20 and never figured out what it was for. Any ideas? Gary Hoyt, via e-mail

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A. It’s marked “Edelbrock CA6.” When I Googled that, I found a Cadillac reference. I expect some of our readers will know which Cadillac engines it will fit.

Q. I own a 1940 Chevrolet with 48,000 miles. It’s stock, with the 216-cid engine, three-speed transmission and 4.11:1 rear axle. First gear is 2.94, second is 1.68 and third is direct. When I drive 35 mph, what is my rpm? Also, what is it at 40 mph. It seems like the car is going 100 mph at 50! Carl Ney, Ashland, Pa.

A. I believe your car has 6.00x16 tires. The tires I have in that size measure about 27 inches in diameter. The circumference, the distance traveled in one revolution of the axle, is then 84.9 inches (27 times the constant “pi,” which is 3.1416). Some simple math starting from 63,360 inches per mile (12 inches per foot times 5,280 feet per mile) shows your car travels 0.001338 miles per revolution. That’s 747.3 revolutions per mile (1 divided by 0.001338) at the axle. Your driveshaft turns 4.11 revolutions for every turn of the axle, so your engine is making about 3,071 revolutions per mile. Forty mph is 0.667 miles per minute, so at 40 your engine is turning about 2,048 rpm. Proportionally, 35 mph would give 1,792 rpm, 50 mph would result in 2,560 rpm. These are all in high gear, of course. Using one of the lower speeds would boost the rpm proportionally. Your car must be the Deluxe model since it has the 4.11 rear. The cheaper Master came with 3.73 gears, presumably for economy. One reason for the feeling of great speed is that we’ve become used to the comparative silence of modern cars, particularly those with “tall” gearing and overdrive transmissions.

Q. In the Dec. 25 “Q&A,” Bob Maske asked about the condenser on the fuel sender in his 1956 Lincoln Premiere. As you stated, this capacitor is not the likely culprit. It lacks the capacity to affect gauge response. Capacitors typically fail in two ways: open or shorted. If it was shorted, the gauge would read maximum, because it would short to ground. These types of capacitors rarely, if ever, shorted. If it was open, the AM radio would have static when tuned to a weak station. That was a more common problem. The purpose of the capacitor was to suppress electrical noise resulting from the sender unit wiper arm moving as the gas sloshed around while the vehicle was in motion. “Tiger Tom” Ehrhart, Annville, Pa.

A. Thanks. I hadn’t really considered why the capacitor was there, but I was pretty sure it didn’t affect the swing of the gauge needle. Bill McLellan, of Pasadena, Calif., however, says that he did use a large capacitor to dampen the motion of the gauge on his 1930 Chrysler CJ, 67,000 microfarads, which is a large capacitor indeed, about two inches in diameter and three inches long. He wired it across the IGN and TANK terminals on the back of the gauge, which directly connected it across one of the coils inside. However, one of the side effects of a large capacitor is that it takes a long time to discharge, and it may do so through an alternate path. In the Chrysler, this results in pegging the gauge for a while before it falls to zero. He says it’s been this way for many years and the gauge has survived the abuse. Maybe so, but this seems like a brute force way of solving the problem, something that goes against my grain. He agrees that the condenser on the sender is for radio noise suppression, and reminds us that similar condensers are found on generators of radio-equipped cars.

Q. My friend is restoring a 1934 Ford flathead V-8 and needs to know the configuration for the spark plug wires to the distributor cap. If you have a diagram that would be a big help. P. Miller, via e-mail

A. I’ve been looking, but I can’t find one for the 1932-’41 two-cap distributors with twin-arm rotors. Later models use a single cap and diagrams for those are readily available. Does someone have a diagram for the earlier cars, or perhaps could make one by “ohming out” a set of wires?

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