If Henry Ford were alive today he would probably be surprised at the impact his Flathead V8 has had on the automotive community, and the place it continues to hold in the hearts of hot rodders. Undoubtedly, an even bigger surprise for Henry would be that, thanks to the nostalgia craze, many enthusiasts are building their first Flathead some 50 years after the last one was cast.

As these old engines are new to some, many of the things taken for granted by V8 veterans aren't known to novices. In some cases that's just as well because on some subjects there's as much bad information as good--ignition systems are an example. While the original ignition systems have weaknesses, they also have strengths, most notably their advance curve.

During operation, the spark requirements of an engine change. Those requirements are primarily a result of engine speed, the air/fuel ratio, and the design of the characteristics of the engine. At idle the ignition timing, referred to as the initial timing, is set to occur at, or slightly before, the piston reaches the top of the cylinder on the compression stroke. But as the engine speeds up there is less time to burn the fuel before the piston is too far down in the cylinder for maximum effectiveness, so the spark must occur earlier. For that reason, most non-computer controlled ignition systems have a mechanical system to advance the timing based on engine speed. As the engine speeds up and the distributor shaft rotates faster, weights attached to the portion of the shaft that mount the breaker cam and the rotor move out due to centrifugal force. That movement causes the points to open earlier and the rotor to align with the spark plug wire contacts in the cap sooner. As a result, the timing is advanced. Flathead distributors from 1932-48 used a pretty typical mechanical advance.

Another factor in ignition timing is the air/fuel ratio--lean mixtures burn slower than rich mixtures. Most carburetors have what is called an enrichment system made up of metering rods or power valves. When the engine is under a light load, and manifold vacuum is high, the carburetor leans the mixture. When the engine is heavily loaded, the manifold vacuum drops and the carburetor supplies a richer mixture. To adjust the timing due to engine load, most carbureted engines have a device called a vacuum advance mechanism. In high manifold vacuum conditions the mixture is lean, the vacuum canister on the distributor advances the timing by rotating the plate the points are mounted on, the points open earlier, and the timing is advanced. In low manifold vacuum conditions the mixture becomes richer, a spring overcomes the pull on the vacuum advance diaphragm, it "relaxes" and the breaker plate moves, and then the timing retards. Front-mount Flathead distributors use a variation of the vacuum advance called a vacuum brake. Distributors for '49-53 engines relied entirely on vacuum to determine timing.

The amount the timing advances, and the rpm where it occurs, is called the advance curve. Due to the turbulent nature of the combustion chambers, the advance curve for a Flathead will look considerably different than most contemporary engines. A small-block Chevy will usually have around 37 degrees of advance maximum, whereas a Flathead Ford will seldom require more than 24.

Despite what anyone says, old Henry's engineers did a good job of plotting the ignition advance curves for the Flathead, and that's one of the advantages of retaining a stock distributor. Very often, when those looking for a hotter spark adapt a later model ignition system to a Flathead, the problem actually becomes limiting the advance. But while the advance curves of Flathead distributors are generally appropriate for stock to hopped-up street Flatheads, that isn't to say these ignition systems can't benefit from modification and improvement. All distributors, even for bone-stock engines, should have the breaker cam checked. Manufacturing tolerances on these parts were incredibly loose when they were new, and a half-century of wear hasn't made them any better. In addition, bushings and shafts should be checked and replaced as necessary.

When it comes to modified and higher compression engines, a hotter spark may certainly be beneficial. However, this can pose a problem for early front-mount distributors. While special coils and electronic conversions are available, the real shortcoming of the '32-41 distributors is the voltage capacity of the rotor. Putting lots of juice through them generally creates more problems than it cures. For performance applications, the best stock distributor is the '42-48 pancake design. The main difference here is the rotor. The large air gap between the coil contact in the cap and the distributor shaft effectively insulates the two. These distributors can easily be converted to electronic operation, and a hotter coil substituted. Of course, there are a host of aftermarket ignitions available, which we will look at soon.

Over the years the '49-53 distributors have been routinely criticized, and while they weren't great, they're not as bad as most think. Generally, the problem is that the vacuum diaphragm is connected to the wrong source. For mild engines a good, properly installed distributor will work fine. For performance use, cut to the chase and use one of the new MSD distributors.

To get the straight scoop on Flathead ignition systems, we recently dropped in on one of the most knowledgeable guys on the subject we know, Jere Jobe of Vintage Carburetion Technologies. Jere rebuilds stock Flathead distributors, modifies them for performance applications, and has one of the slickest electronic conversions around. Here's some insight on how to start a fire in a Flathead.

Cams And Compression This Time We Get It Right

In the Dec. '02 issue of SRM we ran a great piece by Joe Abbin comparing the horsepower and torque produced by two popular camshafts in a typically modified Flathead Ford. Unfortunately, we negated much of Joe's work by not printing the proper charts.

The horsepower and torque comparisons were between an Isky Max 1 (labeled 3/4 Flathead) and an Isky 400-Jr (labeled full-race). While the torque charts and captions on page 215 were correct, the horsepower charts on 214 were not. We got the captions right, but repeated the torque charts in error. Basically, Joe's research revealed that these two cams produced nearly identical horsepower figures, although the 400-Jr made its peak at higher rpm; our apologies to Joe and to our readers.

To make the comparison easier to understand we prevailed upon Joe to do a new horsepower table, and we're including the previously published torque charts. We promise to get it right this time.

And while we're at it, we need to clarify one other item. The relief by Kenny Kloth shown on page 212 was not used on a Bonneville engine, as it was not found to be effective. Joe adds, "I've found that a .100-.125 constant depth relief improves average flow from about 7 percent (with stock 8CM heads) to about 14 percent (with Offy 425 heads). I actually got an 18 percent improvement when I ran the Offy 425 with a relief but without a gasket (equivalent to an Offy 375). I've tested several "ski jump" (my term) type reliefs like the one in the article with disappointing results."

SOURCE
ROADRUNNER ENGINEERING
P.O. Box 53296, Dept. SRM
Albuquerque
NM  87153
VINTAGE CARBURETION TECHNOLOGIES, INC.
Dept. SRM