Full-Size FlounderI have a '6311/42 Ford Galaxie 500 XL with a 390. The engine is equipped with a 428CJ camshaft, (Ford PN C5OZ-6250-B), adjustable 1.76:1 rocker arms, and an Edelbrock Streetmaster manifold with a 600-cfm Holley carburetor. Until about a year ago the car had polished and slightly ported "Police Interceptor" heads and a stock distributor. At that time the heads were replaced with Edelbrock aluminum heads, and the distributor was replaced with a '75 truck electronic distributor and a Ford Duraspark ignition module (PN DY184C), which is listed for a '78 Ford 460 passenger car.
After making these modifications, the performance is not what I expected. In fact I thought the car did better when the old cast-iron heads had been freshly reworked in 1984. Additionally, the fuel mileage is between 12 and 12.5 mpg (now that this a little more important). I would have expected a little better than this even with a 3.50:1 axle ratio. I suspect the ignition module may be part of the problem, as my father made the same modification to his '63 four-door sedan. I hate to ask these questions, but no information is given on these proprietary modules in the shop manual. Any suggestions as to what we are encountering here?Donald B. Kerr P.E.Via e-mail
It sounds like there are two primary possibilities for the disappointing performance you're experiencing, and neither is related to the ignition module. Although it seems like a complicated electronic device, at the core, the module's job is to tell the coil when to spark, and it does this based primarily on the input from the distributor's pickup signal. It's simply a more efficient way of triggering spark than the old mechanical breaker points, but there isn't as much voodoo there as you may suspect. In other words, if the engine runs, the module is probably doing its job unless there is a severe lack of power and frequent misfiring, which you didn't mention.
However, there is a function of the ignition system that could be costing you lots of power: the advance curve. It sounds like you took the distributor from the '75 truck engine and dropped it into the 390 without making any changes. The potential problem here is that the later distributor probably has a much less aggressive advance curve and maybe even less total timing. A truck engine from the mid '70s would have been among the first subjected to the perils of early smog-control measures (not as stringent as on passenger cars of the day, but an issue nonetheless). Included in the factory tune-up specs was probably a really lazy advance rate, and again, probably not much total advance. Cranking the initial timing might serve to crutch this a bit, but it won't make up for the lame curve. A simple test of this theory would be to reinstall your old breaker-point distributor in the engine and see if it feels any better. While you're at it, if you have a dial-back timing light (or a timing tape on your balancer), see if you can chart the advance curve of each distributor for comparison's sake.
Another item to consider is compression ratio. You stated that your 390 had Police Interceptor heads, and we're assuming you mean 390 Police Interceptor, as there was also a 428 version. Most 390 engines used cylinder heads with 71-74cc combustion chambers, and that's the likely reason Edelbrock produces its aluminum FE heads with 72cc chambers. However, some of the high-performance 390 engines had smaller chambers for increased compression. If your iron heads had smaller chambers, they would likely have been in the neighborhood of 65-68 cc's.