Mostly likely, the difference in compression, if there even is one, would be about a half-point, which might not normally make a huge difference in output, but remember that since you've gone to an aluminum head, a lot less heat is being retained in the combustion chamber. With the loss of heat comes a loss of power. As a general rule, when planning an aluminum-headed engine, builders will spec about an extra point of compression to make up for the heat lost to the faster dissipation rate of the aluminum. When aluminum heads for street engines started to hit the market, many consumers seemed to think that the use of aluminum allowed for more compression, but really, it almost requires it. Another general rule is that street-going pump-gas engines with aluminum heads should have right around 10:1 compression.
If your 390 had had somewhere between 9.5 and10:1 with the iron heads and now has only 9:1 with the aluminum, you may have lost a little snap. Determining if this is true will take some investigation on your part. Step One would probably be to measure the chambers in your old iron heads to see how they compare to the Edelbrocks. You should also determine what type of head gasket you used previously-if you took out a pair a steel-shim gaskets and installed thick composite pieces, you probably contributed to the problem.
If you are able to determine that you have lost compression ratio, and that you want it back, you may be able to get some with a simple head-gasket swap, again depending on what you used when installing the aluminum heads. After that, you could remove the heads and have a machine shop mill them to shrink the combustion chambers, but consult with Edelbrock techs before proceeding with this, and try to use a performance-oriented shop to perform the work.
Cop StoppingI have an '87 Plymouth Gran Fury that was once a police car. It has the 318 with four-barrel carb along with the factory oil cooler, 125-mph certified speedometer, and the rest of the heavy-duty A38 police package stuff. I know this isn't a popular body style to work with, but I'm a Mopar guy, and choices are limited for cheap, late-model traditional rear-drive platforms if you like Chrysler stuff. I've done some minor tweaking to the 318, but I plan to swap in a 360, and I'd like to improve the car's handling along the lines of the Pro Touring trend. Ideally, this would be a car for your Real Street Eliminator if it ever reaches my version of completion.
Anyway, I figured that since this is a police car, it would have come with larger brakes, however, my research on these cars shows that only the rear brakes are larger than the standard Diplomat/Fury, not the fronts. Being discs, the stock front brakes are certainly better than the drums we had to deal with on most classic Mopars, but they're not very big-less than 11 inches. I'd like to upgrade, but I'm not sure if there's a bolt-on improvement out there. My wallet's not quite ready for the aftermarket just yet.Gene HansenBranson, MO
You should know that some of us here at CC have a bit of a cop car fetish, so we'd never look down on one of the '80s-era M-body Diplomat/Fury models. They've got the look down but do need help in the horsepower department. Former CC staffer and Mopar fiend Marko Radielovic scored a nice, clean '89 Diplomat a while back and set about giving it proper motivation. His ultimate goal was to run the car in the Silver State Challenge open road race, so braking was a consideration. Like you, Marko found that only the rear brakes were upgraded on the cop-spec Diplomat, which is odd since the front brakes of any car do most of the work. However, we were also able to determine that the front brakes could easily be upgraded using factory parts.
The 10 71/48-inch rotors on your car can be swapped for the nearly 12-inch units used on late-'70s R-body models, which include the Dodge St. Regis and the Chrysler Newport and Plymouth Gran Fury that share the same body style. You can also find these brakes on some late ('70s) B-body models, like the Chrysler Cordoba. The bigger rotors, which can also be purchased new from any auto parts retailer, will mount on your existing spindles. The key to the swap is the caliper brackets that accompany the 12-inch brakes. By swapping your stock caliper brackets for the R-body pieces (a direct bolt-on), the calipers will be repositioned appropriately. You can even retain your original calipers. Since the calipers don't change, there's no need to make any changes to the hydraulic side of the brake system, like the master cylinder. Making this brake swap will require you to use at least 15-inch wheels on the front of your car, though since your cop car came with 15x7-inch steel wheels, this shouldn't be much of an inconvenience. This brake swap works on F-body Mopars as well (Aspen/Volare), because the chassis is virtually the same as the M-body cars like yours.