The real question is: Do your 906 heads really have only 79.5cc chambers? That figure is the official NHRA-accepted minimum chamber volume for these heads, but in the real world uncut production 906 and similar open-chamber heads come in around 85-90 cc. On a standard 4.25-inch-bore 383, milling 906 heads to their "blueprint" 79.5 cc spec yields about a 9.5:1 compression ratio with the typical "pure flat-top" 383 piston (Federal Mogul L2315F or equivalent) and your specified deck height and head gasket (about 9.7:1 on a 0.030-inch-over 383). On 906 heads, every 0.0042 inch removed from the head deck surface reduces the chamber volume around 1 cc. A "worst-case" 90cc starting chamber volume therefore requires milling 0.0041-inch of material off the head deck. For each 0.010 inch removed from the deck, 0.0123 inch must be removed from the head's intake face to maintain proper intake manifold alignment (0.054-inch total intake-face material removal for the worst-case scenario).
Caddy Lacks Power
I own a '77 Cadillac De Ville powered by a 425 with a Rochester four-barrel carb. It has around 130,000 miles on it, but 110,000 were from my grandfather, who really babied it. My questions are:
1. I've ordered a few Cadillac performance catalogs. Most only offer parts for the 500 and 472, but aren't the 500, 472, 425, and 368 all from the same engine family? So shouldn't most of the parts for the two larger engines fit the smaller ones?
2. I can't seem to find performance suspension parts for the Cadillac. Would equipment meant for fullsize Chevys or Buicks fit my car?
3. Is the Rochester carb worth keeping, or is it only holding me back? (All I know is it's a Rochester Q-jet M4ME.)
I'm not really looking to build this car into a crazy street machine, just improve a daily driver. I'm a starving engineering student, and I have a big daily commute!
1. The 368s had the weird "4-6-8" valvetrain, so they're in a class by themselves. In the 425's case, there's about a 90 percent parts interchange with the 472 and 500 engines. One key part that doesn't directly interchange is the intake manifold. The 472/500 intakes (including the Edelbrock Performer listed as fitting those engines) physically bolt on, but there's an air gap because the manifolds' runners are too big. However, 472/500 Cadillac specialist CMD offers its ProCad-3 performance dual-plane intake specifically for the 425 that accepts either Rochester or Holley carbs. For best results, use the intake with a 1-inch-tall phenolic spacer (hood clearance permitting). Add CMD's entry-level Caddy cam plus a free-flowing exhaust system and you're looking at a 50hp gain. Want more? Remove the heads, install CMD's stainless steel, swirl-polished oversize valves with a good three-angle valve job, and mill the head decks 0.035-inch to raise compression by half a point from 8.2 to 8.8:1 (still 87-octane gas-compatible, according to CMD).
2. Cadillac front coil springs were used only by Cadillac, probably because Caddys were heavier than other GM cars. Except for those Cadillacs equipped with the optional centerlink that mounted a steering damper, all the rest of the front suspension components interchange with other '77-and-up fullsize GM B- and C-chassis. If present, the steering damper-type centerlink, while unique, does not effect the interchange of the other components. Big car antisway bars interchange, but the Cadillac had bigger bars than its sisters to begin with. The Cadillac front steering knuckles and disc brake rotors are the same as other '77 GM fullsize cars with the 5-on-5 wheel-stud bolt pattern and 12-inch rotors (typically police cars, wagons, and heavy-duty trailering packages). The '78-and-up 12-inch brakes use a different wheel bearing, but they'd fit if both knuckle and rotor are swapped as a complete unit.