V8-6-4 backwards compatibility...

[Mr Thompson]

Hello, gentlemen!

I have loved Cadillacs all my life. I grew up with them, and bought my own when I could. The "Cadillac way" of engineering has always appealed to me, and I've always been drawn to the big-block years. My own first "real" Cadillac was a '78 CDV, in college, and my most recent was a '69 Calais. (I panicked and got rid of it when gas prices spiked a few years back. )The ride, handling, acceleration, styling...heck, I even think the climate control was brilliant. I also love the V8-6-4. It's entirely unfair, the bad rep it got. The body style of the '81 is NOT, however, the company's best. But enough about me.

Here's my question, for wiser and more experienced mechanics than I. I know there have been threads about putting 368 heads on a 500, and there have been issues with chamber volume and gasket interference. I also know that the rocker solenoids won't just bolt onto earlier heads. So, specifically: I want a 1978 carbeurated 425 Eldorado, with the intake manifold and heads off of a 368 V8-6-4. The Eldo would retain its Quadrajet, and the solenoids would be manually activated, from one switch on the dash. There would be no provision for a '6', just a V8-4. I'm not looking to win any drag races (obviously). What I am looking for is an improvement in constant-speed highway mileage. Is this project do-able? What say y'all?

Thrilled to have found this forum, and hope you can help me out.

[drmenard]

There have been threads about putting 425 heads on a 500 or 472... 368 heads won't fit anything else...I know of one guy that adapted the solinoids to his 500.. but it took alot of maching on the heads and making of parts...

[Mr Thompson]

Well, thanks for that info, doc! I've followed some of those threads; slapping 425 heads on the big engines (that seems to be popular!)--and machining a 500 head to fit the solenoids. That did seem like a lot of trouble, I remember valve spacing was a problem... As I recall, the problem was the 368 head gasket intruding into the combustion chamber of the larger-bored engine, and some related volume issues.

I was hoping (fruitlessly, I suppose) that the smaller bore of the 425 would be more compatible with the solenoid-equipped heads. I know the blocks are essentially the same, with just bore and stroke differences. So why wouldn't a 368 head bolt in? Ah well, if it just doesn't fit then it doesn't fit, and the dream ends there.

The problem is, there just isn't any information about utilizing the V8-6-4 system! Information about disabling it abounds, but precious few people seem to be interested in taking advantage of it. I guess I'm reduced to 1) buying a car solely for the engine, or 2) swapping a perfectly good engine in an earlier car, just to have those dam' solenoids. [Or 3) bodge around a fix to get solenoids in a 425 head.]

[Jed95fwb]

Doesn't the stock 425 get pretty good mileage without cylinder deactivation? I have an 80 Seville 368 and it gets 20mpg on the highway at 75mph. Even the 500 gets in the upper teens on the highway. Own a 460 Lincoln (8mpg-all conditions), as I have and you'll never fault Cadillac MPG.

[Mr Thompson]

Very true, actually. The big block beasts were pretty thrifty, if you account for miles per gallon per cubic inch per cylinder. I'm just one of those guys always trying for a little more just for the heck of it, to see if it's possible. I did note on one of the other forums, a successful transplant of the solenoids onto a 500, using just a moly bar and a little creative shaving. So the dream isn't dead; just on hold for awhile.

[tripleyellow78cad]

Ive often thought about just robbing the whole damned drivetrain out os an 81 deville, wireing and all, and dumping it into an old 1965 chevy 1/2 truck, there light and it would be something VEry different than the plain
old 350-350 combo you always see.
ahh to dream about caddy big v=8s's