Cadillac Jim

Mailbag: 1970 Coupe DeVille - Turbo Build Questions

2 posts in this topic

...I have read some of your post and you sound knowledgeable, well a lot more then me. I was a mechanic in the marines, now I'm out, but me and my son bought a 1970 Coupe DeVille convertible 472 ci that we are going to restore, frame off restoration. I have never done anything like this, but my brother in law does body and paint, but before i start i want to make sure i have a plan so my question is that I have been looking into maybe putting a turbocharger on the engine, its a 472 which can be made into a 500 ci. What are your thoughts on how the turbo would effect the engine block over time? I have a lot of experience with turbos but i have never tried to put one on an engine that was not meant to have a turbo. I am pretty sure i will need an oil cooler, new fuel delivery system, all new upgraded internal engine parts, new upgraded external engine parts as well, but what about changing the transmission, drive shaft and rear axle(its rwd), and updating the brakes? Also i am kind of lost on where to start with this project, as in how do i figure out the proper components to add a turbo, and which component do i want to start looking at first? Or would the turbo be a bad idea for that engine? My main goal is to teach my son mechanics, but i wanted to make it a bad *smurf* Cadillac that has power, we plan to paint it cherry red, white top, white interior with red trim its going to be amazing!

This is the kind of topic that we like to have on the forum, so, I'll start a topic there with your message, with names and other personal ID deleted. I presume that you found me by clicking on my web site on my profile and then on a webmaster e-mail link, because that is how your e-mail came, it seems.

I'm not an expert on big blocks but there are people on Caddyinfo that are. I'm sure that some of them will chime in.

Whether you have problems, and what problems you have, will depend on several things:

1) How much boost you put on. Up to 5 lbs or so, you should be able to run with a hot street setup with no problems.

2) Your use of the engine. If you just drive it on the street with no more than about 5 psi boost, you should be able to use the stock 472/500 cooling and oil systems without modification.

3) Your compression ratio. The 1970 472 has a 10:1 compression ratio, and if you put a 500 crank in it, the compression will be 10.6:1 unless you use 500 pistons (probably necessary anyway), which will hold the compression ratio to 10:1. That's a lot of compression if you want to run boost. You may need a water spray or octane booster to make that work without detonation, even with 5-7 psi boost.

If you plan to take it to the track, even for just a quarter-mile run once in a while, you should consider a water spray, an external oil cooler and fan, and possibly increased cooling. Anything more than that will need a full-bore HD competition oil and water cooling setup.

If you want to run a lot of boost and/or nitrous, you will need an intercooler and the compression ratio will have to come down. I would look at 9:1 forged dished pistons that are specially made for boost engines in competition, for example.

Your existing engine is rated at 375 hp, one of the highest rated outputs for big-block Cadillacs. The 500 is rated at 400 hp. I suspect that your transmission would benefit from a rebuild by someone who specializes in ruggedized clutches for road and track, and an external transmission cooler and fan. I would leave the torque converter alone unless you put a really wild cam in it and don't want to drive it on the street. High stall rate torque converters leave you stirring fluid and putting horsepower into heating the transmission 99% of the time on the street. Note that a stall rate over about 2200 rpm may be OK in the highway but not so good around town, and terrible for in-town gas mileage. Match the stall rate with the cam you select.

Brakes and suspension are a whole other topic. Of course, if you increase the engine output by 50%, you will need to increase the brake capacity by 50%. I believe that this car came with disc front, drum rear brakes. You may want to consider upgrading the rear to disks. PowerStop may have a good street-and-track kit.

Your suspension will need more than stiff shocks to keep the shiny side up if you go to 600 hp with your car. The car comes with L-78/15 tires/wheels, and the alignment specs are a setup for a smooth cruiser; you will need a tight suspension to keep control when things start happening a whole lot faster.


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Yes, I was Jims_97_ETC before I changed cars.

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You need to drop the compression ratio to 8:1 with a nice dish piston. Those big motors don't like a lot of high rpm.

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