I want to play with my 79' 425
is0crazy
06-21-2004, 11:16 PM
I just got a 79 coupe deville with a 425 and a th400. I am in an area of WA (kitsap co) that does not require smog testing on any vehicles. I want to remove the emissions equipment and anything else I can to help it run stronger. I'm also interested in some mods that I can do to the engine that are relatively inexpensive to boost the power. I've only dealt with 60's Ford v8s so this is all very foreign to me. (i'm used to like one or two vacuum lines for the entire engine so I don't want to just go ripping off all the vacuum hoses under the hood because I don't know how it's going to affect the overall operation of the engine...) If anyone's done this before or knows someone who has, please help me!
MagicRat
06-24-2004, 08:40 PM
I posted this reply a few days ago in reply to your same question in the Technical forum:
You have the right idea with the parts, and if you had any other GM V8, it would be easy. However, your engine is a design unique to Cadillac and was built in various displacements (368, 390, 425, 429, 472, 500) from mid 1964 to 1981 (Note the pre mid-'64 engines were also 390's, but a completely different design).
So, no one makes the performance parts you want. There is one small company out east that makes them in small quantities for big $$$ A web search would likely turn up their name, I just can't remember it.
Otherwise your choices are:
1. Work with what you have. Tune it up and make sure everything, carb and ignition parts and settings are correct. Be sure to run ported vaccuum from the carb to the distributor. Many '70's and early '80's carbs had a thermal switch that selected manifold vaccuum at low engine temps and ported vaccuum at normal operating temperatures. These switches usually seize in the manifold vaccuum position making the engine run like a dog that won't pass a gas station. Install dual exhaust, with decent mufflers and no converters. Use an open element air cleaner, or better yet, modify the stock air cleaner to a dual snorkel type with two cold air ducts.
Remove the air pump to the exhaust manifold and block off the lines. Do you have EFI? That can be changed but requires a differnt fuel pump. If you have a Q-jet, just make sure its tuned properly and that the secondary air doors open when they should. Those carbs have a linkage to prevent them from opening when cold. Often, this linkage gets stuck, making the carb run like a little 2 barrel all the time. Theoretically the EGR valve reduces power, but if its working properly, it does not hurt performance much. You can make sure its closed, (not stuck open) and remove and plug the vaccuum line for it.
If you don't have aluminum rims, find some and toss the spoke hubcaps. I shaved 100 lbs of rotational mass off my Cadillac by using aluminum rims off a mid '70's T-bird (Lincoln ones work too,) and some low profile tires. If your AC does not work, remove the compressor, bracket and condensor. That saves another 60lbs.
2. Try another Cadillac engine. Your 425 has relatively small valves in the heads, so the performance potential is somewhat limited. Locate a 472 or 500 cid block. They are a straight bolt in and produce lots more power. A '69 or '70 has the best high compression heads of all, although those engines were made through 1976 in a lower compression form
3. Try a Buick/Pontiac/Oldsmobile engine. All kinds of performance parts are easily available for all 3 brands, although Pontiac parts are the slightly cheaper and most easily available. These engines will bolt right up to your transmission, and all 3 designs came in a 455 cid size, although they are each different in design. Use the engine mounts intended for that engine and its quite easy to weld them in place in your car. You would need the accessory brackets for that engine too.
I have an '81 Fleetwood with an built - up Olds 455 and the stock THM 400 trans. With the tall stock rear end ratio, 2.56:1, its good for 150 mph, no kidding.
4. Install nitrous on your existing engine. So long as its a relatively mild shot, it would work quite well.
Good luck.
You have the right idea with the parts, and if you had any other GM V8, it would be easy. However, your engine is a design unique to Cadillac and was built in various displacements (368, 390, 425, 429, 472, 500) from mid 1964 to 1981 (Note the pre mid-'64 engines were also 390's, but a completely different design).
So, no one makes the performance parts you want. There is one small company out east that makes them in small quantities for big $$$ A web search would likely turn up their name, I just can't remember it.
Otherwise your choices are:
1. Work with what you have. Tune it up and make sure everything, carb and ignition parts and settings are correct. Be sure to run ported vaccuum from the carb to the distributor. Many '70's and early '80's carbs had a thermal switch that selected manifold vaccuum at low engine temps and ported vaccuum at normal operating temperatures. These switches usually seize in the manifold vaccuum position making the engine run like a dog that won't pass a gas station. Install dual exhaust, with decent mufflers and no converters. Use an open element air cleaner, or better yet, modify the stock air cleaner to a dual snorkel type with two cold air ducts.
Remove the air pump to the exhaust manifold and block off the lines. Do you have EFI? That can be changed but requires a differnt fuel pump. If you have a Q-jet, just make sure its tuned properly and that the secondary air doors open when they should. Those carbs have a linkage to prevent them from opening when cold. Often, this linkage gets stuck, making the carb run like a little 2 barrel all the time. Theoretically the EGR valve reduces power, but if its working properly, it does not hurt performance much. You can make sure its closed, (not stuck open) and remove and plug the vaccuum line for it.
If you don't have aluminum rims, find some and toss the spoke hubcaps. I shaved 100 lbs of rotational mass off my Cadillac by using aluminum rims off a mid '70's T-bird (Lincoln ones work too,) and some low profile tires. If your AC does not work, remove the compressor, bracket and condensor. That saves another 60lbs.
2. Try another Cadillac engine. Your 425 has relatively small valves in the heads, so the performance potential is somewhat limited. Locate a 472 or 500 cid block. They are a straight bolt in and produce lots more power. A '69 or '70 has the best high compression heads of all, although those engines were made through 1976 in a lower compression form
3. Try a Buick/Pontiac/Oldsmobile engine. All kinds of performance parts are easily available for all 3 brands, although Pontiac parts are the slightly cheaper and most easily available. These engines will bolt right up to your transmission, and all 3 designs came in a 455 cid size, although they are each different in design. Use the engine mounts intended for that engine and its quite easy to weld them in place in your car. You would need the accessory brackets for that engine too.
I have an '81 Fleetwood with an built - up Olds 455 and the stock THM 400 trans. With the tall stock rear end ratio, 2.56:1, its good for 150 mph, no kidding.
4. Install nitrous on your existing engine. So long as its a relatively mild shot, it would work quite well.
Good luck.
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