: 1979 coupe deville 425 engine tuning

06-23-11, 02:33 AM
hey guys, im brand new to this forum.

i recently purchased a 1979 coupe that is immaculate! it has 60k miles. you can tell the car has been babied is life.

its got the 7.0l 425 carb'd big block...

my question is id like to get as much horsepower out of it without having to tear it all apart and re-build the whole engine.

would you recommend changing the camshaft with a slightly larger one?

im all ears on ideas how to make my engine run to its potential.

06-23-11, 10:51 AM
So tearing it apart basicly means not taking it out? A bigger cam with the edelbrock intake would wake that thing right up. Get those 2.23 gears out the rear end and you will really be moving.

06-24-11, 04:19 PM
thats what im talking about. what size gears would still be practical? 3.73? how many cmfs on the carb? like 650cfms? and in terms of a cam, i should keep it still kinda mild right? what would you reccomend for a cam? when you do the cam, are you supposed to change the valve springs?

06-24-11, 04:20 PM
and are there many aftermarket parts for these engines? is it the same block as the 500 c.i.?

06-24-11, 04:35 PM
Yes it is the same block as the 500...

There are a lot of performance upgrades for that family of engines. There are companies, such as MTS (http://www.500cid.com/) and the CadCompany (http://www.cad500parts.com/), that offer parts for the 500/472/425 engines.

06-25-11, 12:30 AM
The most bang for your buck would be to get rid of the single pipe exhaust with the cat and get a true dual exhaust with no cat installed
Then get the old Quadrajet tuned by someone skilled
New plugs, wires, and a fuel filter wouldnt be a bad idea.

The cool thing about this is it should really wake the car up, and if your still not happy it would complement the 500 or modified 425 quite well
But once you do it youll likely stop there, as this should satisfy :)

06-25-11, 12:52 AM
For a rear end gear, 3.08 gears would be nice. As far as carb, your stock one is a 750, wouldn't go any lower. The intake is your enemy, you have a single plane intake that sits so low, your gas has to go up before it goes into the combustion chamber. That's the biggest change you can do, get an edel intake and bigger gears in the rear. Unfortunately, that's gonna put the bill at a little under 800 for those two upgrades.

06-26-11, 11:28 PM
bang for the buck is the name of the game for me. which would you guys reccomend if you had the choice of a bigger cam, or headers w/ true dual? btw, what size is the rear end in my 79 coupe?

does anyone know the stock cam specs of a 79 coupe?

06-27-11, 12:11 AM
I think your rear gear ratio is gonna be 2.23 but I believe there were other options too.

06-27-11, 12:19 AM
I would start with a duel exhaust system and a less restrictive intake. Throwing a larger cam in can cause issues with the engine's idle and other performance that wouldn't be outweighed by the increase in power. If you want bookoos of horsepower and torque, get a 500 and build it for power, but for a daily driver, letting the engine breath is the best thing you can do.

06-27-11, 02:26 AM
I think your rear gear ratio is gonna be 2.23 but I believe there were other options too.
but what size is the rear end in inches?

and btw, are the stock carbs on these caddys worth a damn? or are they garbage?

06-27-11, 10:37 AM
Rare-ish 8.875 which, I believe, is a 10 bolt also. But I may be wrong, not 100% on this.

06-27-11, 10:48 AM
and btw, are the stock carbs on these caddys worth a damn? or are they garbage?

Rochester is one of the best carburettors around.

06-27-11, 11:36 AM
Sven, I would like your comment but it puts a heart in front of it. Just not right.

06-27-11, 03:09 PM
Rochester is one of the best carburettors around.
is the stock carb a "Rochester"?

06-27-11, 03:43 PM
is the stock carb a "Rochester"?

06-28-11, 02:54 AM
so should i put the stock carb on top of an edelbrock manifold, or should i put a new carb on it? will the performance difference be worth the extra money of a new carb? if i kept the stock 1 id like to have it rebuilt... so they say the 77-79 425 puts out 180hp, after true duals and a new intake, what kinds of numbers do you think im looking at?

and can i get a confirmation as to the size of the rear axle?

and is a 3.73 gear ratio too radical for road trips from the bay area to reno, and daily usage?

thanks again for all this great information guys, ive learned a ton on this site.

06-28-11, 07:45 AM
The problem with all 70's GM engines is the tiny camshafts they put in them and the single exhaust choking them down. I would rather find a 500 than modify a 425. 3.73's with no overdrive will make some high cruising rpm's. I wouldn't like it. I have 2 425's and I just drive them as they are, nice and slow.

06-29-11, 12:36 AM
i thought the poblems with 70s gm v8s were the low compression ratios? i want my car as a cruiser, i just want to be able to jump on it an burn a little rubber and have it sound like a big block should.

07-03-11, 10:18 AM
There's plenty of ways to make a sleeper out of this car if that's your goal. Personally, having both a Diesel and gas version in the big RWD Caddies, I'm always happily surprised at the power my 425 in my '77 makes, bone stock. They were designed as a torque monster, long stroke engines, a camshaft profile resembling what you would find in a RV. They DO NOT like high RPM unless SUBSTANTIALLY modified to accept it (read as - aerate the block - yard sale the guts of the engine). The stock axle ratio was a 2.28:1, optional was 2.73:1 and 3.08:1. If you get much over 3.08:1 or 3.27:1 and plan to drive on the freeway, expect to get Christmas cards from OPEC, they're gonna love you. I agree the intake wasn't designed for a lot of horsepower, the air has to flow up then down, but it was designed to provide adequeate clearance under the hood too.

If you want a good big block street car, consider a Chevelle : )

Don't molest a Cadillac for power! I never had a problem pulling out to pass a truck on a 2 lane road in my '77 or getting on the freeway double quick. Thats what the 2 extra bores at the back of the carburetor are for. Double check that the engine is tuned properly and the carb is set right. That Quadrajet is an engineering masterpiece, but in the wrong hands can be easily loused up. When I got my '77 I was horrified to see what they did to the carburetor. I spent full day rebuilding it back to factory spec, primarily because whoever ravaged that thing before me bent rods that should never had been touched! The car runs great now.

I do not know the exact CFM rating on that carb, but I do remember the '70 model was 875 CFM, the largest stock 4-bbl ever offered. The beauty of that carb is with minor mods it can work on a big block or a V-6, it's only going to use as much fuel as the engine will allow it to since the rear barrels are metered based off how much flow is going through them. They may never ever fully open on a stock engine.

07-11-11, 03:34 AM
yeah, i was reading, replacing the intake manifold with an edelbrock manifold will raise the carb by 3"s... i dont think i have enough clearance for an air cleaner too? do i?

my goal isnt to "molest" it for power, its just to use all 425 cubic inches to their fullest.

07-11-11, 04:35 AM
I used the edel intake but I had to get an aftermarket drop base air cleaner.