cheap power improvements to the 425...

[Dead Sled]

Originally Posted by abcdefg

My cadillacs 1977 425 is a gutless wonder. Im amazed at how i "cruise" up hills in this thing. With the gas pedal about 3/4 on the floor (in 2nd gear going 20mph) it barely holds its speed, and this is a fairly new engine.

When i put it in Low and "floor it" its pretty good untill about 25-30 then its like im towing a school bus behind me, it accelerates faster in 2nd after that than it does in 1st. I busted out my cadillacs service manual, and looking at a cutaway view doesnt give me a suprise as to why this engine is neutered on the top end.

The intake manifold looks like a snake, you can even see the tight bends just by popping your hood.

I do have a muffler and exhaust tip off a 12 liter kennworth diesel, but i think the caddy would drag bottom with either thing. Plus i dont want it to look like a "ricer" (5" 4.5 foot long chromed pipe hanging out)

The easiest route to go would have been to buy that 500 cid on ebay. It had a blower, 8 port nitrous fogger, heavy duty performace connecting rods pistons and all, hot valve cam... but i dont want to blow up my THM400...


Is the 425 cid really this wimpy?


If im lucky enough to find a 425cid or 500cid caddy with fuel injection, and figured out how to hotwire it into my carbureted wiring, would this give it more balls at the top end (past 2000rpm)? I see the fuel injected deville is rated at 190hp, versus the carbureted 180hp. I suppose a die grinder and carefull planning would give the fuel injected manifold more oomph?

start with little things first the 425 isn't really that bad just needs a little help breathing. make th eintake and exhaust flow. things like a hotter coil and ignition advance kit for the dizzy good plugs and wires. small things like this will really liven it up. cad motors arent a high revving motor like chevys or fords. Cads are a torque motor you may only have 190 horse but the torque more than makes up for it.

how your tranny doing? color and smell of fluid, when was it changed. its hard to blow a trubo 400 but you may be

[lux hauler]

abcdefg,
Sounds to me like you need some tuning. As dead sled said, the 425 is not that bad an engine....if it's tuned right.
Remember....these engines are designed to make their power at lower rpm's. If your looking for an engine that'll pull to 5500 or 6000 rpm's with factory parts, you don't need a big block caddy. Stock....4000-4400 and these things are done. That's the way they were designed.....low rpm torque.

[abcdefg]

im not looking for a high revving motor. I need a motor for crusing in, and i've already had everything checked on it. My exhaust finnaly fell off (whole 8 feet after the catalytic converter) so i ripped it out and drive it with the pointed peice out of the back of the converter. It now can do a U-turn where it wouldnt before (some help from spinning the tires). But it still doesnt have what it takes at the low end to move this car from stop signs and up hills.

What i meant about the THM400, i didnt think it would hold out to a supercharged 500cid with nitrous. I will unhook the catalytic converter when i repair the exhaust, and see if that helps the bottom end.

How would further advancing the timing help? im running 18DBTDC, and with vaccume advace it should be at least 22-24 degrees. The whole engine looks like it will jump out of its engine mounts from the timing. And it does drive more sluggish with less advance.

[7.0,hellyeah]

It aint made for high end at all. A good exhaust system wouldn't be bad for the thing either. Torque comes from massive intake and low exhaust outlet. Fabricating bigger exhaust manifolds would make it awsome. New intake manifold too. Those two things and the car would run like a champ! The 472 is a much better engine, I'm sorry to say that though as an owner of a 425.

[davesdeville]

You want more power, bolt on an Edelbrock and some headers and a big enough exhaust system to handle it. As for timing, the big brothers of the 425s seem to like 10-12* initial, 32-34* advanced, the 425s are probably similar.

[7.0,hellyeah]

Slapping on 472 intake manifolds will give more power right, more high end I least I thought. Will the 472 manifolds bolt right on or do they need to get ported?

[abcdefg]

I got it up on the highway with only the catalytic converter for exaust. It probably has enough power to pass someone, but once it hits about 50 its just about dead, then it shifts into 2nd somewhere near 60. The 425 just does not have what it takes to move an areodynamically challenged 2 1/2 ton car at 60mph up hills.

I can get ahold of a 472/500cid intake manifold. Im sure my current gaskets will be trashed if i take the 425 manifold off, so if it will make enough difference to make me want to spend money on gastkets, sure...

What about a bigger air cleaner box? I might be able to get one off a truck at an auto junk yard. I will definatley look into buying a 2nd catalytic converter for dual exhaust. Its just a matter of spending money for exhaust piping.

And i know the largest factor would be my differential. 2.8:1 standard (spins one wheels only). 8.3mpg town/highway combined with a 2.8 is bad enough, so i will leave that alone.

If i put a ricey exhaust tip on the caddy, i suppose i could get 300Hp out of a fart can, but i wont go there.


I am parking this car over the winter, so now is the time to fix the 8 track player. Any other things i will have to get started on ASAP.

[Night Wolf]

my '79 pulls like crazy thru 1st.... which ends around 53mph... then pulls nice in 2nd as well...

... the '79 actually accelerates faster up a steep grade better then my '93... either it is perceived by me, or the fact that it has alot of torque at a very low RPM...

once it hits 3rd around 85, it slows down... I know since i gutted the cat (drained the cataylist beads) that helped alot.... ntohing like going up a hill 40mph in 3rd, flooring it, secondaires opena nd a downshift to 1st... it literally thorws you back in the seat too...

for the time period, and the push for lower emmisions, and better fuell economey, I say the engine is not bad... if you replace the most restrictive parts of the engine (deisgned for better millage and low emmisions) which are the intkae system, cam and exhaust system... you will have a lot more performance form an otherwise stock enigne...

[Night Wolf]

also the standar rear end on these cars is 2.28... I think the limos got 3.08 or something similar...

... last time i check, I got about 12mpg around town and.... surprisingly 20mpg on the highway... I was only on the highway for about 30miles though, when I checked that... but still...

I'll be checking millage again.. I need to patch the hole int he bottom of my cat before I cruise in it... droning sound gives me a headache, and makes the car sound bad... (basically a big exhaust leak on top of the slight exhaust leak form the left exhaust manifold)

[caddy4yaass]

I just bought a 79 caddillac coupe, 425 in it. and i am looking to upgrade it [ engine power wise ].i have got the whole thing taken apart already all the way down to the camshaft. i would like to to make the engine run good street wise but still have some power to it. now i want to get a new carb intake, and exaust for sure,but were is a good place to get the parts from. thanks for any reply.

[lux hauler]

Summit and Jegs both sell the Edelbrock intake. The stock Quadra-Jet is good for overall power/throttle response and mileage. The exhaust will have to be custom.

[abcdefg]

Maybe 20 years from now i would like to turn this into a "drive it once a week to burn rubber for 2 blocks straight" car. Nice looking, quiet, smooth riding, fast...

My cadillac is acting weird. I floored it at a stop sign on a steep hill, and the right rear tire chirped. I was very dissapointed, i was being pushed back in my seat about the same as a 15hp riding mower. Before, at one time, i was revving my motor in drive with the brake on, next to one of my friends in a toyota. I didnt have any exhaust after the catalytic converter, so it had a bit of a rumble to it. I also liked how the whole car was rocking over to one side. I revved it at about 1 /2 throttle and the tires broke loose, so i floored it and let off the gas. I was smoking the right tire for a good 30 feet, before i let off the gas for a tight turn. In a round-a-bout, the right tire would do its stuff when i gave it too much throttle.

But i had to adjust the choke to rich, because the engine kept dieing before it was warmed up.

On ebay, i typed in 425 cadillac, and turned up with "Torque maximizer III" cams. I have no idea how special they are, and what good they would do me. The rpm range seems to be a little bit high for what i want, but everyone is looking for more hp options, we should just put a 350.

http://cgi.ebay.com/ebaymotors/ws/eB...spagename=WDVW

Once for 2000$ starting bid, there was a roots type blower that said it was made to use on a 500 cadillac engine, but i wont have a 2ft tall intake system sticking out of my hood.

Lets just say i had the intake ported larger (Not polished because its suppose to be rough) , lighter pistons, stronger crankshaft, good breathing heads and fuel injection, would that make a big difference (like 200hp difference?).

I need to write an angry letter to the cadillac motor division. Who ever thought of cramming the 425 in the back near the firewall so I cant get at anything. How am i supposed to remove the catalytic converter without ruining it, if i cant get a wrench on the bolts?

Maybe someone who knows what their doing should start an FAQ. Not many people know anything about the older cadillac engines. Theirs unhooking the exhaust, but after that things get vauge.

Many people around here suggest stupid stuff. "rebuild the engine"... That would leave me with what i have, a rebuilt 425 with under 1000 miles on it. K&N Air filter... I could use a riding mower air filter on the cadillac and it wouldnt know the difference. Putting a 350 in it? hahahaha, thats a good one!

[TorqueInc]

Kinda lost me there but..

There is alot you can do to improve the 425 besides just"rebuilding it"
One thing you can do is make the engine larger
A .040 overbore would be a start
Some head work including new valves,port work(nothing exotic),springs and a .020 cut off the face would do wonders.
a camshaft designed for the engine (torque cam 218-224@.050
Good sound machine work and a 472/500 intake manifold(modified)rebuilt Q-jet.
Headers if you have the skill to convert a set or buy some sandersons
You would have a good strong runner that would get decent mileage
I know of several good running 425's that have alot less that what i described done to them and are driven daily.
Nothing to it
a 472 or 500 would be better for power but you would be pissed at the pump as well.

[Night Wolf]

does a 472/500 really use that more gas then a 425?

since i gave up the idea of building up a nice 500 for my '79... I would like to keep the origanal engine, that way I can say "yeah, it has the origanal 425" and i am doing basic and simple mods so far... it seems like with a little wor, these can be opened up pretty nice....

how dificult is it to put a performance cam in the 425?

[sethmartin]

Originally Posted by TorqueInc

Headers if you have the skill to convert a set or buy some sandersons

Will the sanderson headers fit on the 425 in my 78 SDV?