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123K views 100 replies 38 participants last post by  DouglasJRizzo 
#1 ·
Since I decided to push the build up on the 425 out of line for this winter project (NorthStar took it's place) I was thinking what I could do to get a little more power from the beast....

my first thoguht is the cat... it's gotta go... my father mentioend something about thee first gen cats having a "drain plug" that you can take out, and the beads inside fall out, kinda like gutting the cat? On his '79 Coupe, he had a test pipe on it.. but I can't seem to find them anymore.... I don't think I could take it to a shop , even though it is now over 25 years old, and there is no emmisions testing... so what would the best thing seem to be?

While under the car, the whole exhuast system seems to be restriced... especailly all the bends going over the rear axel... what is this, 2.5" piping (my 4.9 has 2.5") but I am not messing with the exhaust system now... I was thinking maybe a cheap after market muffler... might flow better? something like $40 or something? also give it a nice tone... but not too loud... would I even benifit form that? or just keep what it has.... Also the resonator... from what i coudl find out, they don't seem to restric the flow of exhaust at all.. do they?

Then it would be some sort of an intake... I bought, and tired to fit a 14" open element on it, but it hit the AC compressor... somebody menitoend about a 1" spacer? should I do that or see if a 9" kit will work (but probably look bad)... also would I gain anything from the open element over the factory air cleaner assembly?

the tune-up I first did on it consisted of:

new air filter
new fuel filter
new spark plugs
oil change

I suppose I should rrepalce the cap&rotor and wires also... just for the extra touch, and maybe even the plugs, although they are new... they are basic AC Delco, $2/each... nothing special (platnium, double tipped etc...) do you think they are ok to keep in there? should I replace the cap/rotor and wires, or leave them? the wires are 8mm Whitiker....

What else can I do for a tune up? I know my carb needs to be rebuilt as it acts funny when the enigne is cold... anything to restore some power?

I do need to change the coolant, as it is really ugly... but that really isn't anything for performance...

my AC dosn't work (leak in the lines) no big deal, but should i remove the AC belt... less rotating mass?

Abotut he carb... here are the problems... when the enigne is cold.. after I first started it, if in park I put my foot to the floor for a second.. it'll sputter, run rough and want t quit... if i drive it.. there is an off-idel dead spot where it'll shutter and have a hard time accelerating, and if the engine is cold enough.... stall out... also until the engine has fully warmed up, if I try to floor it... well, to the point where the secondaires would normally start to open... when it gets to that point, the enigne looses alot of power, the car shakes and shutters, engine RPM really dosn't build, it may back fire, then depenging on how warm the enigne is, either rev up in RPM like normal, or just keep doing that.... maybe a problem witht he secondaires?

Everythign else with the carb is fine, when the engine warms up, there is no delay or anything, runs great... if it hasn't been ran for a day or 2, a cold start can be hard.. I heard all Q-jets have a drip down problem? I pump the gas 3 times, crank it, stop, pump twice then crnak and it'll start... although when warm, it'll start up within a split second of turning the key...

Maybe if the carb is out of adjustment, and I fix it, that coudl improve power and gas millage.... IIRC my father said the caps on the adjustments have been removed, so it is adjustable now...

I am just looking for some cheap ways to tya nd improve the power a little... I know the enigne isn't running how it hould be (well, have the power it should... she runs nice) so maybe I can get some help from the crew :)
 
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#2 ·
I had a similar problem with my 425... The carb probably does need an overhaul, not very much trouble if you are mechanically inclined...the accelerator pump seal you will probably find dry rotted, causing that hesitation, and these fuel pumps have a problem with a check ball to return excess fuel at idle... you should probably throw one of those on, too...they run about $35...also be sure to clean the secondary rods when you have it apart! Let us know how it goes!
 
#4 ·
Night Wolf said:
my first thoguht is the cat... it's gotta go... my father mentioend something about thee first gen cats having a "drain plug" that you can take out, and the beads inside fall out, kinda like gutting the cat? On his '79 Coupe, he had a test pipe on it.. but I can't seem to find them anymore.... I don't think I could take it to a shop , even though it is now over 25 years old, and there is no emmisions testing... so what would the best thing seem to be?:)
There's about a 2" plug on the bottom of the cat. It's a pain in the ass to get it out, and if you can get it out there's a good chance you'll have deformed it so it won't go back in there. If you know someone with a welder you can just weld some sheet metal over the hole. I did this a few weeks ago and it did seem to make a good differance. I left the hole open for awhile and it was pretty loud...
 
#5 ·
I was thinking about going ot the local Midas, and saying that the car is over 25 years old, no emmissions testing etc... if they can cut the cat out and put a stright pipe in it.... if he says no, I'll ask about slipping him a $20...

Did it make a nice difference? I would imagine it helping, as the whole exhaust system seems to be restrictive...

... what about using JB Weld to keep a piece of aluminam over the hole?

Also, when you pulled the plug, did the beads just drain, did you have to shake it etc....?
 
#7 ·
lux hauler said:
Chances are, a "chain-type" muffler shop isn't going to touch it. I've heard there are big fines if they're caught. Best bet would be to find a small local shop or just buy the stuff and do it your self.
most will make you sign and offroad use only waiver

but agree start with exhaust make it flow then intake make it flow. the easier you can make air move in an engine the more power you can make
 
#8 ·
Night Wolf said:
Did it make a nice difference? I would imagine it helping, as the whole exhaust system seems to be restrictive...

... what about using JB Weld to keep a piece of aluminam over the hole?

Also, when you pulled the plug, did the beads just drain, did you have to shake it etc....?
It did for me - but then again my FW was in pretty bad shape and had run real rich for awhile, so the beads were fairly clogged with carbon. Cats get pretty hot, I'm not sure how well JB weld would hold up under several hundred degrees. I put a paper bag under the hole, the beads took their time to come out, I used a rubber mallet to help them along.
 
#9 ·
well, I am not looking to spend alot of money for this.. but besides dumping the cat, what else could I do to free up the exhaust? the piping dosn't look too big, but I am not replacing it, as mine is fine... that leaves the muffel and the resonator...

..for intake, maybe I'll try the spacer and get the 14" open element again... I really don't want to spend *alot* of money into the 425 (yet) but just something to make it a little more snappy....

that is true with the chain stores.. there is a local private shop that does exhaust work,t he kid goes ot my school... the only thing i owrry aobut is, they beat the hell out of cars.. customers cars, cars they sell on the lot etc... so I suppose I could just wait there and make sure thing bad happens... I know they woudl do it too...
 
#10 ·
I notice sometimes after running the enigne, or is I stand outside the car while it is running, the exhaust will smell like rotten eggs.... mixture too rich?

Should I bother to replace the cap+rotor, and wires, or no... also are the basic AD Delco plugs good enough for the job for now? they are practically brand new...
 
#11 ·
The rotten egg smell may be indicative of a cat that's already going bad. It might go away after the catalyst beads are gone.

My friend had some exhaust tubing and a perfectly good chrome tip so we cut off the resonator and replaced it with that. It didn't change the sound at all but I never took it on the freeway to see if it increased the drone.

Take a look at the cap and rotor and if any contacts don't look good replace. The plugs should be OK, I mean it won't hurt to change them but it won't do much good. At least visually inspect the wires for cracks, and take a look at them in the dark with it running, if you see any electricity 'leaking' from them it's time for some new wires.
 
#12 ·
hmm, maybe the cat is starting to go... only (almost) 86k, but it is 25 years old....

... the only wear point on the cap was where the finger touches the metal plate.. my father said it is in pretty good shape... no cracks on the wires, nothing strange at night either.... I never heard of the brand... Whitiker (sp?)they are orane and 8mm....
 
#13 ·
if you wanted to remove the cat you could try getting a couple of 2 and1/2 to 3" clamps along with about 18" peice of pipe and cut that cat off and clamp the pipe on tighten it down should work fine also I replaced my muffler a while ago as the weld had totaly rusted away with a turbo muffler from dynomax but its 3" very open almost straight through sounds excelent with a good low rumble until the secondarys open then it gets loud very loud muffler was only round 40 to 50$ definitly improved the acceleration also replace that cap and ignition coil in there I replaced my cap with one from acdelco along with a ignition coil form acdelco with 8mm wires and plantnum bosch plugs all for under about 60 or 70$ the plugs i think were the most expensive plus get that 14" air cleaner you'll need to get some eltra spacer peices and longer stud but that stock air cleaner has an opening of about 4" whereas the edelbrock is 14" round and 3"high and will definitly let her breath in a lot better also look in to replaceing that carb ABC gets a performance rebuild
Q-jet from a place in california cost around 250$ but works wonders over the factory carb
 
#14 ·
As far as all of this exhaust talk goes, it gave me loads of powr just ripping everything off after the cat. It keeps it quiet enough that police won't immedietly cry no exhaust but it gives the car more snap. The 14"X3" will only take in loads of hot air. Get a later models fuel injection air intake assembly. That will be able to handle a 14X3 K&N and it will get you cold air from the front of the car. No riser is needed either. If you do use the 14X3 assemply and riser make sure to plug the vacuum tube coming out of the original assembly. That caused me really bad timing problems later on. Dist. coils give good power too.
 
#15 · (Edited)
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?
 
#16 ·
abcdefg said:
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
 
#17 ·
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.
 
#18 ·
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.
 
#19 ·
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.
 
#22 ·
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.
 
#23 ·
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...
 
#24 ·
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)
 
#25 ·
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.
 
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