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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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#78 ·
i know a couple cheap improvements that work excellent for my 425 but everybody knows this..a brand new air filter,oil change,belts,oil filter,i know this works great for a fact cause i live in phoenix and i go up north quite a bit and one time a drove up north without no oil change,new airfilter,oil filter and i top'd my car off and when i got to my destination(1hour 45 min away) my caddy was already out off gas ...just on the way up their...then this christmas i went up north again got everything done and i still had 3/4 tank their and back...treat your car good and it will treat you good..thats for sure:helpless:
 
#79 ·
Well... no more 425 for me.... damnit I miss that car so much now... never realize what ya have....until its gone...

Anyway, my list of, well, cheap power improvements to the 425...

- full tune up. This includes fluids changed, plugs, wires, cap and rotor, air filter, fuel filter, PCV valve etc....

- 14" open element air cleaner. Mine was made by Edelbrock, it looks really nice. Free flowing, and sounded sweet, especially when the secondaires opened :). It also greatly helped to clean the engine bay up.

- drain cat. conv. If your car still has the origanal cat... its plugged. Pry out the drain cap on the bottom and drain the beads... make sure they all come out... then take it to a shop that has those plugs... when I got an inspection on the car, the guy had a bunch of them laying around (old GM part) and charged me $5.... it was loud, but got annoying fast with the plug missing.

- rebuild carburator. The Qudra-Jet is a sweet carb, don't let anyone tell you otherwise... but obvisouly a ~30 year old carburater is going to have problems. Rebuild it, and if you know about carbs, or a shop does, then dump the emmisons settings from factory and go for a little performance, a stock Q-jet can be worked over to squeeze more out of it. Keep in mind this is an 800CFM beast that fits the engine perfectally, keep it. A good book for the Q-jet is Doug Roe - Rochester Carburators.

- find a 1968 472 intake manifold. This intake manifold, stock, flows alot better then the 425 intake. The '68 dosn't have EGR either, so the 425 now dumps EGR. While I don't mind EGR, with an old valve and lines, it is more of a problem then anything. Then, take the manifold to a machine shop and have them totally cut the center section under the carb out, so it is 1 big opening. You now have an intake manifold that flows 95%+ of the Edelbrock intake, for a whole lot cheaper. Sure it still weighs 45lbs, but it looks stock. This intake manifold will flow just as well as the Edelbrock until around 5,500RPM.... seeing as how these big blocks don't rev past 4,000RPM stock, you have to do alot to really get out of the effective range of it. It is also a low-riser intake, no more hood clearance problems. Perhaps the biggest thing is, you can still use the factory AC, not so with the Edelbrock. The A-4 from the 425 has a different mounting bracket, so to correctly mount the compressor, you need the 472 AC compressor bracket. Swap that over and she bolts right in. Revs smoother, sounds more healthy at WOT and noticeable power boost. The BB Cads are so cool, there is no coolant to mess with :).

Another non-engine related performance boost that really helped the car in handling and overall driving, was installing a factory rear sway bar. Mine was from a '79 Fleetwood, but I have been told 70's G-bodys use the same bar. Very nice mod.

The stock '79 DeVille with out 472 intake or rebuilt carb ran a 17.5@78mph. She was noticeably faster after words. I gotta say, the car was actually fast by the time I was done with these minor things. Floor it at 30 and she'll downshift into 1st, put you in the seat and you see the nose of the car jump up.

Other cheap things I had in mind was a shift kit for the trans... that would really help since the trans is calibrated for excessive slipping for smooth shifts.

Another thing that was going to be sweet, since the factory 2.28 gears don't offer much for performance, I was going to swap a cam in... not a hot cam, but a cam that brought the power band lower then stock... an RV cam. More low end torque at the expensive of high RPM HP. The neat thing is, the cams power band is idle-4,000RPM... well, it would shift at 3,900RPM going in to 2nd and 3,400RPM when going into 3rd, when in drive. It was perfect. The cam is the MTS #3, stock valve train is fine. It is also the gas saver cam and is very possible to get 20mpg+ on the open road... that was going to be the project. I wasn't ready to swap the rear end just yet, and so being stuck with the 2.28 gears, I was going to make a torque beast, a whole lot meaner.

Also, find the cross member from a Caprice 9C1, that way you can easily run true duals. True dual 2.25" exahust on a near stock 425 would be sweet, unleash a few ponies too, and with the MTS #3 cam, really open it up...

while on the exhaust note, the 425 manifolds are junk. Find a set of 472/500 manifolds, while not the best, they flow alot better, direct swap in for the 425, that and the true dual exhaust would be sweet.

There are all the basic plans I had for my '79... I had lots of plans. Hopefully this will help anyone else that just got a 425 and wants to bring the life back. Mileage dosn't effect the BB Cads much at all... meaning a 425/472/500 with 100k, if mainted thru its life, will show minimal wear and be ready for any plans you may have. Same goes for the Turbo 400.

- BTW from an appearance standpoint, something that REALLY helps the '77-'79 DeVille is to run a 2nd pin striping down from the fender and follow the body line that ends at the door. That right there is the missing link, without it, the car looked bland and missing something... I added that and DAMN those 2 pin stripes really set the car off like nothing else, it looked soo nice.

She will be missed....

 
#80 ·
One other performance mod, if you have your engine out for a refresh or rebuild is to have the block decked. As I understand it - at top dead center - the pistons are down in the holes a bit which could cause detonation. Besides preventing that - it should give a slight bump in compression which would be good for power.
 
#81 ·
Decking the block is usually done to get a good quench distance (piston to head dist.) Depending on the crushed thickness of your headgasket, you'll usually want around zero deck for a tight quench. Or you'd want to run the pistons way down in the hole, like .120, for a very loose quench. In between has a tendency to see detonation problems.

If you want a bit more compression have the heads milled. .040" which is about all I'd be comfortable doing, should get it to about 8.75:1 by calculations (which might be wrong since it's 1am and I should get some sleep.) Still would do well on regular gas, and might net you a dozen ponies or so.
 
#84 ·
:bouncy:

That is awesome... I still have my '68 intake manifold... I pulled it when the car was wrecked. You'll need to dump the EGR system though, normally I woudln't do that, but ah well.

'78 rear sway bar? you'll notice a big difference.... very good difference too, that was a really nice addition to my car.

And... cross member from the '96.... the double hump? I was looking for one of those... thats the only thing that kept me from getting a nice, new dual exhaust... the stock exhaust system is pretty bad.... some nice headers and dual 2.5" exhaust the whole way back would really wake it up too.
 
#85 ·
I have a 77-96 B/D Body rear bar that will be up for sale soon if anyone is looking. Should work in the 73-77 A/G body cars too. It is the large 9C1/F41/Impala SS/FE2 Fleetwood bar.

Yes, all 94-96 (all LT1) cars are double hump.
 
#87 ·
Post pictures of the '77 somewhere!

About the 425,000 mi Deville, that's nothing. The world record for the car going the most miles without an overhaul was held until very recently by a 1979 Sedan DeVille with just over 650,000 on the clock.
 
#88 ·
Hello i have rebuild my 425 i have put a 472 intake on it from 1976
, but i have a carb thats belong too the intake can i used that or must to buy a original 425 carb, mayby the 472 carb its too big

thanks for your opinion.

the carb its a rochester quadrayet 17056230.

I have not 425 carb but it was earlyer a injected.

thanks sleutelgek from the netherlands (europe)
 
#92 ·
I got a line on hopefully some 68 parts. How can I confirm that the intake and exhaust manifolds are truly the correct parts?
I had this data in my spec lists - no guarantee that I have it right, but this manifold works on my car:

1605061 is a 472/500 stock cast iron with EGR – replaced 160955 stock 425 “negative rise” piece.

I also had this from a web poster:

‘68-mid’70 engines used the first generation heat operated choke, with an exhaust passage around the front of the carb. These intakes have a cast rectangular box-shaped choke well. Mid’70-’73 engines do not have the heat passage around the carb, and have a round choke well with what looks like a freeze plug in the bottom of it. (This describes mine) Many ‘68-’70 engines also had no EGR valve. All OEM 472/500 intakes are dual plane.

I surmise that I have a mid '70-'73 intake...

I don't have the parts numbers for the exhaust manifiolds. You want those that have a nice thick front runner (by this I mean the exhaust tube for #1 and #2 cylinder). The ones to avoid are the stock 425 pieces which have huge dimples cast right in on both the left and the right front runner.
 
#93 · (Edited)
Wow! This post has a lot of history and a lot of info. There was someone selling crossmembers he manufactured for A bodies. The sugestion made here for the later C body looks good. Still available from GM?
ONe thing I thought of, You can change your cam's performance by adjusting cam timing by a couple degrees. You don't neccessarily have to give your money to a cam company for a mild timing change.
 
#95 ·
The after market cam guys like Comp Cams advise advancing the cam timing by like 4 degrees for low-end power, and retarding the timing by about 4 degrees for higher RPM power. There is no magic power boost in fooling with cam timing in general. Most folks install new cams "straight up", meaning the timing marks on the cam drive sprockets are lined up like an overhaul manual specifies, as opposed to setting it up with a degree wheel etc, for getting all available power.
 
#99 ·
The after market cam guys like Comp Cams advise advancing the cam timing by like 4 degrees for low-end power, and retarding the timing by about 4 degrees for higher RPM power. There is no magic power boost in fooling with cam timing in general. Most folks install new cams "straight up", meaning the timing marks on the cam drive sprockets are lined up like an overhaul manual specifies, as opposed to setting it up with a degree wheel etc, for getting all available power.
Your correct here. My statement, "changing performance", was wrong. Advancing or retarding the cam's degree of timing is a torque curve adjustment.
In the 1980s or sooner GM started changing the torque curve of the cams to enhance mileage, which made performance off the line anemic. Degreeing a cam is not magical, but you have to understand valve train dynamics and other aspects of your engine/drive train to maximize your performance / economy.
 
#101 ·
The EFI on 75 - 79 cars was primitive by todays standards. However, waking up a 425 EFI motor cant be too hard. People here have mentioned degree-ing in the cams and I think that would do a lot. One thing I've noticed about Cadillac camshaft profiles is that they are hyper mild.

Oh, one other thing. The EFI on the regular car line was different from the EFI on the Seville.
 
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