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1979 Coupe Deville, 1995 FWB & 6 Other Non-Cadillacs.
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Discussion Starter #1
I'm caught between a rock and a hard place because my 78 Fleetwoods 425 was rebuilt 4 years ago. I spent close to 3K building it and had different intentions in mind when doing so. Now I'd like to squeeze some performance from it, but don't feel like breaking a motor with less than 3,000 miles back down. I'm also not crazy about the rochester because the car was originally Fuel Injected and I've never been able to get the roch to produce a smooth idle or take off (I've had it rebuilt twice in the past 3 years).
Anyway...I'm wondering if there is a way to obtain a fuel injection kit/parts designed for this motor, or if I should just dump the 425 and get a built up LT/LS1 and bolt it up.
 

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'80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
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2,096 Posts
You will not be able to duplicate EFI idle and throttle performance with a carbonator. The only thing that comes close is a Holley and that will gum up your intake valve stems and crap up your gas mileage. There is only one way to prove this and we have both done it.

Almost all performance increasing schemes that you could apply to the engine will shift the torque curve upward. This means more horsepower at the expense of low end torque. Say goodbye to fuel economy.

How about leaving the engine alone and trading your rear axle for a numerically higher unit. Then retrofit a 4L80e overdrive transmission with a stand alone compuker from a diesel vehicle. Careful shopping will leave you with a car that still gets good economy without spending a ton of money trying to make your 425 into a racecar engine. A 3.23 gearset with posi will wake your car right up. Headers are goos because they can actually increase economy and performance if used properly. Problem is they are expensive and installation is usually pretty nasty.

Google megasquirt when you are good and tired of that toilet bowl venturi sitting atop your engine.
 

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1979 Coupe Deville, 1995 FWB & 6 Other Non-Cadillacs.
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759 Posts
Discussion Starter #3
I should of mentioned that i'd like to keep my TH400 and 2:28's. I am interested in that Megasquirt EFI unit though. I'd like to find someone who knows what they're doing to build it up for me.
 

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'80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
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If you are not interested in swapping transmissions and axles then drop in a beater 500. Source one in a junkyard. The whole car will wake up with extra torque and more weight underhood. A nice feeling. You will not be able to use all the power from a standstill without a posi though. Wet roads will become a chore. Mileage will decrease but not all that much if you baby the car.
 

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1992 Eldorado Touring
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I agree, pick up a 500, 472, or even a Buick 455 (if you can find one cheap) and it will bolt up to the TH400.

The 425 is a horrible motor IMO. My Dad's '78 Collectors Edition Eldorado had the 425 and with the FWD, it was such a slug... all the displacement for nothing.
 

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1979 Coupe Deville, 1995 FWB & 6 Other Non-Cadillacs.
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759 Posts
Discussion Starter #6
I'd be thrilled if I could find a fuel injection setup for the 425. It was already removed when I bought it, so I would need the computer and injectors I assume.
I was wondering if anybody knows of an aftermarket setup that would be compatible with the 425.
 
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If you still have the EFI pieces the Megasquirt will be a good choice. Even if you don’t a TBI will work too. You can buy them pre built, but you still require some skill in getting it all working. If you are one of those non-computer type people you should probably leave it alone.
 

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Find someone who knows what they're doing with that Quadrajet. They're the most complicated but the best street carbs around in my opinion. Then run something like an MT5 or 10 cam. The stock cam is what really kills these motors from the factory (all of them, 500-368,) there's nothing inherently bad about them other than that toothpick cam.
 

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1979 Phaeton Coupe, 1990 Brougham d'Elegance 5.7 liter
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4,416 Posts
Anybody that says that the 425 is a horrible engine must have had some real carb trouble, once I got mine straightened out, I love it! But it took six months and two mechanics to get it just right. It isn't "fast" but it can still roast a rear tire on level dry-land quite nicely. And I never have trouble keeping up with traffic, opening the secondaries at 30 throws me back in the seat, how can I complain? If my 425 ever dies, I'll consider tossing in a fresh 500, but until then I just couldn't bear to ditch my reliable baby big block for a few more ponies.
 

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'80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
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Anybody that says that the 425 is a horrible engine must have had some real carb trouble,

425 was an excellent engine for what it was designed for. Deliver the best economy possible while used in a 4000 lb car while still having good driveability. No overdrive in the development budget at the time. Only barebones electronic engine management and THAT was even nixed.
That said, the 425 is not as beefy as a 472 or 500. They went on a diet. Still would outlast many other engines built at the same time. High nickel content block and based on a design which was more modern and an improvement over other V8s of the era.

No they didn't make the earth shake. Try selling something that would in the late 70s.
 

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79 coupe deville d'elegance (76k og), 84 coupe deville
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114 Posts
also in the same boat, but not looking for a engine swap. in the middle of a overhaul with my 425. never seen anything this nasty before!
 
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