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1979 Sedan Deville
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448 Posts
Discussion Starter #1
I've got my stock L33 (425 carb'ed) in my car now. Runs pretty nice, added 500 EFE valved manifolds with a 3" cat split to dual 2.5" exhaust. Could be better with a carb rebuild, new cam, and some head work, and I also have a bunch of parts I've collected. I have an aluminum 368 carb intake, a dual plane 500 intake, and a 425 EFI intake complete with distributor and computer. Also have a set of V864 solenoids and valve covers.

But, I really want my car to get 25+ mpg highway. I don't care about city, just highway, and the more power the better. I love tinkering with old stuff and seeing what I can make it do, and I'm a web developer and I'm hot with computers, wiring, etc, but I don't know how much time I want to spend on all of this, and then still not quite get there. I refuse to do anything that will add weight to the front of the car, so 472s and 500s are out, plus they would just get worse mileage anyway. Also, living in California means I still have to smog this thing, although a few years ago a bill went up for a vote to move the cutoff year to 1981, but it didn't pass.

Sure, maybe I could build a pretty hot EFI 425 with a built overdrive behind it, and get real close, but with all the time and money and passing smog worries, why not just swap an LS3? Tons of Impala SS guys are doing it, you can buy engine mount adapters, and there's all those build threads to refer to. It won't sound exactly the same, but I kind of like the way those engines sound in the Escalade, and these cars have a long enough exhaust that it might be close. I would imagine in a wild dream that with AFM and keeping a tall rear end, I could easily beat 25 mpg, if not 30 if I cruise at 55.

Votes?
 

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95 FWB 81SDV 96 FWB 94 Fleetwood
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1,474 Posts
To me the only way you can get to your 25 highway miles is get a motor and trans from a 94 or 95 fleetwood with the PCM to run it all.. Then with your 2.28 rearend you might get there.. Just find a donor car.. To me there is no way your going to get 25mpg with a 425 in a full size Caddy..
 

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70 Deville 77 Fleet 78 Seville 92 Deville 03 Deville
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3,310 Posts
It's an LS world now. All the cool kids are doing it. They sure are homely though. Just have to keep the hood closed. I walk right by those ugly engines at car shows. The power is probably fun in the end but it seems there are a lot of little odds and ends needed for the swap that nickel and dime you to the finish. I can't see it beating a Northstar in fuel mileage either. That's what makes hotrodding though; building your car the way you want it. As always, keep us posted.
 
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