The 425 isn't going to be much cheaper to operate than a 500 if used in the same chassis. A good 500 driven conservatively will get pretty much the same fuel economy as a 425. If you pound it the story changes. The bottom line is vehicle weight, gearing and aerodynamics. You cannot obtain highway mileage with a '77-79 RWD Cadillac in excess of about 21 MPG without changing one or more of these. I've owned several cars over the years that started with 425s and landed up with 500s. Each rides better due to the additional weight of the 500. It's more than 40 Lbs difference. I have not weighed any but I'd guess it to be more like 100-125 Lbs difference. GM put their engines on a diet during the 425 years. An interesting swap would be a drivetrain from a 96 or so FWB into a 10% lighter HT-4100 powered RWD Fleetwood. That would probably reach the limits for fuel economy.
500 C.I. equipped RWD vehicles used a Cadillac rear axle that was different than corporate axles. The gear ratios were something like 2.93:1 or optional 3.15:1 .

For the largest pre-war engines, check here: en.wikipedia.org/wiki/List_of...e_superlatives
Go to the very bottom of the page. Largest is a 21.5L 4-cylinder!

Sadly, even the 500 has been surpassed recently. By 1%... 2003 Viper with 505ci.

anyways, back to the thread...

A 500ci motor will make more power and torque than a 425 given equal emissions equipment and exhaust. 75ci of extra fuel and air being pulled in each intake stroke will make a difference. It's the same reason that people stroke their motors to go faster. Just because the factory only had 180hp out of a hugely de-tuned 500ci motor does not show the motors potential. By the same token, you could swap in any number of V6's made in the last 20 years that also make 180hp and then build that up, but it wouldn't get you the goal you are after.

I might just have to snag a 425 out of the junkyard and weigh it... I actually re-checked my data and found that the 472/500 motors are more than 60# heavier than a 350 SBC, but if a 425 comes in light enough. hmmmm...

I guess the direction of this thread shows the fate of the 368. Red-headed stepchild of the bunch I suppose. Only a product of the fuel economy requirements that started in the late '70's and probably never would have existed otherwise. Cadillac probably could have kept the 425 motor up with emissions regs for longer with better EFI systems, but that combined with fuel economy requirements forced Cadillac to make do with shrinking down their big block untill a new engine family could be designed.

Logically, I suppose one would put in a 400ci Chevy smallblock before a 368. The 400 motor weighs less, costs less, has tremendous aftermarket support, etc... But, that wouldn't be a Caddy motor then, and that's the whole point here. I'm pretty sure you already have a bellybutton haha.
I suppose if you just can't find anything larger than a 368 to replace the 4100, then go for it. Coming from the 4100, it will probably feel like a sports car anyways. 1.9L more - it's like you've got an extra engine under the hood! Of course, be prepared to long for even more power and the urge to swap in a larger Big Cad when you find one

<br><br>Post edited by: Pariwish, at: 2007/07/30 09:22

Quote:
For the largest pre-war engines, check here: en.wikipedia.org/wiki/List_of...e_superlatives
Go to the very bottom of the page. Largest is a 21.5L 4-cylinder!

Sadly, even the 500 has been surpassed recently. By 1%... 2003 Viper with 505ci.
Nope!

I said \"largest production V8 used in a car\" and the 500 still holds that fame. None of those pre-war engines mentioned were production, they were all low-quantity hand built jobbers.... same witht he Viper... that is not a production car, or engine.... the 500 was.

Shes still got the claim to fame .

The internet can be a dicey place to get correct information. The 472/500 can be found in several different places weighing anywhere from 625# to 720#. Same charts show the sBC at 575#.
Here's one:

Scroll down when you get there.

en.wikipedia.org/wiki/Cadillac_V8_engine#425

The thing that worries me is they say that the 472 was heavier than the late 429. Anyone weigh a late 429???

429 was more heavy then the 472.... thats one of the many \"advancements\" they didhad with the \"modern\" engine at the time..... dunno why it would say that...