[dogwalkin]

Im gonna try and keep this short.
1950 pontiac with '68 350/700r4 3.73 rear
This is my daily driver I built a few years ago. I "refreshed" the engine but its pretty tired. Runs good but low oil pressure at idle.

Here is where the question comes in.
I have a good 400 that I could drop in. How would that affect mpg? Im getting around 17-18 mpg with mixed driving.

I have a gen 4 5.3 that I had thought about carbureting and dropping in.

I DO NOT WANT TO START A PONTIAC V8 vs. LS DEBATE.
I simply was looking for opinions. Thanks

[Bruce Meyer]

I dont think you will notice much difference with the 400 as long as you have a mild or stock cam in it,

[Jay S]

It depends on how the 400 is set up. What year. Which heads. Compression. cam. ect. I have several cars with bigger 455+ cid engines that are decently economical to drive, up around 20mpg. Not really much different that my modern platform cars.


Would love to see some photo’s. We have 51 Pontiac.

[JLMounce]

For a given weight, drag and gearing, the amount of power to sustain driving costs roughly the same, regardless of how much power the engine itself makes.

If nothing else is changing and you aren't putting a 400 in it that is more radical than your 350, it's unlikely that you'll see less fuel mileage. If the 400 makes more power and you find yourself accelerating harder more often to use it, that's where a fuel mileage decrease would come from.

[Formulajones]

I've daily'd my stock RAIII 400 formula for years. It's a 400 turbo and 3.31's and it gets 17 mpg highway and 14 around town. If it had an overdrive it would easily do over 20.

I built a 383 recently. Chevy example but similar cubic inch. It's semi mild (470hp) with AFR 195 heads and a smallish 224 @ .050 roller cam, 750 Holley on an air gap intake. I put a 4L60e behind it and 3.50 gears. It gets over 15 around town and a hair over 20 highway. So a decently modified engine in the 400 ci area can do exceptionally well on gas.

[mgarblik]

If the 400 vs the 350 are very similar builds, cam, compression and so forth, the gas mileage will be virtually the same. The 400 will make more power when needed. Driven the same, MPG should be the same. As far as the LS swap. Do your homework before you make any decision. I have had 2 students build LS engines for old Hot Rods and wanted to go with a carburetor set-up. The backward swap was VERY expensive. By the time you buy an intake manifold, a decent carburetor, and finally a stand alone controller, cam and crank sensors, 8 coils, wires, a low pressure fuel pump. It really adds up. They had thousands invested in going backwards. Would have cost much less to use an aftermarket injection system.

[Half-Inch Stud]

Over drive makes such a good difference. 2.54:1 or 2.73:1 also good for mpg.

[dogwalkin]

Im thinking my best bet is to just use the 400.
I've got several sets of heads and a 2800 cam to use.
Ive got the '68 15 heads. Ive got a good pair of 17 heads on the 350 and a nice set of 6x4 heads. Also have a set of goo 670 heads. But I dont want the compression too high because I wanna run cheap fuel.

[b-man]

Put the 6X-4 heads on it for 8.2 to 8.5:1 compression.

Along with the 2800 cam it’ll be a nice powerful 87 octane friendly daily driver.

Essentially the same engine I once used daily in a ‘69 GP:

https://forums.maxperformanceinc.com...1&postcount=22

[F ROCK]

My oil pressure was low at idle when warmed up and i was quite concerned. I found that my old sending unit was bad. It SEEMED to work perfectly; but someone suggested that I try a new one because they are inexpensive. The new unit was like $15. Now my hot Idle is reading 20lb higher.
My engine didnt SOUND like low oil pressure... but that meter was scaring me.
Might be worth trying a new sender. I hope that's all it is.

[steve25]

Don’t even consider using the 670 heads as there chambers burn very inefficient and proof of that not only is there BSFC as seen during a Dyno test, but there need also of 3 to 4 degrees more timing to make the same power at any given rpm as a 68 and up Chamber.

The deeper chamber of the 6X heads promotes better intake pressure recovery then the earlier higher comp heads.

If gas mileage is being talked about then you need to look a efficiency and where that all starts.