[Mike S]

I am going to take my 72 back to stock. It did go 11.70's but haven't raced it in years. In the next few years, I am planning a coast-to-coast trip with it.
The 455 will be coming out along with the th400. Plan is to leave the 3.73 gear and install an overdrive auto.
As far as engine I have a few choices I need opinions on.
1.) I have a 455 block and was thinking of getting a 4" stroke crank to get a 428 with 9,5 compression with the 6x heads that are on the car now.
2.) I have a 389 that I can build and have 8.5 comp with my heads.
3.) I have available a v6 engine and trans out of a 06 gmc truck.

Which engine do you think would be better for a road trip?
I am thinking the 389 but don't know if it would get near 20 mpg.

[Schurkey]

The modern V6 and transmission would be the best for highway cruising.

Also best for desecrating the car.

[nUcLeArEnVoY]

As long as the engine is built somewhat conservatively and within reason (i.e., streetable), any Pontiac mill should work fine. Maximum cruising efficiency and comfort is more contingent on gearing and transmission, so think about maybe going down to a 3.23. You're playing it smart by upgrading to an overdrive trans. That'll make the biggest difference of all. This calculator is useful for determining what RPM you'll be making at a given speed. Just plug in your rear gear, the ratio for the given transmission gear, your desired speed, and your total tire diameter and it will give you the RPM you'll be running at. If you're already above 2000 RPM by the time you hit 50 mph in your last gear, she will be a bit of a screamer on the freeway and engine temp will elevate quicker (but by no means should it overheat).

[Richie Hoffman]

I built a 400 for friend yrs ago 70 gto 400 turbo 400 3.08 gears. 9.25 comp custom hyd roller cam
Cast intake Qjet got 20 to 21 mpg

[Goatracer1]

Does your Fbird have 3 bolt motor mounts? If so the 389 would require adapters. Maybe you can swap for a good 350. That with OD and EFI would be a great highway combination.

[Charlie Brengun]

Or leave it a 455 and go 3.55 gears with an overdrive? I had a 73 caddy with a 472 and it was good for a cruiser, I bet with a slightly higher compression ratio and well chosen cam and properly rebuilt quadrajet it would have gotten 20 mpg. I just never got it right but didn't know what I've since learned here..
I'm sure a Pontiac 455 with the right cam for gear ratio and compression ratio and perhaps a 355 geared rear end with a 4l80 with a good torque converter will get you a very nice cruiser..

[Formulajones]

The overdrive and gear selection will be the biggest key to mpg.

About any V8 will get you where you want to be mpg wise. A mild 455 will do it, 428, 400, what ever you decide.

I like to set them up so cruise at 70 mph is about 2000 rpm. That seems to return me respectable mpg and cross country trips are easy, even if you want to pick up the speed a bit on those long highway stretches.

I set the Z up with 3.55's and a .64 overdrive, and with a 27" tire, 70 is 2000 rpm. It gets 21 mpg and doesn't care if I go 65 or 85 mph, the gas mileage is the same. That's with a pretty fat running 780 Holley that I haven't bothered to lean out at all.

Dad's GTO, with a 571 that makes over 700hp is getting 15.6 mpg highway. I consider that pretty good for what it is. Very simple, 4L80E behind it with a lockup converter and 3.42's out back with a 28" tire. This one has EFI throttle body so I can fine tune the AFR in specific cruise areas, which I haven't even refined yet. I think there is a pinch more in it.

I did another 4L80E in my 600hp 454 Chevelle, lockup converter again, but I still have 4.10's in it with a 28" tire. So 65 mph is 2400 rpm. I generally cruise 70 mph at 2600 and it's gotten me almost 16 mpg. If I knocked 400rpm off the cruise speed and refined the AFR a bit, I'd see a nice gain in mpg.

Just for reference, my stock 400 RAIII firebird with 3.31's and a 400 turbo will get 17 mpg if I'm easy on it, cruising at 70 mph. I stuck a 28" tire on that one too. An overdrive would certainly get me 20 or more.

If you're planning an auto overdrive, you might want to knock the 3.73's down a bit, most auto overdrives don't offer much gear reduction to get you where you want to be.

[Formulajones]

Originally Posted by Charlie Brengun

Or leave it a 455 and go 3.55 gears with an overdrive? I had a 73 caddy with a 472 and it was good for a cruiser, I bet with a slightly higher compression ratio and well chosen cam and properly rebuilt quadrajet it would have gotten 20 mpg. I just never got it right but didn't know what I've since learned here..
I'm sure a Pontiac 455 with the right cam for gear ratio and compression ratio and perhaps a 355 geared rear end with a 4l80 with a good torque converter will get you a very nice cruiser..

[Mike S]

Just checked your right about the 389 motor mounts. Took the v6 out of the decision. I would like to keep the 455 for a spare for the race car. I am going to look at a 74 400 a buddy has tomorrow. It is complete carb to pan and all the brackets for $500 supposably rebuilt.
Need to find a 700r4 trans. Don't want a computer controlled trans.
Decided to change the gear to a 3.31.

Also going to install a vintage air system and a Dakota digital dash.

I put the Dakota digital dash in my 70 camaro and really like it.

[Formulajones]

That would be a nice setup. 3.31's with a .70 overdrive and a 27" tire would get you 70 mph at 2000 rpm with a converter locked up. Should easily be 20 mpg when dialed in.

As long as the engine isn't so radical that a lockup converter isn't an issue. The only problem with TV cable style auto overdrives is there lack of adjustability when it comes to lockup. The vacuum switches only swing so far with a small window of adjustment. So as long as the camshaft is mild enough it's not an issue.

That's one area where the electronic transmissions shine, with infinite lockup adjustability, it works with even the rowdiest of camshafts.

[77 TRASHCAN]

I vote for the 389 or 400. My TA, when stick got 20 MPG and did as good as 22 MPG. many times, mind you though with a c 2.41 gear.

389-400's have enough torque to easily move a fairly heavy around with ease. Steve Barcek, years ago built A well thought 301 Pontiac, I believe to put in a 66 Lemans, to haul water with? I don't remember the MPG with the 301, but believe it somewhat missed the mark?

He put a 455 in it with a 068 cam and got 20 MPG. He said ( Steve, where are you?)

I posted on that thread about me having a 283 built. And even dyno"d for my very first car, a 1967 Chevy Bel Air. Spent good money on it, still got it..it did 19 MP, but had more power (2.73 gear). In my opinion, the engine just didn't have quite enough torque to EASILY move the car around, especially at highway speeds. I unknowing passed up the chance to have a small block 400 built for.my car, LOL I gave it to a good friend.e had it built by the same engine builder for his 1966 Chevy C10 pickup. That thing ran like a bat-outta-Hooterville. Got 12 MPG with a 3.42 gear. And a Muncie M20 and weighed 5000 pounds.

I will build ANY engines, anymore smaller than 400 cubes.

[Steve C.]

Stated... "As long as the engine isn't so radical that a lockup converter isn't an issue. The only problem with TV cable style auto overdrives is there lack of adjustability when it comes to lockup. The vacuum switches only swing so far with a small window of adjustment. So as long as the camshaft is mild enough it's not an issue."

X2

I can highly recommend the Bowtie Overdrives TV and Throttle Cable SYSTEMS.
That is we used on my 200 4R

http://www.bowtieoverdrives.com/

.

[Singleton]

Originally Posted by Mike S

Just checked, your right about the 389 motor mounts.

389 shouldn't require an adapter to go into a 72 Lemans. unless its a early 389

[Sirrotica]

Originally Posted by Singleton

389 shouldn't require an adapter to go into a 72 Lemans. unless its a early 389

If the car was a 455 from the factory, the mounts on it would be the later 3 bolt style mount, 400, and 350 used the early style 2 bolt mounts 70-72. It all depends on what engine came in the car from the factory.

The other alternative is use the 70-72 frame mounts for 350/400 to use the early 2 bolt frame mounts.to install a 2 bolt style early block. If the car originally came with a 350/400, then it already has the early style 2 bolt mounts on the frame. So if it was originally a 455 equipped car you would have a problem, if not, the 389 mounts are a direct bolt in.

[Singleton]

Originally Posted by Sirrotica

If the car was a 455 from the factory, the mounts on it would be the later 3 bolt style mount, 400, and 350 used the early style 2 bolt mounts 70-72. It all depends on what engine came in the car from the factory.

Agreed, but either way an adapter isn’t required, just the correct frame mounts, if needed, which are easily changed.

[android 211]

You can still buy a lot of gas with the money needed to make all those modifications. The best bang for the buck would be the low compression 74 400 with a mild cam and something taller than 3.08s out back. Any trans you like.

[geeteeohguy]

FWIW, the original 400 in my '67 GTO convertible, with 87cc #15 small valve 455 heads, stock Q-jet, points distributor, with a TH400 and an 8.2 2.56 Safe-T-Track out back consistently gets 21 mpg at a steady 75-80 mph.
Better mileage than my Tundra. I've gotten 23 mpg at a steady 65, but I drive at traffic speeds, which here is about 80 mph. With a 389 or 400 and an OD auto trans, you should be able to get decent mileage. Another advantage is a cool running engine and very little noise going down the road, which makes long drives much less tiring.

[Formulas]

i wouldnt discount the use of a 350 engine either with a 200 or 700 OD. trans
set of 9:1 heads smoothed ports Qjet small tube headers 2800 cam 3.30 gear.

one time i ran that engine with a muncie wide ratio and 2.73 gear. i didnt science out mpg because no gps at the time and i assumed there was no trans speedo gear for it anyways but it sipped gas on freeway

[TA63]

Originally Posted by Richie Hoffman

I built a 400 for friend yrs ago 70 gto 400 turbo 400 3.08 gears. 9.25 comp custom hyd roller cam
Cast intake Qjet got 20 to 21 mpg

What were the specs on your cam and who made it? Doing something similar myself.

[Mike S]

Just picked up a complete carb to pan 74 400 engine. Tore it apart. It must have had some kind of rebuild. Stander bore with stock pistons but the crank was cut 30 on the rods and 10 on the mains. It has new freeze plugs and looks like new cam bearings. Think I am just going to get a rotating assembly from Butler.
The heads are 5c-8 101cc screw in studs if anyone wants them they can have them.