[aguila1]

Tested 1/31/07 - 40 degree ambient temp. Drove very conservatively on secondary highway and got only 8 mpg. Noted K&N A/F meter reading somewhat rich. Specs:

1979 Trans Am, no AC, 3.42 gears, TH-350, 1973 455 w/6X heads (9.25:1), Crane Energizer 272 H10 (454 lift, 1.5 rocker ratio, 216 degree at .050, 160 degree thermostat and blocked heat risers, Edelbrock Performer intake, Q-Jet carb, open shaker hood; stock log-type w/H-pipe, 2.5" duals, Dynomax turbo mufflers, 10W40 oil.

Before changing to 3.42s from 2.41s best was 15-16 mpg. Plan on overdrive - either 200 4R or 700 R4.

Should gas mileage improve by putting in a hotter thermostat?

Can the carb be made to run leaner while cruising, but still put out maximum power at WOT?

Shooting for 18-20 mpg. Is this possible with this combo?

Thanks for your input!

 

[pasadenahotrod]

You'll be lucky to get 12-14mpg with that big block. unblock the heat risers, put in a 180 degree thermostat and you should get some improvement.
If you're looking for 18-20mpg you'll need fuel injection and an OD tramission.

 

[hotrod389]

you didnt mention ignition.. install a MSD kit a hotter spark will give you a more complete combustion.
what series q jet are you running? is it an 850cfm?
you got a 455 for a reason.... and that reason is not for fuel economy.
i get 8-10 mpg on my 389..
if you drive mostly on the freeway, i would convert to a 6 speed manual.
good luck...
and if you want 15 MPG, i will trade you that 455 for a 68 354.

 

[aguila1]

Fuel economy vs. maximum efficiency

Clearly a 455-powered muscle car is not built for economy, but I'm seeking to optimize the combination without any loss in WOT power. Others claim as high as 22mpg with this engine/car combo. The ignition is factory HEI, unsure about what series Q-Jet, though it is probably stock 1973-1979 400 vintage... Just bought a 195 degree thermostat to help combustion efficiency. The heat risers are blocked with aluminum poured in the 6X heads. I am hoping the open shaker will cool the charge while the combustion is more complete with higher operating temperatures. I will post the results of this change and performance figures once tested. Thanks.

 

[428ho]

Not sure you'll ever break 20 with a 455, not on a consistent basis anyway.
The Overdrive would help, re jetting the carb might help. With your combo and keeping the secondaries closed I'd think 14-15 would be average.

Looked at your pics, was that taken at Jocasse?
If so, it's no wonder you only got 8 mpg, no one can keep their foot out of it on Hwy 11. :mwink:

 

[grandpaws 57]

mpg

The best fix is an OD ,HONE OD would handle power good,and you could split gears.I just put an AOD in my 57 ford ,gets 20mpg+ with 351w.

 

[pasadenahotrod]

Yeah, I've often heard those big block rodders claim 20-30mpg going from Houston to Colorado and back at 80mph.
I didn't believe them then and don't believe them now, even at 55mph.

A friend bought a motorhome back in the 70s with a 350 SBC in it. The MH was a 22foot Class A and was pulling a fully loaded parts trailer to swapmeets nationwide. Average mileage with trailer 6mpg, average without trailer 10mpg. Wore out the 350 in 3 seasons, built a 400 SBC and installed it. Average mileage figures didn't change but vehicle had substantially more pep. After 2 seasons, bought a factory fresh 454 from a rollover Corvette and installed it.
Average mileage with trailer 4mpg, average mileage without trailer 10mpg. Changed torque convertor to RV special and installed RV grind camshaft. Mileage increased to 8mpg pulling trailer and not pulling trailer. Never wore that one out.
New motorhome is late 70s 26' Class A with 500cid Cadillac. Mileage averages 8mpg. Can't complain for a carbureted engine pushing a wall of air down the road with the Caddy just loafing along. It's nice to have restaurant, motel, and bathroom with you on a trip and plenty of WalMarts to park at overnight.

 

[aguila1]

Not an RV

428ho - right about Hwy. 11 - it is a bit hilly and still a great-looking road and fun road!... I wouldn't expect a heavy unaerodynamic brick to get decent mileage, as Pasadena mentioned about RVs, but a TA is a whole lot slicker. I think 15 mpg with the 3.42s would be a great accomplishment and goal... Spoke to my carb guy and he suggested turning the mixture screws in a quarter of a turn. On crank-up today, after the cold weather run two days ago, black carbon and condensation spewed onto the garage floor. That would be indicative of an overly rich mixture, would it not? More testing when the weather gets nice. Hope to have it a little more dialed in for the drive to the Atlanta World of Wheels show coming up.

 

[rondv]

Pontiac 455

Hi aguila1,
Sympathize with your goal. It certainly can be done. I've had many ponchos over the years. My last one - 1975 formula 400-4sp got 20 mpg driving easy on the highway(60 mph). Had 342 gears and turned 13.9 in 1/4 mile. Should mention that's on a Canadian gallon which is bigger ; would probably equal 18 mpg U.S.
I now drive a '70 SS454 with 9.5 comp., 540 lift cam, holley carb, 700-r4, 3000 stall w/lockup, and 3.31 12-bolt. Took a lot of carb work but get 18 mpg (probably 16 mpg U.S.) @65 mph with air on. Also got 12.82 1/4 mi but that was with my Holley 850 dp installed.
Upgraded HEI should do you fine. I don't think you can get a stock type 700-r4 for a BOP unless you buy a racing bell (called a super-bell) to put on it ; can be done, but expensive.
You'll definitely be looking for a 200-4r, which will be easy to find, (olds motored RWD gm's from 86 - 92. If you're going to drive it hard parts are available to make them quite strong but probably not enough for a strip car because of the 455's torque. Also, cheap gm parts can make a computered 200 work in a non- computered car like your,s. If you upgrade the convertor be sure to retain lock-up ; this is where your mileage will come from!
I liked the Holley because it is so easily adjustable and probably because I know them well and have parts. The quad can definitely work just as well and probably get even better mileage but, IMO, are not as easily tinkered with..
Don't know what intake you use. Stock is a fine design but is VERY heavy and holds a lot of heat especially in warm weather. A stock Pontiac aluminum intake or an aftermarket aluminum is a better choice. My experience was that single planes were just as driveable and efficient as a dual plane in the Holley/ Edelbrock lines (never had a Warrior though).
Hope my lengthy response didn't offend you ; good luck on your project.
- Ron D.

 

[aguila1]

Malfunctioning electric choke

First, I'd like to thank all who contributed their thoughts on gas mileage improvement. Mark Byars, my carburetor specialist, checked the function of the electric choke and despite getting juice to the wire, the new piece was inoperative, causing the choke to stay closed, creating the overly rich mixture. The factory set-up worked with heat from the intake, but all that is blocked-off in the search for a few extra ponies, so an electric choke was used in this application. On the trip down to Atlanta, the car improved to just under 12mpg, buzzing the motor between 3,000 & 3,500 rpm. On the way back, I was losing a lot of oil from the valve cover breathers, so I cut the speed way back to 2,500 to 3,000 and did around 14.5 mpg, letting everyone pass me on I-85. I'll keep fine-tuning the motor and post the results here... On another happy note I took Best of Class in the ISCA World of Wheels in Atlanta - a totally unexpected outcome for this home-built at its first show! Now to work on getting that sweet overdrive...

 

[home brew]

Congrats on the Best of Class in the ISCA World of Wheels in Atlanta . Had to go to your photo journal (I don't know how I missed it before) to see some pics of the car without the eye candy in front of it? I appreciate the Hooters but I wanted to see the car too! Great looking ride!!! :thumbup:


Another member on the board - risaacp - did pretty good at the ISCA car show in Abilene:

http://www.hotrodders.com/forum/journal.php?journalid=18542&action=view

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[jray]

Was there a car in that picture?? :thumbup:

 

[xntrik]

Nice trophies!!!!! Congrats.

To get decent mileage, you need to cut your cruise rpm in half. Six speed?

I have ridden in those cars that get claimed 22-25 mpg on the highway.
With me along, they get 16. Guess it's the extra weight.

 

[Bryan59EC]

my 79 chev c-10 with a 454 will get 14-15 on the hiway
truck 4-spd and 3:08
At about 70-75 it is turning 2700 (according to stock tach)
The instant the secondaries start to flutter---fuel mileage is gone.

6-8 pulling the trailer

Bryan

 

[ChevelleSS_LS6]

I love those T/As! Yours is a fine example... regular OD trannies aside,

Gear Vendors OD... basically a 2speed unit that goes behind the regular trans. I guess get it geared to run like it has 4.10s or something in one gear at the track or whatever and on the hiway like it's got 3.07s. Kinda pricey though.

 

[vintageautoparts]

mpg increase

I agree with chevelle go with the gear vendors unit I've heard nothing but good things about them as to dependabilty and durability even in high HP/ tourque applications a bit pricey but probably no more than a trans swap

 

[aguila1]

Latest update

2.41 gear swap from 3.42s nets 15-mid16s on the highway. The TH-350C slips and the lock-up is not hooked-up. So, there's a little more mileage to be gotten - 18? I don't know.