454 Big Block MPG

[Wumbo]

I get 10-12mpg with mine.

3.73 gears, stock tire size, SM465

Engine has 049 heads, comp 11-235-3 cam, cast iron oval intake, original quadrajet.

 

[staticattic]

I was first thinking this thread is like Fire and Ice. Big Block and MPG just don't go in the same quote.

yeah, i figured it would raise some eye brows. i tried to be very careful how i asked my question. granted, i am new to the square body world, but i’m pretty sure no one buys a square body with the idea that are going to make that little dutch girl smile. they buy them for the coolness factor. prior to my square, i had a jeep. on a good day i could almost squeeze out 13mpg. but i knew that going in. no one buys a jeep for fuel economy, they buy it for the coolness factor.

everyone in this thread has provided good ideas and a good idea of what i *should* be getting. i’ve got a lot of work to do.

 

[Catbox]

My kid drives my truck with its 454 and 4 speed.

He complains he gets 8 mpg around town, but he is 17 and likes the noises it makes.
That is probably my fault as I have always had fun cars that sound good.

If I could get him to quit winding the thing up and dipping into the secondaries I am sure he would be able to get it up to 10-11 mpg.

Soon we have to pull the intake as it is leaking.
At that time we will be revising all the vacuum lines as we have a better intake for it.
Maybe that will help the mileage out, probably not.

Then we are going to install headers and new exhaust.

 

[idahovette]

My kid drives my truck with its 454 and 4 speed.

He complains he gets 8 mpg around town, but he is 17 and likes the noises it makes.
That is probably my fault as I have always had fun cars that sound good.

If I could get him to quit winding the thing up and dipping into the secondaries I am sure he would be able to get it up to 10-11 mpg.

Soon we have to pull the intake as it is leaking.
At that time we will be revising all the vacuum lines as we have a better intake for it.
Maybe that will help the mileage out, probably not.

Then we are going to install headers and new exhaust.

Sounds like you are adding fuel to the fire!!! ….LOL

 

[JohnRM]

78 GMC Shortbed, Sepside, 454, M465 4 Speed, 308 Rear Gear, Sniper EFI. I get 9 MPG. It doesn't matter highway, city, pulling a loaded car trailer all the same. With a carb I was getting 8 MPG.

 

[Kevin S]

Might mention the possibility of a fuel leak and all vacuum hoses and gaskets.

I have fresh rebuilt 454 SM465 with 3.21 rear end & 66mph@2250, getting 12.21mpg.

The rebuild included increased compression to 9-1 with new pistons, changed from peanut port to oval port heads, Isky 112LSA 208/218 cam/dual plane edelbrock intake, new HEI distributor. Built for torque, using Thorley Tri-y 303cy headers. Very nice combo making ~325hp/450 torque@flywheel (chassis dyno showed 260/385 2200-peak power @2900rpms. Very happy for a modest torque build and driving easy & using the power down low, staying out of the secondaries.

Next step in the plan for me would be an Edelbrock Pro-Flo 4 MPI fuel injection sytem, and my dream of a adding a Ranger Torque splitter auxilliary gearbox for 30% overdrive, and changing the rear end to say a positrac 3.73. Lower gears for towing, more gears to choose from, and at highway speeds of 75mph could keep the engine about 2200rpm at the bottom of the torque peak for efficiency. The way its geared now it is already there, so hard to really justify the expense of 2nd gear box as I have nothing to tow... yet.

I think as budget permits ($2K) the fuel injection is a good idea and the hope would be to make it easier to cold start, more drivable, always in tune with elevation changes. My hope would be the mileage could be taken closer to 15mpg if not towing, half that depending on load.

 

[clendon1]

My 76 C10 stepside, 468,alum heads, big cam (?), TH400, 456gears, 31x16 tires, 750 Edelbrock, gets 6-7 if I stay out of it, I consider that OK for what it is.

 

[80BrownK10]

I don't have a big block. I have a k10 350/350/with205, 33s and a 3.08 rear. I get about 10 normal driving and if I baby it and stay 60mph or slower and just highways I get 12. Towing my tractor on a 16 ft trailer total weight about 5k or a full load of logs about 8-10k I will get 8mpg. I think when I towed my 24ft pontoon with it down the highway it was in the 7-8 range, but that thing is like towing a wall back there.

He may be out of the powerband and lugging it but not at 55-60mph too bad.

I would say that your driving style a d set up I would think you could get 7-10.

 

[HotRodPC]

Might mention the possibility of a fuel leak and all vacuum hoses and gaskets.

I have fresh rebuilt 454 SM465 with 3.21 rear end & 66mph@2250, getting 12.21mpg.

The rebuild included increased compression to 9-1 with new pistons, changed from peanut port to oval port heads, Isky 112LSA 208/218 cam/dual plane edelbrock intake, new HEI distributor. Built for torque, using Thorley Tri-y 303cy headers. Very nice combo making ~325hp/450 torque@flywheel (chassis dyno showed 260/385 2200-peak power @2900rpms. Very happy for a modest torque build and driving easy & using the power down low, staying out of the secondaries.

Next step in the plan for me would be an Edelbrock Pro-Flo 4 MPI fuel injection sytem, and my dream of a adding a Ranger Torque splitter auxilliary gearbox for 30% overdrive, and changing the rear end to say a positrac 3.73. Lower gears for towing, more gears to choose from, and at highway speeds of 75mph could keep the engine about 2200rpm at the bottom of the torque peak for efficiency. The way its geared now it is already there, so hard to really justify the expense of 2nd gear box as I have nothing to tow... yet.

I think as budget permits ($2K) the fuel injection is a good idea and the hope would be to make it easier to cold start, more drivable, always in tune with elevation changes. My hope would be the mileage could be taken closer to 15mpg if not towing, half that depending on load.

Great point about the fuel leak. Check the tops of your tanks right behind the cab. Those hoses dry rot and will leak just enough you don't even realize it until it gets bad enough it's dripping off the tank under the truck. IIRC, I had to loosen my bed bolts and slid the bad back about 8-12 inches to gain access because I damn sure didn't want to drop the tanks. But I replaced several of my hoses and my gas did last a noticeable amount longer. That was before I ever did an MPG check and had my speed and odo checked for accuracy so I can't say how much I gained but it was noticeable to the wallet.

 

[Kiely]

I’ve been getting 8-10 mpg on my 454 with a 4 barrel carb. That is with the trailering package which is geared for power, though a 74 454 was only about 230 hp.

 

[dsteelejr]

I am also looking at replacing portions of the ignition system as well.

Not knowing what you have or have not done thus far, I would start with the timing. It is often overlooked and underestimated what it can do for power and fuel economy. Timing is not just a set-in-stone number. Each engine has a sweet spot for where it will run best based on the carburetor, compression, etc. and although it takes some time and a few test drives, finding the sweet spot will help a lot. I have a 454 in my '80 GMC that is out of a mid-70s truck and was desmogged before I got it. Because it has been modified I can't reliably go by the timing settings in the manuals or emission decal. Based on my experience before you start throwing money at new ignition parts take some time to play with different timing settings to find out what works best for your setup, unless you are in an area that has smog check and you have to keep it at a specific setting. Also, pull off the cap and rotor and check for carbon build up on the electrical contacts and look to see that the mechanical advance is working, specifically check the flyweights. I found that mine were not free. They were gummed up and not flying out or returning in freely. I had to clean them up and lubricate them and it made a world of difference at highway speeds because I wasn't getting advance at higher RPM and that led to wasted fuel and a lot of power left on the table. Use a dial back timing light to check your advance at high RPMs to make your advance is all in by around 3,000 to 3,200 RPM or so.

 

[emoze]

I am thinking I should be able to at least squeeze 10 to 12 MPGs. Is that about normal?

my C20 454 camper special with turbo400 & 3.73 axle & 16" tires has gotten 10-12 consistently for last 35 yrs no matter if pulling 4-horses or a 6000lb car trailer or running empty (city vs hwy irrelevant - always the same)

(& that's with both the oem cam & a rebuild @ 205,000 miles with RV-10 cam, Edelbrock manifild & dual flowmasters)

- i recently installed an old school MSD6A & that makes it start/idle much better, but have not checked mpg since (i don't drive it anymore for enuf distance to do a proper check)

 

[HotRodPC]

Might mention the possibility of a fuel leak and all vacuum hoses and gaskets.

I have fresh rebuilt 454 SM465 with 3.21 rear end & 66mph@2250, getting 12.21mpg.

The rebuild included increased compression to 9-1 with new pistons, changed from peanut port to oval port heads, Isky 112LSA 208/218 cam/dual plane edelbrock intake, new HEI distributor. Built for torque, using Thorley Tri-y 303cy headers. Very nice combo making ~325hp/450 torque@flywheel (chassis dyno showed 260/385 2200-peak power @2900rpms. Very happy for a modest torque build and driving easy & using the power down low, staying out of the secondaries.

Next step in the plan for me would be an Edelbrock Pro-Flo 4 MPI fuel injection sytem, and my dream of a adding a Ranger Torque splitter auxilliary gearbox for 30% overdrive, and changing the rear end to say a positrac 3.73. Lower gears for towing, more gears to choose from, and at highway speeds of 75mph could keep the engine about 2200rpm at the bottom of the torque peak for efficiency. The way its geared now it is already there, so hard to really justify the expense of 2nd gear box as I have nothing to tow... yet.

I think as budget permits ($2K) the fuel injection is a good idea and the hope would be to make it easier to cold start, more drivable, always in tune with elevation changes. My hope would be the mileage could be taken closer to 15mpg if not towing, half that depending on load.

I had a 454/TH400 with 3.21's in it. Set the cruise at 65 on stock tires, the tall skinny ones from the factory. I got 13.2mpg and the speedo is 100% accurate according to GPS and odo correct according to mile markers. You should be doing better with an SM465. Tire size matters alot thogh if you have bigger than stock size tires. People don't realize even just a bit wider tire is heavier, and it's also more friction on the pavement and it will certainly make a differece on MPG.

Did you just recently buy that truck in your Avatar @Kevin S ? I think I seen your post on one of the Square groups on FB. Glad to see you made it here. Welcome aboard, feel free it hit any of us up for help. You'll get much better detailed responses here, not to mention alot more credible knowledge. My brain explodes everyday seeing some of the **** on FB that people tell others to do to fix their square.

 

[SquareRoot]

Shitcan it. LS swap.

 

[Ricko1966]

Put a vacuum gauge on it watch the needle look at the readings. Where's the needle sit at idle? Does the needle bounce around? What carb is on it? Is the choke fully open when hot?