Thread: Big block 540 gas mileage

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Lets say you built a mild 540. To the tune of about 1hp per cube. Built it with low to mid range power in mind. Then, used an over drive tranny, such as a Richmond 6 speed and geared it so the engine would turn about 1500-1800 RPM at 60-70 mph. Could you get OK gas mileage with this setup? Could you get 20mpg?

Hmmmm.... I doubt it! My question would be why would you want a 550 HP big block and be concerned with fuel economy? My mild 502 (with a 6-71blower) could average about 4 MPG.. if I took it easy on the loud pedal, and that was running $3.65 / gallon race gas. I have no idea what it will get after the rebuild is finished, but I can always add a larger fuel cell.

contrary to popular belief, gas mileage is more closely correlated to total vehicle weight and aerodynamics.
While accelerating from a stop the only work being done is overcoming mechanical drag and accelerating mass. (F=ma)
More mass=more gas.
When you're on the highway (flat highway) the only work your
doing is overcoming mechanical drag and wind resistance.
Gigantic SUV frontal surface area=more gas.

If you stuck a suburban engine in a civic it could get 30mpg.

Its not quite that simple.

Gas milage is directly proportional to the efficiency at which you are burning fuel. There are obviously a number of factors contributing to that efficiency - but timing and spark reliability reign supreme.

Bert

Big block 540 - Decent gas mileage

Those seem to be mutually exclusive statements to me.

While I agree with tcodi on the mass and aerodynamics to a certain extent, I can't buy the 30 mpg big block Civic. Volumetric efficiency also plays a factor. Assuming optimal Air Fuel ratio is about 14-1, air to gas, if you bring more air in to fill the cylinders you are going to bring in more gas too. Modern fuel injection and computer control has gone a long way to improving mileage, but in most cases a bigger motor is gonna use more fuel. The exception would be if you underpower a heavy vehicle.

Back to your question, could you get 20 mpg on the highway. I don't think so. Could you get 12-14? Possibly if you keep your foot out of it, but where's the fun in that? Perhaps a better choice is to build one of the new generation EFI motors that will run hard and still pull down 20+ mpg.

Pat

548 10 1/2 cr two 750 edelbrocks on top of a b&m meg blower with a 714 714 lift roller cam 260 at 0.50 on 110 cen 400 trans with a 10 inch stall 389 gear 50 chevy coup 12 mpg all day long butt you have to keep foot out of it

I helped someone move from the Austin area to mid-NC. He drove the rentatruck, I drove his Avalanche 4WD 350, with which he averaged about 12-14 MPG, mostly freeway. I averaged just over 20 on freeway driving, but most of that was due to my watching what the traffic was doin' far in the distance. It's amazing that people can think that the laws of physics are changed in a car.....

He's one of those "digital drivers" -- either flat on the gas or hard on the brake. And even the mighty 350 Chebby can't do much rapid acceleration in a three-ton Tonka toy. He's also convinced that the only truly valid form of auto racing is NASCAR.

Oh, well.......

But I think the answer to your question was answered previously. "A big-block 540 getting decent mileage?" Why would that be a consideration with a 540? You want an econocar, get a turbodiesel

HAHAHA!!!

You have crossed over from MPG to GPM.

Just my 2 cents....i drive a 94 cavalier for the fuel economy, im building a 71 monte with a supercharged 502 for the power, i have had a few people come up to me and ask "what kinda gas mileage is that car gonna get?" i tell them im not building a supercharged 502 for the gas mileage, who knows?, who cares? really? you know? Just my opinion but you know what they say about opinions...

people ask me all the time about gas mpg on my cars and it is not that bad but i never built them for it and i have had some nasty big block on the street all rollers 548 blown 572 13to1 548 14 to1 620+ 10to1 and smaller ones 477 12to1 468 12to1 460 11to1 all of them were driven on the street most of the time they idly a round town so im am not in them all the time

Here's why I asked this question. I had a 454 in a 1974 Vette. Mostly stock, with the exception of some closed chamber heads to increase compression a bit without having to get into the lower end. A Stock class drag racer did some tweaks to the Quadrajet. I put a slightly higher lift/duration cam in it (I don't recall the specs, so I'll use the ubiquitous term ‘RV cam’ to describe it), and I had a Doug Nash 5 speed. High gear was 1 to 1, and I think I put a 3:55 rear gear in it. One summer I went on a trip and got an honest to God 17 MPG, which I was extremely pleased with. I just cruised; keep my foot out of it. I’ve read about big inch big blocks making tons of torque right off idle, so I thought this might equate to decent MPG. Yeah, I want it to go like that 1974 when I want, but want a bit of economy too. Thus, the bigger engine idea with the new Richmond 6 speed with the OD sixth gear. My co-worker has a new Z-06, and has this luxury. He says if he gets on the freeway and puts it on cruise, he’s in the high 20’s MPG wise. But he has great performance too if he hits the loud pedal. I have a big block Nova. While it’s fun to hit the gas, it’s not so fun to go to the neighborhood Blockbuster in it and return with ½ a tank less gas! Finally, I can pull off a 540 project. I can do the engine, tune up, drop in the engine, etc. I don’t feel ready (or willing) to try a LS1 or LS6 swap to get what my Z-06’s buddy’s got. But maybe the new computerized engine management system is the key to the Z-06, and an old school big block is going to make the HP, but not the MPG. Thanks for all the replies. It's better to talk about now, as it's just theory, than do it, spend the money, and to wind up unhappy and having to start again.

Tcodi's point about aerodynamics is a good one.

My 462" Buick powered 2400# Deuce highboy roadster gets 11-16 mpg.
Most times 11-12 mpg in combined town/highway driving.
The 16 mpg bit is on a long flat highway with virtually no stops and running the vacuum level as high as I can.

The 32 is about as aerodynamic as a brick.

The full size Buicks with similarly built engine - mild cam, single 4 bbl etc. - get the same mileage I do.

it is rpm and how the engine timing and carb is set up and a good exhaust .aerodynamics ?? how is a 50 chevy my friend had a buick g body with a hot small block 400 we both drove to the track and race them they got a bout the same mpg i hand a big block they both had the same rear gears the weight i thinnk it was close

You are all right. Weight, axle ratio, how a vehicle is driven, how slippery it is all play into the equation...

A little practical experience. My neighbor had a 1980 Chevy 3/4 ton pick up on a 1 ton chassis. It's 350 was tired and not pulling his trailer as well as it use to. We installed a 454 with about 1 hp per cube.

Mileage in town with the 350 was 10-12. Mileage in town with the 454 was 13-14... Highway was even more dramatic. He got 13-15 with the 350 and 16 pulling the trailer with the 454.

Same mass, coefficient of drag, tires and driving style. The 454 just did the work easier. Of course he could have driven the truck harder, faster with the 454, but that wasn't the goal. So apples to apples, at least for him the 454 paid for itself in fuel savings.

I was merely making a generalization about mileage. I.C.E.'s have way too many influencing factors to be exact about it.
But from a dynamics and physics standpoint, the only factors going into the energy required to move your car are weight and drag.
The only work your doing when you accelerate is to accelerate the mass (that includes overcoming inertia of rotating mass in the drivetrain). The only work your doing at constant speed is overcoming air drag. AND mechanical drag in the drivetrain, so obviously a big monster engine will need a monster tranny and rear, all things that have drag associated with them.
But that is IT!

Any difference is gas mileage between different engines in the same car are a result of mechanical drag, volumetric efficiency, and the driver.

If the mass is the constant, the same power put down to the ground will accelerate it at the same rate, whether or not that power comes from a 4 cylinder @ WOT, or a big block @ 20% throttle.

Same deal on the highway only the aerodynamics is the only factor (ignoring mechanical drag).


This is just a generalization, I'm not saying anybody's gonna get good mileage with a 540. It probably takes 100hp just to spin your drivetrain @ 6000rpm's.