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Attention rocket scientists, I have a question for ya.
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Posted by: Mr. P

Say you have two engines, one big block and one small block, and they're both rated at the same power (300-hp). If they're doing the same work (pulling the same load) at the same RPM, same gearing, (lets just say they're both running at 3000 rpm), which one will be consuming the most gas?

An idle mind is a terrible thing to waste, lol.

Mr. P

Posted by: LS1JAY

Big Block!

Posted by: 65fairlane

How is the Air to fuel ratio tuned for each? What are their relative displacements?
Assuming equal state of tune and displacement the amount of gas used would be the same I would think.

Posted by: Mr. P

Quote:

Originally Posted by 65fairlane


	How is the Air to fuel ratio tuned for each? What are their relative displacements? Assuming equal state of tune and displacement the amount of gas used would be the same I would think.

Say a 454 versus a 350, both tuned for 300-hp, both running 3000 rpm, both hauling the same load in the same gear. We know the big block would be pumping more air, and would appear to need more fuel, but would it????

Posted by: 65fairlane

Intuitively I would think that the big block would take in more fuel since it would require more air.

Posted by: Mr. P

A big "detuned" big block, with a RV camshaft, lots of torque and relatively low compression will be loafing at 3000 RPM and will be right on it's max torque curve.

A small block might develope 300 hp like the big block, but it might need an additional 1000 RPM to reach that number. If the small block has higher compression ratio, bigger cam, and is putting out the same horsepower, I honestly don't know which one burns the most fuel.

I'm with you, it would appear the big block air pump is pumping more air, but it could be low compression air, while the small block could be needing a more dense charge. I thought John knew everything, why doesn't he weigh in on this one, or how about Matt, hell Jay, MikeT, Ole Man??? Give a brother a hand, sheesh.

Mr. P

Posted by: LS1 Mopar Turbo

HMMM, I wonder, but I don't know, Lets take Harvey the RV 24 foot class A motor home VS my Neighbor across the street Class C 24 foot Motor home....Harvey Big block 454 3 speed, Neighbor 350 3 speed, he averages about 8 mpg, I get almost 12. I weigh in at 11,000 with everything in it. He is about 10,000. So that is real world.
I would think the BB dosen't have to work as hard to move the same amount of weight as the SB, but I would have thought the BB would use more gas but, if it isn't working as hard and all things are equal...who knows.

Posted by: VETTKLR

If the A/F Ratio is the same on both engines, the BBC will consume more air/fuel.

Posted by: Mr. P

If the AF ratio was the same for both motors, they both wouldn't be producing 300 hp, would they? Also, I wonder what horsepower each motor is actualy producing at 3000 rpm. Big torque could be efficient??

Wonder if the small block would require the 4-bbl to be opening in order to maintain that pace?

Mr. P

Posted by: 65fairlane

I was thinking nice and easy along the lines of the formula given in the engine math article John wrote and posted. This is actually an interesting discussion.

Posted by: LS1 Mopar Turbo

Quote:

Originally Posted by Mr. P


	If the AF ratio was the same for both motors, they both wouldn't be producing 300 hp, would they? Also, I wonder what horsepower each motor is actualy producing at 3000 rpm. Big torque could be efficient?? Wonder if the small block would require the 4-bbl to be opening in order to maintain that pace? Mr. P

Yes the 4 bbl is open in the 350 in similar weight RV's to maintain the same speed, my 4 bbl is not open when maintaining the same speed as the 350 powered RV. Big torque is very effiecent in the RV, I don't have to push it as hard as the SB my Neighbor has.

Posted by: John

You need so much more to determine fuel consumption at 3K RPM's than just compression ratio, cam timing and lift and gearing. You need to know what the air/fuel ratio is. Optimum is something like 14:1 but not all engines run at that exact ratio. Especially under heavy load. They will often run rich so that they don't burn up and lean out at high RPM. You also need to know what the volumetric efficiency of the engine is. That right there is he largest determining factor of fuel consumption.

However, I think (notice the operative word there being THINK) that given everything else is equal, a 300 horsepower 5.7 liter engine would use less fuel than a 300 horsepower 7.4 liter engine. Mainly because even if volumetric efficiency is equal, it would still stand to reason that a smaller displacement engine would be running fairly well to make 300 horses but a large displacement engine with similar cam, port flow and fuel curve settings would be running like crap to be making only 300 horses. So, when that big engine runs through 1 complete cycle, it will have pumped 7.4 liters of air/fuel through the engine. The 5.7 liter engine has less displacement and would most likely use less air and fuel to reach the same goal.

And what the fuck is this supposed to mean?

Quote:

Originally Posted by Mr. P


	I thought John knew everything, why doesn't he weigh in on this one, or how about Matt, hell Jay, MikeT, Ole Man??? Give a brother a hand, sheesh. Mr. P

Posted by: John

Then again though, if volumetric efficiency is equal, the engines would be using the same amount of air/fuel. But just because the big block is almost 2 liters larger in displacement, it will, by default, use more air/fuel because the physical proprotions are different. Therefore, the only way volumetric efficiency can be equal is if the big block uses different cam, timing, port flow and gearing specs than what the small block uses.

So I guess, if all of the blueprinted specs are equal on both engines, the big block will use more fuel and air. Also, the small block will be running more efficiently because this is one time where the power per liter of displacement comparo is valid. A 5.7 liter engine tuned to make 300 horses is running well. A 7.4 liter engine running the exact same specs as the 5.7 liter engine is not going to be running well because the specifications are optimized for the smaller, physically different 5.7 liter engine and they are not going to be optimal for the larger engine.

Posted by: MadScientistMatt

The big block would have to be throttled back a bit more. At least in theory, the throttle causes a bit of a loss of efficiency because it cuts into your cylinder pressure and effective compression ratio. In practice, things like cylinder head design differences would probably have a larger effect.

Posted by: DeckSetter

You'd have to figure in where they are in relation to peak torque. If one engine is at it's torque peak, it would be running more efficiently and I'd think it would burn less fuel per displacement. Hmmmmm...........

Posted by: GMR

Quote:

Originally Posted by Mr. P


	Say you have two engines, one big block and one small block, and they're both rated at the same power (300-hp). If they're doing the same work (pulling the same load) at the same RPM, same gearing, (lets just say they're both running at 3000 rpm), which one will be consuming the most gas? An idle mind is a terrible thing to waste, lol. Mr. P

Too many variables to come up with an answer.
If a small block is producing the same amount of work as a BB at the same RPM and gearing, that would mean that the SB is more efficient than the BB right?
Maybe theyre using the same amount of gas, the SB while smaller in CI is doing the same amount of work as the BB, but it will have to work harder, using more gas. The BB doesnt have to work as hard, using less gas.
However, it doesnt seem possible given the load,RPM, gearing theyre working at.

LS

Posted by: No Rice Allowed

The SB weighs less...So it would get the best Fuel Mileage.....out of the two with the same HP...

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