Fuel burn at wide open throttle

[Truckedup]

To settle a discussion.... I have always read that reasonably tuned 4 stroke engine burn 1/2 pound of fuel per hour per HP...Or about one gallon an hour for 14 HP....This formula has been used for aircraft and race cars I believe...

The question...Do any of you know or have facts on the wide open throttle fuel burn for a DOHC VS a 2 valve pushrod engine making the same HP at wide open throttle?
Example, 4 liter DOHC V8 making 400 HP compared to a 5 liter wedge head two valve V8 making 400 hp.....I say the fuel burn is very close between the two engines? Thanks

[stealth]

Man that’s a loaded question...

Why would valve train make a difference?

Energy content in fuel
Internal friction
Heat loss
Efficient use of fuel
Least number of misfires
Etc.

These are far more important...but may be overlapping from one engine design to another.

[Truckedup]

Yes, a few variables.. But I think the fuel burn for the same power level is close regardless of engine design...

[statsystems]

.5 BSFC is a bit on the high side.

Now I forgot the rest of the post. Damn...that's why I quote.

[statsystems]

To settle a discussion.... I have always read that reasonably tuned 4 stroke engine burn 1/2 pound of fuel per hour per HP...Or about one gallon an hour for 14 HP....This formula has been used for aircraft and race cars I believe...

The question...Do any of you know or have facts on the wide open throttle fuel burn for a DOHC VS a 2 valve pushrod engine making the same HP at wide open throttle?
Example, 4 liter DOHC V8 making 400 HP compared to a 5 liter wedge head two valve V8 making 400 hp.....I say the fuel burn is very close between the two engines? Thanks

I agree, the BSFC will be very close if all things are relatively close to the same.

[englertracing]

The one that makes less power per liter will have slightly higher frictional losses.

[Circlotron]

BSFC figures for a flathead might be interesting too.

[Truckedup]

BSFC figures for a flathead might be interesting too.

I have a few detailed books on piston aircraft engines.....Flathead engines disappeared from aviation in the 1920's because of higher BSFC than OHV engines.A few very small 40 -60 HP engines lasted longer.....BSFC's of .6-.7 are mentioned for flatheads...

yes, I said .5 and it is a bit high... after doing looking around it may be as low as .390 for some N/A engines...

[MadBill]

.5 BSFC is a bit on the high side.

Now I forgot the rest of the post. Damn...that's why I quote.

While you are typing a reply, you can scroll down and see all previous posts, last one first, from the current page.

[user-23911]

It comes down to efficiency.

Most engines are most efficient at peak torque,less efficient at peak RPM and idle.
A diesel engine is more efficient that a spark ignition engine.
A DI petrol engine is more efficient than a carburettor petrol engine.
A high compression engine more efficient than a low compression engine.

[mag2555]

Can you even compare a Diesel engine to a spark ignition engine when due to them not being air restricted they are at max VE for any rpm they are at where they are not at the limit of there Intake and Exh tracks?

[John Ross]

An easier rule of thumb for gasoline engines is to take horsepower produced and divide it by 10. That gives the approximate fuel consumption in Gallons Per Hour. The electronic fuel flow meters in airplanes are very accurate because they are calibrated by the pilot by filling the tank, flying for a few hours, and then filling the tank again. The flow meter in my Pitts Model 12 shows 35-37 GPH at sea level and WOT on a 60-75 degree day. The engine is rated at 360 HP.

You can easily use this formula to work backwards to figure out actual power used in part-throttle operation. My well-tuned 1996 Viper GTS would return 22.5 MPG at a constant 75 MPH on the interstate. 75 MPH for one hour at 22.5 MPG burns 75/22.5 = 3.33 gallons per hour. The above rule of thumb tells us the engine is making about 33 horsepower at 75 MPH in high gear (about 1500 RPM.)

That same Viper had a top speed in 5th gear of 184 MPH as verified by some timed two-way runs. Horsepower required goes up as the cube of the speed. 184/75 = 2.453 times as fast, so 2.453 cubed is 14.76 times the power required to maintain 75 MPH. 14.76 x 33 = about 485 HP, which is right in the ballpark.

JR

[Truckedup]

I'm interested in fuel burn comparisons of spark ignition engines at maximum power...Info on the Internet is mostly aircraft and a few newer OHC small auto engines...Somewhere there must be GM info on the LS engines and info on German or Japanese hi out put DOHC engines..There must be NASCAR info also...

[John Wallace]

Try reading some of this:

Piston Engine Technology

He does a good job of describing things.

[user-23911]

Can you even compare a Diesel engine to a spark ignition engine when due to them not being air restricted they are at max VE for any rpm they are at where they are not at the limit of there Intake and Exh tracks?

No, they're not at max VE.........they still have a torque curve which drops very low at max RPM due to the injectors limiting the fuel .