Engine required GPH

[fnaramore]

Hello guys,

So I calculated my required fuel pump gph from engine demand to be 27.5. How much, if at all, am I taking a chance on hurting myself by still running a 25 gph carter fuel pump? When does this calculation really come into play? Just at long WOT would be my guess.

Thanks in advance

 

[AJ/FormS]

That's right. You calculated the fuel requirement in pounds per horsepower per hour and converted it to gph.So the requirement is for WOT.
Now the pump rating is usually stated as output at a certain pressure. But when the pump runs without a restriction, it usually puts out a bit more.
Sooo, if you were to run the engine at max output for a long enough period of time, that the floats drop far enough that the only restriction is the wide-open float valves, AND you ran no pressure regulator nor a return system,then the pump might put out more gph. Only a flowtest will prove that.
But heres the thing you gotta know;
At 27.5 gph, thats 1 ounce per second. I think there's probably 50 to 60cc of fuel in a standard Holley bowl. At 7cc per ounce, thats over 7 ounces per bowl so total is 14 ounces. Quick question if your fuel pump quits when the tree turns green, will your car make it to the finish line on 14 ounces? Well at 27.5gph, the math says you have 330hp so about a high 13 second car.So,Ima guessing yes.With a second to spare.
I wouldn't try it tho, heehee
And I'll tell you why; cuz not all of those 14 ounces are coming out at full gallop.

But at 25gph, your pump is putting out 91% of the calculated requirement. And at every shift the rpm drops , and so does the engine output, and so does the pump output requirement.So it has lots of time between shifts to catch up.
Now I made one assumption in all this; that you are asking about an electric pump.

 

[diymirage]

and extended WOT situations, like going down a drag strip

I ran my 408 with a stock style mechanical fuel pump for a half a year before going to the track for the first time
never had a problem in daily driving , highway on ramp sprints...whatever

until I went to the dragstrip, clocked the 1/8th mile at 74 MPH, drained the bowls and clocked the 1/4 mile at 72 MPH

I replaced it with a 120 GPH mechanical pump, but I haven't had a chance to try it out yet

 

[fnaramore]

That's right. You calculated the fuel requirement in pounds per horsepower per hour and converted it to gph.So the requirement is for WOT.
Now the pump rating is usually stated as output at a certain pressure. But when the pump runs without a restriction, it usually puts out a bit more.
Sooo, if you were to run the engine at max output for a long enough period of time, that the floats drop far enough that the only restriction is the wide-open float valves, AND you ran no pressure regulator nor a return system,then the pump might put out more gph. Only a flowtest will prove that.
But heres the thing you gotta know;
At 27.5 gph, thats 1 ounce per second. I think there's probably 50 to 60cc of fuel in a standard Holley bowl. At 7cc per ounce, thats over 7 ounces per bowl so total is 14 ounces. Quick question if your fuel pump quits when the tree turns green, will your car make it to the finish line on 14 ounces? Well at 27.5gph, the math says you have 330hp so about a high 13 second car.So,Ima guessing yes.With a second to spare.
I wouldn't try it tho, heehee
And I'll tell you why; cuz not all of those 14 ounces are coming out at full gallop.

But at 25gph, your pump is putting out 91% of the calculated requirement. And at every shift the rpm drops , and so does the engine output, and so does the pump output requirement.So it has lots of time between shifts to catch up.
Now I made one assumption in all this; that you are asking about an electric pump.

Thank you very much! This is some great information! It is a mechanical setup, but if I understand correctly the main factor with that is that at higher rpms that it can fail to pump as efficiently.

 

[fnaramore]

and extended WOT situations, like going down a drag strip

I ran my 408 with a stock style mechanical fuel pump for a half a year before going to the track for the first time
never had a problem in daily driving , highway on ramp sprints...whatever

until I went to the dragstrip, clocked the 1/8th mile at 74 MPH, drained the bowls and clocked the 1/4 mile at 72 MPH

I replaced it with a 120 GPH mechanical pump, but I haven't had a chance to try it out yet

Yeah it wasn't daily driving that had me concerned, but doing hot passes. Out of curiosity, what pump did you replace it with?

 

[diymirage]

Carter m6270
I did install a fuel pressure regulator with it aswell

As soon as I can make it down the track again I'll report back

 

[AJ/FormS]

Me too on the 6270, no regulator, tho, and no problems.

 

[AJ/FormS]

Thank you very much! This is some great information! It is a mechanical setup, but if I understand correctly the main factor with that is that at higher rpms that it can fail to pump as efficiently.

This would be a problem for Nascars.
Not a problem for streeters who seldom are stuck at WOT and over 5000 rpm for more than two or three seconds atta time
And for street/strip cars the first 4 seconds are sub 5000 rpm as well, and then with ideal gears and an automatic, there will be at least 2 or 3 more seconds in a near-14 second run during which the engine will be at sub 5000 rpm. So the pump has lots of catch-up time.The total time spent over 5000 rpm with a street type TC, is probably only 7 or 8 seconds with 2 catch-up periods.
Unless you have the crappiest street suspension and the unsitickiest tires which might allow you to spin to 80 miles an hour, or more, like I do, heehee . I leave the light at 3000, and the tach immediately hits 7000 and stays there till I let off, Hah, what a rush. Unfortunately, it ain't particularly quick. S'ok I'm not there for the money, just the hot-dogs.....
Oh wait I have a bit more power. My fuel requirement is 35gph