Do blown motors need more fuel pressure?

Posted By: Ramrod39

Do blown motors need more fuel pressure? - 08/07/13 06:05 AM

A buddy of mine is putting a blown big block together. He is figuring out his fuel system. Is 7 psi a good fuel pressure for a blown motor? That is what I've always heard is about right for normally aspirated motors.

Posted By: Cab_Burge

Re: Do blown motors need more fuel pressure? - 08/07/13 06:55 AM

Quote:

A buddy of mine is putting a blown big block together. He is figuring out his fuel system. Is 7 psi a good fuel pressure for a blown motor? That is what I've always heard is about right for normally aspirated motors.

Pressure is not the thing to concentrate on, you want enough volume to feed the motor to make sure it has more than enough fuel for the intended HP it has

Fuel can aireiate(SP?), get air bubbles into the fuel stream, making pressure but not enough volume

Is he going to suck through the carbs or blow through them? How many and how big of carbs. and which type and size blower? He will need a big enough fuel line pickup in the fuel tank, big enough fuel filters and fuel line to feed the motor, big enough fuel pump and regulator and big enough needles and seats to keep the fuel bowls full enough to not starve the motor at wide open throttle

Lots of gremlins out there

, and then you need enough ignition to fire the sparks plugs under pressure

I have thought of making a blown pump gas BB wedge motor(Indy 440-1 CNC ported, 500 C.I.) that would have right at 9.0 to 9.25 to 1 comp ratio and use a 10.71 blower and under drive it so it would have between 3 to 5 lbs pressure at 6000 RPM, two big carbs and get after it, that would probally need a aftermarket block to be safe though

Posted By: patosmith

Re: Do blown motors need more fuel pressure? - 08/07/13 02:44 PM

I have a 56 Chevy wagon with a stroked 383 small block with a 671 blower.
I run 2 holley carbs and around 8.5 to 1 compression.
The blower drive is 1 to 1 and I use a mechanical holley 110 GPH fuel pump. I have close to 9 pounds of fuel pressure.
This car is for show and it sees the strip at nastalgia drags.
I can get into the high 11's and it does not starve.
Like the other poster says, its not pressure , it is volume.

By the way, I am also a Mopar fan. I am currently restoring a 72 Challenger.

Posted By: feets

Re: Do blown motors need more fuel pressure? - 08/07/13 05:29 PM

Quote:

A buddy of mine is putting a blown big block together. He is figuring out his fuel system. Is 7 psi a good fuel pressure for a blown motor? That is what I've always heard is about right for normally aspirated motors.

Tricky question.

Carb above the blower? 7 psi is fine.

Blow through setup? Fuel pressure must increase with boost.

Posted By: 70Duster440

Re: Do blown motors need more fuel pressure? - 08/09/13 09:44 PM

I'll echo what others have already said. Depends too on what carbs you're running. Even 7 psi can be at the edge of what a Holley needle and seat can handle before being overpowered.

I'm running a 6-71 on a 440 with dual Edelbrocks. Carter/Edelbrocks are particularly sensitive to pressure so they're regulated to 5 psi fed from a mechanical HV pump - and constantly monitored with a pressure gauge. I've found this to be plenty adequate for a street car.

For reference I'm running 8.5:1 compression with 7.3% overdrive which results in 6 lbs. of boost and a calculated 12:1 dynamic compression - so it runs happily on pump gas and I don't have to sweat detonation. 6 lbs. of boost may not sound like a lot but it'll keep a smile on your face.