Fuel flow requirements, GPH

Chuck Moreland · 06-05-2003, 08:05 PM

Short version -

I'm trying to determine the peak fuel flow requirements of my 2.4 with Webers such that I can properly size the fuel pump.

Long verison -

If you've been following along, I had a fuel delivery problem on my '73 about 6 months ago. Turned out to be a flakey Facet fuel pump.

I replaced it with a 100 gph holley that worked fine. But it was very noisy, it could be heard over the engine at idle with a loud muffler. Plus it was heavy, not a good choice for my lightweight rocket.

I returned it and got a Carter 100 gph. But before I got it installed I tore the car down to the tub. As I put things back together I found I had a new Facet fuel pump in my pile of "stuff". It's rated at 30 gph. It's quite and weighs about 1.5 lbs.

So I now have the new Facet pump in the car. I'm just a bit concerned that 30 gph may not be enough at 7300 RPM full throttle.

There never was a factory pump that would be directly comparable since the 2.4 came with MFI.

jluetjen · jluetjen's Garage

If you can get a copy of the factory HP charts from the shop manuals, I think they generally also plotted the fuel consumption.

jluetjen · jluetjen's Garage

Chuck;
I pulled some of the Specific Fuel Consumption graphs that Porsche usually included with their HP charts.

The MFI'd engines seem to peak at about 280 g/HPh (gallons/HP/Hour????) at peak rev's. The T's seem to run in the same zone. The carb'd 2.0S though seems to get closer to 290 g/HPh, so I can only assume that the carb'd engines need a little more gas flow then a comparable MFI. This would be consistant with the fact that carb'd engines almost never get as good mileage as an MFI'd engine.

So does anyone know what Porsche meant by "g/HPh"???

304065 · 06-06-2003, 08:24 AM

Grams per horsepower-hour. The Frere book has some detail, I think.

jluetjen · jluetjen's Garage

Thanks John C. You know -- sometimes it would be good if I just read what I wrote. If it was 280 Gallons/HP/hour, a 911 would need a fuel line the size of an urban sewer pipe to keep the engine fed. Duh!

CRH911S · 06-06-2003, 08:59 AM

Can you post pictures of the completed pump installation when your done. I am in a similar situation and have yet to decide fuel pump to use. I am leaning toward the Holley Red with the PMO pressure control unit. I can't expand on g/HPH. Thanks in advance.

Chuck Moreland · 06-06-2003, 09:13 AM

I did some calcs and unit conversion.

Density of gasoline is 680kg/m^3 = 2574g/gal

I'll use the same 290g/HPh for my carbed 2.4 as you list for a carbed 2.0. I believe the HP in that equation will compensate for the displacement difference, therefore no need to adjust this raw number.

If I expect my motor to make 200HP (a little optimistic) then:

290g/HPh X 200HP x 1gal/2574g = 22.5 gal/HR

If all that is correct then the Facet at 30 gal/HR should do it.

However the Facet 30 gph is surely a free flow rating. In my car it has to pump through a filter, regulator, lines and float valves. I'm probably OK but I don't think I'd use a 30 gph on a more powerful motor.

304065 · 06-06-2003, 12:57 PM

Chuck, here's a thread from many moons ago, when I was re-engineering my fuel system (added a fuel cell and wanted to beef up the lines). The Maestro, Roland Kunz, came to my aid, posted some interesting stuff.

MFI Fuel System Re-Engineering Questions

I know you are doing carbs, not MFI, but the BSFC number doesn't care what kind of induction system you use, right?

rudy sanchez · 06-06-2003, 01:58 PM

chuck,

there was a recent discussion on electric ful pumps over on opne of the bmw 2002 boards. as I just swapped out my lectric fuel pump yesterday, some of that info was fresh in my brain.

some people like the facet and some don't due to reliability issues and noise. some people use a walbro that has an internal pressure regulator and it's a high volume low pressure pump that is super noisy (sounds like a jack rabbit trapped in a shoe box). the walbro has POOR reliabitilty as I have gone through TWO of these.

I went w/ an airtex! bought it from winchester auto parts for $59.95 there were a few folks who had good things to say a/b this pump and nobody had anything bad to say.

so far I'm happy w/ it. think it came w/ a 3 yr warranty

there is an airtex website some place

good luck,

paul schuster