vac fuel pumps

[Tim Kurcz]

Though I've never actually measured, a carbureted naturally aspirated racing two stroke should burn something in the 0.7 Lb/Hp/hour range (crankshaft). So a good 49.9 FE/850 mod or SST60 engine delivering say, 105 HP will consume:

105 HP X .7 Lbs/HP/Hr = 73.5 Lbs/hr / 6.0 Lbs/Gal = 12.25 GPH, or about 1.225 Gal for a typical 6 minute heat (plus milling).

Are these numbers consistent with your experience?

[Mark75H]

Not for me with the 850 class (60ci) Merc ... we use a little more than a gallon and a half for most heats, but it could be that the Merc is dramatically less efficient

[Tim Kurcz]

Sam has a good point. A megaphone scavenged deflector Mercury may burn more fuel than a pulse tuned OMC looper (lbs/fuel per HP/hour). Then again, it has 10 more cubic inches. Also, judging fuel burn by the race course is an overly subjective measure: Length/shape of course, milling time, etc all add into the equation. The only way to check for sure is to measure total burn on a dyno or test wheel @ RPM over time. Sounds like a waste of time!

[Mark75H]

cubic inches is less important than horsepower when considering fuel consumption

it matters less if a motor is 10 cubic inches or 50, if both are making 20 hp the consumption will be closer than any motors making 100 hp or 1 hp

[Tim Kurcz]

Not sure what you are saying. Stated another way: 60 less efficient cubic inches will consume more fuel to deliver the same HP than a more efficient 50 cubic inch engine. Is that what you mean?

[Mark75H]

I'm saying a fuel pump for 100 hp is a fuel pump for 100 hp ... doesn't matter that much if its 30 cubic inches or 300 cubic inches ... efficiency varies less than power does

[Tim Kurcz]

Ahh, I understand you now. If the engine efficiency is equivalent (lbs fuel/HP/hour), then the pump(s) must be up to the fuel delivery task. It/they must deliver X pressure and X flow to accomplish the target HP - agreed.

Are you aware the efficiency of the diaphram fuel pump is affected by port timing? Be aware a vacuum pulse diaphram fuel pump designed to run with stock porting may not deliver the same performance in a port modified engine. For this reason all new mod engines should be instrumented until pressure and flow is proven.

[Mark75H]

I agree

[ima75man]

finger ported, solid molded, SST60, i'm going with one on each end of the manifold.

freddie, your pm is full, clean out a spot for me,, thanks out

[Fast Fred]

, ya not good at that pm thing, should work now