vac fuel pumps

[Fast Fred]

wanted to run twin vacuum pumps on an OMC, wanted to know how everyone runs theres,
do thay run two pumps on the same cyl, or should i make an other vac barb on another cyl so each pump pulses on its own cyl.
thanks
FF:cool:

[Powerabout]

definately one pump per cylinder

[Fast Fred]

:cool: ya was thinkin that, all factory pumps are set that way, thanks, anybody gots any pic?

[Powerabout]

and run the fuel flow in series

[17W]

This engine uses 2 fuel pumps, one was for bring fuel from the fuel supply, the other to deliver fuel to carbs via the oil mixing unit.

Maybe J. Nerstrom will chime in on this.

[Fast Fred]

so drilled in on number two intake on the linkage side, ( theres a threaded boss for a dashpot, used the hole closest to the crank). so not side by side? was thinkin fuel line in to a Y fittin then into the pumps, then from one pump to a Tee between the top and the center carbs, pump two Tees in between the center carb and the lowest carb. lookin for volume to the carbs, seen this set up on a V6:cool:

[Tim Kurcz]

Hey Fred,

There are two set-ups you can use.

1) The puller & pusher pump set from an OMC 30 or 35 HP triple work well. These are piped in series to the fuel manifold. Or,

2) Two standard pusher pumps piped into a common fuel manifold one from each end.

In both cases pumps are run individually from different cylinders. I've used these systems for race boats with electrics deleted for weight savings.

Good luck,

Tim

[Fast Fred]

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

[Bill Gohr]

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


On a 3 cylinder? How many Lbs. of fuel do you need for 100hp?

When I ran the 2.6GT's (V6), I used, as many did, what we called half a VRO which is a South American 200 (3.0L) fuel pump, spun 8300RPM, no problem..................

[Powerabout]

a cross flow big bore to 8300..wow!
and about 10gph so less then 10lbs/hr

[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]

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

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

[Fast Fred]

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

[wolfgang]

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.

Absolutely. And pressure as well as fuel flow increase with rpm. I am currently retrofitting two Merc double diaphragm pumps to my Force 4-cyl. One diaph. gets fed from a transfer port cover, the second diaph. from the tapped transfer port of another cylinder - sort of suck and blow (I figured). Each pump feeds its own twin throat OMC carb. I think plumbing both pumps into one manifold might lead to the pumps "hunting" i.e. the pressure pulses may double in intensity or cancel themselves out. What do you think?

[Tim Kurcz]

Hey Wolfgang,

You raise a good question about the possibility of pulse cancellation with alternating twin diaphram pumps. My guess is this is unlikely due to the baloon/dampening effect of rubber fuel hoses unless the lengths are perfectly the same.

My concern about independent fuel systems is if one goes south - you burn down a cylinder. Piping to a common manifold eliminates tuning issues and provide redundant supply.

Good luck with your project. Please post pics - sounds cool!

Tim

[wolfgang]

Hey Wolfgang,

You raise a good question about the possibility of pulse cancellation with alternating twin diaphram pumps. My guess is this is unlikely due to the baloon/dampening effect of rubber fuel hoses unless the lengths are perfectly the same.

My concern about independent fuel systems is if one goes south - you burn down a cylinder. Piping to a common manifold eliminates tuning issues and provide redundant supply.

Good luck with your project. Please post pics - sounds cool!

Tim
thanks Tim - on the strength of your experience I will install a manifold.