marine fuel pump question

[Johnny248]

I'm helping a friend with an '89 Mercruiser 7.4 carb engine. The fuel pump died and he needs a new one. The pump that a previous owner installed was a Holley Red Marine pump rated at 7psi max, 97gph free flow, and 71gph at 4psi. Does anyone familiar with this stuff know if he needs that much fuel pump? Do you think a 5-9 psi 30gph pump would work? Or does t need the extra volume.

Thanks in advance for any suggestions
Limo

 

[hattaresguy]

Just buy the Quicksilver pump and be done with it.

 

[wag123]

To answer your question, a 30 gph fuel pump will not get the job done.

 

[Trav]

Not taking any losses into account you need about 72 gal per hour minimum.
If it is a return system with 3/8 lines you can go 80-100 GPH but depending on the HP even that may cause it to go slightly lean at WOT. I would go with the Holley black pump 712-815-1, plenty of headroom.
It is ethanol E10 & E85 compatible, the red and blue are not, they will fail using even E10.

Voltage plays a major role in fuel pump performance. The black is a 120 GPH pump at 9 psi and is adjustable.

 

[1 FMF]

Originally Posted By: hattaresguy
Just buy the Quicksilver pump and be done with it.


that sounds like an iboats answer

general rule is gph required = hp x bsfc / 6.
for naturally aspirated engines bsfc = 0.5 and for your older 454 that bsfc might even be lower (produce less hp per pound of fuel used) around 0.45. anyway for your ~350hp 454 you would require about 30 gph if the motor were putting out 350 hp which is wide open. but if you are running 1/2 to 3/4 throttle and only pulling 200-250 hp from the motor then you'll be using less fuel obviously. so if the electric fuel pump your looking at truly does 30 gph having at least 2 psi then it'll work fine.
what'll happen is going wide open throttle you'll slowly lose fuel pressure at the carb if the fuel pump can't keep up, until the point the engine doesn't get enough fuel and loses rpm and power. fwiw i've found over the years various no name brands of electric fuel pumps can be poor performers and not live up anywhere near their rating. what also hurts electric fuel pump performance is they don't like to pull fuel so you don't want them installed much higher than fuel level in the tank, that plus if there's a fuel/water separator upstream that'll reduce it's real-world output in which case your rated 30gph won't come close to that and you would need a performance pump like a holley red to be safe.
it might be worth your while to pick up a mini fuel pressure gauge from summit and plumb it inline at the carb to monitor fuel pressure, if you can observe it while motor running.
given pricing, you might be better off putting a mechanical fuel pump on the 454, you can find them on the web and ebay pretty easily and are probably cheaper than an electric pump or a new holley red anyway and you don't have to worry about electric or safety switches.

72gph minimum not taking losses into account ... funny. maybe you meant liter/hr ? the fundamental math relating fuel required to hp is not that complicated and you're an ASE Master?


http://www.holley.com/data/TechService/Technical/Fuel Pump Tech Info.pdf

 

[Donald]

Not sure why you would go with a fuel pump smaller than the one that came from the factory?

 

[Trav]

First you knock my numbers then confirm he needs a Holly red.
Originally Posted By: 1 FMF
that plus if there's a fuel/water separator upstream that'll reduce it's real-world output in which case your rated 30gph won't come close to that and you would need a performance pump like a holley red to be safe.


Why would he want to use a non ethanol rated pump? Only the black is ethanol rated. Is the block mounted pump diaphragm ethanol rated?
I can see you goal here, you want to discredit someone else while making yourself look good.
If that's what makes you feel good have at it.

 

[hattaresguy]

Originally Posted By: 1 FMF
Originally Posted By: hattaresguy
Just buy the Quicksilver pump and be done with it.


that sounds like an iboats answer

I like factory stuff, it tends to work. Its also the safe answer having worked on boats and sold parts for years with customers who are not mechanically inclined and don't want to pay lots of labor to figure stuff out.

Look up the right fuel pump with the Merc serial number, replace both if its a twin engine boat, and be done for another 10-20 years.

Also FYI depending on the hours and fuel burn it might pay to replace the carb, they do wear out and in twin engine applications I have seen 2-3 GPH savings with new carbs per motor. Rebuilding for whatever reason doesn't seem to be as affective.

 

[Donald]

can we move this to boating??

 

[1 FMF]

there's gph that the motor requires, and there's what the electric fuel pump is actually doing real world which is highly dependent on how/where it's installed and what else is in the system before it. if i came off a ****, i'm sorry. can i paypal you a hug? but your minimum value was a little off. the poor guy has a 454 he's got enough trouble with the fuel bill i wouldn't go telling him he "needs" a holley black pump. e10 is legally "gasoline", up to 10% ethanol is compatible with everything and the holley red/blue and mechanical pump diaphrams are fine with e10... not that i'm saying get another holley electric pump. but again, if he gets an electric pump he should evaluate his fuel system, and also be wary of the electic fuel pump ratings.

 

[Trav]

I am aware of the motor requirement calculations, I have all of them here plugged into my equipment.
I also deal with 300+ marine fuel injectors and fuel systems a month in the high season.
The OEM block mounted pump is rated IIRC at 110 gph. A 72 gph free flow pump mounted externally pre filters and on 12v will be lucky to deliver 60% of that.

E10 is not normal gasoline when it comes to older boats, having a carb I would guess this is not a newer model. Holly states for the red and blue..
Originally Posted By: Holley
NOT compatible with alcohol or methanol fuels

These are low pressure pumps for carb applications, they do not state any percentage of ethanol, to assume they mean e10 is okay and others are not is nothing more than an unsubstantiated assumption on your part.

You claim to be an engineer so tell me how the black pump can possibly increase fuel consumption over the red or any other pump? The black is 100% compatible and will only deliver what the engine can use, all it will do is provide more headroom.
We don't know if this is a return system, how much voltage is at the pump, the filter set up, line size, distance from the tank, amount of lift required, etc.

Even if these conditions were marginal the 120gph black will be sufficient for this application with a margin of safety.
Originally Posted By: 1 FMF
the poor guy has a 454 he's got enough trouble with the fuel bill i wouldn't go telling him he "needs" a holley black pump.


I see the e10 problems everyday in marine engines, they make up 80% of my business.
Injectors, lines, fiberglass tank issues, carbs, pump issues are all common.
You cannot compare ethanol in a marine environment to a car.

http://www.fuel-testers.com/e10_gas_not_safe_for_marine_engine.html

 

[OVERKILL]

There's nothing wrong with leaving plenty of headroom for the pump, it'll account for filter loading, separator loading, perhaps less than optimal operating voltage....etc. Which seems to be Trav's point here.

He's going to be spending the money anyways, the difference between the black and the red is $110.00. Might as well just get the bigger pump, it certainly isn't going to hurt anything.