Fuel Pump Pressure and marine vs auto

Hi, I have a 1987 gm 4.3l merc 2 barrel carb.

I need to buy a new electrical fuel pump but don't know which one I need. I am looking at one that has 30gph and 2.5 - 4 psi. I was told that this would be fine because most carbs. like running at 3 - 4 psi. Actually I was told this would be perfect.

Are there any performance advantages to go to a higher psi like 4 - 6 psi.

What is the difference in marine and auto universal fuel pumps.

Thank you all!!!

Re: Fuel Pump Pressure and marine vs auto

Why don't you just get the proper mechanical fuel pump for your engine. It didn't have an electric fuel pump to start with.
Besides, fuel pressure means nothing as far a performance goes.

I was told that this would be fine because most carbs. like running at 3 - 4 psi. Actually I was told this would be perfect.

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"I WAS TOLD" and you said it twice.
Doesn't mean much if the one that told you that really doesn't know what he is talking about,
I was told to say that. Wanna guess who told me to say it?

Re: Fuel Pump Pressure and marine vs auto

Wanna guess who told me to say it?

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I guess Mercury....
What do I win

Re: Fuel Pump Pressure and marine vs auto

Fuel pressure is handled at the carb with the inlet needle and seats. Excessive pressure will push them open and flood you out or at least make it run rich. Max pressure for a carb is around 7 psi. 5 psi is about perfect. A proper mechanical fuel pump will give you this. Honestly-you dont want to start messing around with electric pumps, mechs are way more reliable (IMO), cheaper and should be a direct fit on your boat. Once you get into electronic fuel injection the electric is necessary to "control" the engine through use of the electronics. On a hi perf carbed boat, the electric can provide a greater volume of fuel to a hungry engine, not quite the case on a stocker.

Re: Fuel Pump Pressure and marine vs auto

The big problem with an auto fuel pump is, when it leaks in a car the fuel drips onto the ground and you move on,,, when it leaks in a boat it drips into the bilge and when a spark (alternator, starter, static, etc) hits the fumes the boat goes BOOM

DonS is advising to put the fuel pump and engine back to the way Mercruiser had it configured from the factory, so if the fuel pump leaks the excess fuel dumps into the carb, and gets burned up in the engine.

Re: Fuel Pump Pressure and marine vs auto

Here are some specs I found way back in hotrodding days and use when needed. It shows how much fuel flow is needed to feed a particular level of horsepower. It does not matter what type of pump provides the flow. (Seems to me your current block is not fitted for a mechanical unit, but I may be confusing yours with another).

Flow in GPH = HP * BSFC *(1 / fuel weight)
For BSFC(brake specific fuel consumption) rated in lbs per horsepower-hour, use:
0.45 to 0.5 for naturally aspirated engines
0.55 to 0.60 for supercharged engines
0.60 to 0.65 for turbocharged engines
For fuel weight, use
6.216lb/gal for regular unleaded
6.350lb/gal for premium unleaded
For example, here is the calculation for a naturally aspirated 250HP engine on regular unleaded gasoline.

250 * 0.45 * (1/6.216) = 18 GPH

Adding a 30% safety factor a 24 gph fuel flow would be recommended.


Your HP is lower than 250 so a 24gph flow is sufficient. Carb inlet pressure should be at least 3psi at wide open throttle, and as mentioned earlier 7-8 psi is at the high end of what the needle and seat can handle.

Re: Fuel Pump Pressure and marine vs auto

Thank you all for your responses.

I order a fuel pump that was 2.5 - 4psi. I told the owner of the site was i was using it for and he sent me a pump that is 5 - 9 psi (without even asking me and charged me an extra $20). I told him that I wanted the 2.5 - 4 psi one but he still sent the other. He told me the other one wouldn't work.

I am sorry for asking this some many times but will this know be acceptable.

I know that Maclin mentioned that 7-8 psi is on the high side for a carb but will this pump ever run at full 8-9psi.

What is meant by 5-9psi; Is it 5psi idle and 9psi full throttle?

Thank you and again I am sorry for beating this over the head.

Re: Fuel Pump Pressure and marine vs auto

The 5-9psi is the range the pump will work in when it has full current available. It regulates itself and will not drop below 5 and will not let the pressure get above 9. It does not know if you are at idle or not, it only knows when it needs more current to keep the pressure up. If enough current is available it can stay above 5psi.

Use the one he sent you, I am sure he understood it was for a boat and probably has more knowledge about what it does in the real world, and the psi rating is probably equivalent to what you had that quit working. Pressure only builds when the needle and seat are near closed or closed, and as long as they are in good shape and the carb floats are set correctly you will not have a problem. I was staying on the low side so as to not introduce a potential carb overflow situation. Sometimes a carb's needle and seat can get used to the falling pressure from an older pump and then when a fresh pump with up-to-spec flow is installed the needle and seat can get overwhelmed. The 5-9 rating will work, especially with 3/8" lines as pressure the carb "sees" is less than with the typically smaller 5/16" lines used on a car.

Automotive applications also have to push the fuel a long ways and when the car is accelerating the push is even harder as the car tries to shove it all back down the line. On a boat the opposite is usually true, it does not have to push a long ways and rarely has to push against the boat's travel. This is why you need more of a fudge factor for a car's pump rating.

Good luck, sorry about the extra $20 but I am sure the fish are waiting....

Re: Fuel Pump Pressure and marine vs auto

Quote from Don S

Why don't you just get the proper mechanical fuel pump for your engine. It didn't have an electric fuel pump to start with.

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What your not getting is any electric fuel pump is wrong. your putting yourself in danger.
on my boat if the engine doesn't start the fuel pump turns off. The difference is your fuel pump will just keep running gas into the bilge and your boat can explode
Don S is THE most experienced guy here man if he talks, listen.

Re: Fuel Pump Pressure and marine vs auto

What your not getting is any electric fuel pump is wrong. your putting yourself in danger

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That's a load of dung. Lots of boats have 2 or 3 electric fuel pumps on them. It's all in how it's wired... that's all.

Re: Fuel Pump Pressure and marine vs auto

coastalcruiser said:

Don S is THE most experienced guy here man if he talks, listen.

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^^^ YUP!

There are some auto parts (mostly internals) you can get away with using on a marine engine, but a fuel pump is not one of them.

I expect that 5-9PSI pump is going to cause more problems than it fixes.

Re: Fuel Pump Pressure and marine vs auto

JustJason said:

That's a load of dung. Lots of boats have 2 or 3 electric fuel pumps on them. It's all in how it's wired... that's all.

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True,,,

But does the OP have the electric fuel pump wired correctly, kind of doubt it.

Re: Fuel Pump Pressure and marine vs auto

Lots of boats have 2 or 3 electric fuel pumps on them. It's all in how it's wired... that's all

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How do you know how is it wired? Did you read about how the motor originaly came with a engine mounted fuel pump?