What should a guy expect a mechanical pump to do when he puts the patriot pedal to the freedom floor? Mine bounces around 7-9 at an idle (40gph unit) and drops to 4 under load. I need to drive it a little more to make sure it continues to hold at 4, but if it does can this be considered normal for a mechanical pump? Or should it stay closer to that 7-9 range it operates at at idle? Yes, 7-9 seems a bit high but it's what the pump does out of the box with no regulator. It's a China made Delphi pump.
Mechanical Pump Pressure Drop at WOT?
- Thread starter Supercharged111
- Start date
Pressure and flow are 2 different things. Sounds like there is not enough flow.
Maybe the cam eccentric is no good.
Maybe the cam eccentric is no good.
So I shouldn't be dropping to 4? If not, my first course of action would be to source a more rigid section of rubber suction line back at the tank. The 40gph pump does better than the 30gph unit it replaced, but I feel that the 30gph also should have been more than enough. The hard line from the tank to the pump is new.
What are you trying to supply fuel to? 40 seems kind of low for any performance motor. I normally start with a 110gph pump for a street car.
Delphi claims to be OEM replacement quality and I've had decent luck with them before but you can try a Carter, I've had much better luck with them. Or, the performance style mechanical pumps I've ran without any issue are the Holleys with adjustable base that start at 80 gph and go up from there. They run about 8 psi and should be paired with a regulator. I agree that 4 psi ain't right when trimming your toenails in the fan blade, even though carbs don't like more than about 5 or 6 psi before you can start to overpower the float. The lines may be sucking themselves shut but I think a slightly larger pump with a quality regulator set at 5-6 is your answer. It's absolutely possible that the eccentric on the cam is wearing down, as is the possibility of the pump pushrod being slightly bent or worn. If you should remove it again, roll it on a piece of glass to check straightness. The length should be 5.75".
It got a new pushrod back in October but I didn't measure length, I got it from AutoZone for a 454. I was under the impression that 40gph was gonna be overkill? Didn't GM put 30gph on the old 396SS cars back in the day? It's a peanut port big block with a 256 cam, idles like stock and completely done at 5000. And I'm at 6500' elevation, so whatever is going on will be exacerbated when we move to MI later this year. Back in November I replaced the entire suction hard line, it's all 3/8". Before that it'd drop the pressure to 0 if you stayed in it for a few seconds. It was worse with the 30gph pump that came out, but acted similarly so I'm not convinced either pump is junk. According to this link, the 40gph should be plenty.
Or am I missing something? Also, how much lift should the pump eccentric have? I'm leaning toward it being OK as the cam has hardly any miles on it.
Or am I missing something? Also, how much lift should the pump eccentric have? I'm leaning toward it being OK as the cam has hardly any miles on it.
You'd think that 40 gph would be plenty, and the numbers back that up. I've found that it helps to always err on the side of caution when it comes to fuel flow though, as some people's numbers are outright guesses. Has that Delphi pump proven that it flows 40 gph wide open? If not then we can't trust the claim. It's like buying amplifiers and speakers- "It can do this momentarily, This all day, and our numbers are a complete lie because we developed our own testing techniques".
I'm usually the first to say "Go Bigger" and I realize I'm saying that now, but before I go and mess up your project let's think of anything else that could be the issue. Are you 100% certain that the fuel pickup sock in the tank is free flowing and clear of any obstruction? And I'm not familiar with your build so I have to ask since I had a similar issue: What's the condition of the hoses atop your tank? Mine were a gigantic leak. Is your pressure gauge trustworthy?
I'm usually the first to say "Go Bigger" and I realize I'm saying that now, but before I go and mess up your project let's think of anything else that could be the issue. Are you 100% certain that the fuel pickup sock in the tank is free flowing and clear of any obstruction? And I'm not familiar with your build so I have to ask since I had a similar issue: What's the condition of the hoses atop your tank? Mine were a gigantic leak. Is your pressure gauge trustworthy?
Flow will give you pressure. Sounds like something is limiting the flow somewhere to me.
Sender was new Dec 22, tank is also silver in color. My current suspicion is the line getting sucked shut back at the tank. It's in good shape, isn't 1000 years old, is EFI pressure rated I think. But it's too easy to just swap it out and be sure, I just need to find someone who has a more rigid rubber line for this. I'd done a Google search and some folks were preaching that the pressure drop was all fine and dandy as the pressure will drop once that needle lifts off the seat, but I figured halving my pressure was excessive.
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