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Discussion Starter · #1 ·
Ok I already have my preference but I wanted to run this by you guys and get your thoughts. I have this buddy yeah him the guy who knows more then you do but still asks you questions.So he is running the 565 big block in a pro-street car with a 1050 Dominator for my money a bit too small but not exactly the reason for the post. He is running a mechanical fuel pump because his engine builder who according to him walks on water says electric fuel pumps are no good because if you ever stall the car electric pumps keep pumping fuel into the carb and thats how fires start. Ok maybe back in the days of Fred Flintstone I could slightly understand that logic but obviously he's never heard of a fuel pressure regulator or how to wire a fuel pump. Any fears about that happening can be simply addressed by running the circuit through the oil pressure switch to a relay. Engine dies> pressure drops> relay opens> power is cut to the pump.

But and there's always a but he asked me about running both pumps the electric to feed up to the mechanical then to the motor, I asked him why and his response was because some guy told me that how you can prevent vapor locking. I think more the truth of the matter is thats the only way that a mechanical pump would be able to handle the demand of that big a motor full out. I simply told him if mechanical pumps were better then why do they run electric pumps on race cars?

Your thoughts?
 

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The way I understand it is, in a carb, fuel level and overfilling is controlled by a float, needles, and seats. As the fuel level in the bowls comes up, it raises up the float, which pushes a needle that slides into the seat and seal, effectively closing off the pressure and stopping the flow of fuel. Which in theory would shut off flow, now if something fails in there all bets are off, but stalling is the least of your issues at that point because it would overflow even while driving.

 

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But and there's always a but he asked me about running both pumps the electric to feed up to the mechanical then to the motor, I asked him why and his response was because some guy told me that how you can prevent vapor locking. I think more the truth of the matter is thats the only way that a mechanical pump would be able to handle the demand of that big a motor full out.
There's a lot of stuff to do and undo here. Let's get going.

A mechanical pump is actually very good about preventing vapor lock as a general statement. It has good suction pressure where an electrical pump does not. An electric pump with a proper installation will do well too. What you need is gravity feed to the electric pump inlet (because it makes no suction pressure). This means that the pump must be mounted below the tank and that there is a sump or other means to assure there is fuel available to the electric pump at all times (especially acceleration in this case).

With either pump and a 565" engine, the fuel lines must be sized properly, especially the lines feeding the pump.

Depending on the power level of the 565" engine, there may or may not be a stock type pushrod operated pump that will have enough reserve to feed that engine at WOT. But there are belt drive and direct drive pumps (off the cam drive) that have enough capacity.

As far as the safety aspects like fires, etc., I don't see a lot of difference, because they are caused by dumb stuff like bad tuneups, badly adjusted regulators and carb floats, loose fuel fittings, etc.

As far as race cars having electric vs. mechanical fuel pumps, drag racers, at least with NA engines, seem to prefer electric pumps. A lot of circle track race cars use a mechanical pump. Either a belt drive or cable drive. Blown drag race cars tend to use direct drive mechanical pumps.

There's no pat answer. What's common among all is doing the system correctly.
 

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"There's no pat answer. What's common among all is doing the system correctly."
Rick L's statement is very correct.

However, some do run both type pumps together. An electric pump will overcome vapor lock, as well as push fuel through a blown mechanical diaphragm. A mechanical pump will draw fuel through a non-functioning electric (sorta) also. Running both is a back-up move to keep the car running to get parts or to get you home. I've done it myself before. When I've run only one type I always kept a replacement handy somewhere in the car. It's kinda like always keeping a new replacement fuel filter handy in an EFI vehicle....ALWAYS.
 

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I think if you're going to run both, you still need to observe the gravity feed rules for the electric pump. In this case it's just a transfer pump, and doesn't need any pressure capability.

I also don't see how one can be a backup for the other without replumbing. You can't reliably pump (or suck) through an inoperative pump. At best it's a big restriction. At worst it's like a plug.
 

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One thing to consider if running both electric and mechanical pumps - if the mechanical pump diaphram fails, the electric pump will pump fuel into the crankcase past the broken diaphram. Ask me how I know.
 

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Discussion Starter · #8 ·
But what he fails to realize is if he does run both then he's back to his original fear of if the car stalls the electric fuel pump keeps pumping fuel right? So you can't have your cake and eat it too or the best of both worlds. I asked a stock car buddy of mine about belt driven pumps and his op was really good choice if it were a race car only but too much for the street. From what I know if you wire an electric pump correctly and mount it as close to the tank as possible and down stream if you will from the tank outlet your not going to have any problems. If you spend the money on good pump not some off shore $50 piece of junk your going to get a pretty reliable product. Next door neighbor has been running a pair of Holley blue pumps in his 427 Thunderbolt for better then 12 years now without a lick of trouble. So I guess to each his own, but I'll go with the electric pumps
 
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