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Old May 31st, 19, 10:39 AM
Big Dave Big Dave is offline
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Default Re: I canít find a whole lot on my engine

Originally Posted by Elvis View Post
That's a high pressure fuel pump, Dave.
Low pressure pump is the type like Alan mentioned. 5-7 psi.

I was referring to the in tank electric pumps. TBI is a low pressure pump, as the injector compresses the fuel in the throttle body for final atomization. The High pressure pump is controlled by the return valve on the fuel rail as to pressure but the sequential in port injectors where only open and closing not squeezing the fuel so they used a high pressure pump. That was before direct injection EFI was introduced which has to now overcome 170 psi dynamic cylinder pressure before the fuel is injected (atomized since liquid fuel won't burn only gas vapor will ignite).

The mechanical (bolted to the block) fuel pumps are controlled by the return spring pressure and for stock are set to 4.7 psi. I have not seen a stock pump that outputs fuel at 7 psi because that would cause most carburetors to flood. The fuel is not pressurized by the arm moving up on the eccentric, but by the spring pushing the fuel out from under the diaphragm as it resets the arm. The spring sets the pressure in stock style pumps. Not going to talk about the other mechanical fuel pump used by mechanical fuel injection (alcohol or nitromethane cars) as it gets very complex rapidly depending upon injection system and pump size, but pressure is important to keep the engine lit under load.

In a Holley brand carburetor the float generates only enough pressure applied against a .0.137 inlet seat to close it against 7 psi. 7.01 psi will force it off the seat and flood the carb. only a newb would attempt to run 7 psi with a carb because there are bumps in the road that would allow the fuel to blow off the needle and seat. That is unless you are building a custom carb with a bigger float, or a longer float arm to apply the buoyancy of the float against the needle on the seat (you could also restrict the diameter of the seat allowing in less gas to fight the pressure). Seven psi is the maximum allowed and pressure doesn't make more power, or add to reliability.

I have a half inch diameter fuel line fed to the inlet of my Dominator carb with the fuel pressure set at 4 psi. I have never run out of fuel since i changed over to a single Aeromotive A1000 fuel pump (had twin Holley black pumps before).

Here is the 305 bore with a 1.95 inch intake valve installed. I can see lots of valve shrouding with this set up which might make a 1.72 inch intake a better choice.

No body will run a 305 or a 396 because the bores are too small to make any power. The bores shroud the valves which adversely affects the ability to breathe. Bigger bore is always better.

Big Dave

Last edited by Big Dave; May 31st, 19 at 10:51 AM.
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