Bypass fuel pressure regulator systems

Vic Santos · 12-05-2015, 10:38 PM

I really thought hard as to whether to post on this subject but I thought some might be interested in my findings:
During the first 2 to 3 seconds of a run using a Q-Jet, fuel pressure to the carb is never constant no matter what fuel system you have! During launch the Q-Jet inlet valve actually closes multiple times for as much as 1/4 of a second at a time. The reason is the fuel in the bowl is pushed to the back of the bowl (by G force) and that is where the float is. At the same instant the G force acts on the fuel in the line supplying the regulator causing the fuel pressure to the regulator to go down by 5 to 10 psi depending on the G force and the length of the fuel line. At the time the inlet valve closes there is NO FUEL FLOW through the regulator without a bypass. At that instant pressure ratio across the regulator is varying wildly and the regulator mechanism is trying to cope. This does not happen with a Holley, only the Q-Jet due to its small, single bowl and float design.
That being said, will you notice a difference with a bypass? Only if you have a extremely well set up Q-Jet and multiple data logged sensors. The best Q-Jet set up for launch typically is not the best for the rest of the race. A racing Q-Jet setup is always a compromise. You'll have to try it and see. But you should test different setups.
The best thing you can do for a Q-Jet is to make sure your fuel system can refill the bowl as soon as possible and the carb bowl is as large as possible.
BTW the above info is for system that has a regulated bypass from the pump discharge to the tank i.e. a big pump. The pump is putting out constant psi and flow but the carb never sees it until G loads are stabilized.

12-05-2015, 10:38 PM	#10
Vic Santos Junior Member Join Date: Dec 2008 Posts: 27 Likes: 0 Liked 14 Times in 2 Posts	Re: Bypass fuel pressure regulator systems I really thought hard as to whether to post on this subject but I thought some might be interested in my findings: During the first 2 to 3 seconds of a run using a Q-Jet, fuel pressure to the carb is never constant no matter what fuel system you have! During launch the Q-Jet inlet valve actually closes multiple times for as much as 1/4 of a second at a time. The reason is the fuel in the bowl is pushed to the back of the bowl (by G force) and that is where the float is. At the same instant the G force acts on the fuel in the line supplying the regulator causing the fuel pressure to the regulator to go down by 5 to 10 psi depending on the G force and the length of the fuel line. At the time the inlet valve closes there is NO FUEL FLOW through the regulator without a bypass. At that instant pressure ratio across the regulator is varying wildly and the regulator mechanism is trying to cope. This does not happen with a Holley, only the Q-Jet due to its small, single bowl and float design. That being said, will you notice a difference with a bypass? Only if you have a extremely well set up Q-Jet and multiple data logged sensors. The best Q-Jet set up for launch typically is not the best for the rest of the race. A racing Q-Jet setup is always a compromise. You'll have to try it and see. But you should test different setups. The best thing you can do for a Q-Jet is to make sure your fuel system can refill the bowl as soon as possible and the carb bowl is as large as possible. BTW the above info is for system that has a regulated bypass from the pump discharge to the tank i.e. a big pump. The pump is putting out constant psi and flow but the carb never sees it until G loads are stabilized.