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Discussion Starter · #1 ·
Went to a Paint & Performance shop this afternoon to discuss getting my bike repainted & ended up in a conversation about replacing the fuel pump. The guy wanted to replace the stock pump with an external high flow model. He claimed that the stock pump delivers the fuel at around 30 psi & that the high flow model will deliver it at about 60 psi. He "guestimates" that the higher pressure, & hence more gas, gets him about 15hp without any other modifications ( ie. no exhaust changes, no intake mods & no re-mapping the EFI module ).

I don't know, but this all sounds a little suspicious to me. Anybody have any comments?
 

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Discussion Starter · #3 ·
It seems to me like the only thing he would accomplish is possible getting the fuel to vaporize a little more & also dumping more gas into the cylinders which more just make the engine run rich. I don't see why this would do that much for performance.
 

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The pressure is controlled by the fuel pressure regulator. No matter how much pressure the pump develops above the set pressure it will always be the same as the regulated pressure.
As long as pump volume keeps up with consumption there is no advantage to a higher volume pump either.
Mounting the pump outside of the tank would be a mayor safety issue on a motorcycle.

If one were to raise the pressure significantly by means of the regulator obviously you would reach a point where the OEM pump can no longer supply it reliably over time. Then a higher pressure pump would be needed, but you would still mount it in the original location.

The advantage of higher system pressure on a modified engine that requires a lot of fuel is that to some extent you can reduce the time the injectors stay open injector flow being a constant.
When you are locked into a existing system you can get a more favorable injector open timing as related to valve timing this way.

These things are best left alone unless there is a pretty good understanding on how they integrate with the rest of the subsystems.
 

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Discussion Starter · #5 ·
Like I said before the whole setup sounded pretty fishy. I'm sure that this guy was just trying to find a way into my wallet.
 

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The Harley pressure with no mods is 44lbs, you can get up 60lbs by installing a stage 2 ecm reflash. There are after market pumps which go inside your tank that allow you to set the pressure yourself with a built in regulator but are not really neccesary unless you have 117 inches or more,,,,,,,in my opinion the stage 2 reflash is all that is needed.
 

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Stage reflash doesn't change fuel pressure.

43 or 44 psi is where the MM's run, the Delphi's run 55 to 62.
 

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Da Hipster is right. The EFI controls injector pulse "width" or duration in milliseconds not the fuel pressure regulator which is mechanical - not electrical. There is no way that the stock injectors need any more pressure. In fact most of the fuel is returned to the fuel tank via the bypass because at 60lbs (Delphi) very little is actually "injected". Of course if you change the injectors and get a bigger flow rate for that 134inch stroker monster, then all bets are off.
 

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Hippo wrote it best. Most fuel pumps are capable of well over 75 PSI. It is the regulator that sets the pressure, and the ECM that control the amount of fuel (via injectors and pulse width) that enters. Raising the fuel pressure by modifying the pressure regulator is a trick to add more fuel for engine modifications that allow more air flow, but without changing the ECM mapping. It is by far better to control the fuel with better maping.
 
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