How low can I safely set my fuel pressure?

[jimberg]

It couldn't possibly be true that the injector pulse cannot be shortened. If that were the case, the example in the book about running 20% rich and being able to adapt would be false. To do so it would have to decrease the length of the injector pulse. Other than that everything else that he said doesn't contradict what I've been saying.

You still don't seem to understand what I am saying about why you would raise fuel pressure.

(after all if it did then there would be no sense in upping your fuel pressure because the pulse width would shorten)

It makes perfect sense since you don't raise fuel pressure to increase the amount of fuel in relation to air. You WANT the pulse to shorten. I've said all along that fuel pressure has nothing to do with air/fuel ratio across the rpm range. The reason you raise it is if you are pulling in more air for which your injectors can supply fuel. Leaving all of the adjustments aside, let's just say that 19# injectors can support 304 HP at 39psi. You've made modifications to your car that makes your car capable of 336 HP. You start running lean at WOT, though, since your injectors can only handle 304 HP. You need 21# injectors, but there's no such thing. You could always get 24# injectors to support up to 384 HP, or you can bump up the fuel pressure to increase the flow rate of your 19# injectors to be the equivalent of 21# injectors.

Now let's take the flip side of that. You're at warm cruise going steady at 70 mph. The computer is in closed loop mode ensuring that you are getting the best possible gas mileage and producing the least amount of emissions. It has a target air/fuel ratio that it needs to keep. Whether you have 19# or 24# injectors, the amount of fuel it uses to maintain the a/f ratio is the same. The difference is the pulse width of the injector. It's shorter for the 24# injector. If you raised your fuel pressure, the computer would simply adapt by lowering the injector pulse length.

I suppose a lot of guys on here with superchargers would be disappointed to find out that they can only be safe at 40psi.


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351W 89 Mustang GT Convertible