How long does it drop for, and does it stay down?

What is the starting pressure you have it set to?

Do you have a vacuum/boost source attached to it?

Say I am at 60psi. When you nail it it drops to 50-52 for a second and right back to 60psi. No fuel adding under boost its atmospheric from the air cleaner.

Did you set your fuel pressure without a vacuum hose attached to it?

Base pressure is 43.5psi. Not sure if you are aware of that?

Yes the base is 43.5. And vac hose disconnected raised pressure to wanted we wanted. Basically trying to get a small injector to put out more with a higher pressure.

Lets say on a all factory setup @43.5 psi and you have TRD injectors with factory fuel pump and you WFO what would be the average drop if any? (Not my setup by far, but for the sake of conversation lets use this example)

With increased fuel pressure, the OEM pump or any other non high pressure pump, will show a dip in pressure. The OEM pump isn't meant for that.

With an aftermarket pump, it could be that the wiring is too small and causing a dip while it tries to ramp up to demand.


If you have a vacuum hose Tee'd from the same port of the boost gauge, to your FPR, then the pressure should be lowest at idle and highest in boost.

If you have the FPR vacuum hose from the intake tube, it should do nothing.

Do you have an upgraded pump?

Yes its a 340lph racetronix pump. I have thought about going to a relay system for the pump thinking that might help.

Injectors that I have now are the 660's. I have APR fuel rails. I am thinking that it would be normal to see a small drop in psi like 2 psi and a very quick recovery. What do you think?

But you don't have the vacuum/boost hose attached to it?

It could be the vacuum hose, and could be the amp draw on wires that are too small.

I too plan on increasing my wire size.

The vac hose goes to the air cleaner. I have tried to run it without a hose at all, there is no difference.

What is considered a normal drop?

I don't know what a "normal" drop is. I don't have a gauge.

This is where the vacuum hose should go.

http://www.xr-underground.com/vb3/showthread.php?t=50532

I dont think that the boost line should go to my FPR. Toyota works on the Bernoulli's principal. So they are saying that when the excelrator is mashed they are expecting some fuel psi drop. By adding psi after you mash the excelrator you are throwing this equation off.

If anyone can tell me what is the average drop that they see would be great.

When you are sitting at idle your motor is pulling vacuum and applying vacuum to the fpr closes it making the FP lower.. When you mash the gas your motor losses vacuum which opens the fpr allowing more FP..

Applying boost to the fpr allows for more FP..

I don't think there is rocket science behind it...

My truck was running a little lean... I put a boost line to it and it helped keep my AFR a little closer to where I want it...

Alright ya newbs. Fuel pressure with boost should rise 1 for 1. So you are either running out of fuel or something is wrong. That's the end of it. Hope this helps. Its called a boost reference regulator for a reason... our stock one is more than suffice. I never had an issue and mine rose with psi just like it should. That is when that little vacuum hose is supposed to be attached to the regulator!

Rising rate Fuel Pressure Regulators (FPR) and Base PressureWhile many cars may have rising rate fuel pressure regulators that increase the fuel pressure in the fuel rail when the manifold pressure rises due to boosted applications, the base pressure is always the pressure that the injector operates at, since the added pressure due to boost is on both sides (manifold and fuel rail) of the injector. These two “added” pressures therefore cancel each other out and the ECU can carry on assuming that the injector will continue to deliver the same amount of fuel as when it was at “base” pressure.

Bernoulli’s Principle and Fuel InjectorsYou may know that when the pressure in a fluid rises behind a constant size opening, there is a formula that we can use to determine the flow change due to this pressure rise. This was derived from Bernoulli’s principle, which most of us probably remember learning about in school. Applying this formula, we can determine exactly what flow change we can accomplish when we raise or lower the*base fuel pressureThis simple formula:*Q2 = {√(P2/P1)} x Q1Where:*********************************Q1*= Original injector flow rate*********************************Q2*= Injector flow rate at new pressure*********************************P1*= Original fuel pressure*********************************P2*= New fuel pressure*

All quoted from injector PROS. newbie. ....lol

XR said:

I dont think that the boost line should go to my FPR. Toyota works on the Bernoulli's principal. So they are saying that when the excelrator is mashed they are expecting some fuel psi drop. By adding psi after you mash the excelrator you are throwing this equation off.

If anyone can tell me what is the average drop that they see would be great.

Click to expand...

I threw that principle out of the window when I needed my aftermarket FPR to be a 1:1 rising FPR. The OEM FPR work decent enough for those that don't have an aftermarket one.

1:1 larger diaphragm FPR have been used and proven for ever. If you have your base pressure set to 43ish psi, then add the vacuum hose, as shown in the link I posted, and see how it reacts. You gain your first hand knowledge and it is easy to change back to "stock" if you don't like it.

And you can compare to how it is running now.

Well I solved my fuel pressure drop with a Racetronix 40 amp relay kit wired into a JMS Power max. I hold a solid 56 psi with my wonderful corn fuel. My Racetronix 340 LPH is now properly fed with up to 18 volts under boost depending on the knob adjustment. Check it out guys better then the KB boost-a-pump

So wire size was too small for the current draw.

Looks good man. On my To Do list.