What PSI to run with this fuel setup. I will first have:

Walbro 255lph HP Pump
Weapon-R FPR
Stock Injectors
Apexi SAFC

Whats the best i can do basically with all that but still stock injectors? Also what are my boundaries with say 300zxTT injectors?

 

stock 4th gens injecors are 240cc per minute. 300zxTT injectors are 370cc per min. I have seen 4th gen stock injectors running over 13psi of SC boost.

which FMU are you using? your fuel pressure is determined by which FMU you are using. with an 8:1, I would say 6psi of turbo boost. that will get you up to ~70psi of fuel max. make sure you get a fuel perssure gauge and EGT to monitor your car.

 

I don't think he has an FMU... I think that is his question.

What PSI can he run WITHOUT an FMU.

I can't give a good answer, because I do not know. So I will say nothing

 

I was under the impression from his other thread, where he stated that his weapon-r had a vacuum line and was a boost sensative fpr?

96blumax want to clarify exactly what you are wanting to know here?

As far as running without any kind of fmu or fpr, not a good idea.

I wouldn't worry soo much about injectors as everyone else is making it seem - first just get your car up and running on 6 or 8psi, dyno it and see how things are looking and move forward. I know some people are running 12-15psi on stock injectors.

 

A little ricer math, er, I mean racer math, is always fun.

Without a rising rate fmu, the stock 240 cc/min injectors are good for about 220 engine hp (not fwhp), based on an 80% injector duty cycle, 250 hp if the duty cycle is 90%, and as much as 275 hp if you max out the duty cycle to 100%. I don't know what the Maxima ECU will allow for max duty cycle, probably in the 80-85% range. All this is based on a brake specific fuel consumption of .5 lb/hr/hp.

The formula for calculating hp capacity for an particular injector size is:

hp = (S X N X DC) / BSFC / 10.5

where S = injector size in cc/min
N = number of injectors
DC = injector duty cycle
BSFC = brake specific fuel consumption
10.5 = conversion from cc/min to lb/hour

A rough estimate of how much boost you can handle can be made by dividing the maximum hp by the hp your engine presently makes in unboosted form. This gives a pressure ratio from which we can calculate boost pressure.

For instance, my unboosted Max makes about 215 hp at the engine (based on a dyno run where it got 180 fwhp). Assuming a BSFC of .5 lb/hr/hp and an 85% duty cycle (again, I don't know what the maximum possible duty cycle is allowed by the ECU) then my 240 cc/min injectors should be able to produce 233 hp at stock fuel pressure of 43 psi. 233/215 = 1.08 is the max allowable pressure ratio. So the max allowable boost pressure is bp = 1.08 X 14.7 - 14.7 = 1.18 psi. Not much, huh? This is why people use a rising rate fmu.

If we raise the fuel pressure to, say, 85 psi, then we can increase the flow in the injectors by sqrt(85/43) = 1.4

Power is directly proportional to injector flow, so raising the fuel pressure to 85 psi gives a 40% increase in hp to 326 engine hp, or roughly 270 hp at the wheels.

Take all this with a grain of salt. There are some assumptions made here concerning bsfc and duty cycle which have a degree of uncertainty.

 

Nice calculations. Yes my fpr does have a vacumn line, and i had it hooked up for a while and adjusted it NA. The question really was if i was gonna have a problem with being too lean. I think you guys have answered that unless i want crazy power, i should be ok with stock injectors for now. Thanks.

 

Your fpr may have the vacuum line so it can sense when you're at WOT and bump up the fuel pressure from 34 psi to 43 psi (or whatever you have it set to), just like the OEM fpr does. It does not mean that it is a rising rate fpr that raises fuel pressure based on boost pressure.

The way to test is to disconnect the vacuum line and connect a MityVac to the fpr. You also have to have a fuel pressure gauge (which is a must have if you're boosted). Turn the car on and pump some air into the fpr with the pressure port of the Mityvac and see if the fuel pressure rises.

 

Ok, my best guess would be that it only triggers for WOT. I might end up getting a FMU anyways. You think 8:1 is good enought for the disc?

 

you have to run it with an FMU unless you have you ECU settings re-mapped for turbo setup. if he really wants to know how much boost he can run without the FMU....I would say 1-2psi because at 2psi of boost, it gave me around 50~psi of fuel, which is what we see when we WOT our ex-NA maxima.



96BLUMAX: 8:1 is enough and that's what am I using now. I am running very rich with 8:1. you can adjust your AFR with your S-AFC. But S-AFC will not lower the fuel pressure and once you turned up the boost, it might max and lock-up your injectors if you decided to go with an 8:1. I would get a Cartech adjustable so you can get a highest safe fuel pressure to run your car with. a vortech FMU is around 140 and a calibration disc is around 30....you can get a cartech around 185 on ebay.

 

So i might need the cartech to do rising psi with boost and the FMU to increase the fuel presssure. Do i need the weapon-r fpr to adjust the fuel pressure at idle pr can the cartech do all that?

 

cartech only adjust fuel p. under boost. vortech's super FMU adjust idle fuel pressure and boost fuel p.

 

So could i go with the cartech for the boost pressure control and the weapon-r for the idle control? Wouldnt that be the same as the Super FMU from vortech?

 

Yes.