Fuel pressure question? AFPR, with Walbro 255 pump, TBI 60 pound injectors.

[96lt4c4]

I am working on my 94 Blazer and I am going to change things up a bit. The truck has a 383 in it with a very mild cam, World Product SR Tourqer heads, running the stock intake ported and 2" TBI bores. The TBI has 2" bores with all the spacers...etc running 454 injectors (80 pound per hour). The pump in there now is a GM later model truck pump for a Vortec. I have the tank out and I am going to swap in a Walbro 255 pump, pull the injector pod off and mod the FPR to make it adjustable, also going to stick the stock 350 (55 pound per hour) injectors back in. It seems like I have fought the tune on this truck since day one. Mainly cold idle, winter time, runs really rich. My question is what would be a good starting point to set my fuel pressure to with this setup. After its set I will leave it alone and work on the tune.

[96lt4c4]

Guess I should have looked at the FAQ at the top of the page. I am going to guess and say I have at least 300 hp, so it looks like I need about 16 pounds of fuel pressure. We shall see what happens.

[jameslleary]

Ooofff....60 PSI on 80lb injectors. Yikes. The 94-95 TBI 454 was the only one to use a high pressure pump, regulated to 30psi, but it used 40pph(@13psi) injectors, which ended up being 75pph, with the higher pressure.
What supply pressure are those 80pph injectors rated?

[96lt4c4]

Ooofff....60 PSI on 80lb injectors. Yikes. The 94-95 TBI 454 was the only one to use a high pressure pump, regulated to 30psi, but it used 40pph(@13psi) injectors, which ended up being 75pph, with the higher pressure.
What supply pressure are those 80pph injectors rated?

Sorry, I meant 60 pound per hour injectors, not 60 pounds of fuel pressure. I am running stock 13 psi fuel pressure with 80 PPH injectors right now. I am going back to the 55's and raising the fuel pressure.

[EagleMark]

Guess I should have looked at the FAQ at the top of the page. I am going to guess and say I have at least 300 hp, so it looks like I need about 16 pounds of fuel pressure. We shall see what happens.

Did you find the Injector Sizing Spreadsheet? You can enter fuel pressure, injector size engine displacement and see if it will support estimated HP. One big thing to factor in and included in spreadsheet is BPW to all this or your screwed...

[96lt4c4]

Did you find the Injector Sizing Spreadsheet? You can enter fuel pressure, injector size engine displacement and see if it will support estimated HP. One big thing to factor in and included in spreadsheet is BPW to all this or your screwed...

Yes I found the spread sheet, thanks. I will take a look at the Base Pulse Width.

[96lt4c4]

Did you find the Injector Sizing Spreadsheet? You can enter fuel pressure, injector size engine displacement and see if it will support estimated HP. One big thing to factor in and included in spreadsheet is BPW to all this or your screwed...

Which spreadsheet calculates base pulse width? I must have downloaded the wrong one?

Nevermind, downloaded them all and found it, man this site is sweet...like a kid in a candy store...LOL

[96lt4c4]

I do have another question though, why is it the stock tune shows the injectors as 60 PPH and they are known as 55 PPH? I always called them 60's because thats what the stock tune shows.

I just recalculated using one of the other sheets and it shows I need to run the 60's at 25 psi.

[EagleMark]

Once an internet rumor is started it can never be taken back!

Your 55PPH and 60 PPH are 61PPH...

[EagleMark]

I just recalculated using one of the other sheets and it shows I need to run the 60's at 25 psi.

I was waiting for that!

This also brings on many more tuning issues as the pressure is way outside of what the injector was designed for, can be done but way to much work.

But your 80PPH injectors, at 85% Duty Cycle and .5 BSFC and 17 PSI (more realistic) will support 311 HP, now the key to get a good tune with this spreadsheet is the BPW is 102. Or at 16 PSI = 302 HP and BPW 105.5. I er to a little bigger, easy to take away fuel in tune, hard to add if it was never there... VE numbers can come down but can only go to 100 and should really never pass 95, but having WOT only need 85 is sweet....

This is really KEY to tuning a modified engine with old 160 Baud ECM.

Now you probably have a newer 16197427 and it can be fudged a little after this calculation by changing injector size, smaller = more fuel and larger = less fuel, but you still need enough fuel to feed the beast at WOT or all your idle and drive ability tuning has been a waste of time. This fudging will only work so far and you'll be able to tune in low end or high end, usually you end up short at high end and injector duty cycle goes 100% or static... so only fudge a little after calibration.

I like to calibrate a tune correctly to start. If it does not have enough fuel to support WOT High RPM, then why start?