Brent

iTrader: (1)

I see, from time to time, folks asking about injectors, fuel pumps, FPR's etc and there seems to be some misconceptions floating around.

1 Horsepower requires approx .5 lbs/hr of fuel on an N/A engine. Yes, new cylinder head designs are capable of 1 HP per .45 lbs/hr and less but its better to estimate too large when picking your fuel system.

Lets say we have a combo that can make 330HP. This requires 165 lbs/hr of fuel. With 2 injectors that 82.5 lbs/hr each. Thing is we can't run the injectors at %100, maybe %80-85. So, we end up needing 2 injectors capable of 103 lbs/hr.

In TBI land the "best" injector we've got is the 17112560 (Yes there is a "90"lbs/hr unit but that is subject to some discussion) which has been tested at 75 lbs/hr @ 10psi. To get our 103 lbs/hr we'll need to raise the fuel pressure to 19psi. If we choose a smaller injector, say 60 lbs/hr, we'll need 30psi fuel pressure, which may be fine, on the other hand it is 3 times the stock spec and we may have trouble. Seems to me the closer to factory specs you can run your parts the better.

Incidentally the 17112560 came in the big block trucks which made around 235-240hp.

I choose the 330hp example because this is the highest HP TBI combo GM sold. Its the HT 502 kit for trucks. It uses the 17112560 injectors and the VFPR which provides 19-20psi @ WOT.

Once you determine you injector and fuel pressure requirements you can look at the fuel pump. Its almost a given that the stock 11 year old (in the best case) pump will not provide 165lbs/hr @ 20 psi. The bottom line is you will probably need to replace the pump with a high pressure unit.

RBob

iTrader: (1)

Would you happen to have the color code for this injector?

Thanks,

RBob.

Brent

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Blue/Black engraved with #17084304

and I should have mentioned that W. Sherwin gets credit for testing....

Chuck!

What about 13 psi on 45 lbs injectors? lol... I hate my car.

Brent

iTrader: (1)

I assume you are refering to stock L03 injectors...

I'm gonna go out on a limb and say there is no such thing as a 45 lbs/hr GM TBI injector as that size unit would require 100% duty cycle to make 170HP.

They are probably closer to 55lbs/hr.

That said, if all your engine requires is 3-4 psi over stock, and your fuel pump is keeping up (no pressure drop at WOT) then there isn't any issue that I can see.

Anyhow, here is another example...

I'm installing a 250hp combo in place of my L03. How should I get the nessesary fuel? Let's assume the L03 injectors are 55lbs/hr for the sake of argument.

There are several approaches:

1. Raise the fuel pressure on the stock injectors from 10 to 20psi.
We might find that the fuel pump won't supply 125 lbs/hr @ 20psi so we'd need a new FP. We may also find that the idle pulse width gets too small and things get unstable.

2. Install 17112560 75lbs/hr injectors and run them @ 11-12 psi. We might find the pump is inadequate here also but chances are better, due to the lower pressure, that it has enough capacity. The idle pulsewidth may be an issue here also.

3. Install the VFPR which raises the pressure on the stock injectors to 20psi at WOT and lowers it back towards 10-12 psi at idle (depending on your cam and manifold vacuum). This method may require a new fuel pump but it will help to avoid the possible short idle pulsewidth issue.


Perhaps the best thing to do is check the capacity of your fuel pump before making any parts purchases. You'd need a way to regulate the pump output to your target psi. Then you'd have the pump fill a container and time how long it takes. This would give you your pumps capacity.

Chuck!

I had the understanding that the LO3 did come with 45s - Source

That said with the mods below I dont think Im getting enough fuel, my pump just wont put out more than 13.5 psi with the afpr wide open. For a stop gap until I can change the pump, running ~80 pph injectors might do the trick, at a lower psi? Im looking for about 250-275 horse which I think is realistic given my setup.

Good thread

btw, where in Ohio are you?

Brent

iTrader: (1)

I've seen all the sites reporting 45lbs/hr but it doesn't add up.

170hp needs 85lbs/hr of fuel @ .5bsfc (two injectors flowing 42.5lbs/each) You don't run injectors at 100% DC. 55lbs/hr injectors will flow roughly 42.5lbs/hr @ 80% DC.

I personally wouldn't spend the money on injectors until I had a good pump installed.

Nate's 88

ok Brent, since your teaching I'll learn. Question: I have a 355 with a 440/454 lift cam with the stock injectors off the 305. have done nothing to the chip, TBI or much of anything else truthfully. I'm pretty sure that I'm getting nowhere near the needed fuel. Do you have any suggestions?

Brent

iTrader: (1)

First we need a reasonable estimate of your engines HP. What are the engine specs?

HP * .5 = fuel requirement

GNVAIR

I have a Vortec engine in mine and was looking to replace the injectors. I was considering getting the 65lb injectors and just upping the fuel pressure since mine would need approx 150lbs of fuel to feed approx 300 horses.
My question is this:
Is it better to run low pressure and very large injectors OR run higher pressure and small injectors?

Brent

iTrader: (1)

I think it depends on your situation..... Hows that for straight forward. argh

Some "tuners" claim high pressure results in better atomization. At part throttle some say this may be moot because the "wind shear" past the throttle blades and the hot manifold floor are what atomizes the fuel ....

Theoretically you could fuel 300hp by supplying two L03 305 injectors with 28psi. What that much pressure does to the injectors response at idle or otherwise I don't know. OR you could fuel 300hp by supply two L05 350 injectors with 20 psi. (fastbroker reported some dyno testing to this effect a while back) Or you could use two BBC 17112560's at 15psi.....

With 300hp, you are probably looking at a new fuel pump either way, so maybe it would be cheaper to try higher pressure regulator spring before buying new injectors. The only way to be sure would be to test the pumps capacity at different pressures.

Not very helpful eh

GNVAIR

No, that helped put things in perspective Brent and the info was much appreciated

r90camarors

I have recently gone through much pain with my fuel system for my 383 that should be putting out over 400hp. My guess would be that I'm at around 350 right now-do to lack of air. Anyways, this is what I have learned.

1) The stock fuel pump is very weak and cannot supply much pressure, the best I could get was 15psi, and that would drop off signifigantly under load. A Walbro 255 high pressure pump was the cure. Supplies all the fuel I need, at the pressure I need it at. Just keep in mind, the sending unit is at least 11 yrs old, and probably has some shotty lines. You might want to replace that at the same time.

2) I use the Delphi injectors w/ my holley, rated at 85pph @21 psi. I'm running 22psi right now, and have plenty of fuel, but can always bump up the pressure if I need too. The car idle nice too, a little rich right now, but that is because I need more tuning.

3) I've gotten the impression that GM injectors run better at lower psi, while the delphi are better suited for higher pressure.

4) I really believe that holley underates their injectors. I had the holley 670 w/ 57pph(@21psi) injectors on my 305. Only I was running 11psi, same as stock, which in theory would have put the injector rating somewhere near 40pph if I remember right. However, I was constantly running rich using the same chip, with blms in the 115-118 range

From my personal experiance, The TBI will run out of air long before it runs out of fuel. 670cfm @ 3"hg just isn't a lot of air flow

r90camarors

By the way, this is an excellent post Brent. Lots of good info here, with the key being matching the injectors to the engine needs.

kevm14

I have this theory also. A 91 L03 Caprice BIN shows a BPW of 132. According to my calculations, that works out to 55 lb/hr injectors. To assure that I didn't make a huge error, I ran numbers for MY car, which is supposed to have 68 lb/hr injectors. I got within 2 units of the BPW in my BIN (122 calc'd vs 120 BIN).

GNVAIR

Are the 85lb injectors what the Holley came with or did you add them. What application was the Holley for (was it a 454 truck?)
What do you guys like better? The Holley or 454 throttle body?
Is the Holley regulator externally adjustable without disassembly?

r90camarors

The Holley is Part 3 502-5 advertised for '87-'89 305 trucks. I origionally used it on my 305. It came with 57pph injectors, though other applications come with different injector ratings. I had to order the 85pph injectors when I decided to go with the 383. I think the 454 tbi is rated at 600cfm, where as the Holley is rated at 670cfm. Both use the 2" bores. You may be able to get a little more out of each with a polish and injector pod spacer. The Holley regulator is externally mounted and very easy to adjust.

GNVAIR

What is your opinion of the quality on the Holley throttle body? Any major modifications required to install it?

r90camarors

Personally, I love the Holley TBI. The injector pod is smaller and really easy to make a spacer for. The regulator is right there and really easy to adjust. The only thing I didn't like about it was that the little **** the throttle cable fit on the throttle lever is opposite of the stock one, but it is really easy to over come. And i needed chip tuning due to the injector size being different. Other than that, it is a direct bolt on, though it is really pointless unless you are running a carb intake with an adapter w/ 2"bores or the Holley TBI intake I am going to get my engine dynoed after a little more chip tuning, and hoping for around 360hp at the flywheel. I'm pretty confident the 2bbl can get there. When I get some money saved I'm gonna go with a 4bbl tbi and hopefully will be on my way to 400+hp

Knyghtmare

A buddy told me he could get mine to 400HP which would round out to about 350 or so... which is pleanty... I keep hearing that the L03 305 TBI can never ever produce that... why is that exactly? My buddy swears it can be done... "its the same block" and all this stuff he rattles on about... is it just the TBI holding her back after all cam, heads, intake and exhaust mods are complete?

Check this thread:
https://www.thirdgen.org/techbb2/sho...hreadid=185624