Hutch

Here's the deal... coupla months ago I installed a bit bigger cam in my LT4... swapped out my Comp 210/224 for a Crane 222/230 unit. Runs like a champ and feels strong on the street but dyno power increase just wasn't as much as I was hoping for. As I recall from the dyno, peak power (364 RWHP) comes on around 6200 vice 5700 for the previous cam.

But the real test is on the track, right? So the other night I ran it for the first time with the new cam. All is well until the last 1/8th mile and it just felt like it was not getting enough fuel. No surging or bucking, mind you, just felt like it wanted to go faster but couldn't.

When the cam was installed, the fuel pressure was cranked up a lot higher than it was before to get peak power. That got me thinking that perhaps it was trying to over compensate for low fuel flow through the injectors. Logical?

Any way to tell for sure? I don't want to drop $500+ for larger injectors if I don't have to. After all, the LT4 has bigger injectors to begin with, right? Just don't know where to go from here.

Oh yeh, it has CNC heads, CNC intake, 52mm TB, ram air, shortys, power effects, random cats, 4.10's, Ed Wright program, etc, etc.

Jimmy69

John,

Here is the formula for injector sizing vs. HP.

Engine HP = ( Injector Flow Rate lb/hr ) * Number of injectors x % max duty cycle) / (BSFC)

Your application looks like this:
430hp = (28 lb/hr) * ( 8 x % max duty cycle) / 0.5

Which calculates to a Max inj. duty cycle of
96%. I'd say from that some bigger injectors are in your future.

Also cranking the fuel PSI does increase your injector size, however you need to tell the PCM that you have bigger injectors. Time for OBDII LT1-Edit (or send it back to Ed).

Jimmy

GlennS87

If I were you I'd bring it to a dyno shop with a wideband O2 (preferably one upstream of the cat) and check your A/F ratio. I would also get scanner info and send it to Ed Wright.

Good Luck

Hutch

Jimmy -- Great little formula there. I better hang on to this one. What is BSFC and what % inj. duty cycle should one aim for?

Glenn -- I *did* have it at a reputable dyno shop. In fact, they did the cam install. Its all greek to me, mind you, but they claim the A/F ratio is OK. Called up Ed Wright and he said my current program (one of his) should be OK. Go figger.

All I know is that I paid more money and now I'm going slower. Somehow I think I'm doing this all wrong.

Thanks guys.

-Hutch-
hutch@hashmarks.com

LPDesRoche

For what its worth, in my 90 I had 30# injectors. And I was making 360 RWHP and 411 RWTQ. So our number are kind of in line. I know they were not maxxed out but I'm sure that 24's weren't going to get the job done. I like having the insurance. Not to mention a little room for growth.

Lee
90 Coupe 383 (sold)

65Z01

gs633, BSFC is Brake Specific Fuel Consumption. For normally aspirated engines it is .45-.50; for TPI ususally taken as .45.

Normally 80% is a good max duty cycle to use since the injectors can hang up above 85%.

Fuel flow varies as the square root of the fuel pressure:
NewFlow = SQRT(NewPress/OldPress) * OldFlow

I have a table on my site for one BSFC value that gives HP supported by various injectors at several fuel pressures.

Chris@SD

When you crank up the fuel pressure, you are effectively changing the flow rate of injectors. I will have to dig up the formula, but GM 24# injectors are rated at 42 PSI. When you turn them up to 55 PSI, they are now ~26# injectors. You need to compensate for that in this formula! Also, 80% duty cycle is ideal for the injector. Going beyond that is not really going to hurt it unless you are sustaining 90-100% duty cycle for extended periods at a time.

At that RPM and HP, you should be fine with the injectors and higher pressures. Have you changed your fuel filter? Have you checked pressure at WOT? Tap into the schrader valve on the back of the fuel rail with a gauge and lay the gauge on the windshield, then take if for a spin. The Dyno does not mimic real world conditions so you need to check it this way.
Good Luck,

Hutch

Thanks for the input, guys.

Looks like I should consider some 30#'ers, I guess.

I guess my first step should be to check the fuel pressure at WOT, like you said. Any idea what pressure it SHOULD have?

Maybe a dirty fuel filter? I thought that was changed not long ago. Dunno, I'll have to check. What about fuel pump? I guess if the pressure is OK I shouldn't have to worry about that, eh?

I suppose my 52mm TB should still be sufficient, yes?

Thanks!

-Hutch-

95vettski

I think you should be fine with your injectors. I have a similiar combo dynoed 370rwhp and that's with the stock 24lbs injectors. If you adjust your fuel regulator the pcm will compensate, best way is via programming perferably on a dyno with a wide band. :cheers:

AquaMetallic94LT1

I would consider going to 36#s if I were you. LT4 comes with 28# injectors. You could go to 30# but its not that big of an increase over what you have now. Consider a larger fuel pump and make sure the fuel filter is not clogged.

Here is some food for thought: BSFC @ .50, Duty Cycle @ .8

Stock LT1 is rated at 300 hp - the injector formula works out to be
(300 * .5)/(8 * .8) = 23.43 lbs, stock inj is 24 lbs.
Stock LT4 output is actually 360 - formula says
(360 * .5)/(8 * .8) = 28.13 lbs, stock inj is 28 lbs.
Your mtr is approx 430 - formula says
(430 * .5)/(8 * .8) = 33.6 lbs

A duty cycle of .8 is the safe value used for stock and most aftermarket pintle style injectors. There are disc type injectors that can work safely at .9 duty cycle. These are much more expensive ones.

I am using 36#s in mine because I figured I might get up to 450HP which using the formula works out to 35# and leaves me some room for extra hp. The downside to injectors over 30lbs is that the price goes up quite a bit.