Kingtal0n

okay ive got this equation.

(HP X BSFC) / Duty Cycle / #injectors.

thats easy. injector Lb. Per hour are rated @ 45 Psi no?

So lets say that equation spits 24 PPH at me.
How do i check what PSI I would need at, say, a 19 PPH injector to get it to flow, 24 PPH ?
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okay with THAT covered, lets say I've got a stock TPI setup. whats the max safest pressure for stock 19 PPH injectors?

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and as far as a MAF setup is concerned, will a speed density ECU still work? I have a ECU from a 1992 TPI car. the MAF tpi setup is from like 87...

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Heres a good one. its probably NO but what the heck ill ask anyways. Can I use a MAF sensor with the holley stealth ram?

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alright heres the last one, before I do somthing stupid. please tell me Im not crazy but...
Switching from my current performer RPM / holley setup to a stock TPI setup isnt THAT bad of an idea... Is it? I mean, to really get good fuel economy... without going the expensive route...

cp87GTA

You can use a stock, or modified, maf sensor with a stealth ram.

V8Astro Captain

iTrader: (1)

Just set up a proportion. (p/v) = (p/v) where p=pressure and v=volume

so X is the fuel pressure you want to make 19 pph injectors act like 24's. So you'd need around 56 psi.

Kingtal0n

Thats what i thought but i kept telling myself no it cant be that easy....

doc

Everything written above on equations is just plain WRONG.

This stuff has been posted many times, do a search.

But, I will save you the trouble,,, there are two main equations concerning fuel injectors,,, they are:

1) inj flow rate = [(FWHP) * (BSCF)] / [(# of inj) * (duty cycle)]

2) new inj flow rate = SQRT[(new pressure)/(rated pressure)] * rated flow rate

BSCF = 0.50 for L98 engine
.........= 0.45 for LS1 engine
.........= 0.60 for a supercharger

FWHP is FlyWheel Horse Power

For example: the flow rate of SVO 24#/hr injectors in an LS1 car:

= SQRT[58psi/39.5psi] * 24.0#/hr
= SQRT[1.468] * 24.0
= 1.21 * 24.0
= 29.1 #/hr

Note that the stock pressure in our 3rd gen cars is about 43.5psi,, those same SVO injectors would flow about 25.2#/hr in our cars. At a fuel pressure of 50psi, the SVO injectors flow 27.0#/hr.

The MAP and MAF stuff are not interchangable. Either you use a MAF with the appropriate ECM ('85 thru '89) or you use the speed density (MAP) system ('90 thru '93).

At 29.1 #/hr flow rate, the 24#/hr SVO injectors will be large enough to make approximately 414 FWHP using 80% duty cycle.
This is most likely what I am going to install in my '99.

Kingtal0n

whats the maximum safest duty cycle for injectors? I always assumed it was like 90% since thats how holley rates their systems. they use a BSFC of .45 and a DutyCycle of 90% to determine if the injectors supplied with their KIT will be sufficient.

doc

90% duty cycle max sounds good to me. I do know from personal experience that fuel injectors can run at static (100% duty cycle) for a short period of time, such as, a 1/4 mile drag strip, but this is not the best thing to do to your fuel injectors.

Kingtal0n

Why should it hurt the injectors? I thought it was bad because they were open all the time or somthing, and that was like a ton of fuel sloshing on top of your intake valve or somthing.

Does 100% duty cycle mean open continuouselly? or is it just open when its supposed to be, except longer. eh?

doc

At 100% duty cycle, the injector is open ALL the time,, its also called going static.

In order to open a fuel injector, electricity is applied to it,,, this electricity causes the injector to get hot. If the injector is too hot alot of the time,, you kill the injector.

For our batch fire cars, I think the a pulse width (PW) of 10msec at 6,000rpm means that the injector has gone static,, 100% duty cycle.

Does this help?

Kingtal0n

yes, and is it true that the stock injectors may lockup if you go over a certain PSI?

Is it also true that aftermarket injectors can handle alot more PSI?

doc

I dont know about the fuel preesure stuff. All I can say is that (if I remember correctly) I ran my factory injectors as high as 56psi which seems to be the upper limit on fuel pressure on my car.

Vader

I don't believe that the injector flow function is quite as liner as one might presume. It probably has a relatively linear plot to a point, but becomes asymmetrical as it approaches peak pressure. I.E. - linear - with a curve. If that isn't confusing enough, peak pressure varies depending on the injector used.

And for more confusion, if my information is correct, our ECMs won't operate the injectors at more than 80% duty cycle. That's peak output timing for the ECM, so the injectors don't fry. Tha could also explain some of the rated flow differences commonly quoted for the same injector in either a Ford or GM application. I'm not sure is Ford EEC-IV systems exceed 80% duty cycle, or by how much, but it would be poor engineering to run them in a static mode. Then again, we're talking about Ford....

Yee-Haah!

doc

Our ECMs will definitely operate the injectors at 100% if so commanded either by the PW calculation the ECM makes or by the called for PW in the fuel table ($32B).

Since our 3rd gen cars are batch fire, at 6,000rpm we achieve 100% duty cycle with a PW of 10msec. Sequential fire would make a huge difference here.

Vader: As far as batch fire is concerned,,, I understand that each injector fires once for each full rotation of the crank.

Vader

Yes, the injectors fire once per revolution, or twice per cycle.

IIRC, the maximum duty cycle in stock programming is 80%. Am I in err? (again?) Or are you talking about altered tables?

doc

When you sit down to calculate what size injector is right for your use, you should use the 80% rule,, that is 80% max duty cycle.

But I know that in actual use, fuel injectors are run sometimes at 100% duty cycle if that is what the computer calls for. This is not good for the injectors,, but injectors can be run at 100% duty cycle for a very short time period, such as, a run down the drag strip.

On my LS1 car, I have seen 20msec for the PW at (or near) 6,000 RPM,,, being a sequential system that means the injector is at 100% duty cycle. By rights, I should install bigger fuel injectors.

Also, I have seen 11, 12 and 13msec on my '87 IROCZ at around 5,000 RPM. This car has gone 100% duty cycle also.

I do not believe that there is any programming in the ECM which limits the injectors to 80% duty cycle which is what I think you are saying. Simply put, there is no checking by the ECM on duty cycle.