A buddy of mine has a H/C/I stroked 5.0. He has Stage 2 heads and cams and a 4.6 stroked to a 5.0 with full bolts ons. He is currently running the stock 19# injectors. He made only 305rwhp on a dynojet.

My question is how much are his 19# injectors hurting his power? What kind of gains could he expect to see with going to 24# or 30# injectors?

 

My buddy has a HCI 95 Cobra that put's down 300 rwhp on a dynojet, an he has 24's IIRC. Considering that you buddy is making 305 rwhp (which for his mods sounds right on the money to me) i'd say that he would gain nothing buy going bigger.

Do you know if they had to stop the dyno short (not revving to full capacity) because of his injectors maxing out? If not it sounds like there would be no gain in upgrading.

 

He didn't have to stop his dyno short. I don't get how 19# injectors can be at 85-90% duty cycle at the stock power levels and not be maxed at well before 300rwhp.

 

He needs larger injectors. That combo should be at around 320rwhp, not 305. Whats his torque??? Running lean will cut down on torque. But tell him to up it to 24-30lb injectors and get it retuned and it should come out to be more power. If not, he knows he will have better driveability than before due to the injectors working less and having less duty cycle

 

This sounds like one of those questions for Cliffy lol. I didn't know the 19's were 85-90% maxed out at stock power levels. I know my stock 21's were good up to 330 rwhp at 5400 rpm on the juice, but they were 100% maxed at that point.

I'm basing my guess on the fact that 305 rwhp sounds about right (at least on dynos around here) based on his mods, but who knows.

 

yea, he is running stock 19's...which is bad!!! The reason they went to 21lb injectors stock was that the 19's were almost maxxed out with the PI combo. He definitely needs more injector for the combo/power, especially since he will be seeing high rpm's with the head/cam combo.

 

What did the AFR look like at peak HP? If it's not leaning out then installing larger injectors will not add any power, this said however I'd get a set of 24s just to provide some headroom--read more about this below.

What sort of fuel system is he running, do you know what the pressure drop across the injectors is?

If this is a standard 4.6L 2V returnless fuel system, with no modifications, then at WOT and peak HP the 19lb/H injectors are likely very close to going "static". I.e. being open all the time, 100% duty cycle.


Fuel injector ratings are determined, by industry standards, at a 43.5psi pressure drop across the injector. By increasing the pressure beyond that the injector's capacity will be increased according to this formula:



So, 19lb/H injectors operated at 60psi would flow 22.3lb/H.

As to power, here is the formula for calculating how much horsepower a given injector can support:



The big "fudge factor" here in the BSFC, Brake Specific Fuel Consumption, which is fuel consumption in lbs/HP. Classic values for this are 0.5lb/HP for natural aspiration and 0.6lb/HP for forced induction--however, for modern EFI engines I have seen values more commonly range from 0.40lb/HP n/a, to 0.55 for f/i.

So, plugging in the 19lb/H injectors, and choosing 0.42lb/HP BSFC and a 95% duty cycle we get:

(19 * 0.95 * 8) / 0.42 = 343.8 fwHP

Here are a couple charts showing the horsepower various common injector sizes will support, at BSFCs of 0.40 to 0.62.

These are not etched-in-stone values, but rather places to start when selecting injectors. Ideally injectors should be sized so that their maximum in-service duty cycle is 80% to 85%--never below 65%, and never above 90%.