Fuel Injector Questions

[car12586]

I am a new member to this board, but followed many builds on JeepsUnlimited. I started my stroker project nearly 3 years ago and sadly just now got it together. I took alot of info from Dino's site and others from JU to get it built.

Either way it is in my 2001 XJ. Its a 4.6L Low Buck stroker with with P&P head and a Crane cam. I just finished the break in period and now im wondering about what injectors to put in it. It seems to run ok with the stock injectors, but i dont believe its running at full potential. I also have a slight hesitation around 3K Rpm's. From what I've read, it seens that i should maybe use 24# injectors, but according to FiveOMotorsports it seems i should be running 27# for my 2001 XJ.

Any ideas or help would be great. Thanks in advance.

[SilverXJ]

I have to disagree with fiveo. I think they forget that you have a 49 PSI fuel rail. I tried those injectors in my 4.6L and it ran to rich. I have exchanged them for a set of injectors that run at 23.6 PSI IIRC from them. I can't remember the number off hand. However I can't tell you how well they work for the engine since I have not installed them yet. What are your AFRs like with the stock injectors? I hope you aren't pushing it into WOT too much as it will most likely run leak with the stock fuel map and stock injectors.

[car12586]

I am not positive of my AFR's as i have no way to monitor them. I haven't pushed it too hard yet as i just got done with my break in period. Basically i needed to get it running so just threw it all together. What do you recommend for AF monitoring?

[Comanche91]

I have to disagree with fiveo. I think they forget that you have a 49 PSI fuel rail. I tried those injectors in my 4.6L and it ran to rich. I have exchanged them for a set of injectors that run at 23.6 PSI IIRC from them. I can't remember the number off hand. However I can't tell you how well they work for the engine since I have not installed them yet. What are your AFRs like with the stock injectors? I hope you aren't pushing it into WOT too much as it will most likely run leak with the stock fuel map and stock injectors.

Agree. I'm using the 96-98 Mopar #53030778, 23.2lb/hr @ 49psi. Work great.

[gradon]

A couple ?s comanche: Did you have the 23.2s in there before the unichip? If so, how did it run? I know on my 96, the 23.2s didn't cut it--way too lean(esp in the 2700-3300 range). The 25.5# I have on there now still need the pcm to add 10+% to the STFT to make the 14.7:1. Has anyone questioned how you're getting 300+ hp out of 23.2# injectors(I still don't understand how those injector calculators basically recommend 30# injectors for 300hp on a six cyl)?

[Comanche91]

A couple ?s comanche: Did you have the 23.2s in there before the unichip? If so, how did it run? I know on my 96, the 23.2s didn't cut it--way too lean(esp in the 2700-3300 range). The 25.5# I have on there now still need the pcm to add 10+% to the STFT to make the 14.7:1. Has anyone questioned how you're getting 300+ hp out of 23.2# injectors(I still don't understand how those injector calculators basically recommend 30# injectors for 300hp on a six cyl)?

The 23.2s came originally with my Hesco crate engine four years ago. It's a 96 block and head, and still OBDI. If anything, it ran rich at first according to plug color (didn't have an A/F meter at the time), and I switched from the Champion RC9s to a bit hotter RC10 plug. Then I did my dual exhaust upgrade and a few other mods and went back to Hesco for another dyno run. The engine had about 12K on it then. It was still running a bit rich over 3K RPM according to the dyno, and Lee set the FP down from 46 to 40 PSI and the over 3K numbers improved. Ran like this for another two years and fitted an A/F meter on in the meantime and all was well. Then recently installed the Unichip along with the 99 intake detailed in another thread. This dyno run is detailed in another thread and showed good improvements on the numbers. Best readings were attained with the FP set at 44 PSI. I had to return to Hesco once for some Unichip tweeks due to slight stumbling under hard acceleration, and drive about 150 miles or so until the ECU "learned" the new settings. My A/F ratio has been averaging 14.5/14.6:1,a little on the rich side still, so we're in the ballpark. It's running great.

I personally don't give a damn about tables and guidelines. These are just generalized recommendations, and do not apply to every engine or everyones driving style. The only real way to really know what's happening is to get it dyno tuned, which includes optimizing the A/F ratio. "Has anyone questioned how you're getting 300+ hp out of 23.2# injectors?" I don't know what you mean by that, but check out the dyno results on the other thread.

[gradon]

One more? What AFR did hesco set the wot to?

[Comanche91]

I asked Lee the same question. The A/F ratio @ WOT after he finished tuning the Unichip was 12.8.

[SilverXJ]

I personally don't give a damn about tables and guidelines. These are just generalized recommendations, and do not apply to every engine or everyones driving style. The only real way to really know what's happening is to get it dyno tuned, which includes optimizing the A/F ratio.

All those calcs say we should be running 27 lb injectors, and that is what fiveo said would work. However I found out that that isn't correct at all. I don't know if he was going off other engines of similar displacement or simply didn't realize my fuel pressure is 49 lbs. I suppose our brake specific fuel consumption is better than most.

[car12586]

What are some specific part #'s that will work for me? 27# is too much, 23# maybe not enough. Can i find something used? I've read something about Cobra injectors may work? Or there are some Accel parts that might work? What can i find that will work with the stock harness? Thanks for the help.

[gradon]

I read of somebody running 25# cobra injectors on a 2000+ XJ--I believe five0 or precision injectors carry them. Just make sure it is the ev-6 type plug.

[SilverXJ]

I read of somebody running 25# cobra injectors on a 2000+ XJ--I believe five0 or precision injectors carry them. Just make sure it is the ev-6 type plug.

Noooo... those are the 27lb injectors that I tried that run too rich. Did you get the 25# cobras from fiveo? The injectors that I have here tat flow around 23.5 or so are the Bosch XF2E. I haven't tried to run them yet, but hopefully shortly after the new year I can.

[gradon]

I'm running LT1's that are rated 24#@43.5psi. On the 49psi rail, that's 25.5#. Even with these, the pcm still adds 10+% to the fuel trim to make the 14.7AFR(so I could've went with the bigger 24# blue tops that flow 27# on the 49psi. I didn't think it was you Silver, but somebody said/wrote that they were running 25# ev-6 Cobra injectors(I haven't tried looking them up, so I took his word for it).

[car12586]

How are injectors rated? Are they rated at a specific fuel pressure? Whats the conversions to the 49psi that im running? What is the ideal flow rate for a mildly built stroker like mine? Sorry all this is slightly new to me.

[SilverXJ]

How are injectors rated? Are they rated at a specific fuel pressure? Whats the conversions to the 49psi that im running? What is the ideal flow rate for a mildly built stroker like mine? Sorry all this is slightly new to me.

Injectors are rated x lb at y psi. http://kgparts.com/index.php?page=fuel-calc for the calc

or if you like math: new flow = squareroot ( new-pressure / old-pressure ) x old-flow-rate