Drivability with larger injectors

[JBrobeck]

I am wondering what kind of drivability I can expect out of a set of 21 or 24lb injectors, on an otherwise stock 4.0L. Reason I am asking is that my injectors are pretty shot, and starting to leak (at the body itself, not the o-rings) and I am going to be building a stroker come mid summer. I figured rather than buying 2 sets of injectors (one set for now and larger ones when the stroker happens) I would buy a set of ford motorsports 21-24lb injectors and use an adjustable fuel pressure regulator to tone them down a bit if necessary till the stroker motor is built. Have any of you out here in 4.0L land done such a thing, and what were your results?

 

[Fryphax]

First off, I haven't done this so take my word with a grain of salt.

I believe the stock computer could handle *slightly* larger injectors with a AFPR, 24 lbs/hr might be a bit on the high side.
Chances are you would end up running rich, how rich I can't say. I believe you would see a slump in fuel economy, depending on what else you have done to your engine. If you are running the stock cat you may have problems with clogging if your running too rich As far as driveability, you might have difficulties getting it to idle well with that much fuel being dumped in, other than that I can't really say.


I really don't know what im talking about so take it how you want.

 

[Dr. Dyno]

Your engine will definitely run too rich with 24lb injectors but don't fret. I've done a lot of performance mods on my 4.0 and it still ran too rich with 24lb injectors. I corrected it by making a homebrew MAP adjuster and I turned down the MAP sensor input voltage from 5.0v to 4.2v. It worked like a charm and you could do the same.
The stock fuel pressure is only 39psi so if you lower it much further by way of an AFPR, your injectors may not atomize the fuel properly. The MAP adjuster is a better idea and it'll cost you less than $10 in Radio Shack parts. Take a look at my write up (link's in my sig.).

 

[MaXJohnson]

Your engine will definitely run too rich with 24lb injectors but don't fret. I've done a lot of performance mods on my 4.0 and it still ran too rich with 24lb injectors. I corrected it by making a homebrew MAP adjuster and I turned down the MAP sensor input voltage from 5.0v to 4.2v. It worked like a charm and you could do the same.

Dino, are you running an oxygen sensor? If not, I fail to see how your mods serve as an example for a sensor equipped 4.0

 

[tnomud]

I used to work for Accel DFI. Your ECU has a 10-15% corrections factor. Thos new injectors will more than likely be trimmed back out by the ECU and you probably wont notice a difference. I have gone from a 19 lb to a 24-26 on some cars and the ECU is still capable of trimming it back.

 

[5-90]

All 4.0 engines are sensor equipped. The purpose of the adjustable MAP is to trim the air signal to correct the air-fuel ratio and save possible clogging of the catalytic converter with carbon from unspent fuel. It's a workable mod, and isn't really a bad idea for bone stock engines, either, if you are into tuning (I just haven't gotten round to it myself yet...)

Check his homepage - he's got a lot of sound info and homebrew mod information you might like!

5-90

 

[Dr. Dyno]

Dino, are you running an oxygen sensor? If not, I fail to see how your mods serve as an example for a sensor equipped 4.0

No I don't have an O2 sensor nor a cat so your point is valid in so far as the PCM will be able to trim back the fuelling from oversize injectors when the engine is running in closed loop (deceleration, light throttle cruise).
It might not trim back the fuelling enough though when the engine's in open loop mode (acceleration, warm up) because the PCM ignores the O2 sensor in these situations, so the engine could run too rich under these conditions.
That's where the MAP adjuster can help out. If you reduce the MAP sensor input voltage below 5.0v, it basically fools the PCM into thinking there's more vacuum than there really is so the PCM shortens the injector duty cycle and trims back the fuelling when the PCM is in open loop mode. When the PCM's in closed loop mode, the voltage output from the O2 sensor will ensure that the A/F mixture remains stoichiometric.

 

[5-90]

Hey Dino - are emissions regs nonexistant in Riyadh? I wish they were here...

I either didn't know or had forgotten you'd stripped the HEGO sensor (you did strip it, didn't you? Or is in non-equipped on export XJ's? Further question - did you buy it in Saudi or in Cyprus?)

5-90

 

[JBrobeck]

you guys are awsome, I knew I would get some quality answers here. I was kind of thinking the computer may be able to adjust for the larger injectors, but I completely forgot about closed loop mode, If I run a MAP adjuster with the larger injectors, I am assuming the 02 sensor will be able to adjust for the lower voltage in open loop, and the lower voltage will lean it out a bit on the closed loop circuit... Is this logical thinking? I do drive quite a bit, so my MPG is fairly important to me, just wish I could build the stroker sooner (hmm.....those two don't really go hand in hand do they?!)

Hey 5-90......I can't speak for DR. Dyno, but its reeaalllyyy nice not having emissions testing/inspections in MN!!:yelclap:
I don't run a cat, egr, or any other emissions stuff either, I had a cat start a small grass fire out wheelin one time, so when it came time for replacement....well.....

 

[5-90]

I can imagine - I grew up in IN, where we could do pretty much whatever we want with our engines as long as there was no visible smoke coming from the tailpipe.

Bloody california...

5-90

 

[Dr. Dyno]

Hey Dino - are emissions regs nonexistant in Riyadh? I wish they were here...

I either didn't know or had forgotten you'd stripped the HEGO sensor (you did strip it, didn't you? Or is in non-equipped on export XJ's? Further question - did you buy it in Saudi or in Cyprus?)

5-90

Yeah, emission rega are just about non-existent over here. As long as your vehicles passes the emissions test during the annual inspection, you can pollute the atmosphere as much as you like the rest of the time. I passed mine last month with a CO reading of 0.95% and a HC reading of 120ppm at idle.
I bought my Jeep here in saudi. It came from the factory without an O2 sensor and no cat (Gulf export model).

If I run a MAP adjuster with the larger injectors, I am assuming the 02 sensor will be able to adjust for the lower voltage in open loop, and the lower voltage will lean it out a bit on the closed loop circuit... Is this logical thinking?

Te lower MAP voltage will take care of the A/F ratio during open loop, while the O2 sensor will correct for any potential leanness/richness during closed loop. Therefore you can have your cake AND eat it.

 

[JBrobeck]

Well.... Looks like I'll be ordering some new injectors soon here, this makes it nice cause I can essentially build my stroker, bit by bit and also improve my existing engine performance.