4.0L injector upgrade (math and science)

Hey all.
I recently decided it was time to upgrade my injectors is my mostly stock 98 4.0L HO.
I was going to to with the 703's from a 2.4L Chrysler, but could only find 4. I stuck'm in anyway just to see what they'd do when paired with 2 stock injectors, and much to my surprise, my mileage did in fact go up (back up to 20mpg with 6.5" lift and 33" shoes).
But, as I could only find 4 and I really wanted to do this right and I knew that running a mismatched set wasn't going to be healthy in the long run, I decided to order a set of 6 injectors from a guy on Ebay. I asked him for PN's but he said he had a bunch of different PN's and that he'd just send me a set that were flow matched and rated for the application.

He sent me some Bosch 700's (19#, without the clips). I did my research and while lots of guys say the 700's work great, the math just doesn't add up to me. Perhaps I'm missing something, but can someone who actually knows the math/science chime in?

Stock Jeep 4.0L HO (98) injectors flow 248cc/min or 23lb/h at 49psi.
The 700's flow 19lb/h at 43.5psi, which works out to 20.2lb/h at 49psi.

I understand that the ECM can adjust pulse width and that the Jeep injectors were never used to 100% duty, but the above math says that 700's cannot drive the 4.0L HO even at 100% duty (and they're only rated to 80% duty anyway).

The Chrysler 703's flow 22.5lb at 43.5 and 23.9 at 49psi. This seems like a much better fit.


Someone correct me if I'm wrong. I've contacted the ebay seller to discuss (always safer to have a dialogue open as soon as possible), so I want to make sure I'm not missing anything here.
From what I can tell (from the math), there simply is no way for the 700 injector to provide adequate fuel at higher RPMS and certainly not at WOT. At WOT, we'd actually be running at a deficit of fuel in the range of 13%.

Please, do not respond with "well i've got 700's and they work fine" or "i installed the 700's and got better throttle response, 20mpg increase, and 800hp. Even if the 700's were an improvement to your engine, it means nothing because we don't know the condition of your old injectors. I'm only interested in quantifiable evidence and data.

No experience with the 700's,but with the lower rating/flow I would not waste my time.
There is a member in the Sierra chapter, 831_xj, that clean/refurbs 703's and 784's. His price is very reasonable, not sure how much more shipping would be to Canadia. But since you already have a few 703's you would have few extra.

The 700's would probably be fine if you are at any kind of altitude since the PCM would lean them out anyway. If not, I would try to switch back to refurb stock or the 703's. The risk of getting just 2 is having mismatched flow rates.

The big risk with undersized injectors is not as much running the injector over the duty cycle (which will shorten the life of the injector for sure), but having the injector pulse still going while the intake valve is closing. Typically at that much on the positive side of trim the PCM will be close to throwing a mixture code.

This upgrade has mostly been blown out of proportion by something called a placebo effect.

It's not much of an upgrade whatsoever. You're just putting the same amount of fuel in your cylinders with a different spray pattern. The injectors themselves don't control the amount of fuel that goes from your gas tank to your motor. You're not going to save mileage with this. I've done the swap from clean/perfect original injectors and went to shop re-built four-port injectors. No real differences whatsoever, and definitely not worth the price.

thatxjguy said:

This upgrade has mostly been blown out of proportion by something called a placebo effect.

It's not much of an upgrade whatsoever. You're just putting the same amount of fuel in your cylinders with a different spray pattern. The injectors themselves don't control the amount of fuel that goes from your gas tank to your motor. You're not going to save mileage with this. I've done the swap from clean/perfect original injectors and went to shop re-built four-port injectors. No real differences whatsoever, and definitely not worth the price.

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I've heard this and understand the Jeep upgrade placebo effect very well, which is why I asked about the math instead of opinions . We can all list off at least 10 bogus upgrades that people swear by.

That said though, I couldn't find any problem with my stock injectors, spray pattern looked normal, they were pretty close to flow matched, but when I put those 4 703's in there I did notice a MPG increase by almost 2mpg, confirmed both by ECM data as well as at the pump. My driving style is very consistent so I'm confident it wasnt' a factor, especially as I was hammering on it pretty constantly after installing the 703's.

Again though, could just be evidence of bad stock injectors. I do think the spray pattern SHOULD help, but without some wideband O2's on each cylinder, it's really hard to precisely quantify any difference.

My "opinion" at this point, is that the 703's did increase MPG slightly, did increase HP/Torque slightly, and did smooth out my idle slightly. All in all, I stole the 703's because the wrecker kept ****ing me around and has no idea where their stock is at, so for a free upgrade, I think it was worth it.
Now, because I could only find 4x 703's, I paid $100 for the 6x 700's and they definitely ran a lot leaner until the ECM figured itself out. The math suggests that they'll be lean at WOT and anything over 3700RPM, but I have yet to definitively confirm this. Idle is definitely smoother with the 700's as well though.

blistovmhz said:

I've heard this and understand the Jeep upgrade placebo effect very well, which is why I asked about the math instead of opinions .

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The best math I can give you is that with the regular injectors, I would get 250 miles until gas light, and when I put in the new injectors, still 250 miles until gas light. Driving style was the same in both cases. Overall unnoticeable effect.

Anytime you change injectors you should reset the PCM so that it learns the new trims faster.

Any yes, the PCM will always trim the injector pulses so that the engine is right around 14.8% AFR. You shouldn't notice anything unless the injector is so far out of range that it throws a code. I only replaced mine a while back because two of the original injectors failed

Simply changing injectors without resetting the PCM will show a temporary change while the PCM learns. Unless there was something wrong with an injector, any long term change will be mental.

I did reset the PCM. I didn't rule that out. After the 50 warmup cycles, the results were like mirror images of each other.

You're right though. Unless you have problems with the injectors, swap really isn't worth doing.

I would never mix different injectors in the same engine. 703's are everywhere $10-15 each.
Buy 2 more 703's or try and install the 6 other ones you have.

A full tank trial to me is too much chance of other factors. I would think go to a gas station, reset your ecu, fill the tank, drive 50 miles one direction, turn around and come back to said station, change injectors/ reset ecu, use same pump to fill and try again. But in reality this isn't a placebo as much as just a non standard repair item much like an air filter that is never changed. If your going to replaced why not try something that might spray better ( and yes I know injectors gave same full circle).

imprezkid said:

A full tank trial to me is too much chance of other factors. I would think go to a gas station, reset your ecu, fill the tank, drive 50 miles one direction, turn around and come back to said station, change injectors/ reset ecu, use same pump to fill and try again. But in reality this isn't a placebo as much as just a non standard repair item much like an air filter that is never changed. If your going to replaced why not try something that might spray better ( and yes I know injectors gave same full circle).

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It's not like I only tested once and then decided it was a crappy upgrade. I filled up multiple different times (10-20 times) and results were always the same give or take 5 miles between fill-ups.

My driving has a pattern to it – I drive the same distance to work each day, and pick up family members from school every other day. There was no wheeling/different routes/unexpected stops/unexpected traveling involved in any way shape or form. I wouldn't be a racecar one day and a granny driver the next. I too wanted to see if the upgrade was worth it, so I drove just like I normally would after the swap. Computer was reset after installation.

I would say that offers pretty concrete evidence that the swap really didn't do much for me that was worth noting. Actually – it offered enough evidence for me to just sell the new injectors I paid $100 for and go back to stock and say "good riddance" after having gotten my hard-earned money back.

the biggest issue you will run into is high throttle/high RPM flow. The 20lb injectors will be fine for closed loop operation where the O2 sensors work with the PCM to get it right, but under open loop which is typically higher RPM and higher throttle %, you will be running very lean due to lack of O2 sensor feedback. I cant confirm for sure on the Jeep 4.0, but open loop fuel adjustments with narrow band O2s (what the Jeep 4.0 has) is nearly impossible and as such I have never seen a PCM that uses narrow band O2 sensor feeback in open loop whatsoever, which also means that there are 0 long or short term fuel trims in open loop. How lean you might run in that scenario is going to depend on the dynamic flow characteristics of the injectors, ie: how well the 700s flow at 49 PSI and 25%, 40%, 60% duty compared to the stock injectors at 49 PSI and the same duty cycle.

Short version: While cruising and lightly accelerating they would be fine, while juicing it you will be running lean.