Fired up the stroker today, questions on injectors.

[DaveB]

Well I fired up the 4.6 stroker this morning and it went well, no problems that I know of and no leaks. I upgraded the cooling system with a new aluminum radiator, high flow water pump and thermostat housing and home made shroud. It never got too hot. I got smoked out a bit from the "ceramic" coating on the header which didn't last long at all. I left in the stock 93 known to work well injectors because I didn't want problems during the cam break in. Now that the initial cam break in is done I need to upgrade the injectors to fuel the motor properly. Most stroker web sites say to use 24 # Ford injectors and that is what came with the original stroker I bought which had to be rebuilt. The injectors are 4L8E-A4A-9F593 units which are two hole injectors with EV6 connectors. My Jeep's harness has EV1 connectors on it. I know you can buy an adapter that will let you use these newer injectors but would it be better to see if I can find some 4 hole EV1 24# injectors ? Does anyone know what pressure the Ford injectors are rated at ? and what Ford runs them at ? My OBD1 system currently runs the injectors at 39 PSI. I do have an adjustable regulator that I can use to dial up the pressure if needed. Do you guys take your strokers to a shop and have them test the final mixtures or just experiment till you get something that works ? By the way the specs on the motor are 4.2 crank, .030 over Kieth Black forged pistons, 4.0 rods MSD coil, and Cleggs stage 1 performance cam. I still need to decide if I should upgrade the throttle body or leave it as is.
Any thoughts ?

 

[MikeGyver]

I don't know why people seem to think so, but changing the injector size won't really do anything, there's a lot more to it than that..

Fuel injected engines have an o2 sensor which is used for closed loop feedback of the air fuel ratio. This feedback generates a fuel trim in the computer which applies this error correction to base settings in order to maintain the desired air fuel ratio. The fuel trims are set to only make so much correction, if you let/force it to go past this amount then you're forcing the engine into open loop control, which is just blindly running off some table and has no knowledge of whats going on or ability to change it. It's the absolute worst way to run a fuel injected engine. It's like forcing an engine into limp mode as a form of tuning it.

If the engine has a maf sensor you can get away with a lot more because it is measuring the amount of air going into the engine. But, I believe most jeeps are speed density mode where air is not measured but calculated off of a map sensor and a volumetric efficiency table in the computer. This table represents the physical motors breathing capability at every rpm and map combination, which you have now changed quite a bit.
the fuel trims will allow it to run but once the trims are maxed out it looses control.

The only time bigger injectors will really do anything is when the old ones are running past 100% duty cycle and are therefore starving the engine of fuel. You need to drive it around long enough to develop fuel trims, then measure the max injector duty cycle at WOT with an oscilloscope/multimeter. If it goes above 80-85%(which you don't want) then you can make injector size adjustments, but once you go above the fuel trim correction limit (15%, 20%?) then you're going to be running with maxed fuel trims where the computer has essentially lost control of fueling (and you haven't taken control either).

Now this isn't exactly a critical application, which is why most people just do whatever and it usually works ok for the most part.

 

[hoosier]

ummm... what mike said

 

[DaveB]

Well it sounds like I'll be busting out my oscilloscope. It would be nice if my scope could run on battery power so I could run it while driving under a load. Any idea how long it takes for the new trims to be calculated ?

 

[MikeGyver]

The map vs rpm table is usually split up it to like 16 segments or "cells". You need to operate the vehicle in each cell for a couple minutes to establish their long term fuel trim (once the 02 sensors and all have warmed up and gone into closed loop mode).

If you have a fluke 83-87 multimeter they have a duty cycle setting. Another way is to make an appropriate low pass RC filter which will give you a ratiometric voltage representing percentage duty cycle (dc voltage read/voltage max = percentage).

Another thing, it's all a complete guessing game anyway unless you can actually see what the AFR is at. I'd recommend getting a AEM wideband AFR gauge (like $180 on ebay I think).

 

[Kevin B.]

I was about to ask what point he should be looking at a wideband. Thanks for the posts, Mike!

 

[DaveB]

I have an older Fluke 87, don't know if it has the duty cycle measurement but I'll look. I'll have to look into the AFR gauge. Is there a way to use a scope on the O2 sensor and figure out the AFR ?

I just looked up the specs on the AFR gauges and the stock O2 sensors and I would guess that the stock sensor is a narrow band so it won't provide much useful info. I have hooked my oscilloscope to a Jeep O2 sensor and it was basically a very slow square wave.