AIC Tuning

Last updated: April 3, 2000

Date: Fri, 17 Mar 2000 17:13:47 +0000
From: "David Lane" (dlane@gigue.peabody.jhu.edu)

> I'm in the process of doing my motor with 3mm seals.  I am then
> going with a full 3 inch exhaust and an intercooler.  I will (of
> course) need to upgrade my fuel.  I am thinking about the HKS AIC
> with 2 injectors in the Greddy Elbow.  I will be running 15-20 psi
> of boost on the stock turbos. I'm also upgrading the fuel pump and
> adding an aftermarket fuel pressure regulator. This will all be
> dyno tuned.   Ive heard that the AIC is kinda hard to tune, any
> opinions out there???

The HKS AIC is actually easy to tune. The problem is that the unit itself gives you no frame of reference for your air/fuel ratio--which is what you need if you are going to tune it properly. You say you are going to run 15-20 psi on stock turbos. Even the lower side of that will put you into very dangerous territory unless you have a wide-range A/F rig hooked up for the tuning. Frankly, I agree with Max that 20 psi is beyond the design parameters of the stock turbo system. What I have to say would be best applied for, say 14-15 psi. I defer to 3rd gen owners for better figures.

The system you propose (boost dependent FPR + AIC) is the one I use on my turbo GSL-SE. There are some limitations to this kind of rig.

Neither device "knows" about the other, and your ECU hasn't a clue that it is getting "help," so you have to figure out the best way to integrate them yourself.
The AIC has no feed for engine temperature or ambient temperature--just boost and RPM. Thus, if you tune it on a hot day, it may be a bit off when the temperature drops. This would not be much of a problem in Southern California or Florida, but it is a concern in locations where there is wide variation in temps over the year.
The boost dependent FPR works through the stock injectors, and is thus affected by all the parameters measured by the stock ECU (as translated into the injector duty cycle). The boost dependent FPR also works through the additional injectors, but the AIC only "sees" boost and RPM. You can see how this can become confusing when you are trying to make fine adjustments.
Since the ECU, the boost dependent FPR, and the Additional Injector Controller do not "talk" to each other, and do not operate with the same input, you cannot control your fuel mix as consistently as you can with other fuel enrichment options (aftermarket chip, piggyback controller, Motec, Haltech, etc.). This is not much of a problem with a car like mine, running relatively low boost with a lot of headroom, but I am skeptical about the wisdom of using it for the kind of boost you specified. Your engine would have little tolerance for small variations in A/F mix. Besides, at those kinds of boost levels you would probably need some kind of ignition timing control to keep the engine happy. You get the benefit of integration and timing control in the more comprehensive aftermarket systems--even a relatively inexpensive chip upgrade.
Since the system you propose is relatively imprecise, and can change performance (literally) as the weather changes, you will need some way to monitor it at all times. I am a big proponent of inexpensive A/F meters for cars like mine, but such a meter does not have the resolution to tune the car properly for the kind of boost you want to run. Still, I recommend one as a gross indication of the health of your system. At the same time, a J&S knock sensor is mandatory to keep you safe when small changes bring you too near the edge of detonation. You said your car would be dyno tuned, but that tuning may not be accurate over a period of months and under changing conditions. Boost dependent FPRs are not particularly noted for being consistent over the years. As a final caution, I understand that dyno tuning by itself is not a guarantee of good results. I don't have the details, but one of the true gurus out there told me it was tricky--that what looks good on a dyno is not always the best setting for real life.

With the above background, I will tell you what I have learned about tuning the AIC. The instructions (at least with my earlier version) were pretty vague. They describe the functions of the various controls, but do not give a procedure for step by step tuning. Here is a basic way to go about it. The usual disclaimers apply. This is just what I came up with after living with my system for a few years.

Start with the AIC switched off, letting the FPR alone provide additional fuel as long as it can. Finding this point takes a little experimentation, but is made easier by the fact that since the FPR is only activated by boost, the adjustable parameters of the FPR will have served their purpose by the time the car is at the lowest RPM where full boost is available. I doubt that the FPR will take the car to 15 psi, but let's say (for the sake of argument...I have no experience with 3rd gens) that you can get the car safely to 12psi at 3600 rpm with just the FPR. You can determine this point because if you adjust the FPR for more pressure, you will get a rich condition when you reach max boost.
Now switch the AIC on. Set the boost threshold to activate at 12 psi (where the FPR runs out of poop), and set the Boost Gain around mid point or higher. The Boost Gain control simply adds more fuel as a function of boost pressure, so set it to provide additional fuel starting from where the FPR leaves off (12 psi in the example), and ending at highest setting you think you will use (say 15 psi). Said another way, the Boost Gain control is responsible for the different fuel needs reflected when you run the engine at 12 psi and at 15 psi. As with the FPR, boost pressure will max out at relatively low RPM, so the effect of this control will be most obvious in that range.
Set the RPM Threshold control to activate at the point the FPR starts to run out of steam (3600 RPM in the example). Put the RPM Gain control to maximum. This is the control that is responsible for taking the car from full boost (lowest rpm) to full boost at redline. With the RPM Gain control maxed out, you should find the A/F mix getting richer as revs build. Back off slowly until the A/F ratio holds steady.

As you do your dyno runs, you can fine-tune the car. If it tends to go rich or lean as revs build (but after full boost is reached) adjust the RPM Gain control accordingly. If the car goes rich or lean as it reaches full boost, adjust the Boost Gain control accordingly.

If you run into a rich or lean spot where the system transitions from the FPR to the AIC, adjust the appropriate threshold for Boost or RPM.

Once the A/F ratio is satisfactory over the range of boost pressures with which you intend to run the car, and once the A/F ratio stays stable as revs built to redline, you are done. The only control you might touch during day-to-day operation is the Boost Gain control, which will adjust the whole curve richer or leaner at max boost. As I said earlier, this kind of "shade tree" messing around with the system is of little consequence on a car running conservative boost. It could be disaster if you are trying to push 15 psi, and err a little on the lean side.

Bottom line:

AICs are not cheap, and you could probably get a suitable chip for your ECU that would give you better results with more control for the same money. If you really MUST get to 20 psi, I should think you would need an entire aftermarket engine management system, and you would have to ditch the stock turbos. Max Cooper covered that topic much better than I ever could.

On the other hand, if you are looking for a relatively low-tech approach for fuel supplementation with an aftermarket turbo, or for safety if you are going to push stock boost pressures by a couple of pounds, a boost dependent FPR coupled with an AIC is a reasonable approach. Just don't forget to add a J&S knock sensor to your budget. Sooner or later you WILL experience a lean running condition--either as you initially try to tune the system or later in its life when something goes on the fritz, clogs up, or comes loose. Maybe you will just get a little frisky with boost pressures to see if you can't just get a little more power out of it.

Better to see the lights on the J&S flicker than to hear the popcorn sound and see the smoke.