Last updated: April 3, 2000
Date: Fri, 17 Mar 2000 17:13:47 +0000
From: "David Lane" (email@example.com)
> 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
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
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.
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.