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Adjusting MIVEC Air/Fuel Ratios

September 2005

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Back in September 2003, the web-based automotive magazine AutoSpeed published a very interesting article on a simple voltage interceptor kit they called the "Digital Fuel Adjuster" (DFA for short). The thing that really caught my eye was the expected price tag... under $150!

The unit was designed to adjust the output from certain types of airflow meters. Despite the fact that my MIVEC FTO uses a different system to provide "air volume" information to the engine ECU, I was beginning to formulate a plan. I thought this DFA kit might give me the ability to adjust the FTO's air/fuel ratio. I had never considered aftermarket engine management gear to be really worth the cost - not for an FTO, anyway - so even a rudimentary degree of AFR control would be a step up from nothing!

The DFA was to be sold at Jaycar. I resolved to buy the kit as soon as it was available in my local store. Unfortunately, "soon" turned out to be early 2005. Even then, it would be some time before the completed setup was taken anywhere near a dyno.

Mitsubishi 6A12 Engine Air/Fuel Ratio

Below is an image of a dyno run performed in late 2004. This shows power (HP at the wheels) and Air/Fuel Ratio (AFR). Click on the image so see a larger version.

The MIVEC rpm range is highlighted. Above 5400rpm, the MIVEC 6A12 engine uses high-lift, longer duration cams and more advanced ignition timing. This provides good, usable torque right up to the 8000rpm redline.

During this run, the factory ECU keeps a precise 13:1 AFR in the non-MIVEC rev range. The mixture can be seen to become briefly richer after MIVEC cutover. Above 7000rpm, the engine ECU is progressively enriching the mixture (probably intended as a safety buffer). By the time 8000rpm is reached, the air/fuel ratio is approximately 11.8:1.

Could my FTO's 6A12 engine benefit from changing the fuel mixture in the MIVEC range? Could leaning out the mixture result in greater torque? Or was the mixture a little too lean already, and require enrichment?

This Digital Fuel Adjuster kit could provide answers to such questions. It might also be able to wring a few more kilowatts out of the FTO. There was only one way to know for sure... Time to get busy!

Initial Construction

With my basic electronics skills, I set to work building the kit. To begin with, the unit was put together exactly as specified. Well, almost. I made life needlessly difficult for myself by getting an IC orientation incorrect! It was reasonably easy to see that something was not quite right, as the instructions mentioned nothing about smoke coming from one of the 10 Ohm resistors! Amusing now, but I don't remember it being very funny at the time...

After correctly fitting the IC (and replacing the abused resistor), the build was done. The image below shows the finished kit...

Great! But how does this thing actually work? What does it do, and how do you control it?

Theory of Operation

The DFA is an extremely simple tool. By way of a jumper setting or two, it is calibrated for a particular input DC voltage range, eg. 0-5V. It then takes an input voltage, applies a programmable "plus or minus" adjustment to it, and produces an output voltage.

It is important to note that there is only one input - the voltage being intercepted. There is no separate "RPM" input. The DFA simply carves up the input voltage into 128 "points", and applies a programmable lookup table of adjustment values. For an input ranging from 0V to 5V, "Point 1" would be 0V, and "Point #128" would be 5V. At each of these points, the voltage can be increased or decreased by a "delta" value. The programming is performed using a push-button hand controller with an LCD display.

The diagram to the right shows a simple example - an adjustment around "Point 64". The horizontal axis graphs the input voltage, and the vertical axis graphs the resultant output voltage. If the input range was between 0V and 5V, the illustrated adjustment would produce a slightly higher output around the 2.5V mark.

It can be seen that the DFA is not an RPM-based voltage interceptor in any strict sense. It cannot adjust input voltage based on engine rpm. Well, not unless the input voltage itself is directly proportional to engine revs.

This explains why the DFA is so useful as an airflow meter interceptor. Certain types of airflow meter produce a simple DC voltage that is proportional to airspeed. Lower rpm means lower airspeed, which means an output voltage at one end of the range. As rpm increases, so does airspeed, so the output voltage changes to reflect the increased engine load.

Used in this manner, the DFA can tune an engine by "fooling" the engine ECU into thinking there is more (or less) air being consumed at any given engine load. The engine ECU will react by adding (or removing) fuel accordingly, and may adjust other engine management parameters as well, such as ignition timing.

Airflow Meter? What Airflow Meter?

Now for the Bad News. The FTO GPX uses a Manifold Absolute Pressure sensor instead of an airflow meter. In the words of the DFA kit manual...

"In a naturally aspirated car which uses a MAP sensor to determine fuelling, altering only high-load mixtures may be difficult. This is because the manifold vacuum will drop to zero when the throttle is fully open - irrespective of whether the revs are at 1500 or 6000 RPM.

"Modifying the voltage output signal of the MAP sensor will therefore lean the wide-open throttle mixtures right through the rev range."

If such behaviour was undesirable in a normal engine, having two distinct sets of intake cams, exhaust cams, timing and fuelling maps could only make things more complicated. Welcome to MIVEC!

To address this, my plan was to utilise the MIVEC cutover itself. The FTO's engine ECU switches over to the high-lift cams by driving a pair of solenoids (which then direct oil pressure to the cam switching mechanisms). Given that there was a simple on/off signal from the engine ECU at 5400rpm, I planned on using this to switch in the DFA interception for high load only.

The result of such a setup would be to keep all non-MIVEC engine management absolutely stock-standard, while allowing a degree of fuel mixture tuning for the MIVEC range. From 5400rpm to 8000rpm, it should be able to change the air/fuel ratio to either run rich or lean.

This idea appealed to me for a number of reasons. First, the mixture adjustment would only be applied during high-lift, high-duration cam operation. This was likely to be more straightforward than changing mixtures across two distinctly different styles of engine operation. Second, I didn't imagine it would be all that complicated to find and utilise the ECU's MIVEC switching signal.

The first point would only be proved or disproved on a dyno. But as for the second point... Well, that took some doing!

Modifying the DFA for MIVEC Duty Only

The first thing I had to learn about was how the ECU switches stuff on and off. Instead of supplying +5V or +12V, it works by grounding the circuit in question. For example, when driving a relay, the "+" pin would be wired up directly to +12V, and the "-" pin to the relevant ECU plug pin. When the ECU wants to switch on the relay, it simply pulls the link to ground, completing the circuit.

Inside the DFA, there is already a 12V relay, conveniently used to switch between its "signal bypass" and "signal intercept" modes! Originally, this was designed to provide a delay of sorts, just in case some engine management computers took an instant dislike to the DFA before the car was even started. I rewired the "bypass" relay so that one pin was connected directly to the Digital Fuel Adjuster's +12V power input. The other pin was connected directly to the MIVEC Engine Control Relay wire from the ECU.

For those with an FTO Electrical Workshop Manual, the ECU connection that controls the "Engine Control Relay, Left Bank" is B-33, Pin 35 - Blue with Black stripe.

The next change made was to rewire the input signal so that it was always being processed by the DFA. One unwanted side-effect of the non-MIVEC bypass was the complete and total inactivity shown on the hand controller below 5400rpm. It was impossible to see if the unit was actually functioning correctly until the engine revs passed 5400rpm. It also seemed cleaner, somehow... now, the relay only needed to switch the output from bypass to interception, as opposed to switching both input and output.

The final modification was to add a couple of LEDs to the unit. A red LED now lights up when the unit is bypassed. Above 5400rpm, the red LED switches off, and a green LED shows that the DFA is intercepting.

I bought some 12V LEDs, as they would be easier to wire up. I had previously freed up half the relay pins when I had directly connected the MAP sensor input to the DFA processing circuit, so I had switching to spare. It was simply a matter of wiring up the relay to provide +12V to one LED or the other, depending on its switch state.

It all sounds so easy now, but getting everything working proved to be an exercise in frustration, to say the least! On several occasions, a perfectly good benchtest was followed by a woefully unsuccessful test drive. In the end, it came down to one final late-night attempt. I vowed that the unit needed to work flawlessly throughout the test drive, or it would not see another sunrise with its chips intact.

Did it work? You bet it did. Hardware kits always listen when you threaten them with the Spare Parts Bin! Well, nearly always...

The next morning, I booked the FTO in for some Quality Dyno Time.

Dyno-Testing Fuel Mixture Changes

Dyno testing of the FTO was performed by Glenn at Hyperdrive Motorsport in Malaga. I had no idea what to expect from the DFA, and Glenn had not seen one before. He was surprised that it had no clue about engine RPM, and could only affect the MAP sensor voltage based on "load points" from (you guessed it) the MAP sensor voltage.

The first test was to try and enrich the mixtures by a measurable amount. Having driven with the DFA installed for a week or so (not programmed with any adjustments), I knew the MAP sensor range that occurred at wide open throttle... from 108 to 115. We logged an initial "standard" run first, then programmed up the DFA with a plus-4 adjustment in the relevant voltage point range. This would alter MAP sensor voltage when flat-out, but would not change anything at part throttle. By setting plus-4 values in this range, we hoped to trick the ECU into believing that the atmospheric pressure was greater. Hopefully, this would prompt the engine management system to inject a little extra fuel to compensate, resulting in an enriched AFR.

That dyno run showed the Digital Fuel Adjuster was living up to its name. It was indeed adjusting the fuelling if the FTO's 6A12 engine...

Below the MIVEC cutover point, the red LED shone on the DFA, and the air/fuel ratio went unmodified. When MIVEC switched in, red LED turned to green LED, and the mixture on the graph became richer. It was not a big change, but it didn't need to be. It showed the DFA kit was capable of doing the following:

  1. It intercepts only when the MIVEC timing and cams are in use.
  2. It can demonstrably change the AFR by adjusting MAP sensor voltages.
  3. It can remap MAP sensor voltages corresponding to wide-open-throttle only.

On the performance side of things, there was a slight power drop with the air/fuel ratio set richer.

Lean and Mean?

The next test was to apply some negative adjustments to the MAP sensor voltage.

As excessively lean mixtures can result in detonation, this was going to be an exercise in conservative tuning. Adjustment values of minus-4 were tried first, then, with no detonation occurring, minus-6. This would tell the ECU the air pressure was lower that it actually was. Hopefully, the computer would react to this by reducing the quantity of fuel being injected.

Four test runs were tried - two pairs of repeat tests, in the following order:

  1. Modified MAP sensor: minus-6 adjustment from Points 107 to 116
  2. Cleared all adjustments, back to standard factory mixture.
  3. Modified MAP sensor: minus-6 adjustment from Points 107 to 116
  4. Cleared all adjustments, back to standard factory mixture.

These dyno runs showed that the modified tuning adjustments were definitely causing the engine ECU to lean out the mixture. The DFA was again working as expected. This little unit was capable of enriching mixtures or leaning them out... and only for wide open throttle in the MIVEC range. Not bad for something so inexpensive!

The air/fuel ratio graphs of the "retuned" runs were very closely overlaid, so the fuelling changes were consistent and repeatable. Below them on the graph were the two "factory tune" AFR graphs. The "retuned" AFR graphs were quite distinct from the "standard" graphs:

Most notable, however, was the lack of any change in performance when the leaner mixtures were used. The power curves from all four runs were indistinguishable. Indeed, dyno runs 1 (lean) and 4 (standard) recorded the exact same 150.3 HP peak power.

Fascinating! And, initially, quite disappointing!

This project had finally given me the ability to adjust the FTO's fuel mixtures, but it seemed there was no additional torque to unleash by doing so.

We discussed the possibility of testing a mixture that was leaner still. However, at 6400rpm, the "tuned" AFR was already touching on 13.5:1. Glenn commented that the FTO's "untuned" mixtures were already exactly where he would want them (up to 7500rpm, anyway). In the "standard" tests, the ECU was keeping the engine's AFR between 12.8:1 and 13.2:1 with great precision - it then enriched the mixture progressively from 7500rpm to redline.

If more MIVEC torque was available simply by running a leaner mixture, it would have been apparent in these dyno tests. It may simply be the case that, for most of the rev range, this FTO's mixtures are already dead-on for generating maximum torque. If so, further leaning out of the mixture would do little more than generate more heat - not to mention increase the risk of detonation.

So how can it possibly be the case that the factory engine management is running AFR at the "sweet spot" for maximising torque? Weren't stock ECUs meant to be extremely conservative in their EFI programming?

The answer was staring at me in the face. Leafing through a file of old dyno runs, I came across a comparison graph from 2004, and the penny dropped.

Previous Modifications to the FTO

In the engine department, this FTO GPX is not quite stock-standard. Close, but not exactly As Mitsubishi Intended. For a start, its induction system has been redesigned. While it still draws cold air from the front spoiler cavity and uses the factory airbox, its airflow piping is far less restrictive. An aftermarket exhaust Y-pipe replacement is also fitted, replacing the stock item that favoured quiet operation over performance. Both these changes resulted in measurable gains in torque... especially in the MIVEC range.

On reviewing my old dyno graphs, it is evident that the above changes had already resulted in a leaning out of the air/fuel mixtures. The following image shows a "before" and "after" test of the UAS FTO Y-pipe (also called "downpipes"). The dark curves are from a June 2004 run with the induction system fitted, but with a totally stock exhaust. The fainter blue curves show a dyno run with the exhaust Y-pipe fitted too...

The AFR was definitely leaner with the UAS Y-pipe installed, and torque was up from 5700rpm to redline.

Also, compare the "After Exhaust Y-Pipe Fitted" curve to the previous image's "Standard Air/Fuel Ratio" curve. Despite the car's mechanics being unchanged, the most recent dyno run showed slightly leaner mixtures again. Bear in mind we are talking about tests performed nine months apart... It could come down to the scheduled servicing performed, the cheap oil catch can I fitted recently, ambient air temperature, etc. It does seem to be a quirk of the MIVEC range only, though... below 5400rpm, the old Y-pipe test still tracked 13:1 AFR quite accurately. Interesting... this car just keeps running better and better as it gets older!

Anyway, it is fair to say that the mixtures are leaner now than they were a couple of years ago. The most recent tests seem to indicate that it is running at an ideal AFR for peak torque across most of the rev range. Adding more fuel resulted in lower torque, but taking fuel out gained nothing. So the "bad" news is that my FTO's MIVEC fuel mixtures are just about perfect as they are!

So where does that leave the DFA?

The MIVEC DFA - A Tool for the Future?

My goal of finding a cost-effective mixture adjustment tool has been achieved. That's great, but the first thing the tool did was to indicate no such tool was required!

But that's only true today. Who knows what the future holds for this FTO? When the remaining standard exhaust bits start falling apart, I'll be looking to replace everything from the catalytic converter back. If such a system sends the mixtures further into lean territory, I might need to use the DFA to enrich the air/fuel ratio to compensate.

The final verdict? The Digital Fuel Adjuster is indeed a useful tool. It gives a measure of control over the FTO air/fuel ratio, where previously there was none. I'm going to keep the DFA installed in the FTO, just without any remapping values set. It will be there when I need it...

Hmm... maybe I should buy that exhaust system... or perhaps some aftermarket MIVEC camshafts, and add some more fuel in... I could get the head flowbench tested and ported, and adjust fuelling to suit... Yes, I can see the inexpensive DFA is going to "save" me lots of money!

Additional Notes

The DFA was designed and developed by Silicon Chip Magazine.

The Jaycar product code for the DFA is KC5385. The LCD Hand Controller product code is KC5386.

If you would like more information, please feel free to post any queries on the FTO Drivers Club Forum.

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