Please note: This writeup was taken from an evolutionm.net thread. It is NOT specific to SR20s but will help set you on the right path.
Before you begin, you should have an idea of what you're aiming for. For EVOs, 19psi (131 kPa) seems to be a safe setting based on what people on the forums have found since it is close to what the stock boost pressure is, yet there is an increase in power due to the Greddy unit keeping the boost close to 19psi while the stock boost tapers off as the RPMs increase. I will henceforth refer to what you're aiming for as "desired boost pressure".
Definitions and things you need to know before you start:
SET This is how you set the boost pressure. Rather than setting it in psi or kPa, the Greddy unit allows you to adjust it as a percentage value, from 0% (greddy unit essentially turned off) to 100% (greddy unit will set the boost as high as it can). This setup demands a certain amount of trial-and-error to properly configure it since you have to make adjustments, then drive under WOT (Wide Open Throttle) and see what the maximum boost pressure achieved was throughout the entire RPM range. SET SHOULD BE SET TO A CONSERVATIVE VALUE WHEN BEGINNING TO TUNE YOUR GREDDY UNIT. 30% SEEMS TO BE A CONSERVATIVE SETTING BASED ON MY TESTING AND BASED ON OTHER REPORTED NUMBERS FROM EVO OWNERS AND TAKING INTO CONSIDERATION THE LINK BETWEEN SET AND GAIN (see GAIN below).
GAIN is defined in the manual as the value to adjust the "boost consistency". You don't really need to know exactly what that means. You should set GAIN to 0 when beginning, and you will then test the car under WOT while paying attention to the boost pressure. If the boost goes up and then falls off at higher RPM, you will want to increase the GAIN by a conservative amount (5% should be relatively conservative to begin with, then when you want to fine-tune it, you can go down to intervals of 1%). When you increase the GAIN value, the corresponding boost that you will go up to will be higher even if you leave the SET value alone. GAIN SHOULD BE SET TO 0 WHEN BEGINNING TO TUNE YOUR GREDDY UNIT.
START BOOST (also known as SET GAIN because that is what is displayed on the unit when adjusting this setting) is the lowest boost that the Greddy unit will begin increasing the boost from under WOT. You want this to be as close to the SET value as possible, since you want to keep as close to your desired boost as possible. However, setting it too close to the SET value will cause the boost to spike. You should set this to a conservative setting when beginning to tune your Greddy unit. Then you can fine-tune it later to get it as close to the SET value as possible without causing the boost to spike. Fortunately, you can set this in psi or kPa, thankfully Greddy didn't decide to let this be adjustable in % like the SET value. START BOOST SHOULD BE SET TO YOUR DESIRED BOOST PRESSURE MINUS 4 PSI (about 28 kPa).
WARNING is the maximum boost that you do not want to exceed. Fortunately, you can also set this in psi or kPa like the START BOOST value. When the boost exceeds the WARNING level, it will kick in the LIMITER, which decreases the boost a certain amount that you can set. WARNING SHOULD BE SET TO YOUR DESIRED BOOST PRESSURE PLUS 1 PSI (about 7 kPa).
LIMITER is the boost percentage that the Greddy unit will lower to when the WARNING boost pressure is hit. LIMITER SHOULD BE SET TO YOUR SET VALUE MINUS 4%.
PEAK is the peak boost value that the unit has seen since the last time it was cleared. To clear it, go to the peak boost display, and hold down the set knob until the unit beeps and "---" is displayed. IT IS A GOOD IDEA TO CLEAR THIS BEFORE YOU BEGIN JUST IN CASE YOUR UNIT HAS A HIGH BOOST ALREADY RECORDED.
LAST BOOST shows you the last boost that was recorded every time the accelerator is released for 3 seconds. TURN LAST BOOST ON BECAUSE IT IS A GOOD DIAGNOSTIC TOOL WHEN TUNING YOUR UNIT.
Keep in mind that when displaying in kPa, it does not show it technically in kPa, but rather misleadingly in bars, which Greddy inconveniently tries to justify by sticking x100 kPa next to the display. Therefore, 100 kPa will be displayed as 1.00 x100 kPa. Psi will also unfortunately be displayed in psi x10 so that 19 psi will show as 190, adding to the confusion.
Another very important thing to keep in mind is that when you first power on your car or the Greddy unit, WARNING will be set to 14.5 psi (100kPa, or 1 bar) until you interact with the Greddy unit by pressing any button. This "feature" is not documented in the manual.
Also keep in mind that atmospheric conditions affect the operation of your boost controller. When it is hot, you will get different results than when it is cold. One possible way of solving this issue is tuning your Greddy unit under the "Lo" mode for when it is relatively cold, and under the "High" mode for when it is relatively hot. Unfortunately, two modes are hardly enough for somebody that needs to account for very different summer and winter climates, and also for more aggressive settings for when increased performance is desired.
The maximum boost that you will see is also not consistent throughout the gears, which adds even more to the confusion. Unfortunately, if you've already increased your start boost to the maximum setting that doesn't give you surging, then there seems to be no way to get around this variance in boost pressure from low to high gears. I don't know if this is a limitation of the greddy unit specifically, or if it's something inherent to electronic boost controllers in general. The only two things that you can do to compensate is the following:
1. Set it to the "safest" of the settings that does not trip your limiter. To do this, tune the unit to your desired boost pressure in fifth gear.
2. Tune the "Lo" and "Hi" settings corresponding to having the boost maximized during the low gears and during the high gears. This would require you to manually hit the button to switch to the "Hi" setting when you shift to third gear or whatever you started tuning your "Hi" setting at. This is why Greddy makes the wireless remote switch that straps to your steering wheel to switch between "Hi" and "Lo" settings.
The following steps should be taken in exactly this order, taking into consideration all of the previous information:
1. Change boost pressure units to psi if so desired (see manual).
2. Set WARNING to your desired boost pressure plus 1 psi (about 7 kPa) (see above).
3. Set START BOOST (SET GAIN) to your desired boost pressure minus 4 psi (about 28 kPa)(see above).
4. Clear PEAK boost value (see above).
5. Set LAST BOOST to ON (see above).
6. Set GAIN to 0 (see above).
7. Set SET to 30% (see above).
8. Set LIMITER to SET minus 4% (26% if you followed #7).
9. Test for boost falloff at high rpm. You should probably do this in a wide open area with no other cars nearby and preferably no cops. It is also good to have somebody in the car with you that can watch the gauge while you concentrate on not wrecking your car. If there is no boost falloff, then go to #10. If there is boost falloff, then increase the GAIN by 5% and test again. Keep in mind that when you increase the GAIN value, the corresponding boost that you will go up to will be higher even if you leave the SET value alone. Repeat until the boost pressure does not decrease, or until you feel surging. If you feel surging and the boost pressure still decreases (not sure if this is possible) then decrease to the last level that you did not feel surging at.
10. Increase SET by 2% and adjust LIMITER accordingly, then test again. Keep increasing by 2% until desired boost level is obtained.
11. Increase START BOOST (SET GAIN) by 1 increment and test until surging is felt or the WARNING level is hit and the display turns red, then decrease to the previous setting.
Once you have followed these steps, you will have roughly tuned your unit. To fine-tune it, repeat steps 9 and 10 except this time only increase or decrease by 1 increment.
