Again a disclaimer – I am not a Tuner, just a guy with a lot of time. This post describes how I tune boost control and nothing more. This post is in response to the myriad of questions regarding overboost, underboost or GM/Prodrive/Perrin EBCS tuning related questions. There are a few people who believe what I say about tuning, but none of them would call me a tuner either. The images of certain tables are taken from Enginuity.
Disclaimer two – Your car has a value reported by the ECU I like to call “The Happy Value”. Classically known as the IAM( Initial Advance Multiplier). This number is a value that generally goes up when the car is not experiencing knock, provided the right conditions are met, and will drop if knock is detected. Logging this Value is a must at all times when tuning your car. For 16 bit ECU’s, the ideal value is 16. For 32 Bit ECU’s, the ideal value is 1.000. If your logs reflect a drop in this value, there is a problem with your map or the conditions you have placed upon it. These values MUST BE CORRECTED before proceeding with any modification. The information below does not address corrective action for dropping IAM’s due to timing or fuel related issues. If you are inexperienced with tuning and cannot correct knock related issues or are using an OTS map with no support and experience dropping IAM’s, your best bet is to reflash the stock map until you can have your map tuned by a professional.
On to the show…
First off let’s define what is going on with the boost control system in the most basic terms.
The ECU has a “Boost Target” map. This map may be under different names for different model years and models but it will look the same. This map sets maximum boost levels based on Throttle and RPM.
The ECU also contains a map called “Max Wastegate Duty (MT/AT)”. Some cars have several tables associated with this map; however for the 02-03 WRX there is only one which is applicable. This map sets Wastegate Duty Cycles (WGDC) also based on Throttle position and RPM.
There are two other maps called “Turbo Dynamics Burst” (TDB) and “Turbo Dynamics Continuous” (TDC). These maps add or subtract WGDC% dependant on the amount of boost error the ECU detects.
The WG actuator itself is not a digital device; it is not on/off. The actuator will open the WG flapper door a little or a lot depending on the WGDC and the pressure it sees.
So if we relate all these maps we can surmise that we set a Boost target, and then adjust the WGDC to achieve the target. Here is where the fun begins.
This is the stock 02 WRX Boost map.
It runs max boost of 13.61 PSI at >76% Throttle from 2380-4800 RPM then tapers off. Notice the drop to 6.65 at 6800 RPM. For throttle or RPM positions between the column and row values, it interpolates a number between the two, i.e. 6000 RPM at 100% throttle would be between 11.91 and 6.65 PSI.
This is the stock 02 WRX WGDC Map.
As you can see, the WGDC is almost 100% at low RPM across the board then tapers off again. Notice the 0.00 WGDC at 6800 RPM and how it corresponds with the 6.65 PSI on the boost map. Basically, at this RPM, the car is running WG pressure.
This is the stock 02 WRX Turbo Dynamics Continuous table.
In this map, negative values correspond to overboost and positive are underboost. Huh? You say. Well the ECU bases the boost error on the target map. So when you are under target boost, the ECU boost error reading will show the calculated PSI under that target, i.e. target is 16, current is 14, error is 2. On the opposite end, if you shot over, it will return the negative value.
I came to a fork in the road with two paths….
The Factory Boost Control Solenoid (FBCS) controls boost by bleeding pressure away from the WG actuator back into the intake. With this pressure being bled away, the WG will remain closed until the WGDC drops low enough for the WG to see pressure then it will open. The duty cycle is a ratio of the percentage of the cycle that the FBCS is bleeding this pressure. So a WGDC of 90% would have 90% bleed/10% to the WG. I have not confirmed the Cycles per Second but I believe it is around 10hz.
The two crude images below show the possible paths for pressure with the FBCS.
In this position, the FBCS is energized and the internal plunger has been pulled back by the electricity running through the solenoid. The WG actuator actually has some boost at it at all times in this configuration because the FBCS cannot bleed enough pressure away, hence the very high WGDC using a bleed type system.
Here we see that the FBCS is no longer energized and not allowing any pressure back to the intake. The pressure is then sent to the WG actuator, which opens the WG flapper
When people speak about adjusting the WG arm, they usually mean shortening the arm to allow more overhead of the WGDC. By shortening the arm, the actuator will have to push harder before it opens the flapper door. Since as I stated before, the actuator is not digital, the shorter arm will increase the load on the actuator, or possibly put it in the proper position that the factory doesn’t in the first place. The point of this is that by shortening the arm, you will be able to use less WGDC to achieve target boost in most cases.
A 3 Port (GM/Perrin/Prodrive) works by blocking or “interrupting” the flow of pressure to the WG.
The two crude images below show the possible pressure paths using an interrupt style solenoid.
As you can see, the solenoid just blocks the boost from reaching the WG altogether. We also have removed the “T” in the line and the restrictor pill. This diagram doesn’t take into account actual connections obviously, just the principle of the design.
With no power to the solenoid, the unit will open up allowing pressure to go to the WG and actuate the flapper door. The third port on these solenoids is used for allowing extra air out of the system and is also used in external WG setups. The path of pressure is much more direct in this setup allowing for a much lower WGDC. There is still some latency between the opening and closing of the solenoid and it is still cycling at the same frequency, whatever that is.
What to do, what to do….
So how do we tune the FBCS or a 3 port solenoid to achieve target boost? We start off safe.
First, chances are you won’t be tuning from a stock map. You will have gotten a base map from someone and your boost error will be too high or low depending on the WGDC of the map.
Second, you need to log Max WGDC and Boost Error in addition to the normal values, using Enginuity or whatever program you use, so you can review the logs and see exactly where the targets are wrong. I prefer to do WOT pulls in second and third gear from 2000 RPM to redline, traffic permitting. This is not an endorsement to speed, so if you can do this at a track or a dyno, even better.
If you are not hitting target boost at WOT, raise the WGDC by 5% in the 100% throttle column. Be aware though that 5% of 20 is only 1, while 5% of 60 is 3. So as the WGDC% increases cut back on the % you raise it by. Enginuity has a way to raise value by % using the Multiply (Mul) button as show below.
I want to multiply the entire 100% Throttle column by 5%, so I put 1.05 in the input box, select all the values in the column and hit “Mul. I end up with this…
All values have gone up by 5%
You can continue to do this until your target boost is achieved for WOT. I find it best to tune with a passenger for one reason. They can watch the throttle % and tell me when to stop, which is helpful when adjusting WGDC to achieve target boost for partial throttle. Again, adjust the WGDC at the throttle point according to RPM. If you are overshooting your target at certain RPM’s, drop the WGDC for that RPM at the respective Throttle position. In an overboosting situation, it is best to make a large initial drop to the WGDC then bring it slowly back up, rather than try and tune the WGDC down.
The Up’s and Down’s…
Let’s say that you flash a map that has a max target boost of 16, you hit the road, start logging, and hit the throttle. The boost gauge quickly jumps to 21 then back down to 12 then back up to 20. This is most likely one of two things or a combination of both.
One, too much Initial WGDC and/or two, the TDC values are too broad. The TDC table is used when the ECU detects steady throttle position and the boost error is too large. There is another table which dictates what value the ECU will begin to check for boost error, but I have yet to find any reason to adjust that table.
The TDB table is used for quick response during fast transition of the throttle. If going from 25% to 100% throttle, this map will only be used temporarily, until the throttle position becomes constant and then the TDC map will be used.
When installing and tuning a 3 Port solenoid, modifying the TDC map is a must. The base percentages are way too high and will lead to serious boost oscillations at WOT. A good rule of thumb is cut them in half, do a logged run, and continue to adjust. Bear in mind the negative side when adjusting. You cannot simply multiply this number as the percentage will actually increase.
This is an adjusted TDC table I used for a 3 Port solenoid. As you can see, the numbers are much smaller than the stock table, because the overall WGDC with the 3 Port is lower. With a lower base WGDC 3 Port map, smaller changes to TDC will affect the combined WGDC the same as with the stock system. Compare this table to the stock table posted at the beginning and you’ll see what I mean.
So in short, you lower, then incrementally raise your WGDC to achieve your target boost. Sounds simple. For an average Joe, normal intelligence, you can probably figure this one out. The best bet is always to lower the WGDC map to a point where no target is achieved, and then adjust up until you hit your WOT target, then begin to adjust for partial throttle the same way. I prefer to tune this way because it alleviates the chance that my initial setting will overboost into catastrophic failure.
You will notice with a low WGDC that the car simply will not hit the target. You’ll have a target boost of 16 with 56% WGDC at WOT and the max boost will be 11. Adding 5 or 10% additional WGDC to the 100% column will increase it by some amount, say 1.5 PSI. This is just an example as there is no direct formula for tuning boost. The fact that there is no formula, and that WGDC percentages will change from vehicle to vehicle is why we have to adjust the WGDC maps. Maps you might download from around the internet will be tuned for different conditions and different modifications, so the WGDC tables could lead to boost error regardless of you having the same “required” modifications. The stock map works only because every car comes from the factory exactly the same, in theory, so the exact same settings will work on identical cars. However, modifications, altitude, temperature etc. all play a role in the ECU calculations and the function of the boost control system. So being able to tune for changes to the boost table is imperative to getting the proper performance from the vehicle.
Disclaimer Three – Modifying the boost table, WGDC and associated maps could lead to catastrophic failure of your car. The above information is only to be used as background information regarding the boost control system and the properties of it. At no time should you adjust any value in the ECU unless you are certain that the adjustments will not adversely affect the condition or performance of your vehicle. I take no responsibility for any adverse effects to any vehicle modified based on the information above.