Ol Blue's Boost Control
From SCT SRT Wiki
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[edit] Quick and Dirty Steps
1. Adjust Torque Table to reflect your estimated torque output and Air Flow; best to use stage 3 and adjust by percentages, rescale as needed.
2. Adjust Desired Torque, last row, to reflect your estimated torque / horsepower for your setup
3. Zero out Wastegate Adaptives and high/low limits
4. Datalog and adjust Wastegate Duty Cycle table until boost comes in line; best to multiply applicable cells by percentages by your boost error (desired versus actual). Make all cells below used cells 99%.
5. Fine tune (when boost is within 1psi) using Desired Torque table (not best, you can get 100% there through Wastegate Duty Cycle table).
6. Set boost by gear and boost by mph
7. Set high/low limits to 15, -15 respectively. Reload to stock each adaptive table and adjust until boost comes back in line (optional).
[edit] How Boost is Controlled
Basics of Adaptives
The computer is quite good at controlling boost and adjusting for environmental changes to keep power levels consistent. The environmental adaptives the pcm knows and uses are NOT to keep boost psi the same say in 30 degree weather and 95 degree weather but instead to keep the power the same. Typically you will notice MORE boost in hotter weather since the air is less dense and requires more psi to achieve the same oxygen burn. The opposite is true in the winter. So on a very hot summer day, with a stock tune you should notice 1-2psi more boost.
If and when you finish your tune, you should aim for the same result. That said, many users will find it easier to simply deal with the higher and lower power levels and prefer to just have a preset boost level in all conditions. This can be done easily but the main disadvantage is having the same boost level all the time will have adverse effects on fueling and timing. Yes, it can be tuned for but is much more difficult.
Here’s the example:
Stock Tune
18 PSI in 3rd gear
90 Degrees Ambient Temp
AFRs 11.4-11.7
Same Tune (with environmental turned on)
16PSI in 3rd gear
45 Degrees Ambient Temp
AFRs 11.5-11.8
Same Tune (without environmentals turned on)
18PSI in 3rd gear
45 Degrees Ambient Temp
AFRs 11.8-12.1
So the point is if you can spend the extra time to have an adaptive boost curve to change with the outside environment, you will have a more consistent fuel and timing curve. But for big turbo guys or just someone who doesn’t care so much, you can turn off all the adaptives. Lets discuss HOW boost is controlled.
Before going further, if you plan to try to build out your own waste gate duty cycle table, turn off all the adaptives for now. This will allow you to directly understand and view in datalogs what your dutycycle is. You can know that there are no other factors influencing your boost curve and can see changes directly in the next run you make. Here are the tables to zero out:
Section: Turbocharger
Table Name: Wastegate Adjustment for Air Temp (adjusts for inlet temp)
Table Name: Wastegate Adjustment for Ambient (adjusts for outside temp)
Table Name: Wastegate Adjustment for BP (adjusts for elevation)
Table Name: Wastegate Adjustment for Part Throttle (adjusts for part throttle boost)
Table Name: Scalar – set the Wastegate adaptive high and low limits to 0 (this will actually disable all the above tables, but eventually you want to add them back one by one by reenabling the adaptive high and lows)
[edit] Torque Calculation
Section: Torque Calculation
Table Name: Torque Table
The first step to the computer controlling boost is for it to know how much torque at a given Airflow and rpm that your engine will make. This is where your tune will make or break itself. If you’re stock turbo, Mopar S3, E1, E2, D44, D42 then youre in a good shape. The stock turbo torque table is good to use for with the stock or ported turbo, all the rest I listed are all in the 37-42lb range and can use the stage 3 torque table. If you’re turbo is slightly larger percentage wise, feel free to use the stage 3 table and just multiply the entire thing by the percentage larger than it is. For example, an AGP 50T is right around a 49lb wheel. I have had plenty of luck multiplying by 30% [calculated as (49lb – 37lb) / 37lb]. However, be aware that the lower end of the table will be slightly higher already since the stage 3 makes great torque and you may only have to multiply from 3400 rpms on up, as this is where most big turbos spool up.
Get this table CLOSE to where it should be, and then leave it alone, it just needs to be in range but not perfect.
Section: Turbo Charger
Table Name: Desired Torque
The next area is the infamous desired torque table. This is where YOU, as in the programmer, tell the computer how much torque by RPM and TPS voltage that you want. The entire PCM is written in Newton Meters which converts as 0.737562149277 Nm per foot pound. Let’s look at the Stage 2 Desired Torque table converted foot pounds:
As you can see its pretty tame and you can now see that a stock stage 2 will boost around 18-19psi, which results in around 240wtq peak (85% times the whole last row for parasitic losses). Here is the same table converted to horsepower (torque x rpm) / 5250:
Again, this all looks as expected; the stage 2 is rated from Mopar at around 265hp and 280wtq. Now how to incorporate this to your tune: you want to convert your desired torque to be both what you want for torque and horsepower but also have it be within the range of your turbo. Don’t go setting desired torque to 800 ft/lbs for a stock turbo, it isn’t going to happen and is going to result in either a piston through the dash or just a mess of a tune. Even more important, don’t command a torque at such a low rpm that you know will never happen either, for example don’t set desired torque at 2400rpms for a 50T tune to be 500ft/lbs. Again, you’l l have ridiculous spikes and inconsistent boost. I use Excel and have all three tables above linked together, so that I can type in the torque curve, and it will give me the new NM table to paste back in. Set this table up, and then leave it for now.
NOTE: anything less than the 2.8 row on the bottom is part throttle torque. To be honest, I don’t know too many guys who would want more power than the stock turbo puts out at part throttle, so I typically leave this unchanged and only deal with the last row. It’s not a short cut or messy tuning; it’s honestly what most people want.
Section: Turbo Charger
Table Name: Waste Gate Duty Cycle Table
Now here’s where it all comes together. You computer now knows how much torque you want, and how much flow your car will make at that desired torque. The last step is to tell the PCM the duty cycle it needs to use to produce the boost that will achieve the torque you want. This is mapped out in the Waste Gate Duty Cycle table. It is setup by Airflow and RPM. Here’s how to interpret: Let’s say you’re stock turbo and using the stock stage 2 code. According to your Desired Torque, at 4000rpms you have commanded 380Nm of torque (see above). According to your Torque Calculation table, at 4000rpms and 380Nm of torque, you’re right between the 125 and 150 g/sec rows as highlighted here:
Therefore, when you go into your Waste Gate Duty Cycle table, you can find the cells that correspond to the airflow of 150 g/sec @ 4000 rpms:
So according to this, the baseline waste gate duty cycle to get 380Nm of torque at 4000rpms on the stock turbo using a stage 2 wastegate is around 30 to 45% duty cycle.
Now that you understand how this all works, you would NEVER want to cell by cell try to build out a wastegate duty cycle table, its massively too hard and laborious. The Torque Table and Desired Torque are pretty straight forward. To build your duty cycle table the best way is to datalog and find at each rpm what your desired torque, air flow, and boost is. You can then make 10 or 15% changes to each column by RPM in the wastegate table once you find out your percentage error. For example, say from 2600 to 3400 RPMS you datalog and find that your boost is 19psi and you want 23psi instead, your air flow is 271 g/sec according to the datalog. I would multiply all cells in that range by 20% the first adjustment, and each subsequent do around 5% as it gets sensitive.
Increasing the duty cycle increases the boost, 100% means the solenoid is closed completely, 0 means it’s completely open and letting all boost hit the wastegate (i.e. spring boost).
[edit] Some pointers and hints
Not the entire waste gate duty cycle table is used, in fact only around 5 or 6 cells in each column will really ever be used in your tune. This is because your turbo has a flow capability and subsequent flow curve. If you take a ruler and start at the lowest flow area and connect it to the highest flow area on the wastegate duty cycle table, the cells 2 or 3 above and below the line will be your main cells controlling your boost. Below the line is your spool up, so it is important to find out which of the bottom-most cell is being used in WOT, and the rest below that point should be set to 99%. Here are the cells that are used in a stock tune:
As you can see, anything below the highlighted cells are 94%, so basically if you nail the throttle at 2400rpms which would command a g/sec flow that is outside the turbo’s flow ability (according to your torque table) the pcm knows to just shut the solenoid off to get maximum boost. This is where a lot of guys go wrong, if the torque table isn’t setup right, youre boost will go through the roof as there is a sharp edge to being ok to having too much dutycycle.
If you’re using an external waste gate on a 50T and go WOT at 2900rpms, you know the turbo wont spool up until 3500rpms, so as you sit there with the pedal mashed and you cross through each RPM band in the wastegate duty cycle table, anytime between 2900 and 3500 rpms you would want 100% dutycycle to spool as fast as possible. Once you get to 3500rpms, you should then have an air flow that is accurate and should put you in a duty cycle less than 100% this way it will start to control boost.
