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Old 02-22-2021, 02:01 PM   #1
currancchs
Scooby Newbie
 
Member#: 488200
Join Date: Jul 2018
Location: Brookline, NH
Vehicle:
2006 WRX TR
World Rally Blue

Default Getting Rid of Rev Hang (a solution that applies to NA/Turbo models!)

I have been trying to tune out rev hang for a few years now, on and off, but was not able to achieve the changes I was looking for until very recently. While my changes were to a 2010 Forester with the EJ253, this change should also work on many other Subaru vehicles.

More specifically, the following tables should be modified:
  • Target Throttle Angle Idle Airflow Target Base Minimum
  • Idle Airflow Min Target Decel Adder (RPM)
  • Overrun Initial Injector Enrichment (Pulsewidth)

The "Target Throttle Angle Idle Airflow Target Base Minimum" appears to control (i.e. set a minimum for) the throttle plate opening angle at 0% throttle pedal angle. The 100% values in this table should be reduced to reduce rev hang. Some change the 100% values to the value before the 100% value in the same column, I chose to do so, but also added some vertical and horizontal interpolation to smooth things. You could also play around with reducing all values for more effect. I'd suggest just making sure that the values increase going down and right and work on making them smooth (note that one of the values in my provided screenshot does not follow this suggestion, it was a typo due to my over-reliance on automated vertical and horizontal interpolation).

The "Idle Airflow Min Target Decel Adder (RPM)" table seems to add g/s to the airflow calculation under idle conditions, with this adder being tapered off over a predefined time period at vehicle speeds above the speed threshold in the "idle airflow min target decel adder active veh speed A" table. This causes the throttle to delay closing and, for me, was the most helpful table in reducing rev hang. Personally, I zero'd out these numbers, but experienced abrupt drops in RPM on letoff (sort of what I was going for, just harsher than expected, even compared to cable throttle cars I've driven), especially in the lower gears, so am now playing around with intermediate values.

Lastly, the "Overrun Initial Injector Enrichment (Pulsewidth)" pops off the injectors for the duration specified in the table on initial liftoff/decel. It is my understanding that this helps smooth the transition to 0 throttle, especially in the lower gears. I had initially reduced this value to about 25% of stock, but have since increased it again due to abrupt transitions to off throttle with the lower number.

I have made similar changes, with success, to a 2006 WRX as well, although that vehicle did not have the "Target Throttle Angle Idle Airflow Target Base Minimum" table (there is a table in that ECU that sets the minimum throttle opening, but that was already at 0.12% from the factory).

Also, these changes have made the throttle feel a bit snappier at low RPM (e.g. when rev-matching/double clutching into first). I have a suspicion that the lower values I used in the aforementioned tables allow a smoothing routine to complete faster than normal, and, once complete, throttle response goes back to normal (on my 2006 WRX, I had issues with varying throttle response on repeated taps, especially when coming to a stop/at low RPM, which seemed to be resolved by these changes).

Furthermore, an additional change I made that may have helped in reducing rev hang was to reduce in-gear idle timing at normal ECT/above 2k RPM from 16 to 5 degrees. I had read, on a different forum, that retarding timing on liftoff could allow for quicker RPM drop, but primarily made this change to encourage a burble/crackle on decel/shifts when everything was nice and hot. Unfortunately, this change was made at the same time as changes to the other tables, so I am not positive of its impact, taken alone.

While my changes are still a work in progress, I have confirmed that I can achieve the desired changes (and more!) through tuning of known tables and thought it worth sharing for that reason. Depending on your personal taste, the best values for these tables will likely differ.

If you do find values that work particularly well, please do share them along with details of the vehicle and modifications, if any, for the benefit of others coming upon this thread.

A screenshot of my main changes is included below:


Happy tuning!

P.s. Keep an eye on intake manifold vacuum during decel when making changes. If you notice excessively high vacuum, e.g. below -25inHg, on decel, you may wish to allow the throttle to stay open a bit more, since high vacuum could result in excessive oil consumption, due to oil being pulled through valve guides/oil control rings. Personally, I did NOT see an increase in vacuum after making the changes outlined above, although I did realize that I had very high vacuum while tuning (I saw -27inHg under hard decel!), which, for me, wound up being attributed to a clogged/inoperative PCV valve, which has since been replaced and the problem rectified. Actually zero'ing all of these tables may result in issues though since we don't have Idle Air Control Valves (IACVs) on our DBW vehicles to allow air to bypass the throttle plate during decel.
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Last edited by currancchs; 02-22-2021 at 07:16 PM. Reason: Clarified details of post
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Old 02-22-2021, 03:20 PM   #2
benflynn
Scooby Specialist
 
Member#: 515308
Join Date: Jun 2020
Default

My 06 fx will hang idle when cold and backing out of parking spot. It esp happens if I try and use the high cold idle and not live any throttle while backing. Lil blip and it goes away. It was very noticeable when I first got it, use to it now
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Old 02-22-2021, 03:28 PM   #3
currancchs
Scooby Newbie
 
Member#: 488200
Join Date: Jul 2018
Location: Brookline, NH
Vehicle:
2006 WRX TR
World Rally Blue

Default

Quote:
Originally Posted by benflynn View Post
My 06 fx will hang idle when cold and backing out of parking spot. It esp happens if I try and use the high cold idle and not live any throttle while backing. Lil blip and it goes away. It was very noticeable when I first got it, use to it now
Thanks for sharing! The behavior I was trying to get rid of was present when the vehicle was fully warm; I don't worry too much about cold start/warmup.

On my 2006 WRX, the hang was there just enough to notice, although the different throttle response at low speeds/RPM after repeated inputs did annoy me.

On the 2010 Forester, however, the hang was outrageous, with RPM actually increasing slightly after pressing the clutch in and then hanging there for a second or two before dropping. It was very annoying how slowly I had to shift if I was trying to shift smoothly, rather than using the clutch to pull down the revs. At one point, I was simply letting off of the gas well before clutching, but that was also annoying and slightly difficult to time correctly.
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Old 02-23-2021, 03:06 PM   #4
currancchs
Scooby Newbie
 
Member#: 488200
Join Date: Jul 2018
Location: Brookline, NH
Vehicle:
2006 WRX TR
World Rally Blue

Default Update

I messed around with my "Idle Airflow Min Target Decel Adder (RPM)" and "Overrun Initial Injector Enrichment (Pulsewidth)" tables again today and got some better results. More specifically, I increased "Overrun Initial Injector Enrichment (Pulsewidth)" to 2.5ms (back to stock), multiplied the stock "Idle Airflow Min Target Decel Adder (RPM)" values by 0.5 and then copied adder values above 4800RPM in this table to the remaining, higher-RPM cells (the idea was to get RPMs to hang just slightly when cruising, but less when shifting at high RPM - the '06 WRX ECU is programmed this way from the factory).

While with these settings there is a tiny bit of rev hang, it is not very noticeable at all and I prefer it to the zero'd settings for daily use.
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