R34 Decel Timing and Idle Target RPM

[Smity42]

I am trying to solve an issue with my RB25DET NEO tune where there is a significant amount of 'popping' out the exhaust during decel.

I'm trying to increase the closed throttle decel timing, but the tables "Idle/Decel Neutral Timing (RPM)" and "Idle/Decel Gear Timing (RPM)" don't seem to be being used.

No matter how much I increased the timing at the top of these tables, it didn't seem to make any difference. So I did a logging run to see what was happening.

The first log "NIStune_2016-09-19_1659_49.csv" is using "tune1.bin", which has modified Decel Timing tables to increase timing up to 25deg at 1500rpm (DecelTiming_Modified.png). I modified both the IDLE and GEAR tables to match.

What I can see in the log however, is the closed throttle timing is 10deg over about 2000rpm, then 5deg between about 1800-1600rpm, then 0deg below about 1400rpm. It then jumps up to about 15-20 before returning to 5 at idle. As best I can figure, this table would look something like the image in DecelTiming_Actual.png.

The decel events in this log are at:

00:10 to 00:20 - neutral decel from ~5700rpm down to idle.
00:40 to 00:48 - 1st gear decel from ~5200rpm to ~1300rpm.
00:57 to 01:12 - 2nd gear decel from ~4700rpm to ~1500rpm.
01:24 to 01:45 - 3rd gear decel from ~4400rpm to ~2200rpm.

I then tried returning the Decel timing tables to OEM (tune2.bin - DecelTiming_OEM.png) and did another quick log with a neutral decel and idle. This log is in "NIStune_2016-09-19_1710_36.csv". It seems to do exactly the same thing with regards to timing on decel.

Am I looking at the wrong tables here? Is there a different table used for this decel timing?


Also, I've noticed the Idle target RPM doesn't seem to work quite as expected. In both of these logs it's idling at ~925rpm, which is a couple of hundred rpm higher than the expected target rpm of 688rpm (IdleTargetRPM.png). Is there something else that affects idle rpm?

Idle can be seen from about 14s in the second log, and the following places in the first log:

00:00 to 00:07
00:13 to 00:19
03:06 to 03:18 (end)

ECU is 23710-AA50F.

Thanks in advance,

Martin


P.S. I had to zip the attachments as it seems like it would only let me attach 3.

[Matt]

Popping out of the exhaust during decel is linked to unburnt fuel being passed through . It can be caused by reversion for example, with MAF reporting load on deceleration and ECU injecting fuel and that fuel getting passed through. Not related to timing

Yours logs are showing obvious reversion on deceleration (you can see the MAF line jaggered) so this needs to be resolved firstly. Have a look at our 'MAF tuning guide' on our website for hints to fix that

Secondly your injector latency is very high (1450uS) so on deceleration we are seeing 1.50ms (1500us) of injection time since the ECU uses the latency number for injection

If you are using injection latency to bump up fueling around idle then you may consider dropping this lower (eg 1150uS I use for 1000CC IDs) and increasing the bottom end of the VQ map around idle (there is no Min TP load in this ECU unfortunately)

Looking at the tune, the image is from feature pack, but the flex fuel, timing etc maps should be zeroed out (currently they have AA500 or similar maps)

Notes against tune1.bin

Normally 'Alpha/N increase fuel' table we max out so it references the whole fuel map (regardless of throttle position)
Normally dont need to change after crank or after start enrich (if the crank enrich table is working fine)
RPM enrichment has been increased in low RPMs, why is that?

Some tables only use values in certain ranges, so over those values the y are not used. With this R34 seems to lock onto 10 deg BDTC on decel. There might be another table which I have to hunt down which gets used instead, since I cannot find matching values with those tables

Closed loop idle control is AAC and timing adjustments made together. Air control screw can affect this however by putting more air into the engine, so adjusting this may drop your idle further so it can operate in the closed loop properly

[Smity42]

Hi Matt,

Thanks again, very helpful as always.

I can see what you mean about the reversion in the logs. Having read your MAF tuning guide I assume this is caused by having no BOV. I'm not overly interested in putting a BOV back on - I've run no BOV for years and never had this much 'popping' - what it has now would definitely be described as excessive, and wasn't evident before the latest tune.

I noticed the tuner had increased the latency (this was 1230) - when I queried it he said he it was 'adjusted to suit the new k constant and injection multiplier'. From what you have said, I suspect this may be the cause of the popping.

I also noticed the flex fuel maps had been changed from all zeros - again, unsure why. I'll zero them out.

I don't know why any of the enrichment tables were changed - other than the previous tune was a b*tch to cold start, and now it cold starts really well.

Edit: It looks like the RPM enrichment was decreased at low RPM - not increased?

She's going back to the tuner tomorrow, but I was hoping to get an understanding of what was going myself because there seems to have been a lot of changes to my old tune which are questionable. He told me the popping was most likely caused by retarded timing on decel, which is why I was looking in that area.

Edit: It would be good to be able to change the decel timing regardless, so please let me know if you do find a table for this.

I'll play with the air control screw and see if I can bring the idle down.

[Matt]

I dont think its timing as usually unburnt fuel goes pop, not from timing. Even at 10 deg BDTC I would not see this causing an issue

Raised problem report on timing maps but probably wont get a look in today
http://www.nistune.com/mantis/view.php?id=172

[Smity42]

Thanks Matt,

You were right - it was the injector latency. Set this to 1050 and adjusted the VE map to suit, popping on decel is gone now.

The tuner did note that the horizontal scale on the decel timing tables seems to be out. And as previously mentioned, at higher RPMs it seems to be locked at 10deg. Probably leave this one on the low priority list .