Why do most DI tunes have negative driver demand values??

[jtrosky]

For the life of me, I can't understand why you'd ever want to use a negative driver demand value - especially at pedal positions greater than 0. From what I've read on this forum, negative driver demand values actually cause engine braking. I guess I can somewhat understand wanting engine braking for the 0 pedal position, but why would you ever want engine braking when you are pressing the gas pedal??

For example, on my car (2012 Impala - 3.6l direct-injected LFX V6), the stock tune has negative driver demand values for pedal positions as high as 4% when going as slow as 31mph. If I press the gas pedal - even as little as 4%, I would expect the engne to produce more power than it currently is, not less - and certainly not "negative power"! :-)

The 2106 Z06 Corvette stock tune has negative values up 12% throttle!

From what I've seen, this is the "norm" for direct-injected vehciles (not 100% sure about that, just what I've seen in the tunes I've looked at).

What am I missing here?

Thank you!

[mbray01]

ram in, ramp out. The driver demand has to have smooth transitions, or you will be left with hard bucking, surging etc. Even though it shows negative when throttle position is applied, it is in higher rpms, where the little throttle input isn't making power, but still slowing down, hence the negative number to transition to engine braking

[Higgs Boson]

If you log Driver Pedal Power you will find it is "0" even where the DD table is negative. The stock Z06 isnt accelerating in negative cells, those are all decel cells.

Further, I have had excellent driveability with no negative values however, the change to Axle Torque from Engine kilowatts seems to have thrown a wrench in it (for me at least).

[mbray01]

changing to zero will not adversely effect driveabilty vs negative as you stated as long as its a smooth transition

[jtrosky]

I too have removed all negative values and like the results. With the negative values, when reapplying the throttle during certain situations, it's like it's a deal-pedal feel...

I guess it all boils down to whether or not you want engine braking when coasting? If you do want engine-braking while costing, you use negative values for some of the zero-pedal fields. And if you do that, you kind of have to use negative values for the for few pedal positions to avoid "abrupt" throttle changes.

Personally, I prefer zeros in zero-pedal fields and positive values in the pedal positions over 0.... But I guess I understand it better now - if you want engine braking while coasting, you have to have some negative values, even in the non-zero-pedal-position fields to keep things smooth... Personally, I've never liked engine-braking while coasting (automatics).

Thank you - not sure why I didn't "see" that before posting. :-)

[GHuggins]

I'm curious about this now - does getting rid of the negatives in the slightly higher positions get rid of that coast and back into throttle "over accel" feel that the vehicles that you add a PD blower to can experience?

[mbray01]

hmm, good question. worth lookin into. only takes a few minutes to make a couple test changes

[69camaro5speedefi]

"the change to Axle Torque from Engine kilowatts seems to have thrown a wrench in it (for me at least)."

To do axle tq it looks at gear ratio vs speed from what I have came up with.
That is why low mpr it will be like 12,xxx on some trucks
So doing the math 4.55 first gear x rear end gears makes for a big number
so changing shift mpr ,torque coefficients tables or other tq limit tables can change throttle openings or closing

makes for some weird driving conditions to try and make happy
If you log app tps and say sae tps on a stock truck even they are not very good the throttle blips all the time on gear changes
this will kill a ocd tuner lol

[Higgs Boson]

"the change to Axle Torque from Engine kilowatts seems to have thrown a wrench in it (for me at least)."

To do axle tq it looks at gear ratio vs speed from what I have came up with.
That is why low mpr it will be like 12,xxx on some trucks
So doing the math 4.55 first gear x rear end gears makes for a big number
so changing shift mpr ,torque coefficients tables or other tq limit tables can change throttle openings or closing

makes for some weird driving conditions to try and make happy
If you log app tps and say sae tps on a stock truck even they are not very good the throttle blips all the time on gear changes
this will kill a ocd tuner lol

yeah I have definitely done all the calculations and tried to line them up with gear vs mph and they don't line up, lol. The good news is, there is really no reason to change DD tables. I've made lots of power on several cars with stock DD tables. They key is not in the DD, you don't have to demand more power from the torque model you just have to keep it from limiting it.

[jtrosky]

At least In my situation, changing the DD tables is not to try and gain more power - it's to alter the overall throttle feel. That's where it makes a huge difference on my car.

[69camaro5speedefi]

With my project (cam 427 wipple) shift points had to be changed some times 5 to 8 mpr ,tq management,had to be changed unless you like your timing going to the - at low speeds with a big cam (getting nasty stumbles with lack of cly pressure) I had to use it for drive ability unless you like getting throttle openings on commanded
gear changes at low speeds jerking you around every situations not the same well unless your doing same stuff lol

[Higgs Boson]

I finally had a chance to log both Knock Retard and Total Knock Retard today and they are both exactly the same on my car, so I guess in my case, it really doesn't matter which one I log... My tune does NOT use burst knock at all (stock or customized tune), so maybe that is why they are always the same - maybe they would be different if busrt knock was setup in the tune...

And I also confirmed again that my "Ignition Timing Advance (SAE)" values to not reflect KR (when present) either... Strange...

To be clear for everyone wondering why your car is like that and their's is not.......What car are you referencing again?

[jtrosky]

Ooops - posted in wrong thread. Deleted posts and moved them to the correct thread:

http://www.hptuners.com/forum/showth...nockRetard-PID