The official short shifting bench racing thread

B8 S4
#11

Great stuff Jim, glad to see our numbers are approximately close. I’ll think about implementing the shifting delay, and yea, I scaled the Stasis dyno because it was for stg 1 and seemed relatively tame compared to what I’ve seen. But what I scaled above might be more of a race tune…also, I think the dyno is somewhat wrong, because of how the HP continues to rise up top. Usually, these tunes seem to roll off torque more to get a flat HP curve.

The nice thing about these methods is we can change parameters or equations to see what might change. Interesting question about the rotational mass, I kind of assumed it to be embedded within the dyno pull, but it might make sense to add them to the model.

In any case, I think we’ll be able to make some “approximate” conclusions on what the implication is for this DSG tune. I think .3-.5 secs in ET seems reasonable, will also be nice to see how this varies for a stock vs stg 2 car vs stg 1 - where the claim has been all the returns are up top. But absolutely tuned stg 2 cars should not be running stock times “because the DSG is short shifting”.

I’ll look at this all again tomorrow, again, thanks for your contributions.

#12

Drob, excellent work and info. Thanks

#13

That is really cool stuff! I’m assuming the dyno numbers would be based on a 100 tune? Can you do a sim based on a 93 tune dyno? That would really help some of the folks with 2014 models get a realistic time range to compare.

#14

I think all dynos are somewhat wrong for this type of purpose - if you are looking to see deltas for a given modification, they are a useful tool - but as absolute #'s – not so much.

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The problem is that you don’t know what the overall rotational inertia is. The dyno does have the drivetrain intertias accounted for at whatever omega-dot the single gear pull did, but 1st gear the losses will be much higher since you are accelerating things much quicker - 6th gear the losses will be less, and if you had a dyno that would just hold RPM to measure torque the inertial effects would be zero.

Maybe adding in the wheels/tires and using the delta from an assumed 4th gear dyno pull would be better, but I’m not sure it’s worth the effort.

I agree this is a good tool for comparing things and looking at relative effects – sort of like a dyno - it’s close enough to give you good feedback, even if it’s not 100% accurate.

Glad to help - this kind of stuff is a fun distraction :wink:

#15

drob, do you have a cousin named Arthur?

quick question - can you define short shifting? It wasn’t explicit. Is that 6200? I believe some of the guys get a 6200 shift and a 5800 2-3. Not sure.

Can you do this on pump gas? A bit more relevant as most go on pump.

ideally

pump gas, and use the short shift + super short shift like some of the guys (the 6200 and 5800). Primetime will know what the sequence is. And use shit DA of about 2000 feet because that’s what these guys are seeing in the summertime.

+karma for a few days. Great read. Sure it’s bench racing, but that’s fine. Nobody in their right mind has anything against bench racing or keyboard calculating. Reality is this is what the scientific method is all about.

It’s not like you’re using this to say ‘ABC is better than XYZ’…you’re trying to estimate performance losses due to a particular limitation (shift RPM).

p.s. nerd! :slight_smile:

#16

Thanks! Like I mentioned in the OP, this is really designed to show what the relative difference in performance might be if you assume the short shifting or change in gearing for the B8’s to B8.5’s. These relative changes might vary if we assume a stock dyno or stg 1 or stg 2 or stg 2 100 oct…but I would never use this tool to predict actual times.

The interesting point we make with these simulations is seeing how the differences affect 1/4 mile runs, rather than debates about how it changes the engine torque/HP (for math geeks, we’re studying the solutions of the differential equations rather than simply the vector fields that define them).

But I don’t think we’ll ever approach modeling true performance numbers, mainly due to how nonlinear the tire is at time of launch. The dyno plots are also kind of garbage. You can see we get 60 fts of ~1.8 sec while guys like Ron and Jeff are seeing ~1.6 sec, which is quite a difference.

An aside - another interesting question might be doing the inverse of what we did, i.e. taking highly telemeterized data and computing the engine torque from the actual performance at the strip. This is kind of what a dyno does, but they have the use data from the movement of the dyno rollers rather than actual longitudinal motion. There’s a reason engineers use engine dynos and quote brake HP.

#17

lol I’ll take that as a compliment, I was actually home sick the other day and got bored (also tired about the speculation on short shifting). I have since modified the shifting logic, you can see it explicitly below. This is kind of what I saw at the strip, shift 1-2 at 5900 and then 2-3, 3-4, etc at 6400. Not short shifting implies shifting at 7200. Jim is doing something similar.

I just got source data for a dyno from Jim which he screen grabbed from APR. Before any 1/4 runs are presented, we’ll show the assumed dyno like you see in the OP. The stasis dyno seems wrong based on how the HP oscillates at the end. Going to add the shifting time delay and change the parameters to more what Jim assumed.

Shifting Logic

function [gear, shiftTimer_sec] = checkForShift(gear, engineSpeed, …
shortShift_bool, shiftDuration_sec, shiftTimer_sec)

if shortShift_bool
if (gear == 1 && engineSpeed >= 5900) || engineSpeed >= 6400
gear = gear + 1;
shiftTimer_sec = shiftDuration_sec;
return;
end
else
if engineSpeed >= 7200
gear = gear + 1;
shiftTimer_sec = shiftDuration_sec;
return;
end
end

#18

OK, here’s the simulation making a couple assumptions

https://farm6.staticflickr.com/5553/14985522457_a635ccd970_b.jpg

  1. APR Stg 2 Pump Gas File taken from B8 dyno here;
  2. Car + driver weight is 4100 lbs;
  3. DSG shifts take .1 sec, during which time, engine torque = 0;
  4. Rolling friction coeff = .017 and C_d for air drag = .27;
  5. There is unfortunately no model for DA, just assume APR dyno is actual (it’s wheel torque, right?).
  6. “Launch Control” is at 3000 RPM and always keeps engine at or above this (no bogging down)

Thanks to Jim for using the tool to pull data, I used the interpolation spline and this is the dyno I get with the sampled internal data:

https://farm4.staticflickr.com/3842/14985590027_8eaa3db860_b.jpg

The numbers end up being pretty close to the APR figure, any error is due to either the lack of resolution in the APR figure, or the fact that I interpolated the data Jim grabbed.

Here are the results for a tuned B8.5 vs a tuned B8.5 with short shifting. The legend shows true as 1 and false as 0.

Distance

https://farm4.staticflickr.com/3836/14985650610_0edc49a397_b.jpg

Engine speed

https://farm6.staticflickr.com/5560/15172309435_903caac00a_b.jpg

Force at the ground

https://farm4.staticflickr.com/3863/15171930192_7451886d17_b.jpg

Next I ran a B8 tuned vs B8.5 tuned short shifting (the B8 has shorter gearing)

Distance

https://farm6.staticflickr.com/5565/14985616409_ed779dac40_b.jpg

Engine speed

https://farm4.staticflickr.com/3915/15171975142_d120af9ecf_b.jpg

Force at ground

https://farm4.staticflickr.com/3920/15171975102_6f4cb542a3_b.jpg

Comments: The numbers are a little closer, now that I assume 3000 RPM initial launch. I did this because the times seemed too fast (they still do lol). Probably need some better data, if anyone has a good dyno pull that would be helpful. The B8’s are marginally faster if we assume the engines are the same, on the order of .04 sec in et according to calculations (using difference in gear ratios).

Interestingly, Jim pointed out that the dynos are significantly different on APR’s webpage for the B8’s vs the B8.5’s, specifically showing the short shifting issue. Jim, maybe you could post the gif that you made? Is this a way to for APR to blame the Audi gearing on the facelift problems? I probably need to better understand exactly how a dyno works, as my understanding is it shows the engine torque at the wheels, taking into account drivetrain inertia and friction. A pull is done in 4th gear since it approximates a 1-1 ratio, and they remove the final drive ratio?

#19

Here is the comparison gif (animated - should cycle between B8 and B8.5) - I’m not sure how relevant it is, but I think it’s about the only data we have from APR on the B8.5 vs B8… The B8.5 is DSG, and that prevents it from getting to higher revs. I seem to remember Arin saying that the 6MT also had a little bit lower drivetrain loss than the DSG… So - keep in mind that this is NOT an apples to apples, but despite that there may be some info to be gleaned…

Mike - looks like your spline is bad near redline - HP from the APR data should be 388, but since the blip is so small I doubt it changes the results much.

http://i616.photobucket.com/albums/tt241/jgreat_bucket/S5/Logs/30T_APR_compare_zps3a385e45.gif

#20

Very cool guys… I think the launch rpm’s are a little off as the 60’s seem high especially given the et’s and traps… The last set by Jim seem spot on et and mph wise but 60 still seems high… The whole DA throws a wrench into it but if you look at the top times their done in 0 or negative DA’s so it’s pretty close IMO… It’s early so I need to read it again and digest it better! lol Appreciate you guys doing this, again great stuff!

#21

Thanks for the feedback Ron, yea I think the launch will always be the worst. In the real world, I believe pneumatic tires almost act like slingshots or springs when you first load them up. So even getting max torque at launch (at approx 4000 RPM) still results in slower 60’s than you guys produce. Not sure we can fix that without hacking up the equations to artificially get the result we want (which we already sort of do with the dyno lol).

The DA thing will never make it into this, we just assume the engine does whatever the dyno says. I guess the APR dyno is probably better than actual, since they remove the wheels? But we can scale it however we want (i.e. add in drivetrain loss), so it can be tuned to give a certain desired result I suppose.

#22

Added extra torque during launch to help with the 60ft, now appears to be about 1.7 sec for stg 2. I’ll put up the figures when I get home.

Does anyone have any ideas as to how to present the results to give it some kind of “conclusion” feel?

Anyone have any suggestion to the sort shifting values? Ie 5900 and then 6300 for sort shifting, 7200 at every gear for b8/dsg flash?

Ideally, we can end this thread with some reasonable conclusions…ie something to show Arin to convince him that short shifting isn’t completely responsible for poor performance. Thanks all and would also appreciate criticism of methods/assumptions.

#23

Here is a thought on the 60’ times. Adding extra torque seems wrong to me, but…

Typically good 60’ times (and ET) are achieved by staging shallow. I think there is about 1’ of distance between the beams (?) – so … looking at my sims - 1’ of travel occurs in little over .2 seconds - so that would put the 60’ times as measured by the strip when you shallow stage down in the 1.6 range where primetime and the other dragstrip legends are :wink:

It would also cut the same amount of time off the ET, but not change the trap.

However – trap speed - it’s actually an average over the last several feet I think - we could model it that way as well to maybe make it more 1-1 vs. a timeslip…

#24

Damn Jim, that’s pretty genius :o

Bench racing FTW - shallow staging by 1 ft reduces the 60 ft by .22 sec when considering APR stg 2 dyno.

For those interested, found a nice reference for doing essentially exactly what we’re doing: http://www.wskc.org/documents/281621/308637/ENGAGE_E3S_Dynamics_DragRacing.pdf/6d7a264e-246f-4a70-8f91-677697fc20d4

I liked this line ;D: Even with the simplifying assumptions, the calculated results are
strikingly close to the published results.

#25

LOL - no a genius would have thought of this from the beginning… >:(

Nice reference. Shift RPM = 5500 - and you B8.5 DSG guys are complaining :wink:

#26

staging = pretty close. Maybe 10" is more accurate.
trap speed = average speed over final 60 feet of the track

#27

There is a thread over on AZ in which a gent that owns a 2015 S4 claims his tachometer face redline is 7000 RPM and that is where his DSG shifts. My blood pressure is going up!!!

http://www.audizine.com/forum/showthread.php/614935-So-Everyone-Is-OK-With-Audi-Not-Allowing-DSG-Cars-To-Get-To-Redline

#28

Makes sense about trap speed, since they’d need a way to calculate it to begin with: trip first laser beam slightly before and second beam at end of 1/4 mile, then divide distance by time delta. I’ll change the sim to reflect this.

It lowers the trap speed by about .8 MPH if you average last 60 ft vs taking instantaneous velocity.

#29

He’s just eyeballing it and guesstimating. The 2015 is no different from the 2014…just relax ;D

#30

I hope you are correct. Can’t see how anyone could read misread a tach that badly? I may just go past my dealer today and look for myself. I better take some meds!