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Is a car faster without back seats AND no bulkhead or was Feb 2021 a coincidence?

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Duane Black

Curbs go brrrppp
567
401
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Time Attack
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5-10 Years
Durham, NC
On RS4 tires on a cool February day, after VIR has had little to no traffic in awhile, I ran a 2:12.078 at VIR. I was elated. The tires were brand new, a taller profile, and I was in the zone. 2 months later, I was back to 2:14s. The chief instructor of the group I ran with in February since told me the unofficial Spec Miata record was also set that weekend.

I had taken the passenger seat back out, but left the back seat in since my head was buffeting around with nothing to divide my trunk and greenhouse.

Ever since though, I have never run under 2:14 in that configuration, even on brand new RE71R tires that by all means should be faster back in early March this year.

I have now added a Steeda wing and gotten into the low 2:13s on RE71R on a 70 degree day in early May.


Hypotheses -

1) Track was green and my RS4 just LOVED that fresh surface.
2) The car is faster if you just let your head buffet.
3) that back seat. while light, has a major impact on cross weights
4) I peaked that day.


 

Duane Black

Curbs go brrrppp
567
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5-10 Years
Durham, NC
Clarification, I've been in the zone since and still get 2:13s.

I just looked over more data and noticed....

The top speed was higher than day on the straightaway. My data shows the Re71R just a hair faster in several key corners a few weeks back, but on Feb 6 2021, the car just had top speed on the straightaways was nearly 2 mph faster than anytime before or since. It was about 55 degrees with a green track, but the back seat was out of the car too.

So was that weather, or is the car faster with no backseat.
 
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What does your data say about your speed on the corner(s) leading on to the straights? If you were faster off those corners, that's your answer. If you were slower off the corner but your max speed was faster, then maybe your motor just really liked the cooler temps. Either way I really doubt the difference is the back seat. It's very light.
 

Duane Black

Curbs go brrrppp
567
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Durham, NC
No difference in corner exit speeds. I don't think the difference was weight. I know the aero made my head buffer around without a back seat. I'm wondering if this somehow reduced the drag coefficient of the car
 
118
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MD
VMin at oak tree 33mph vs 47mph on first 2:13 run vs 2:12
VMax on back straight was 131mph vs 134mph but on the 134mph/2:12 run it looks like you hit 120pmh sooner and spent more time above 120mph covering more ground. Also looked like you stayed on throttle longer before the roller coaster and were above 130mph longer.

Pause the videos at 120mph on the back straight and compare from there. Pause the videos at 130mph and compare them. The throttle and speed traces from oak tree to roller coaster should confirm whether this observation is correct.
 

Duane Black

Curbs go brrrppp
567
401
Exp. Type
Time Attack
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5-10 Years
Durham, NC
VMin at oak tree 33mph vs 47mph on first 2:13 run vs 2:12
VMax on back straight was 131mph vs 134mph but on the 134mph/2:12 run it looks like you hit 120pmh sooner and spent more time above 120mph covering more ground. Also looked like you stayed on throttle longer before the roller coaster and were above 130mph longer.

Pause the videos at 120mph on the back straight and compare from there. Pause the videos at 130mph and compare them. The throttle and speed traces from oak tree to roller coaster should confirm whether this observation is correct.

I think you put way too much faith in that gps being accurate...

Screenshot_20220524-194151_TrackAddict.jpgScreenshot_20220524-194216_TrackAddict.jpgScreenshot_20220524-194313_TrackAddict.jpg
 
118
155
MD
Looks like it ha. GPS speed was the only data element in the video to compare so I had to assume the GPS was reading consistently good/bad/same Hz.

2:12.078 vs 2:12.571 speed traces looks like slightly higher avg speed through 4-6 and oak tree on the "faster" lap but the "slower" lap looks to have some slightly faster sectors too. It's minor and the runs are very consistent with only 0.5 second between. Does track addict provide sector times? That should help color in the details a little better. The minor differences among the laps look like the best sectors could lead to a theoretical time in the high 2:10s?
 

Duane Black

Curbs go brrrppp
567
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Durham, NC
2:11.045 with 1hz gps on the 2:12s day.

heres an overlay of 2:13.1 with a wing amd re71r vs 2:12.079 on hankooks last year. the wing seems to add a little drag but faster than i was a month earlier without it net lap time.Screenshot_20220524-210024_TrackAddict.jpg
 
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MD
The 2:12 lap appears to be accelerating or recovering faster from lower minimum speeds in 3 turns; blue lap hits lower min speed but catches and/or exceeds some white lap max. Hard (for me) to say what may be causing it based solely on speed trace. A throttle trace would show if it's related to throttle commitment; doesn't sound like it from videos. The different tires and wing (plus more variables if run on different days) add challenges to interpreting the results 1:1 and may be contributing to the differences.
 

Dave_W

Cones - not just for ice cream
986
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20+ Years
Connecticut
At 120mph, you cover 176 feet in one second. So the GPS is telling you your position once every 176 feet at 120mph. Could be that due to exact timing of the data capture, the GPS fix at the start of the lap vs the end of the lap happened to artificially shorten the lap and give you a slightly better time.

Take a look at the 2:12 video when the car is approaching the S/F line. At the start of the lap, you're accelerating smoothly past S/F, but at the end of the lap, the speed kind of stalls out just before S/F and then jumps about 5mph right after. I don't think the car actually did that, it looks like a data glitch.

Add in the engine being up a few horsepower in the cooler temps, and that could be the difference.

The minimum usable rate for motorsports applications is 5 updates per second (5Hz), however an update rate of 10Hz and above is optimal.
 
343
300
Exp. Type
Time Attack
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Under 3 Years
Bulgaria
The 2:12 lap appears to be accelerating or recovering faster from lower minimum speeds in 3 turns; blue lap hits lower min speed but catches and/or exceeds some white lap max. Hard (for me) to say what may be causing it based solely on speed trace. A throttle trace would show if it's related to throttle commitment; doesn't sound like it from videos. The different tires and wing (plus more variables if run on different days) add challenges to interpreting the results 1:1 and may be contributing to the differences.
Weather. Coyotes if not cooled down properly will have that performance on straights where on a hotter day you will speed up slower because of pulled timing because of high CHT. With that in mind I think OP hit sweet spot of good cold day with brand new tires and the magic happened.
 

JDee

Ancient Racer
1,797
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W2W Racing
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20+ Years
5 miles from Mosport
Track conditions can make a huge difference too. Some days it's like magic, other days not so much. I find if the track's been used for racing within a few days of a track day it's often on the slow side. Because stuff happens on race day.....
Then there's weather, that too makes a huge difference, cooler dense air is good for the motor. Or at least it was in the old days, maybe now with modern engine management not so much? Mid/late morning is often the fastest time of day for me, afternoon when the track is getting hot and slicking up it's not so great.
 
343
300
Exp. Type
Time Attack
Exp. Level
Under 3 Years
Bulgaria
Track conditions can make a huge difference too. Some days it's like magic, other days not so much. I find if the track's been used for racing within a few days of a track day it's often on the slow side. Because stuff happens on race day.....
Then there's weather, that too makes a huge difference, cooler dense air is good for the motor. Or at least it was in the old days, maybe now with modern engine management not so much? Mid/late morning is often the fastest time of day for me, afternoon when the track is getting hot and slicking up it's not so great.
This is why it's really important to have a note with track conditions for each session much better to figure out why you were faster also a good ODB2 bluetooth adapter attached to TrackAddict can add so much useful data like ECT and IAT for example so you don't have to second guess was the track optimal or not. So many variables in a lap.
 

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