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Old 10-16-2020, 12:07 PM   #1
IROCZman15
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Join Date: Dec 2004
Location: Roxbury, NJ
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Determining best launch and shift points, using math and datalogs

Not expecting everyone to bust out their calculators and notepads to crunch numbers, but if anyone with a bunch of drag racing experience had any scientific knowledge that they might care to share, it would be very much appreciated! I'm aware, none of this is exciting to dive into, but perhaps myself (or someone else in the future) can see this stuff and learn from it.

My goal is to scientifically use the datalogs and timeslips I have got in the past 2 months to help me truly dial in my launch and my shift points so that I can best optimize everything for the quickest N/A drag passes possible. I have the WOT tune dialed in to near-perfection and the WOT timing is staying at 35 deg, with a 12.5 target AFR at torque peak and a 12.7 target AFR for hp peak. Dyno sheet is from February when the engine was built. For full blown details on the car see my build thread: http://www.njfboa.org/forums/showthread.php?t=69033 but essentially it is a sbc 406 with a 3,400 9.5" converter, probuilt 700r4 and a moser 12 bolt with 3.55 gears. I run MT ET street SS drag radials at the track, and do have a multi-use setup of suspension parts because the car also does street driving and autocross.

videos of my runs can be found at my youtube channel in case it clears up when i am shifting and how the launch/shifting looks and sounds: https://www.youtube.com/user/IROCZma...=dd&shelf_id=4


For the Launch:

Through trial and error, I have determined that the car launches the best with a soft rear shock setting and the drag radials at 19psi. Leaving off a 920ish idle has worked well, but I am not scientifically sure that that makes sense. I do not have a trans-brake, so when I try to foot brake, my brakes don't hold past 1,700-1,800 which is when I start to creep forward through the beam. I have a line-lock but have never tried launching off of it at the track, but when I experimented on the street with it, it seemed slow to release the front brakes. I would love to cut a low 1.6x in the 60 foot so if anything jumps out on this chart I made, feel free to shout it out to me. The car is in 1st gear until way past the 60' cone, so that takes any shifting variables out of the equation.







For shifting:
Launching from idle and only needing to make the 1-2 and 2-3 shift during the 1/4 mile is good. I am making both shifts prior to reaching the 1/8th mile so the back half of the track is just climbing up 3rd gear. When it comes to shifting and revving each gear out, I have yet to determine what is best. I know it has to do with not only how high to rev the engine, but also what rpm the shift lands at in the next gear. One school of thought is that you want the shift to land in the rpm where it is just climbing the peak of the torque curve. Another is to have the rpms land after the torque peaks and when the hp rpms are climbing. TYPICALLY, my shifts land right around the 4,700 rpm range of the next gear (with some variation based on how high I rev the previous gear). I have included my VE table (basically same shape/slope as the torque curve) as well as my engine dyno sheet. A chassis dyno won't help here because a dyno pull is only done in 1 gear (3rd gear for a 1:1 ratio).

So, I am crunching numbers and timeslips, but in an effort to optimize each shift I am wondering if I should shift the 1-2 shift at a different (lower) rpm than the 2-3 shift. I am NOT changing rear gear ratio, converter, trans, or any of that either. The rpms go real quick at the top of first gear, in fact when I do the datalog math, it takes only .33 seconds to go from 6,000 to 6,500 rpms in first gear. Obviously the rpms climb slower in 2nd gear, so I have more time to get that shift perfect, where the 1-2 gearshift can be off a few hundred rpms in just a tenth of a second.















I have reviewed all of my many datalogs too, and I can post screenshots of those also if needed. Mechanically the car is dialed in and probably really close to its limit/optimal performance without any big mechanical changes happening. I plan to go to Island dragway next Friday, Oct 23 and that might be the last drag day of the year for me. A 11.7x or even 11.6x second pass at 117 would make me super happy going into the winter time season and would put this car closer to where we all think it should be running for the hp/setup it has.
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Last edited by IROCZman15; 10-16-2020 at 12:18 PM.
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