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AngryCorvair
AngryCorvair MegaDork
4/29/19 4:33 p.m.

[smart-ass filter]

aside from the start and finish, and the sections of track immediately before and after the start and finish,

[/smart-ass filter]

what is the lowest speed in the Challenge autocross?   Assume A7's so about 1.25g lat if that much detail changes the answer.

mazdeuce - Seth
mazdeuce - Seth Mod Squad
4/29/19 4:40 p.m.

They are second gear courses in normal cars, so......30ish? 

Dusterbd13-michael
Dusterbd13-michael MegaDork
4/29/19 4:40 p.m.

Well, ours was slightly more than walking, but much lower than almost anything else. devil

 

 

(It honestly depends on actual grip, not theoretical. Assuming standards, about 15-25mph realistically)

alfadriver
alfadriver MegaDork
4/29/19 6:04 p.m.

One of these challenges, someone needs to put in a data acq system, so we all will know that.

Bear in mind, many years ago, GRM decided that those systems don't count against the budget if you make the data public.  IIRC.

Pete Gossett
Pete Gossett MegaDork
4/29/19 7:43 p.m.
mazdeuce - Seth said:

They are second gear courses in normal cars, so......30ish? 

I think that might be a bit optimistic, especially remembering 2017’s course with the “oval” near the start. Or was that 2016? 

mazdeuce - Seth
mazdeuce - Seth Mod Squad
4/29/19 7:48 p.m.

In reply to Pete Gossett :

I've only been twice and only as a spectator/person who talked to a lot of people. I did walk the courses and 2017 and 2018 didn't have anything that would have required first gear in a normal car. 

DeadSkunk  (Warren)
DeadSkunk (Warren) PowerDork
4/29/19 7:52 p.m.

In reply to Pete Gossett :

That oval was the 2017 Challenge. I ran through that entire course in 2nd gear in the Miata.

Robbie
Robbie UltimaDork
4/29/19 9:04 p.m.

I'll take and publish data this year if folks are interested. With simply a gps unit and a phone it can swap between lots of cars quickly. Maybe I'll even make some holder for both devices planning to hot swap between cars...

Robbie
Robbie UltimaDork
4/29/19 9:06 p.m.

But to OP, the autox courses are very similar to what you will find at local regional autoxes. 40-50 second courses, 2nd gear, etc. If you are familiar with autox in general the courses will feel normal.

Stampie
Stampie PowerDork
4/29/19 9:54 p.m.
AngryCorvair said:

[smart-ass filter]

aside from the start and finish, and the sections of track immediately before and after the start and finish,

[/smart-ass filter]

what is the lowest speed in the Challenge autocross?   Assume A7's so about 1.25g lat if that much detail changes the answer.

The slowest I've seen was a team pushing their car through. 

ProDarwin
ProDarwin UltimaDork
4/29/19 10:08 p.m.
alfadriver said:

Bear in mind, many years ago, GRM decided that those systems don't count against the budget if you make the data public.  IIRC.

If its an OBD2 car you can datalog vehicle speed (or RPM) for ~$10 anyway...

AngryCorvair
AngryCorvair MegaDork
4/29/19 10:12 p.m.

I’m looking at ring and pinion options, considering my tire diameter and trans gearing and max RPM, and my desire to avoid shifting into 5th on dragstrip.  The info provided here has been kinda helpful, so thanks!

mazdeuce - Seth
mazdeuce - Seth Mod Squad
4/30/19 6:29 a.m.

In reply to AngryCorvair :

Top speed is probably more of a concern. You don't want to max out your second gear too early and have to decide between bouncing the rev limiter or having to shift back and forth between 2nd and 3rd. Make sure your 2nd ends somewhere north of 60ish and that should be the right gear. If rotaries and Vtec Hondas can deal with the slower end of it, you should be fine.

AngryCorvair
AngryCorvair MegaDork
4/30/19 7:31 a.m.

assuming 6k rpm redline with the LT4 cam and 25.7" tire OD, Vmax per gear should be as follows:

1:  37 (in a row?)

2:  66

3:  89

4:  118

5:  153

6:  186 (LOL won't have high enough power or low enough Cd)

i think i'll have enough hp/lb to get pretty close to running out of 4th at the drags.   my prevous V8Vair trapped at 104 with less power / more weight / worse aero.

T.J.
T.J. MegaDork
4/30/19 7:43 a.m.

I assure you we're open.

mazdeuce - Seth
mazdeuce - Seth Mod Squad
4/30/19 7:58 a.m.

Without more 1/4 testing, you're probably right where you need to be. What are your other options?

AngryCorvair
AngryCorvair MegaDork
4/30/19 8:18 a.m.

In reply to T.J. :

bunch of savages in this town!

AngryCorvair
AngryCorvair MegaDork
4/30/19 9:34 a.m.
mazdeuce - Seth said:

Without more 1/4 testing, you're probably right where you need to be. What are your other options?

So, those numbers were using 1st and 2nd from transaxle A plus 3rd thru 6th and ring/pinion from transaxle B, with 3.75:1 r/p.  here's an .XLS snip since i'm an engineer:

Mix 1:

  • PRO:  because it is a FWD box, it is lighter than all other options.
  • PRO:  it stretches out 1st and 2nd.
  • PRO:  it's totally doable.
  • CON:  t might require a shift to 5th on the dragstrip.  maybe wishful thinking, IDK.

Mix 2 :

  • PRO:  stretches out 3rd and 4th, no chance of needing a shift to 5th with current power
  • CON:  because its an AWD box, it's maybe 20 lbs heavier
  • CON:  not sure if AWD 9T pinion will play nice with FWD 30T ring.  Diff carrier is common, so it *could* work, but longevity could be an issue.

So i'm watching Copart for complete 3.0 / 3.2 6MT cars but will likely buy whatever comes up first, then JY the other bits as required.  All of these transaxles are available under $300, but a complete parts car makes the most sense for clutch bits + shifter bits + pedals + clutch master + halfshafts

i'm gonna cross-post this to my build thread so i don't forget it.

Robbie
Robbie UltimaDork
4/30/19 9:51 a.m.

In reply to AngryCorvair :

Do you drag race often? If not I think you are safe planning to trap less than 118. 118 is MOVING compared to all but 3-4 challenge cars each year, and those 3-4 are the ones brought by drag racers. I'm not saying you can't do it and I will be happy to watch you prove me wrong, but this feels like a novice showing up to an autox expecting to PAX in the top ten. Keep in mind you are trying to trap 118+ with 3 shifts already. Those 3 shifts will take a lot of distance to complete. (just for fun calculate how many feet of the 1320 you will not be accelerating because you are shifting - esp the 3-4 shift since it happens at ~90 mph). Keep in mind that traditional fast drag cars have 1 or 2 automatic (ie fast) shifts in a 1/4.

Not to mention you theoretically will be accelerating faster in 3rd and 4th with mix 1 and therefore the higher average speed might outweigh the extra few MPH you might gain at the end. 

and yes, I know, play to win, plan to win, etc, etc. I truly think you will be faster with mix 1 unless your power to weight puts you at an ideal trap speed of like 140. 

Robbie
Robbie UltimaDork
4/30/19 9:56 a.m.

oh, cool! 90 mph happens to be 132 feet per second. So 10% of the 1/4 each second. 

a half second shift is 5% of your 1/4 mile right there. 

mazdeuce - Seth
mazdeuce - Seth Mod Squad
4/30/19 10:02 a.m.

I think mix #1 is the way to go for the reasons you stated. Running out of gear at 118 would be very impressive. 

AngryCorvair
AngryCorvair MegaDork
4/30/19 11:04 a.m.
Robbie said:

In reply to AngryCorvair :

Do you drag race often?

several times per day, LOL.

118 is MOVING compared to all but 3-4 challenge cars each year, and those 3-4 are the ones brought by drag racers.

agreed

Keep in mind that traditional fast drag cars have 1 or 2 automatic (ie fast) shifts in a 1/4.

Ronnie Sox would cock-punch you for saying that!

Not to mention you theoretically will be accelerating faster in 3rd and 4th with mix 1 and therefore the higher average speed might outweigh the extra few MPH you might gain at the end. 

low ET is what wins the race, no doubt.  i did note "wishful thinking" about running out of 4th.  ;-)

Patrick
Patrick MegaDork
4/30/19 11:12 a.m.
mazdeuce - Seth said:

I think mix #1 is the way to go for the reasons you stated. Running out of gear at 118 would be very impressive. 

I’m on target to run out of gear, rpm, or connecting rod bolts at 121.  

AngryCorvair
AngryCorvair MegaDork
4/30/19 11:13 a.m.
Robbie said:

oh, cool! 90 mph happens to be 132 feet per second. So 10% of the 1/4 each second. 

a half second shift is 5% of your 1/4 mile right there. 

nevermind Ronnie Sox, i will cock-punch *myself* if i can't bang-shift a modern trans quicker than 500 ms.

my previous V8Vair (9.4:1 327, mild flat-tappet cam), which i had to walk out of the hole (2.001 60ft) because corvair diff, ran 13.89 at 100, on 26" tires, carrying more weight and pushing worse aero.  i was doing 7.9 at 80 in the 1/8 at the top of 3rd and trapping at 100 after hitting 4th just after the 1/8.  changing to 22" tires cost me 0.1 sec and 1 mph (8.0 @ 79) in the 1/8 because i had to hit 4th earlier, but improved accel in 4th allowed me to trap at 104 with no other changes.  i'm confident, but will refrain from chest-thumping, that more cubes / roller cam / stronger diff / less weight / better aero will make things interesting.  clutch grip is likely going to be my biggest concern.

AngryCorvair
AngryCorvair MegaDork
4/30/19 11:14 a.m.
Patrick said:
mazdeuce - Seth said:

I think mix #1 is the way to go for the reasons you stated. Running out of gear at 118 would be very impressive. 

I’m on target to run out of gear, rpm, or connecting rod bolts at 121.  

in what year?

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