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Autocross gearing question

 
Old 01-26-2019, 11:03 PM
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Default Autocross gearing question

Hello. I recently swapped to a 6 speed and ever since have been wondering how it would affect autocross. I have a 4.3 diff and was wondering if anyone else has the same gearing. And if so, what have been your experiences at autocross with that gearing? Have you had to shift the 3rd and then back to 2nd within a few seconds? Can you ride the course in 3rd effectively? My old 5 speed let me go 2nd all the way through. Just wondering if a 6 speed is a good idea for autocross with it's shorter gears. Thanks
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Old 01-27-2019, 03:33 AM
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Here's a gearing calculator from KMiata. You can use this to calculate your top speed at each gear.
https://kmiata.com/pages/gearing-cal...2C1%2C0.843%5D
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Old 01-27-2019, 04:46 AM
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Already looked at that lol. Just wanted some real feedback not a simulation type thing
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Old 01-27-2019, 07:41 AM
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4.3 w/ 6 speed is not going to be good for most Auto-X courses - you will be forced to either bang the limiter A LOT or shift between 2-3 a few times. You'd probably have the same problem with 4.1 too.

If you make 200+whp you want 3.63 gears with 6 speed. 3.9 otherwise.
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Old 01-27-2019, 02:54 PM
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Thanks, was afraid people also might think this but I think I'll try it out to experience it. Love how the 6 speed feels faster with its closer and shorter gears
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Old 01-27-2019, 03:08 PM
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Good thing you asked us!
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Old 01-27-2019, 04:48 PM
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My msm had 6spd and 4.1 and it sucked for autocross. Think 2-3-2-3 shifts all the time. When in most autox courses you want to stay in 2nd gear.
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Old 01-27-2019, 05:25 PM
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Gotcha. I'm hoping the 4.3 is a little better. The next autocross is going to be a bigger setup at my local track so I'm hoping it won't be too bad. But I know I'll run into it eventually
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Old 01-27-2019, 05:28 PM
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4.3 is even worse than 4.1
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Old 01-27-2019, 05:40 PM
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I know it's shorter but it might not get me stuck in between 2nd and 3rd as much vs the 4.1 because 3rd would be even shorter with the 4.3 than with the 4.1, and it could almost act as a 2nd in certain cases
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Old 01-28-2019, 11:34 AM
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Lots of missing data here. What size tires and what rev limit? Boosted or NA? What are the average speeds of the courses you're running?

With a 6spd/4.3 combo and a raised rev limit (7500-8000) you might be able to get away with launching in 2nd and leaving it in 3rd most of the course (depending on how open your courses are and what kind of torque you're making).
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Old 01-28-2019, 02:46 PM
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Yes you're right I forgot about that stuff. Stock BP5A Japanese engine (which I just found out revs to 7500 stock). 205/50r15 tires. Average speed is going to be ~45mph for this autox.
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Old 01-28-2019, 02:48 PM
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Originally Posted by Melvin View Post
Yes you're right I forgot about that stuff. Stock BP5A Japanese engine (which I just found out revs to 7500 stock). 205/50r15 tires. Average speed is going to be ~45mph for this autox.
Yeah, but does YOURS rev to 7500?

Either way if it's stock, it's not gonna have the ***** to pull out of a tight corner in 3rd.
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Old 01-29-2019, 10:42 AM
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Not directly apples to apples, but I'm running an NC with a 2.5 swap, and the stock 4.10 rear end. With the 6 speed, i was constantly shifting to third, and often having to go back to second. I decided to swap to a 5 speed transmission in december, and I cant wait to get out to the first autox event in a few weeks.... the ratios in the 5 speed are much better for autocross, though that may be different on a 1.6 motor with less torque.

In all likeliness it should be fine, you will need to practice carrying your speed, and be willing to ride the rev limiter, or practice those shifts to 3rd.
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