Selecting differential ratios

Summit Point, VIR, Nürburgring
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ericw
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Selecting differential ratios

Post by ericw » Mon Jul 28, 2014 9:12 am

Posted this on other forms with no response... anyone?


So, here I have some data from this past weekend at VIR. The green lines are myself (1.6l with 5spd 4.3 rear end) the blue lines are someone else (1.8l with 5spd, 4.1 rear end).

Based on this data and the attached gear speed charts (I believe these are the ratios available from Mazda Motorsports), which rear end would be the best choice for this track and why?

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It appears there is a 4.78 from the rx7 that would work and is available cheap on ebay? So, 4.78:
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silver 681

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Re: Selecting differential ratios

Post by v10climber » Mon Jul 28, 2014 1:36 pm

If you haven't read this little CVD bit by Jason Rhoades (multiple time national auto-x champ) then it would be good. This part is about acceleration and showing how it isn't ultimate power/weight that affects acceleration but also gearing. He has a spreadsheet where you can plug in FD ratios and see how the car is accelerating. You need a dyno plot to fill it out though. I'd put together some graphs for your most used gears (3/4 I'm guessing) with different differential ratios and see what the thrust curves look like through your normal speed ranges.
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ericw
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Re: Selecting differential ratios

Post by ericw » Tue Jul 29, 2014 10:15 am

Thanks. No, I haven't seen that article before. . I'll give it a more thorough read later.

I think the crux of the matter is shorter gearing will be quicker as long as I'm not running out of gearing at the wrong time. Right away you can see 5.125 is probably too short for the straights.

The only other points of concern are the ~80 mph peak after T1 and the 105 peak at the top of the climbing esses (~7750x). Possibly running out of third or fourth there. The solution presented to me is raising the rev limit. I've gears on their way.
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Re: Selecting differential ratios

Post by hufflepuff » Tue Aug 12, 2014 6:26 am

I like the 4.78. It looks like you'll top out around 120 mph. My integra reaches about 120mph at both VIR and summit main going into turn 1 at both and also going into the roller coaster at VIR. At 120 mph, my integra is approaching the rev limiter with my 4.7 final drive gear.

It's painful cruising all the way to VIR at 4000 rpms at 70 mph, but the gearing does make a big difference in acceleration.

I climb the esses in 5th, and leave it there through 10. My speed falls to around 90 mph. Carrying more speed through a given corner may mean you don't have to downshift. At summit, i have an unfortunate upshift to 5th and back to 4th for turn 3, which may be solved with a higher rev limiter and stronger rod bolts.
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