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Old 07-26-2014, 09:53 PM   #1
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Question Selecting differential ratios

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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Selecting differential ratios-ericvsjohnniebravo.jpg   Selecting differential ratios-430.png   Selecting differential ratios-444.png   Selecting differential ratios-478.png   Selecting differential ratios-4875.png  

Selecting differential ratios-5125.png  

Last edited by ericwh; 07-28-2014 at 10:06 AM.
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Old 07-28-2014, 10:07 AM   #2
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Were my images not working? Is that why no one replied? Here's the rest, don't know what happened...

It also appears there is a 4.78 from kia sportage that would work and is available cheap on ebay? So, 4.78:





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Selecting differential ratios-478.png   Selecting differential ratios-4875.png   Selecting differential ratios-5125.png  
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Old 07-28-2014, 10:32 AM   #3
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4.44 with 6speed FTW.

4.875 with a 5speed would be a little short on longer tracks with some power, but our tracks are tight, so it works for me.
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Old 07-28-2014, 11:08 AM   #4
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6 spd is not an option for future classing. I was thinking 4.8 would run out of gear exiting T1.

4.77 might be pretty close to running out of gear in the same spot but I think I might give it a shot.

I'm going to get data on the other track in our region and see what the speeds look like there.
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Old 07-28-2014, 01:28 PM   #5
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NASA PT, I'm assuming? 4.875s are what you want but they are all but unobtainable now. 4.78s are virtually the same thing and available cheap (new or used, I paid $300 for my brand new set).

a 7k limiter is a bit of a boon with the 4.78s. You'll end up in 5th more than you'd like, with at least one 3-4-3 shift. The 4.78s get good when you can lift the rev limiter to the 8000-8200rpm range, which gives you the same in-gear speed as 4.30s but lifts the RPMs everywhere. Essentially like adding power to the car.
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Old 07-28-2014, 01:59 PM   #6
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Thanks.

I have a MS so I could change the rev limiter. I found this info by searching (piston speed):

Quote:
1.6's 83.6mm stroke (by itself, no other inputs) shows stock redline of 7200 to be 3950 ft/min.

6400 is 3500 ft/min
6850 is 3750 ft/min
7100 is 3900 ft/min (same as 1.8 ft/min @ factory redline)
7300 is at 4,000 ft/min
7750 is at 4250 ft/min.
Seems like 8000 RPM would be pushing it... ??
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Old 07-28-2014, 02:07 PM   #7
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Oh, and changing transmission ratios costs points but the diff gears are free?

EDIT: NM confirmed this elsewhere.
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Old 07-28-2014, 04:16 PM   #8
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7400 is the safe limit for a stock engine. 8k assumes you've done the required internal mods to spin the motor there. Since you've got a 1.6 chassis, you'll be better off dyno classing and moving to a 1.8L motor if your budget will allow it. Just the swap to a '99 1.8L will give you a huge performance jump, but the real benefit of dyno classing comes when you open the motor and massage, well, everything. I've built some pretty wicked dyno-classed PT motors, and while the days of the pump-gas 145+wtq 2.0L VVT motors are gone, the current rules still allow for a pretty stunning powerband.
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Old 07-28-2014, 06:25 PM   #9
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Quote:
Originally Posted by Savington View Post
...Since you've got a 1.6 chassis, you'll be better off dyno classing and moving to a 1.8L motor if your budget will allow it. Just the swap to a '99 1.8L will give you a huge performance jump, but the real benefit of dyno classing comes when you open the motor and massage, well, everything.
Why did you just say 99 instead of the vvt motor?
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Old 07-29-2014, 09:55 AM   #10
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Buy a Kia Sportage 4x4 95-02 front differential. It has the r&p you're looking for.
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Old 07-29-2014, 10:04 AM   #11
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Quote:
Originally Posted by Dunning Kruger Affect View Post
Buy a Kia Sportage 4x4 95-02 front differential. It has the r&p you're looking for.
Yah I just got the Kia R&P on ebay...
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Old 07-29-2014, 04:40 PM   #12
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Quote:
Originally Posted by ericwh View Post
Why did you just say 99 instead of the vvt motor?
Rule changes for 2014 make the 99-00 motors more desirable. That's about all I want to say in the public forum. Feel free to call me at the shop for lots more info.
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Old 08-02-2014, 10:16 PM   #13
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Quote:
Originally Posted by Dunning Kruger Affect View Post
Buy a Kia Sportage 4x4 95-02 front differential. It has the r&p you're looking for.
This right here.

And being a local, I can tell you having done the 4.78 as well as turned thousands of laps at the place, unless youre clearing 160whp or more, you wont be out of gear, Savingtons comments notwithstanding because they are correct.
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