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Old 03-28-2015, 03:55 AM   #1
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Default Race ratio, how to pick the right gears.

Hey guys,

(If there is a great thread, please link me I couldnt find it)


The white NA6 I play with has recently made 350whp, and unfortunately with a little <1600cc engine its proving hard to drive across the boost threshold, this can be remedied with careful boost control tables however with its 6 speed and 4.1 diff gear its just so much wheel torque and a lot of gearchanges.

Apart from the simple answer of "put in a 3.3", how do I decide what diff ratio is best?

I figure that going to a 3.3 is simply going to result in the first gear being taller and the rest of the gears driving as if they are a gear lower than they are.

If this is the case, what would the advantage of a taller diff gear?

Thanks for everyones help,
Dann
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Old 03-28-2015, 04:03 AM   #2
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Here is the ratios of 4.1 vs 3.3..



I think if the drop between gears is (I know its not constant between each gear but if it was) 20% and you go to a diff gear 15% taller, then you will be forced to go down a gear and then you have effectively geared the car shorter again despite the taller diff gear. Is this right?

Thanks,
Dann
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Old 03-28-2015, 07:00 AM   #3
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I figure that going to a 3.3 is simply going to result in the first gear being taller and the rest of the gears driving as if they are a gear lower than they are.

If this is the case, what would the advantage of a taller diff gear?
A taller gear will let your engine work a little more between each shift.
The stock rations have bigger drops on the lower gears and you will use them more with a taller FR.

I have the opposite a thinner band of torque and would benefit of 6speed with my 4.875, to get as small drops as possible (spending most of my time on track in 5th).
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Old 03-28-2015, 10:21 AM   #4
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I've also got a ratios question- my 6 speed is dying, and I'm wondering if a 5 speed would be noticeably slower on track(180whp, 4.1 rear end). It feels like I shift a lot on track- to the point where I'm not hitting corners as well as I could because I'm always shifting.

At my power level I'd imagine the 6 speed is still faster, but I could probably run a 5 speed without breaking it and they are significantly cheaper and easier to find.
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Old 03-28-2015, 12:36 PM   #5
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I've also got a ratios question- my 6 speed is dying, and I'm wondering if a 5 speed would be noticeably slower on track(180whp, 4.1 rear end). It feels like I shift a lot on track- to the point where I'm not hitting corners as well as I could because I'm always shifting.

At my power level I'd imagine the 6 speed is still faster, but I could probably run a 5 speed without breaking it and they are significantly cheaper and easier to find.
5 speed is plenty for 180rwhp. You would be shifting less and have a taller top gear.
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Old 03-29-2015, 01:05 AM   #6
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https://www.flyinmiata.com/tech/gearing.php

Nearly everything you need to know in life has already been figured out. You just need to know where to look.
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Old 03-29-2015, 02:45 AM   #7
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Can someone please explain if there is a benifit of changing the rear ratio by aproximately a single gearshift?

Seems like you are just trading 1st gear for a 7th gear.. virtually.

Dann
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Old 03-29-2015, 06:35 AM   #8
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Can someone please explain if there is a benifit of changing the rear ratio by aproximately a single gearshift?

Seems like you are just trading 1st gear for a 7th gear.. virtually.
As mentioned, check the rpm drop for the gearshifts you will use, they will not be identical ( since the gear set is not 20% drop for all).
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Old 03-30-2015, 05:39 PM   #9
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Helps me a lot to visualize my current gearing vs. the proposed change.

I make tables in a spreadsheet for current vs. proposed:
Takes a bit of back and forth, but using a regular ol' online gearing calculator, figure wheel speed @ redline for each gear, and rpm in next gear up at the wheel speed achieved in previous gear. Enter it all in a table, then graph it.

-Ryan
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Old 03-30-2015, 07:55 PM   #10
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How about the simple answer of "put a 1.8 in it"?

Serious answer: The 6-speed is better than the 5-speed would be for a highly tuned engine since the gears are more tightly spaced and you are less likely to find yourself between gears on track. You need to identify which specific corners are giving you trouble, and then decide whether going to a shorter or taller ratio will solve this problems. In general, you want to use the highest set of ratios (i.e. 4-5-6 instead of 3-4-5) if possible, since the 4-5-6 gears will be more tightly spaced and give more average power application to the wheels.

Dave, the 5-speed will be slower.

If one more person uses the "tall gears let the motor work more" argument my head is going to explode
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Old 03-30-2015, 08:45 PM   #11
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This is what has been done, graphed on excel and then we have used vids to check Vmin corner speeds at each point in the main 3 places it goes.

3.3 has us use 3/4 almost exclusively with a little 5th. 3.6 is 4/5/6 but its got crappy ratios for a lot of corners we will hit and a LOT more 4/5 shifts.

Do you think the difficulty of the 4/5 and back shift in a 6 speed is worth worrying about here?

Dann
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Old 03-30-2015, 11:40 PM   #12
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On a track that is exclusively 4/5/6, I'd actually think you would want a 3.909 or 4.10. Theseus was a 6sp/4.10 car up to 270whp, and it wasn't a 3.909 car until 300+whp. What is your specific gripe with the 4.10s? Is there a specific corner that's a little too fast for 3rd, or a specific straightaway where you're running out of gear?
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Old 03-31-2015, 12:05 AM   #13
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1/2/3 are completely useless and there are a lot more gearchanges than a 3.3 would need.

Am I overthinking this?

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Old 03-31-2015, 01:12 AM   #14
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For me with over 300hp a 4:10 would make 3rd gear almost useless on most tracks I run. A 3.909 is probably best on most tracks, a few tracks prefer the 3.636. a 3.3 would only be useful if it were a street car and I was more concerned about better fuel economy while cruising at 75mph on the freeway.

The 3.636 was making second gear useful for autocross so that is what I ran. now trying to run a 4.778 with a Quiafe wide focusing on autocross for a season. I'd run it with a 3.909 for track. it's pretty close to a 6 speed with 1 gear placed in the middle of 1st and second compared to the 6 speed.

This is what breaks transmissions. I run it at lower boost than this for reliability on tracks though.
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