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Old 06-08-2015, 10:32 PM   #1
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Default Ideal rear gear for my setup?

I have a '99 miata, supercharged to 23 PSI, making around 300whp, maybe a touch more or less. I currently have a 6 speed and 4.10 gears. The motor can rev to 8,500, but it makes peak power around 6,800-7,000, and seems to hold kinda flat-ish (doesn't fall off to hard) to about 7,800 or so, after that it falls off enough that shifting around 8K seems fastest vs upshifting sooner.

My question is, would dropping to a 3.909 gear in the rear make the car faster in a straight line? Say 1/8 mile or 1/4 mile times.

I know for daily driving, a 3.636 would be sweet, but I don't know if it would actually be faster vs the 4.10 I have now.
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Old 06-08-2015, 10:35 PM   #2
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3.3
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Old 06-08-2015, 10:36 PM   #3
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3.3
Why? Also, I can't buy a 3.3 that I know of. At least, not a miata diff.
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Old 06-08-2015, 10:37 PM   #4
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I was being facetious. But I drove a turbo Focus SVT with very similar gearing to a miata 6sp with 3.3... it was very nice. My miata has a 3.9 + 6sp... sucks *****.
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Old 06-08-2015, 10:42 PM   #5
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I Would not hesitate to go to a 3.6 with that much power.
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Old 06-08-2015, 11:32 PM   #6
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The answer is that it totally depends on the course, how long it takes to shift, your torque curve, redline, etc. For 1/4 mile, you probably want a rear end that'll put you over the finish line just before you hit redline in 4th. Playing around with 1/4 mile calculators and FM's gearing calculator suggests that a trap speed of 115-ish is probably reasonable and you can get that in 4th with a 3.9, 6-speed, and 8250 RPM redline. Putting a 3.6 in that same car will make it slower, because you'll have less torque at the wheels. Putting a 4.1 in that car will also make it slower, because even though it's going faster in gears 1-4 (ignoring wheelspin issues), the need to shift to 5th is probably going to cost more time than you'll make up.

For autocross you want something that's going to put you in second for most of the course, so it's going to depend on your power level, local site availability, and the kind of course your club likes to make.

For a road course it depends heavily on the particular course -- F1 teams used to swap out the rear end and transmission ratios multiple times per weekend to optimize it. (rules don't allow that any more).

For around town, it depends on your personal preferences for how you like to drive. I like the 6-speed with a 3.636 in my 300-ish rwhp Miata.

--Ian
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Old 06-09-2015, 12:35 AM   #7
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The answer is that it totally depends on the course, how long it takes to shift, your torque curve, redline, etc. For 1/4 mile, you probably want a rear end that'll put you over the finish line just before you hit redline in 4th. Playing around with 1/4 mile calculators and FM's gearing calculator suggests that a trap speed of 115-ish is probably reasonable and you can get that in 4th with a 3.9, 6-speed, and 8250 RPM redline. Putting a 3.6 in that same car will make it slower, because you'll have less torque at the wheels. Putting a 4.1 in that car will also make it slower, because even though it's going faster in gears 1-4 (ignoring wheelspin issues), the need to shift to 5th is probably going to cost more time than you'll make up.

For autocross you want something that's going to put you in second for most of the course, so it's going to depend on your power level, local site availability, and the kind of course your club likes to make.

For a road course it depends heavily on the particular course -- F1 teams used to swap out the rear end and transmission ratios multiple times per weekend to optimize it. (rules don't allow that any more).

For around town, it depends on your personal preferences for how you like to drive. I like the 6-speed with a 3.636 in my 300-ish rwhp Miata.

--Ian
Thanks for the reply, good info. For your 1/4 mile numbers, a 3.9 looks nice. I currently have a 4.10 on a rx7 lsd. I also have a 4.10 msm torsen diff/axles I'm thinking about installing. Also have a 3.909 gear on another RX7 lsd in a box. Thus why i was thinking I could put that 3.909 on my msm torsen and run it. I think in 1/8th mile I'll be in 4th for sure crossing the lights, I think i'd be grabbing 5th in the 1/4 mile with this gearing.
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Old 06-09-2015, 01:23 AM   #8
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For most tracks I’d say 3.909. And you wouldn't ever use second gear on track. A 3.636 would be better for autocross and daily driving because of the speed you can reach in second. A 4:10 or a 3.3 both seem stupid to me. with a 4.10 you can’t go fast enough in third coming off corners on the track and you end up having to up shift early and second gear is way too low to be useful for autocross or around town, and a 3.3 makes 6th useless for anything but good fuel economy at high speed freeway driving which seems like a stupid concern if you built a 300+ hip Miata that handles well. A 4.30 might make 3rd useful for an autocross gear first will be good for pulling stumps.

Another thing I believe to be true is shorter rear end ratios require less tooth load in the transmission. A transmission turning a 4.778 rear end is less likely to strip teeth than one turning a 3.3.
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Old 06-09-2015, 04:14 AM   #9
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I though the 3.3 gear was a myth? Bob you run a 4.778 for auto-x right? Do you launch in second and then drive mostly in 3rd?
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Old 06-09-2015, 04:41 AM   #10
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I though the 3.3 gear was a myth? Bob you run a 4.778 for auto-x right? Do you launch in second and then drive mostly in 3rd?
Someone has a group buy going to make one, although IIRC it's been massively delayed.

You can also get 3.3 if you buy a V8R kit and put in a Getrag.

--Ian
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Old 06-09-2015, 04:52 AM   #11
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Yeah the miataroadster group buy. The price kept going up until a bunch of people backed out. But there is a mythical Mazda truck 3.3 that I think has been proven not to exist.
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Old 06-09-2015, 05:39 AM   #12
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This is only for track cars only... this does not apply to freeway cruising. Using the calculator for the 1/4 mile time isn't really a good guide as to how fast the car would be on a given track.

I'm currently running a 6-speed and 4.1 ratio and I much preferred the Na6 4.3 ratio I had before I upgraded the diff. I have a 4.78 r/p I'm going to try it and see if my times drop.

For me it's not so much the time lost or gained by gear changes but rather a better selection of usable gears for the tighter / slower tracks we have around here. My car has a narrow power band so gear selection is fairly critical and currently on most of the tracks I'm in either too low revs in 3rd or too higher revs in 2nd. A change in diff will mean I'm in the powerband in 3rd in the same corner and will actually be able to use 6th down the main straight. Whilst I have to shift more, I'm pretty sure the corner speeds and control will be better to more than compensate.

Who doesn't like rowing through the gears anyway?
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Old 06-09-2015, 10:28 AM   #13
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My 2 cents:
1) actually dyno your car to know what it actually puts down
2) actually take it to the quarter mile to know what gear it crosses in

With both of those being guesstimated it's almost worthless to talk about what you actually have and what you actually need. Even if you don't launch the car you'll still easily find out what kind of gearing you need by doing even just 1 run.

Lastly, I'm pretty sure a clutch type will be much better suited for drag racing than a torsen
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Old 06-09-2015, 11:55 AM   #14
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Quote:
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My 2 cents:
1) actually dyno your car to know what it actually puts down
2) actually take it to the quarter mile to know what gear it crosses in

With both of those being guesstimated it's almost worthless to talk about what you actually have and what you actually need. Even if you don't launch the car you'll still easily find out what kind of gearing you need by doing even just 1 run.

Lastly, I'm pretty sure a clutch type will be much better suited for drag racing than a torsen
First off these numbers are all for drag racing or best acceleration, not for a particular track or auto-x.


1. One day. Doesn't matter exactly what the numbers are, 1/8 mile mph will be a decent judge of power anyways.
2. Working on that, already went twice. First time had problems with the car when I got there. Went again last friday, fan relay died on the way there and had to turn around after it boiled over. Hoping to run it in the 1/8 mile this weekend if nothing unforeseen happens/it doesn't rain. I'm trying!

Guesses is all I'm gonna get here, but there are some smart people with experience so I want to hear opinions. I still wonder cause with this gearing I'd for sure hit 5th in the 1/4 mile, and probably be somewhere in 4th for the 1/8 mile. I mostly run 1/8 mile so I'm guessing gearing to top out 4th is probably best for 1/8 mile, makes sense to avoid that shift.

I'm wondering about which LSD is better, I rode in a 99 miata with a torsen and you can really feel it lock up pulling out of a turn, more than my clutch type. I have a MSM lsd so it SHOULD be stronger than the regular torsen, so I'm thinking I'll swap it sometime and see how it does. Plan is to run the clutch type this weekend, and then swap and run it with the MSM torsen at the drag strip to see which works better. But I have a 3.909 gear I could put on the MSM thus this thread. A 3.636 is 500 plus shipping, I don't really have that in the budget right now.
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Old 06-09-2015, 12:15 PM   #15
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Fair enough.
Yeah a torsen locks up better, but trust me off the line it sucks lol
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Old 06-09-2015, 12:15 PM   #16
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Quote:
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I though the 3.3 gear was a myth? Bob you run a 4.778 for auto-x right? Do you launch in second and then drive mostly in 3rd?
I run a 4:778 with a Quaife wide gear set. That setup gets me a second gear that is slightly faster than a 6 speed in second with a 3.636 1st gear is also much closer to second so it will carry out further. Its a good gear for autocross with my torque curve and rev limit. Problem is it's like a 3 speed on the track with limited top speed.
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Old 06-09-2015, 01:28 PM   #17
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<p>Torsens break with wheelhop too, ask me how I know.</p>
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Old 06-09-2015, 01:56 PM   #18
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<p>Torsens break with wheelhop too, ask me how I know.</p>
My question is, is the MSM torsen any stronger than a regular torsen? It's bigger/different. I have never read a report of a MSM torsen failing.

Either way I have it, so I'm gonna try it and compare to the RX7 LSD at the dragstrip to see which works better. I hope it don't break, if it does it will be quite an expensive experiment.
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Old 06-09-2015, 02:04 PM   #19
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the axles are beefier. dont think the torsen is any stronger but dunno
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Old 06-09-2015, 02:06 PM   #20
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the axles are beefier. dont think the torsen is any stronger but dunno
See that's the thing, it's DIFFERENT. It's BIGGER. But I have no idea if it's actually STRONGER.

My guess is if mazda went to the point to make it different, and larger, and use bigger axles, the torsen is PROBABLY stronger. Wish I had a stock torsen to compare it to.

But like I said, I've yet to find anyone who broke one. That alone seems significant.
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