Boosted folks with taller final drives... happy?
 

Gents,

For those of you pushing serious power, have you swapped for a taller ring/pinion set? For the NC's, the 6spd was the stronger transmission, but the gearing is so low as to be nearly pointless.

I'm strongly considering a swap to the 3.72 final drive. Sure, it's only ~10% different than stock, but that's enough to see 60mph in second gear and certainly enough to shave a few hundred RPM off cruise speed. Since my car will spend most of its life as a street bruiser, this seems like a no-brainer.

Thoughts? Flames? Abuse-the-n00b time? :giggle:

You have an NC? I'm not sure what final drive ratio you're coming from.

All the 2009++ NC's have 4.10 final drive.

NC's came with 4.10's... which is 10% higher than 3.72.

As a boosted guy with an NA 5 speed, 1 feels too short and you could probably space the others out a bit to compensate. When/if I upgrade to a 6 speed, I'll go to at least 3.6 or maybe 3.3 rear gears.

Not sure if I'd go so low as 3.3's, but good to hear someone's opinion. Also this thread demonstrates that I need to set up my autosignature...

My setup at 18 PSI, 4.10's are shorter than I'd like with a 6 speed. I think a 3.636 gear would suit this car very well at the power it's making now. It's driveable with 4.10s, but you're shifting a lot. If (when) it has more power I think I'm going to install a numerically lower R&P gear.

When did you last have a revlimiter lower than 8k though? lol

3.3 gears don't exist for NA/NB miatas.

MiataRoadster definitely sold some for a while. Came from a diesel B2000 if I'm not mistaken.

Actually at 7.5K right now. :) Look at me trying to be nice to my engine!

Yeah, the 7500RPM redline in the NC MkII and later helps a little bit. Looks like the 3.72 might be a good way to go.

I had a 4.10 with the MSM 6-speed. It was horrible.

Did the 3.63 over the winter. At just 230 whp I'm happy I did it. When I up the boost I will be even happier.

For people with NA/NB's, it'd be silly to do anything other than the Getrag. I see it this way: I had an MSM torsen, which is easily worth $700. I bought gears for $400 (which was a steal) = 1100. Cost me about $300 to install = $1400. And I now have a spare set of 4.10 gears (worth about $3).

Getrag swap is $1500, plus diff probably another $400 from what I've seen people buying them for = $1900. But then I have stronger axles, and I'm left with a full torsen swap (axles, driveshaft, and diff) to sell (worth about $800-1000, maybe more) making this course of action considerably cheaper than dropping the 3.6 in an existing pumpkin.

None of this applies to you. BUT this thread will come up in searches.

TL;DR... go with taller gears. Tell us what's available in the NC world.

The one that came in the B2000 does not fit in the miata diff, Moti@BlackbirdFab found one and posted about it on the locost7 forums, here is the thread. MR or one of the other vendors tried to get a group buy going to get them made. It never happened though due to cost and quality issues.

Just swapped from 5speed 4.10 to 6speed 3.63 and it is a fine combo. The first 5 gears are similar in ratio to previous but there is about a 300rpm drop in top gear at 80mph versus prior setup. My speedo is off now but I can fix that with a Revlimiter adjusted gauge face.

I've got the Getrag 3.28 swap in my car. And coming from the MSM 4.10 w/6 speed which I pretty much hated once I started making power; I absolutely LOVE it.

Running 20psi ~ 275whp, there's still enough power to smoke the RS3s in 1st if they're not warmed up, second feels a little long, but everything else is great.

I dearly miss the ratios of my 5 spd with 3.9. Even at 220 whp. The 6 sp with 3.9 sux, and I upgraded to 270 whp.
I drove a turboed Focus with ratios close to a miata 6sp + 3.3 and it very very nice.

The above is all about street driving.

street bruiser means you want them as long as possible. so as low as you can go on the fd
over 300 the 6sp+3.6 can light up 1st at ease, 2nd with ease, and 3rd depending on temp and tires

I guess you should also define what "serious power" is to you ;)
Many of us very different perceptions on power

Serious power in my case will be 350-ish at the tires and a wet weight of about 2550lbs.

I think the main problem everyone is trying to solve here is that high power turbo miatas are incredibly slow off boost and incredibly fast on boost and the transition happens about the middle of the rpm band.

This means that pretty much every first gear that is short enough to not bog off the line will bounce off the rev limiter once boost hits and any first gear long enough to not spin will either bog or eat clutches trying to get off the line fast. If the miata had a flat torque curve from 1500 rpm to redline, everyone would just run 50 mph first gears and call it a day.

The best solution (short of going SC or v8) is probably to just run a shortish first gear and limit power until 2nd/3rd to prevent spin. How hard would it be to rig up a 1st/2nd gear sensor for the miata? Once you have that, lowering the boost would be easy.

edit- I suppose you could just derive gear from the speedo cable- ie, below 30mph turn off boost control or substitute a pulse width of your choosing (selectable with a dial on the dash?)

edit2- wait a second I just remembered the miata uses a cable rather than electronic speedo cable... so where is my ECU getting the car's speed from? Anyone remember where to get a speed signal on the miata for something like an arduino?