Da Groj

Remember that Miata I bought from Ben, that I keep saying isn't a "project"?

Okay, so it's got a 6-speed transmission and a 3.909 diff. Like a lot of people, I hate 6-speed transmissions. Partly just because I grew up with 4 and 5-speed transmissions, and I don't need no stinkin' 6th gear! I wouldn't mind a 6th gear if it was a "highway overdrive" gear, but that's not at all what Mazda did with it. Couple that with the fact that the 6th speed has a 2nd gear that will barely get the car to 55 mph. Yeah, I'm not a fan. I could live with it, but I'm not a fan.

So, Philip parted out his NB mistake and gave me a great deal on a nice 5-speed transmission. I'm still trying to decide if I want to use it or not. Some research and math was required. In particular, I want to know what the top speed in 2nd is, and what the calculated torque to the wheels is in each gear. And that's what I finally got around to doing today!

The attached sheet has data for:
1999 6-speed w/ 3.9 (what's in the car)
1990 5-speed w/ 4.3 (never a fast car, but always peppy and fun)
1996 5-speed w/ 4.1 (I owned one for 7 years, good data point)
1999 5-speed w/ 3.9 (what I'd end up with if I did the swap)

It's clear that swapping the 6-speed to a 5-speed would affect acceleration by a noticeable amount. But, I'd get 11 mph more out of 2nd gear, which pretty much means I'd NEVER need 3rd gear on an autocross course.

I'd be disappointed with acceleration in the range of the early 1.6 Miata. But, I was never displeased with the acceleration of my '96, and the result would be almost identical to that... only with the ability to run up to 65 in 2nd.

I'm leaning towards doing the swap, and including a lightweight flywheel in the mix. I think it'll work!

Then you could put 14's on it and still get the second gear speed you want!
(yeah, yeah, 14s won't fit the brakes...but, you could always do a swap there, too...).

Native wrote:Then you could put 14's on it and still get the second gear speed you want!
(yeah, yeah, 14s won't fit the brakes...but, you could always do a swap there, too...).

99's came with 14" wheels on Base and Touring models.

Loren wrote:Remember that Miata I bought from Ben, that I keep saying isn't a "project"?

Okay, so it's got a 6-speed transmission and a 3.909 diff. Like a lot of people, I hate 6-speed transmissions. Partly just because I grew up with 4 and 5-speed transmissions, and I don't need no stinkin' 6th gear! I wouldn't mind a 6th gear if it was a "highway overdrive" gear, but that's not at all what Mazda did with it. Couple that with the fact that the 6th speed has a 2nd gear that will barely get the car to 55 mph. Yeah, I'm not a fan. I could live with it, but I'm not a fan.

So, Philip parted out his NB mistake and gave me a great deal on a nice 5-speed transmission. I'm still trying to decide if I want to use it or not. Some research and math was required. In particular, I want to know what the top speed in 2nd is, and what the calculated torque to the wheels is in each gear. And that's what I finally got around to doing today!

The attached sheet has data for:
1999 6-speed w/ 3.9 (what's in the car)
1990 5-speed w/ 4.3 (never a fast car, but always peppy and fun)
1996 5-speed w/ 4.1 (I owned one for 7 years, good data point)
1999 5-speed w/ 3.9 (what I'd end up with if I did the swap)

It's clear that swapping the 6-speed to a 5-speed would affect acceleration by a noticeable amount. But, I'd get 11 mph more out of 2nd gear, which pretty much means I'd NEVER need 3rd gear on an autocross course.

I'd be disappointed with acceleration in the range of the early 1.6 Miata. But, I was never displeased with the acceleration of my '96, and the result would be almost identical to that... only with the ability to run up to 65 in 2nd.

I'm leaning towards doing the swap, and including a lightweight flywheel in the mix. I think it'll work!

Did the same calculation, opted for 4.1 with a 5-speed. At Brooksville, ran out of 2nd gear often. Not yet at the Big D nor Lakeland.

jev61 wrote:

Native wrote:Then you could put 14's on it and still get the second gear speed you want!
(yeah, yeah, 14s won't fit the brakes...but, you could always do a swap there, too...).

99's came with 14" wheels on Base and Touring models.

Perfect.

Joe, I like the 4.1 and 5-speed combination. But, I happen to HAVE a 3.9 diff in the car. So, I'll go with that. If it really sucks, I can always swap it for a 4.1 later.

Loren, I have a local friend that's trying to sell a nice 4.1 torsen. You could easily sell that 3.9 as the turbo guys want those.

I'm gonna hold off for now. I might like it with the 3.9. I can always swap it later, and still not lose any money on it.

I need someone else to buy it so I don't cuz I need an actual LSD lol I'm gonna keep rocking this 4.3 open for another season it looks like.

Loren wrote:Joe, I like the 4.1 and 5-speed combination. But, I happen to HAVE a 3.9 diff in the car. So, I'll go with that. If it really sucks, I can always swap it for a 4.1 later.

FWIW, the stock LSD on 5-speed NBs is a 4.3:1. On the other hand, our rules say you can't modify the ENGINE internals...doesn't say anything about the rest of the drivetrain.

Yeah, gear ratios are fair game in FAST "Stock".

4.3 with a 5-speed would still give a longer 2nd gear than the 6-speed with a 3.9. Definitely gives better acceleration.

When I put a 4.1 into Fred (an otherwise stock 1.6 Miata), the difference from the 4.3 wasn't so much that I hated it. Since I'm not used to driving this car as it is, if I do the trans swap relatively soon, I'm sure it will be fine.

I should start shopping for a flywheel. (again)

I would hold off on the swap for now especially with the smaller sites we are running now, the tight gears will keep you in the powerband from the slow sections sections that may require power to dig out. But I can see it being annoying at a runway site where there are times you'll be in no gear land.

Based on your PDF, the 3.9 five speed seems too tall for the smaller sites. I like the 4.3 or 4.1 with preference with the 4.3, but it seems those look like the best compromise for all around usage at different sites.

Loren wrote:4.3 with a 5-speed would still give a longer 2nd gear than the 6-speed with a 3.9.

That was the point...if you decide to swap diffs anyhow, and you're not regularly running long courses, 4.3s are just as easy to find as NA 4.1s.

Loren wrote:Remember that Miata I bought from Ben, that I keep saying isn't a "project"?

Yep. That lasted about a week. Loren is a veryientertaining man.


**Having another one of my spells.

My favorite online Miata gear calculator.
https://www.flyinmiata.com/tech/gearing.php

Yep, that's where I got my top speeds from.