Mazda NB 6-speed transmission into an NA - questions of course

[MiataQuest]

I am moving ahead next month with the 2.4L EcoTec engine conversion.
The 2.4 liters plus modern engine design & control features should result in more useable low end torque compared to a Mazda 1.6/1.8 (even modified)
I have a 3.90 Torsen rear differential on the way for this engine swap to take advantage of the extra torque, additional strength, and to make it highway friendly.

With that in mind, I have a two part question on possibly swapping in a 6-speed NB transmission

QUESTION #1
Is this a direct bolt in NA swap from the 5 speed? I know it does not have a speedometer gear, but I can deal with that one way or another.

QUESTION #2
I have read pros & cons to the 6-speed. What is your opinion? Go 5-speed or 6-speed with a torquey engine and a 3.90 rear end?

6-speed PROS (may be fact or fiction)
* Stronger than 5 speed
* Better for the track and the twisties.
* Better clutch take-up with the 3.90 rear end

6-speed CONS (may be fact or fiction)
* 1st gear is too low. Need to shift out too quickly.
* 6 speed transmissions do not shift as well as the 5-speed transmissions
* Not good for autocross.
* Final drive ratio negates 3.9 gearing highway advantage
* Slight additional weight, but it is centrally located.

My present thinking is that I will go with the 5-speed because I plan to do a lot of highway driving. I like 80 mph cruise speed (except in Ohio!)
It is 3 hours to good driving roads, 2 hours to visit my son, and 8 hours to visit my daughter. Then there is MATG !
However, comments on the last questionnaire regarding the rear end did change my mind to move ahead with the 3.90 rear end swap now and not wait.

[Greasemonkey2000]

In my humble opinion based on others experience is there are only two GOOD reasons to go to a 6 speed:
1)power goals will exceed that 250-300+ ft/lbs of torque that the 5 spd can start failing
2)intended use(track/auto, etc) dictates the use of closer gear ratios or combination of a certain rear diff ratio

Not sure if you are aware but kmiata has a new alternative that sounds incredibly promising and cost effective, the use of BMW 5/6 speed transmissions: https://kmiata.com/blogs/news/big-ne...for-your-miata

In my opinion it sounds like the mazda 5 speed will be the easiest route given the swap/power goals. I can't recall what is involved in using a 6 speed transmission in a NA to retain the use of the oem speedometer but have seen it done before and not all that complicated. Hope I have been of some assistance Rene!

[— kung fu jesus —]

I am moving ahead next month with the 2.4L EcoTec engine conversion.
The 2.4 liters plus modern engine design & control features should result in more useable low end torque compared to a Mazda 1.6/1.8 (even modified)
I have a 3.90 Torsen rear differential on the way for this engine swap to take advantage of the extra torque, additional strength, and to make it highway friendly.

With that in mind, I have a two part question on possibly swapping in a 6-speed NB transmission

QUESTION #1
Is this a direct bolt in NA swap from the 5 speed? I know it does not have a speedometer gear, but I can deal with that one way or another.

QUESTION #2
I have read pros & cons to the 6-speed. What is your opinion? Go 5-speed or 6-speed with a torquey engine and a 3.90 rear end?

6-speed PROS (may be fact or fiction)
* Stronger than 5 speed
* Better for the track and the twisties.
* Better clutch take-up with the 3.90 rear end

6-speed CONS (may be fact or fiction)
* 1st gear is too low. Need to shift out too quickly.
* 6 speed transmissions do not shift as well as the 5-speed transmissions
* Not good for autocross.
* Final drive ratio negates 3.9 gearing highway advantage
* Slight additional weight, but it is centrally located.

My present thinking is that I will go with the 5-speed because I plan to do a lot of highway driving. I like 80 mph cruise speed (except in Ohio!)
It is 3 hours to good driving roads, 2 hours to visit my son, and 8 hours to visit my daughter. Then there is MATG !
However, comments on the last questionnaire regarding the rear end did change my mind to move ahead with the 3.90 rear end swap now and not wait.

Answer 1: Yes.

Answer 2: If it's a matter of gearing, go with what works best for your configuration and needs.

I had my '92 and a 10ae at the same time. The 6sp never felt as accurate or fun as the 5sp did, even though I tried (rebuilt shifter, engine mounts, etc.). It was more calm on the freeway, but not by much. I know the 6sp are better for the track because of gearing, but I was not convinced the extra cost of replacing them versus the 5sp was worth consideration...for my use. Often times, driving the 6sp, I would forget the 6th gear was there on the freeway and left it in 5th most of the time. Miata transmissions are all quite good in feel and availability, but I like the 5sp over the 6sp. All the BMWs I've had were manuals. They never felt as good as the Miatas without much modification and additional parts. Honestly, the shifter on them feels like a dislocating joint. They are very stout, though, and the reason for buying manual BMWs is for that longevity and reliability. It's actually the only component I don't worry about on them.

As for fun roads, you just gotta move. I left Detroit in '96, MI in '03, never looked back. ;)