Fitting a 6 speed box to a NA8

[Range Runner]

Are there any issues in putting a 6 speed transmission into a NA8? Or will it fit without any modification? I've read that the 6th gear is taller than a 5 speed. Does that give better fuel economy?
Also what's the best ratio diff to put in a NA8 with a 6 speed?
Thanks
RR

[d-mag]

No issues, will bolt up. You'll need a a 3.9 or 3.6 or you'll be revving higher @ 100kph in 6th than you were in 5th.

[orx626]

6-speed install starts about halfway down the page. http://mx5cartalk.com/forum/viewtopic.php?f=57&t=29490&start=60

For me the best diff ratio is 4.3. 3,400rpm @ 100kph in 6th gear is perfect (5th gear on a 5-speed was 3,300rpm @ 100kph)... right in the middle of the torque curve. The car is geared to give a top speed around 200kph. Perfect! If you want performance, not improved fuel economy on the highway then 4.3 or higher is the answer. Fuel economy around town does not suffer because you should be one gear higher with the 6-speed than the 5-speed. ie. 5th is the new fourth.

Cheers,
Danny

[StanTheMan]

Its interesting......How we all prefer different things.
I've dríven a NA6 with a 6 speed & 4.3 diff. & hated it. Well, maybe not quite hated but seriously not my own preference.
But hen I've seen a white NA6 with the same/similar set up in a video up from Queensland. Can't recall who it was, it was someone from here. But it just absolutely flew.

[jerrah]

The 6th gear is not as tall as 5th in the 5 speed.

You mean ZoomZoom? http://www.youtube.com/watch?v=7GdPXJporso

[StanTheMan]

yep....that's him.

[wun911]

Wtih a 6 speed I do 110 kmph with 3 k rpm in 6th gear...

What revs are the 5 speed people when traveling at 110 km ph in 5th gear???

[broady]

NA8, 100km i'm at ~ 3400rpm i think.

[pcmx5]

Its a bit of a dilemna really; most mx5's read 105klph when they are doing 100; if you fit a 6sp to an NA8A the speedo becomes accurate and you are about 200prm less at 100klmph than with a 5sp if you have changed from the standard 4.1 to a 3.9.

I reckon that most later NB's with a 3.69 ratio would gain heaps of driveabilty if they had a 3.9 ratio diff.My mates with NB8C's spend all their time on spirited runs in 3rd and fourth where I am in fourth and fifth and in the power band.

Sixth in an Nb8C is a nothing gear and is so high it must have a negative effect on economy in all but perfect higway cruising, which we have proved on a series of runs..

So a big vote from me for a six speed/ 3.9 combnation, especially with an lsd in any Mx that has more grunt than a 1.6.

Peter