NISSAN S15 6 speed transmission Swap.
 

Ok got a transmission so I can playing with to see what is possible up close withone of these.

First things Observable now that I have them side by side; 1)Input shaft splines extend a full 2-1/8” further from the front of the center section of the transmission. that will take some thought to make work. 2)The output looks like it sticks aft of the center section the same amount but the splines are much larger than on the Miata, custom drive shaft will be required.

Props for actually doing something:bigtu:

Would it be possible to modify the input shaft or would a new one need to be made?
Are the Nissan splines going far enough back to work in the Miata application (so the input shaft "just" can be shortened)?

I suspect it would be fairly simple for someone skilled with a good lathe to machine the input shaft back probably what I will pursue I had thought about spacing the transmission back assuming it might be less than an inch or so. but 2-1/8” is a lot back.

The Pilot bearing is also bigger diameter. Actually the Nissan uses a bushing instead of a bearing for the pilot.

The 1” X24 splines instead of 15/16 X 22 on the input is pretty easy to get a clutch disk for. One for a Subaru or a FRS will fit in with a 1.8l Miata clutch and I found one for a 74 Datsun 510 would work in a 1.6l size clutch.

The flywheel that came with this is pretty amazing the sprung center clutch disks like on a Miata will wind up maybe 10 degrees or so before the springs bottom out. This thing uses a ridged clutch disk but the two piece flywheel I can wind up 10 degrees with just my hand force. It will twist up well over 45 degrees maybe 90 without bottoming the springs and it moves smoothly.

Make some measurement to see if the Nissan splines on the input shaft goes far enough back to have the clutch disk in the right place. Shorting the shaft on a lathe is easy, to elongate the splines is not that easy (but possible of course, with the right knowledge).

You mean like a dual mass flywheel or some other animal entirely?

It's a dual mass flywheel. Huge amount of shock issolation but pretty darn heavy and bulky though.

The Nissan splines go all the way back. Pretty sure it will be fairly easy to shorten. The front bearing cover tube thing that the throw out bearing slides on is less than 1/2" longer than the Miata part otherwise they look interchangeable same OD etc. The Nissan clutch disk engages like twice the length of splines that the Miata does making for some of the added length and the super thick section of the dual mass flywheel requires some more length as well.

Makes me wonder about the BRZ/FRS transmission I think it is the same gears but it doesn’t use the big dual mass flywheel. Maybe the input shaft is not so long.

No need to dual mass flywheel with the clutch you have... its just more shock into the driveline.

I ran a solid hub clutch in my Miata an a few other cars... once your used to it, its pretty awesome actually.

The clutch shift speed happens a bit faster (and a little harder) being that its a lot less weight when you remove all of the heavy complexity that is the sprung hub. However for a street car it kind of sucks in stop and go traffic and you need to be pretty precise with your downshifting.


An option would be to have a custom clutch disc made with stronger springs if you want sprung hub with less rotational slop.

Thats what I like the all carbon friction surface for. Drives like the best of the sprung hub and the unsprung with an unsprung center.

Props for paving the way Bob!

I'm really curious how this works out.

Props. I assume you can't just swap the input/output shafts from the Miata box into the S15 box? IIRC 4th gear on the Miata box is cast into the mainshaft?

The problem is the sprung hub centers for the Miata disks don’t have enough rotational movement and the springs are not stiff enough at such a small diameter and low amount of travel. It's kind of like running your suspension slammed low with no bump travel and expecting a good ride quality because you’re running stock spring rates when in fact no suspension at all rides better than that. The NISSAN dual mass flywheel has only two springs in it and each one wraps half way round the perimeter of the flywheel. It gets enogh torsional spring rate because the springs are near the perimeter rather than the center and it gets lots of travel because the springs are really long. Also seems to have damping but I’m not sure how yet.

All I can say is I don’t believe sprung the sprung centers available for the Miata help longevity much when you put 300 ft-lbs on them. I bet the dual mass thing like the NISSAN has would be effective at reducing drive line shock but it is not light by any means.

here is a vido showing how it goes together.

The spring only need to be able to move a fraction of an inch to do their job in saving all your driveline bits from impact loading and torsional vibration.

This is very interesting! I wonder about the power output and beating they can take, judging by the drift scene it's a lot if they use stock transmissions.
How about mounting? would it need a typical trans mount and PPF delete? with Diff mount? or maybe even use s15 differential too?

one side of the 4th gear is cast into the mainshaft. Also the primary gear is cast on the input shaft and it is a different ratio than the miata.

I do think it would be possible to use the miata input shaft if you match it with a miata primary gear slid on the counter shaft. Ill be looking into that and cosidering doing that.

Some of the advantage though is the input gear ratio on the S15 box sends lower torque through the counter shaft because of the ratio. Almost 4% lower torque gets applied to 4th gear because of the primary ratio differance.

The Miata bell housing and tail shaft will bolt up to the center section of the S15. They are both Aisin AZ6 transmissions. So final installation should look pretty close to stock.

This is not a strong transmission in the NISSAN scene drifters don’t use it they consider it fragile and only road racers use it to take advantage of the superior gear ratios over the much stronger NISSAN 5 speed. In theory however it should still be stronger than the Miata 6 speed.

The other thing is there are beefed up road race versions of this transmission that are available at the lower end of the price range for a very serious solution that do have claimed ratings over 300ft-lbs. closer to a Quafe gear set for the 5 speed price than a some form of sequential but with even better ratios for track performance.

When the springs are coil bound in hard stop due to the torque applied they don't move at all. Works fine for cruising down the highway nice isolation and lower noize but a hard pull full power through the gears and the movement basically acts like a slide hammer every shift while doing nothing to help between shifts.

My wheel hop problems also basically dissapeard when I took the sprung center out of my clutch.

Ah I didn't know they were the same but now that you mention it I do believe the new BRZ/FRS/GT86 uses an Aisin AZ6 as well. I remember seeing their name, miata, and s2000 and IS250/Altezza(fuck this car for making "altezza lights" popular) when doing research on it.