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Questions about 1g NSX 5MT and 6MT transmissions

14 April 2017
South Carolina
As far as I can tell, there are 3 different 5speed transmissions. Below is what I estimate the gear teeth count to be. Please correct me if my estimations are off. I have 2-3 questions about the 1g NSX manual transmissions.
Question #1 - Is the C.D. (centre distance, the distance between the center axis of the mainshaft and the center axis of the countershaft) between the 5-speed and the 6-speed the same?
Question #2 - Are the countershaft spline sizes between the 5-speed trans and the 6-speed transmission the same?

If this is the case, then I believe that it is possible to use some of the 6-speed gears inside of the 5-speed trans by doing one of two things:
1) Using a combination of specifically sized DIN 988 shims along with a slightly modified needle bearing from a BMW transmission (BMW part number 23221224713)

2) Taking a 4th/5th collar and turning the OD of the 4th gear side down from 43mm OD to 42mm OD, and then using 4th gear needle bearing from the 6-speed trans.
23915-PR8-000 Collar, Distance (36X43X62.5) $82.13
91107-PR8-F01 BEARING, NEEDLE (42X47X26.5) $25.49

This would allow the following gear ratios in an OEM 5-speed transmission:
1st = 42/14 = 3.071
2nd = 38/22 = 1.727 (or 36/20 = 1.800)
3rd = 35/25 = 1.400 (JDM Type R)
4th = 36/32 = 1.125 (Modified 6-speed 4tH)
5th = 32/35 = 0.914 (Modified 97-99 6-speed 5th)
Final = 65/16 = 4.0625

[B]NSX 91-94[/B]
1.23 =32/26 ??? 

[B]NSX 91-94 Type R[/B]

[B]NSX 95-96[/B]
1.23 =32/26 ??? 

[B]NSX 6speed, 1997+[/B]
1.125=36/32 ?

Edit - updating theoretical trans to include 1.800 second gear option
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If someone is able to confirm the answers for questions #1 and #2 , then I can provide further instructions on how this can be performed.

It's a low risk, low cost project. Especially if that BMW bearing is still available. Beyond that, the only thing that might possibly be an issue would be if the 6-speed's 4th gear's dual synchro required modification of the 3-4 synchro hub. This is something that is simple to do with a rotary grinding tool (think, air powered dremel). I have done this several times on various trans rebuilds of mine to re-use left over parts.
You're going to be hard pressed to find a member who has a 5-speed and a 6-speed disassembled on their bench. I think a simpler solution is to find a way of getting a shorter 4th onto the JDM gear stack of the 5-speed. Most owners want to keep the long 5th for highway duty as a true overdrive gear. Going with the short 5th like you propose might make sense for a track car, though. On most road courses, a stock-powered NSX will never hit the top of 4th, since the straights are not long enough. On my home track, High Plains Raceway, we have a 2,800 foot back straight and, with the JDM gears, 4.06 final and near stock power, I touch about 120 mph before I hit the braking zone- well below the top of 4th. To me, this means that going with a 4th gear maybe half way between the JDM 4th and the 6-speed 4th might yield better performance. Honda may have thought so too- if you watch the NA1 NSX-R Best Motoring Nurburgring lap and listen closely, they have a "trick" short 4th gear in that car. RPMs are at 7,000 when Gan San shifts into 4th. AFAIK, no production version of the 5 speed ever had a 4th gear that short. This change would give you a nearly identical 1-4 shift as the 6-speed, but you would have a bigger drop to 5th.
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How would my hybrid geared setup have a larger drop to 5th gear than the NSX 6-speed, when this hybrid geared setup uses the same 4th and 5th as the NSX 6-speed?

The ultimate solution is of course to just use my modified J trans, which many have expressed interest in, however someone else, with physical access is going to have to mock up the engine + transmission into an NSX, and measure how the mount will need to be made.

I've got enough information gathered already for axles, and how I'll make the shifter mechanism. I've already gone above and beyond with sorting out all the particulars of this setup, but the only way to move forward is if someone with a 1g NSX chassis can do this last part.

ScienceOfSpeed says that they plan to release a standalone kit, but they're a "performance shop" that has to make the capital from selling overpriced "swap kits". I tried to talk with them, but they didn't want to listen.

Yes, this setup would not have a good highway cruising gear. However it would have basically the same 1-5 as the 6-speed.
If you had an NSX 5speed, that needed a transmission rebuild, and were interested in increasing your acceleration, would you be willing to spend $6000+ to get a used 6 speed (that also probably rebuilt (at least with new brass)) ?? Probably not.

But would you be willing to change your parts order to order different gears than the ones you'd already allocated funds for, but then be willing to add an additional, let's say $200 to get that BMW needle bearing, distance collar, and whatever DIN988 washer was needed, and then $50-75 to get the OEM collar turned on a lathe?

$200 for the modifications, and then whatever the cost is for the additional countershaft gears that you normally wouldn't replace.

It then becomes a question of whether or not you're willing to spend $5500 for that 6th gear.
Updated 1st post