S15 6 Speed Transmission for Dummies

[xpertsnowcarver]

S15 6 Speed Manual Transmission (6MT)

***Sorry if the photos no longer show up. Photobucket turned into a real shitshow of a site and wants me to upgrade to a $400 account. I'll figure out how to host them somewhere else.







Everything you wanted to know about the S15 6MT. A lot of this information is scattered all over the place. I'm just bring it together. I personally know quite a bit about the 6MT. I own and use two of them.

Please do not ruin this thread with a bunch of nonsense. Don't post information unless you are 100% positive that it is correct. Don't post how weak you think this transmission is, especially if you've never used or destroyed one yourself.. Any personal experience with this transmission is encouraged to be posted on this thread. For example, "XXXWHP and XXXWTQ, and I destroyed 3rd gear," or "XXXWHP and XXXWTQ, and she still kicks ass."

THE KA 5MT AND SR 5MT BELL HOUSINGS ARE NOT THE SAME.

I don't know everything, but I do know a lot. I'm not stubborn with what I am presenting to you. If you feel I posted incorrect or misleading information, I would be more than happy to hear you out and further research the issue.

To be covered:
- First look
- Comparison to the '5 Speed Manual Transmission' (5MT)
- Gearing and Specifications
- Gearing Comparison to the 5MT
- Necessities for Use
- VSS and Options
- Neutral Position and Reverse Switches
- Aftermarket Options
- Miscellaneous
- Final Notes
- Fitment on Other Engines
- Sources
- Post Notes

Pre-notes:
- I am strictly providing information regarding the S15 6MT for the SR engine. But, always take information as opinion anyway, and fact-check the information yourself.
- When I say 5MT, it is with assumption that it has a SR bell housing for the S13 and S14 SR engines and not a KA bell housing.
- When I am referring to a specific chassis, I will state so.
- I am strictly referring to OEM parts, excluding the 'Aftermarket Options' and 'Miscellaneous' sections.

Apparently the 6MT sycnromesh was designed by Borg Warner, just as the 5MT.


First Look


Nissan guarantees that this transmission will not fail at or below an input torque of 209lb-ft.

This transmission was designed with a Dual-Mass flywheel in mind. Watch this video for an animation on how the dual-mass flywheel works. The first half shows how it is assembled. The last half shows how it works.



Comparison to the 5MT
Similarities
- Bell housing pattern between the 6MT and 5MT are identical.
- The transmission mount is the same as the 5MT. However, the cross member is not the same.
- The slave cylinder, fork, pivot ball, release bearing, release bearing holder and pin all appear to be the same as the 5MT.
- The starters are identical between the S13-S15 SR engines.
- The 6MT clutch disc and pressure plate are similar to the 5MT. (NOTE: Because the 6MT uses a dual-mass flywheel, the OEM clutch disc is unsprung. The 5MT does not use a dual-mass flywheel, so the OEM clutch disc is sprung.)
- The output shaft to the driveshaft is the same as the 5MT.




Starters may appear different, but seem to mount and operate the same.

Differences
- The 6MT flywheel is much thicker than the 5MT. Consequentially, the bell housing is also deeper by the difference in thickness.
- The copper pilot bearing is located in the 6MT flywheel, rather than in the SR engine for the 5MT.
- The 6MT is divided into three sections, rather than two for 5MT. So, no! You cannot put a KA bell housing on a 6 speed.
- The 6MT shaft is slightly shorter than the 5MT shaft.
- The shifter and shifter plate are not the same between the 6MT and 5MT.
- Shift knob threading is wider, but have the same thread pitch. 6MT = M12x1.25. 5MT = M10x1.25.
- The Vehicle Speed Sensor (VSS) is located on the differential rather than the transmission.
- The 6MT cross-member is opposite of the 5MT. (The AT cross-member is closer to the 6MT, but still will not fit.)


Slight difference in spline thickness.


Thicker Flywheel. (In this picture, both the OEM S15 Dual-Mass Flywheel and Autech Flywheel are shown.)


This is the OEM Dual-Mass Flywheel with OEM Unsprung Disc and Pressure Plate. (If I recall correctly, the OEM Dual-Mass Flywheel weighs about 28lbs.)


6MT Crossmember and Nismo Transmission Mount


The 6MT bellhousing is noticeably deeper than the 5MT.


The shifter plate and assembly are completely different.

VSS and Options
As many of you have heard, yes, the VSS is located in the differential, rather than the transmission.

Some of you have speculated that some 6MTs have a plug covering what looks to possibly accommodate a VSS. This is true and false. Yes, the whole is the right size and shape. No, because the plastic gear that turns the VSS is absent from the output shaft. One would need to purchase the plastic gear (and a steel ball and c-clips that lock the gear in place) and dissemble the transmission to install it.

Some 6MTs don't have a plug. They may be cast molded shut.



Here we see one 6MT has a plug and the other does not.

So what are the VSS options?
1) Purchase and install the plastic gear (as mentioned above) and use the pertaining S13 or S14 VSS. This only works if you have a 6MT with plugged VSS slot.
2) Install a differential that has a VSS in it. This is not limited to the S15 differential. From what I've seen, a Q45 differential has a VSS and may be used.
3) Custom magnetic pick up or gear tooth pick up and sensor attached to the driveshaft.

So what are the signal modifier options for the speedometer to read appropriately?
If option 1, then you won't need anything. If you're using a S13 cluster, then use a S13 VSS. Likewise for the S14.
If option 2, you can either use the S15 ABS control unit (located next to the ECM) or use an aftermarket signal modifier such as the Dakota Digital box or Innovate DL-32.
If option 3, you will need an aftermarket signal modifier.

Neutral Position and Reverse Switches

Wiring the transmission to continue using the reverse lights is very easy. No explanation should be needed here.

Gearing and Specifications


Gearing Comparison to the 5MT




***This last one I estimated where the Nismo 6MT gear ratio would be. The original can be found below.***

Necessities to Use a 6MT
For the SR engine ONLY.
- S15 Transmission Cross Member. The 5MT and AT cross members will not work.
- Transmission Mount. The 5MT and 6MT mounts are all the same.
- 6MT Flywheel. The 5MT flywheels will not work.
- Clutch Disc and Pressure Plate. Any clutch kit for the 5MT will work, but if one is using the OEM S15 Dual-Mass Flywheel, then feel free to get an unsprung clutch kit, even though this may be costly. Sourcing a new OEM replacement clutch kit is rather difficult to find. You can use a sprung clutch kit on the Dual-Mass flywheel, but the dampening will be overkill and shifting between gears may feel sloppy.
- Clutch fork, Release Bearing and Holder, pins, and slave cylinder. I am not 100% here, but they appear to be the same between the 5MT and 6MT.
- Driveshaft with S15 output. (Unsure, but the OEM S15 driveshaft may be used as long as the S15 or similar differential with VSS is used.)
- Shift knob with M12x1.25 threading. The 5MT shift knob won't fit because it is thinner, M10x1.25.

Notes: It is best to install the transmission and engine together. With the 6MT mounted on the engine, it will not touch hit the chassis. But, because it is still fatter than the 5MT, there is less room to muscle the 6MT on or off with the engine already mounted. Don't forget, the flywheel is about an inch thicker, which means you need to pull the transmission about an inch further back. That's never going to happen... I've tried; there is no room for removing and installing the 6MT separately from the engine.

Everything else is straight forward and no different than the 5MT. Be sure to torque all bolts down to spec. They will shear if you over tighten them.

Parts to Reference


Aftermarket Options
- Driveshafts. Driveshaftshop.com seems to be the place to go.
- Flywheels & Clutch Kits. This may get costly, but one can either source a used Autech flywheel (~$400), purchase a new JUN flywheel (~$700), or purchase a clutch kit that includes a new flywheel (~$1100+). The benefit of getting the Autech or JUN flywheels is that you can continue using a 5MT clutch kit, whereas a complete clutch kit such as the Nismo Coppermix may force you to stick with the same disc.
- Gears. Nismo, OS Giken, and possible others make gearing kits for this transmission. These kits are rated to withstand more power.
- Transmissions. Nismo and HKS make complete replacement transmissions. The Nismo transmission costs ~$3500, but only guarantees quality up to 298lb-ft. The HKS transmission costs ~$10500. Both, however, have different gear ratios compared to the S15 MT.


Comparison between the Nismo 6MT, S15 6MT, and 5MT.





Here are some more individual comparisons.

Miscellaneous
- Nismo 6th Main and Cross gear swap

I swapped out the 6th gear on one of my 6MTs for track use. It brought the gearing much closer to 5th. (Gear Ratio changes from 0.764 to 0.8624)

- Closer look at Autech Flywheel



If I recall correctly, the OEM Dual-Mass Flywheel weighs about 28lbs, and the Autech Flywheels weighs about 17lbs. Also, the Autech flywheel was engineered by both Nismo and Exedy. Both of their stamps are on the flywheel. (The clutch disc pictured is the Fidanza 3.2 Clutch Kit).

Final Notes
Some of you guys jump the gun by saying, "forget the 6 speed, its weak anyway.." but most of you never used, touched, or seen a 6MT in your life. Don't continue making this your automatic response when the 6MT is mentioned.

How come this guy with 400WHP hasn't destroyed his 6MT when I only have 340WHP and destroyed mine in weeks???
Assuming you know how to shift, it just depends on two things.
- How much TORQUE are you putting out? Not horsepower... One guy may do 340WHP and have 280WTQ while another may have 340WHP and 320WTQ... (If the engine has a GT2871R, bigger intercooler, and injectors, then it should be fine. If you put a rebuilt the engine from scratch and/or put a 2.2L stroker kit, why didn't you buy a stronger transmission anyway? Even a 5 speed is a shitty tranny to mate with a built SR..)
- What are you using the transmission for? Drifting??? Drag???? This transmission isn't good for smashing gears like a mad man, especially with a very abrasive clutch. If you're engagements are smooth, the transmission will do fine. Most of you, and I mean 95% of you, aren't even close to 300whp. So destroying a 6MT transmission would be the last of your worries.

Nissan designed and guaranteed the quality of this transmission when torque input is no more than 209lb-ft. This is because Nissan (as well as any manufacturer) designed the transmission with a Factor of Safety or Margin. So, under normal conditions or even if one were to push an unmodified Spec-R to its limits, the transmission will not fail. (Assuming the tranny is well maintained.)

I estimate Nissan has a margin of AT LEAST 1.333, so one can expect an actual limit to failure to be a least 270lb-ft (emphasis on "at least", because I personally know someone with 270WTQ and has no problems what so ever). The gears are fragile to sudden impulses. To create an impulse, one could simply clutch kick or drop the clutch from a stand still, but you'd need some very sticky tires and a very abrasive clutch. If one were to transition smoothly between gears, no severe impulse made, and the oil used effectively fills the gaps between the teeth of the gears, then this transmission would be capable of at least 300WTQ. (These are all ball park estimates.) Anything after that, beyond me. Have heard of someone with 340WTQ and not having problems... but this is second/third hand information.

Keep in mind that the OEM S15 flywheel is a Dual-Mass flywheel. This flywheel significantly reduces an impulse, making engagement very smooth and allowing the gears to withstand more torque. The Dual-Mass flywheel is quite nice, especially with the unsprung clutch disc. I honestly cannot tell the difference between a dual-mass flywheel with an unsprung clutch kit versus a regular flywheel with a sprung clutch kit (aside from the difference in rev response). I think the dual-mass flywheel with, for example, with the Fidanza 5.4 unsprung clutch kit would be an awesome combination. With the OEM unsprung clutch kit, there is ZERO clutch chatter.

Fitment On Other Engines
- No.
Hahaha. This just isn't the transmission to bother with that. The amount of work it would take to re-center the input shaft of the 6MT to a KA engine, create a sandwich plate to marry the engine to the 6MT, make sure the clutch fork will pivot in the correct range to release the clutch disc, and align the starter to line up with the flywheel... you might as well do this for a sequential tranny, or some other heavier duty tranny.

Sources
- 1999-2002 Nissan Silvia S15 Factory Service Manual
- 1991-1994 Nissan 240SX Factory Service Manual
- Nissan S15 Specifications
- Octane Report - Nismo 6MT Conversion
- Dual-Mass Flywheel assembly and functionality - Youtube
- NISMO | Silvia•180SX Reinforced Cross 6-Speed Transmission
- Gearing and Assembly Prices and Comparison
- Parts List and Prices per Chassis

Post Notes
I think this transmission is a lot of fun to have, but not for drifting or drag racing. I use the transmission for road racing, and it has performed exceptionally well.

Again, please do not ruin this thread. Ask legitimate questions. Post solid facts and experience. If you have not used and/or destroyed this transmission, you're post/claim may not be valid to me.

Hope this helps a lot of you guys.