Review – Nismo SR20DET Reinforced 6-Speed Transmission
By Staff, Photography by Joey Leh
One of the most overlooked areas in project cars is the drivetrain. When you’re hammering through the gears at the track, the clutch under your left foot and the lever in your right hand matter as much as anything else. Put together a good system and you’ll be too busy chasing short straights and tightening turns to notice anything else. Drop in a few pieces of junk and you’ll be cursing the imprecise feel and shocking engagement as you careen off the road.
The Holy Grail for SR20DET owners is a true six-speed transmission. The promise of shorter gearing and an overdrive 6th for freeway cruising is sometimes just too good to pass up. But most of the six-speed options for the SR20DET aren’t a direct drop in, requiring custom parts or lots of parts swapping.
The previous best option was to swap in the stock six-speed transmission from a S15 Silvia Spec R or one of the special six-speed Autech S15 Silvias.
But that route has its own issues, as the S15 six-speed doesn’t have a mechanical speedometer drive gear inside the transmission casing like the five-speed S13, S14, and S15 Spec S models do. Enter Nismo.
Short for Nissan Motorsports International Co., Ltd., Nismo is the tuning and racing arm of Nissan Japan. From within their Tokyo headquarters, Nismo maintains a Super GT GT500-class program and produces some of the most extensively engineered and tested aftermarket go-fast parts. This includes camshafts, reinforced oil pumps, suspension arms, brakes, limited-slip differentials, and a reinforced drop-in six-speed transmission for SR20DET equipped S13, S14, or S15 owners.
Required – S15 Clutch Kit
The first thing that you should be aware of when attempting this particular swap is the price. The transmission itself doesn’t come cheap (it retails roughly around the $4,000 mark last we checked – Ed.) and if you own a S13 or S14, you’ll have to source a S15 clutch kit due to the difference in depth and transmission input shaft length. Trust us, your busted old S13 clutch won’t work here. With our test car being a S13, we sourced a S15 Silvia application Nismo Coppermix clutch kit.
The single plate Coppermix clutch is rated to hold 391 lb-ft of torque and engages lightly and easily. It’s also 1.4-lbs lighter than the factory SR20DET clutch. The Nismo Coppermix aluminum flywheel is 3.3-lbs lighter than stock for a reduction in rotating mass but the most important thing about our clutch kit is its S15 fitment.
| Clutch | Flywheel | Pressure Plate | |
| Nismo Super Coppermix | 2.4 lbs | 16.4 lbs | 12.2 lbs |
| Stock SR20DET | 3.8 lbs | 19.7 lbs | 13.7 lbs |
Nismo Reinforced Six-Speed Transmission
The S15 Spec R picks up its speedometer signal electronically from the rear ABS sensor and a S15 six-speed transmission swap necessitates a swap to the rear S15 pumpkin and lots of custom wiring to get the S15 gauge cluster to work (the S13 and S14 gauge clusters will not read the S15 speedo signal). So, unless you don’t care how fast you’re going down the street, a S15 six-speed transmission is not a direct bolt in into any USDM 240SX.
S15 clutch required
Starting with the FS6R92A S15 six-speed transmission casing, Nismo fits a custom reinforced gearset and adds a mechanical speedometer drive gear to the mix. Tomei USA also sells the separate Nismo bolt-in kit, which includes the transmission mount, driveshaft, shifter, hardware and required harness extensions.
The only other items you’ll need are the previously mentioned S15 clutch, new shift knob (five-speeds use a 10×1.25 thread, six-speeds are 12×1.25), and gear oil. Unhappy with all the burning hot aluminum and titanium shift knobs on the market, we had our rubber Nissan shift knob machined out to 12mm.
| Transmission | Flywheel | Pressure Plate | |
| Nismo reinforced cross six-speed | 91.6 lbs | 16.4 lbs | 12.2 lbs |
| Stock SR20DET five-speed | 95.4 lbs | 19.7 lbs | 13.7 lbs |
Nismo offers the reinforced six-speed transmission with a multitude of kits to make it a direct drop-in into any S13, S14, or S15 equipped with a SR20DET or SR20DE engine. If you own a S13 or S14, the S15 flywheel, pressure plate, and clutch will bolt directly to any SR20DET engine.
We paid a visit to Steve Rodgers at SR Motorcars in Gardena, California and swapped out our tired five-speed transmission in less than half a day. Rodgers knows the S-chassis and SR20DET engine inside and out and made short work of the installation after lifting the transmission into place with a Herculean effort.
3.8-lbs lighter with 1 more gear
We were pleasantly surprised to discover just how simple Nismo has made this six-speed swap. All parts are labeled and separately bagged and have perfect clearances. The included driveshaft was the perfect length and had zero balance or vibration issues. After we fired up the car, it was ready to go and the speedometer functioned perfectly. This is what you get with near OEM level engineering.
Because it’s based around a standard S15 six-speed transmission, shifter feel is largely the same as with a stock S15 Silvia Spec R. The throws are quite long for performance use but the precision is what matters and the six-speed shows its strength over the five-speed there.
Shifting is much more precise through the gears and the movement is so smooth, there isn’t any need to force gear changes. Nismo claims to have reinforced this transmission over a stock S15 six-speed through the use of larger diameter gears and by changing most of the gear ratios versus the standard S15 box.
For fifth gear, which is the same as the standard S15 (1.000), Nismo has reduced the number of both counter and main gear teeth and has increased the size of the gear teeth.
The Nismo first gear is noticeably taller (numerically lower), with the new gearing helping to cushion the shock load of a hard launch from a standstill. One of the biggest advantages with the Nismo six-speed though is how 2nd, 3rd, and 4th gear have all been moved closer to each other, which helps us stay above the boost threshold when on track. Combine that with a higher torque capacity and you’ve got yourself a winner.
Contact
Tomei USA
www.tomeiusa.com
TIRE SIZE WIDTH (mm) 225
ASPECT RATIO (%) 45
WHEEL INCH 17
TIRE DIAMETER 634.3
TYPE S13,S14 STD 5-SPEED (FD: 4.83)
1ST 3.321 26.45 (speed at 3k RPM) 44.08 (5k RPM) 57.31 (6.5k RPM) 70.53 (8k RPM)
2ND 1.902 46.18 76.97 100.06 123.15
3RD 1.308 67.14 111.90 145.48 179.05
4TH 1.000 87.85 146.42 190.34 234.26
5TH 0.838 104.86 174.76 227.19 279.62
TYPE S13.S14 NISMO-6SPEED (FD: 4.83)
1ST 2.907 30.22 50.36 65.47 80.58
2ND 1.989 44.17 73.62 95.71 117.79
3RD 1.537 57.15 95.24 123.81 152.39
4TH 1.218 72.13 120.22 156.29 192.35
5Th 1.000 87.85 146.42 190.34 234.26
6TH, 0.862 101.87 169.78 220.71 271.64
TYPE S15 SPEC-R STD-6SPD (FD: 3.692)
1ST 3.624 26.81 44.68 58.08 71.49
2ND 2.200 44.16 73.60 95.68 117.76
3RD 1.541 63.05 105.08 136.60 168.12
4TH 1.213 80.08 133.47 173.51 213.55
5Th 1.000 97.15 161.92 210.50 259.07
6TH 0.767 126.60 211.00 274.30 337.60
TYPE S15 SPEC-R NISMO-6SPD (FD: 3.692)
1ST 2.907 33.42 55.70 72.41 89.12
2ND 1.989 48.85 81.42 105.84 130.27
3RD 1.537 63.20 105.33 136.93 168.53
4TH 1.218 79.77 132.95 172.84 212.72
5TH 1.000 97.15 161.92 210.50 259.07
6TH 0.862 112.65 187.76 244.08 300.41
Great gear data – a chart would be even better! Haven’t heard of this unit before, how much does it run? Any better or stronger than a Z32 or Z33 6-speed swap?