The Sentra SE-R is a dichotomy; it's blessed by the nearly indestructible, easy-to-extract-gobs-of-power-from SR20DE engine. On the other hand, it's saddled by a buttery transmission that's so fragile, it can break at the sound of a few expletives.
In my opinion, the transmission is one of the main reasons the SE-R hasn't made huge inroads in import drag racing. Few companies make heavy-duty import transmission parts, and for Nissans, the choices are even fewer.
Considering Nissan's penchant for building near-bulletproof stuff, the SE-R's transmission is amazingly underbuilt. Third gear is extremely wimpy, about half the width of a typical Honda gear, and can easily be broken with a well-built naturally aspirated motor in stock condition. The transmission case cracks easily, the stock motor mounts tear and break easily and the transmission tends to pop out of fifth gear on pre-1994 cars. Although the transmission is equipped with a viscous limited-slip differential, this tends to wear out after a few years of hard use. The viscous limited slip is also not too aggressive and only lightly locks up, still allowing quite a bit of peg leg, one-sided wheelspin.
Project Phoenix is being built as a multipurpose car, a car that can brake and corner as well as go fast in a straight line. Surviving the rigors of being pounded on a road course will be its acid test. Road racing is really tough on a car. The car spends a lot of time at wide-open throttle and running through the gears both upshifting and downshifting. Since you spend a great deal of time in third gear on a road course, high-powered turbo and even low-powered turbo SE-Rs are usually forced to limp around with the boost at a minimum level to nurse the transmission.
In this installment, we cover some of the basics to keep your SE-R's transmission in one piece. These tips will work well on lower-powered cars and will forestall the inevitable destruction of third gear for a while in higher-powered cars. Most of these tips are low cost and easy to pull off.
Gear Deburring
One of the first steps in transmission preparation is deburring the gears. Deburring removes the rough edges of the gears where stress risers can form. A stress riser is a sharp edge where stress concentrates and a crack can start. On a pounded SE-R transmission, the sharp top edges of the gear teeth look like they dug into the face of the adjacent tooth and weakened the whole tooth.
In the first step of gear deburring, take a die grinder with a carbide cutter and break the edges of the top of the gear teeth. Note that we just broke the edge, hardly removing any material. Also, break the edges on the sides of the teeth. If you don't have a compressor and a die grinder, you can use a dremel tool and a carbide cutter, although this will be a bit slower. Next, polish the top and sides of the teeth with a rubberized abrasive wheel on a grinder. Don't go crazy and completely round the teeth off, just smooth the edges out and remove all of the burrs.
Finally, polish the gear teeth on a grinder with a Scotchbrite green wheel, and bring the crown and sides of the teeth to a nice shine. A smooth surface will have no flaws where a crack can start and the smooth crown of the tooth will be much less likely to dig into the adjacent teeth. When running your fingers over the gear, it should feel smooth, like soap, with no rough edges that catch your fingers.
Cryo Treating
Cryo treating involves freezing the gears with liquid nitrogen, then slowly heating them up to the tempering temperature of around 350 degrees Fahrenheit. The gear is cycled like this about three times in roughly a 72-hour period. Cryo treating is an extension of the heat-treating process and typically provides a 100-percent increase in fatigue strength and wear, with some improvements in tensile strength, as well.
In addition, cryo treating is relatively inexpensive, with most parts costing less than $100 to treat. Cryo treating works best on ferrous metals, like steel, that have been heat-treated-like transmission gears. Cryogenic treatment has no negative effects and works well on engine parts, axles, brake rotors and any other highly stressed metal parts.
We've used cryo treating many times in the past to solve all sorts of breakage problems, and with SE-R trannies, this works well. We sent our gears, including the input shaft and the ring and pinion, to 300 Below for its proprietary cryo treatment.
Shotpeening
After cryo treatment, we sent our gears to be shotpeened. Shotpeening is the bombardment of millions of tiny steel balls shot at the part at high velocities. It works by compressing and refining the grain structure of the metal on the surface, kind of micro-forging the metal into a harder layer where a crack has a hard time getting started.
Shotpeening changes the structure of the metal about 0.005-0.010-inch deep. The process usually improves the fatigue strength of a steel part by 100 to 300 percent. Shotpeening works well to solve breakage issues on all sorts of steel parts. We shotpeen cranks, rods, axles and many other parts to help prevent breakage. We sent parts to AFCO, an aircraft-certified shotpeener.
Since gears are made from very hard materials, they need special processing to get all the benefits of shotpeening. We blasted our gears with RC 55-65 shot with a 220 size at an Almen intensity of .020; next, we hit the parts hard with larger 330-size RC 55-65 shot at Almen .020. Our theory is, the smaller shot will get into the roots of the gear teeth, but the larger shot will work the surface harder. Any aircraft-certified shotpeener should be able to duplicate our work. If your shotpeener's eyes glaze over when you give him these specs, do yourself a favor and find one who understands this lingo.
So here is the order in which to do these things: polish, cryo treat, then shotpeen. Remember this order. It's important!
Weld your case
The SE-R transmission case has a weak spot where the axle goes into the case on the bellhousing side of the transmission. The case tends to crack from the outside of the case through the axle hole. Once the case cracks, the tranny will leak gear oil from the void. The cracking can be cured by welding in a piece of aluminum to this area to reinforce the case.
Cut a piece of .25-inch thick aluminum to shape and TIG-weld it in place using a skip-weld technique to avoid overheating and warping the case. The welding may inadvertently distort the axle hole to the point where the axle seal may weep around the outside edge. When reinstalling the seal, smear a thin layer of silicone around the outside surface of the seal.
Get good motor mounts
One reason the case cracks is the poor Nissan motor mounts allow the tranny to smack on the cross member when doing hard shifts or when launching the car. The soft, gushy mounts also put a lot of external stress on the tranny, distorting the transmission case under load. We used Jim Wolf Technology (JWT) heavy-duty mounts on Project Phoenix although Energy, Place Racing, and SR20 Development also make heavy-duty mounts. Energy mounts are inserts and are the softest and most streetable. The others allow a bit of vibration to come through. The wheel hop, crashing and banging of the stock mounts put quite a bit of stress on the transmission's internals and the axles as well. Mounts will go a long way in helping prevent breakage.
Use the least amount of clutch to do the jobA solid-hub, hard-hitting three-puck clutch is a sure way to break third gear quickly. Be realistic and pick the least aggressive clutch that can contain the amount of power your engine produces, and by all means avoid a solid hub, puck-type clutch disc.
We used Jim Wolf Technology's Stage III clutch, which has many fiber tuff pucks on both sides of the disc with a sprung hub. Fiber tuff is a semi-metallic friction material that holds large amounts of power well, but smoothly engages-not nearly as grabby as your typical copper ceramic pucks. We also used JWT's Stage II pressure plate with a higher clamping pressure than its standard plate.
For the SE-R, it's better to use a higher clamping load with a smoother, less harshly engaging pressure plate to avoid shocking the transmission.
Run a light flywheel
A lighter flywheel has less rotating inertia and thus doesn't shock the gears as much during shifts. We used a JWT aluminum flywheel, but Unorthodox, JUN, Stillen and Findenza also make good Nissan flywheels.
Use Redline Shockproof Heavy gear oil
This stuff is incredible. It vastly extends the life of the weak SE-R transmission. Shockproof Heavy is blended by Redline to specifically protect transmission gears and help reduce damage to weak gears. Shockproof Heavy forms a cushioning, thick red film on all of the transmission's internal parts. If you wash away the red film with solvent, you'll find a hard-to-remove gold film under the red. What this means, we don't have the slightest clue, but this stuff seems to work.
When testing this 90w-250-weight oil on the dyno, we found it can absorb up to 6 whp. This probably makes it not the lube of choice for mildly built streetcars, but for a turbo motor, who cares as long as it helps the tranny hang together longer? On NASA SE-R Cup road racecars, this lube makes the transmission more reliable.
Do stuff to avoid wheel hopWheelhop pounds the crap out of a car's drivetrain. Use Energy urethane suspension bushings and pillow-ball-type upper strut mounts to help reduce transmission-pounding wheel hop. On the dragstrip, stay away from wheel-tire combos that cause hop or run real drag slicks. Drag slicks don't hop easily and the soft sidewalls cushion the transmission.
Don't speedshift into third
This is self-explanatory. Lift the throttle and ease it into third. Make sure the transmission is fully into third gear before hitting it. One bad 2-3 shift in a high-powered SE-R can spell doom.
Other stuff we did while we were in there
Phantom Grip LSD
The stock Nissan VLSD tends to wear quickly and become marginally effective at preventing wheel-spinning launches and inside wheel spinning under cornering. While the exotic Quaife and Nismo differentials address this issue, they're also expensive.
Phantom Grip has a unit designed to work with the factory Nissan VLSD to make it a much harder-locking piece and to greatly improve the performance of a typical worn-out VLSD unit for a much lower price. We used the Phantom Grip with the competition green spring tuning pack for even harder lock up. Phantom Grips are smooth in operation and you can't feel them work. You just notice the extra traction out of the hold and off the turns.
Courtesy Nissan fifth-gear pop-out elimination kit
Many 1991 to '93 SE-Rs suffer from fifth-gear pop-out. This usually occurs at 50,000 to 100,000 miles, although one blown 4-5 shift can also cause pop-out. After 1993, Nissan improved many of the components, from the 4-5 balk rings to the shift inserts, to eliminate this problem. All of the cars built after mid-1993 have these improved parts in them.
Courtesy Nissan, a leading mail-order Nissan parts house and performance-friendly Nissan dealer, has assembled all of the improved parts into an easy-to-purchase, no-research-required kit. If you have an older SE-R and are rebuilding the transmission, or if your SE-R suffers from fifth-gear pop-out (they all do sooner or later), this kit is a lifesaver. Courtesy Nissan is also a performance-friendly dealer that both stocks many high-performance parts and actively supports the Nissan performance community as well.
While we had the tranny apart, we replaced all the bearings and seals. We inspected the synchros and balk rings and found them all to be in excellent shape, so we reused them.
An SE-R transmission prepared in this manner will have at least twice the service life of a stock transmission. Transmission life is very dependent on operator skill and driving style. What this means is that some people will never break the tranny again. Others will experience significantly better service life, like one transmission every five road racing events instead of every weekend.
One low-10-second, 550-hp SE-R had a transmission prepared like this last more than 100 passes and an entire season before breaking. Shift briskly yet smoothly for best transmission life and don't speedshift into third. Lift that throttle.
The entire cost of all this work is less than $500, including the Phantom Grip LSD. That's money well spent if you have an SE-R pushing more than 200 whp. In the next edition of Project Phoenix, we'll step into the exotic world of building a bomb-proof, cost-is-no-object tranny that will hopefully hold everything we have planned for our car! Stay tuned.