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.