Project Nissan 350Z

From Nissan 350Z & 370Z Wiki

What do you do with a car this good? The 350Z is so stunningly effective in stock guise, we've been struggling for months to come up with a first step for our project car. Struggling, in this case, means long road trips for "research," burning up tires in twisty road "brainstorming sessions" and countless powersides for inspiration. We completely destroyed our first set of rear tires before we had any clue what part one was going to be.

When it finally hit us, the answer seemed obvious. Make powerslides easier, of course! The 350Z's stock suspension tuning is quite good, with very high limits, but when you summon the courage to reach those limits, the handling balance is biased strongly toward understeer. The VQ35DE's massive torque, of course, makes up for this somewhat, allowing the driver to break the rear tires loose at will.

But, after the initial thrill, we found ourselves yearning for better balance. On a twisty road, you want the car to rotate naturally and predictably in a corner. Oversteer should come only if requested, and then should arrive smoothly and progressively. In stock trim, oversteer has to be provoked, but it arrives with a bang.

Part of this hairy edge may be unique to our car. After last year's "Z Thrash" in which we threw suspension, brakes, exhaust and staggeringly sticky Toyo RA-1 race tires on one of the first four 350Zs in the country, the diff's limited-slip effect has never felt the same. The RA-1s had so much grip that when we attempted to do a big, glorious powerslide out of Ziel Motorsports' driveway, the stock viscous limited slip didn't seem to be up to the task of spinning the outer tire. Instead of a powerslide, we made one deep, dark, very black stripe. That car became our project car, and ever since, oversteer has been a bit spooky.

The stock viscous limited slip works by immersing a series of round plates, alternately attached to an axle or the differential housing, in a fancy viscous goo. When you go around a corner, the outer wheel will turn faster than the inner wheel, but because the differential housing itself is going at a speed halfway between the two wheels, the differential housing sees the outer wheel turning forward and the inner wheel turning backward.

The plates can be stacked various ways, but let's assume that on each side of the differential there are eight plates, with half of them attached to the axle, and the other half attached to the differential. Call the axle plates "A" and the differential plates "D", and they will stack up like DADADADA.

The KAAZ diff is easy to disassemble once it's out of the car. Good thing, too, since adjusting its lockup characteristics requires a complete teardown. The KAAZ 350Z diff comes with 24 clutch discs, an unusually large amount, which is appropriate for a car with room for unusually large tires.

The plates in a viscous limited slip don't actually touch each other. Instead, they are separated by a thin film of this viscous goo. When you go around a corner, and the A and D plates start turning at different speeds, the goo trapped between them heats up. Some chemist somewhere figured out how to make a magic fluid that expands and thickens under this shear load, making it more difficult for the plates to go different speeds.

While the fluid tries to lock the plates together, the tires try to rip them apart. At low speeds and gentle turns, the fluid doesn't get thickened much and the tires win. Boot it and try to spin one tire and the plates suddenly are going dramatically different speeds. The fluid thickens just as dramatically and, with the stock tires, the fluid wins, locking both rear tires together and letting you have your glory slide. A little less throttle and you have the smooth, seamless acceleration Nissan engineers had in mind. If you have really sticky tires, though, the tires win, and the long, gooey molecules of the stock diff's viscous snot get sheared into smaller, less thermo-reactive visco-thickening stuff; stuff that would make that mysterious chemist blush with embarrassment.

The spider gears, shown here, are ultimately responsible for transfering torque to the axles. When the throttle goes down, it's their resistance that shoves the clutch plates closer together.

After our molecule-shearing attempt at an RA-1 powrslide, applying power in a corner results in a moment of inner tire spin before the sheared and demoralized fluid gathers the courage to rein in the inner tire. Recalcitrant snot is confidence-sapping at the limit, so we needed a stronger limited slip.

Viscous limited slips have inherent limitations in how much torque they can transfer through their funky fluid medium. Conventional, clutch-type limited slips, however, can be built to handle the massive torque loads of racecars on slicks. They tend to be less refined and seamless than viscous units, but they are more capable.

The vee-shaped groove is the key to making a clutch-type limited slip tighten under load. As the blocks accelerate to the left, the shaft will resist to the right, trying to slide down the gooves between the blocks. This shoves the block apart, which pushes the 24 clutch discs together. Instant powerslide.

The clutch-type KAAZ limited slip we installed in our car works on a similar principal to the viscous limited slip described earlier, but instead of being separated by magic fluid, the alternating clutch plates are actually in contact with each other. Most KAAZ limited slips use eight plates per side, for a total of 16, but the Z, with its massive differential and the potential for huge torque and enormous tires, was endowed with 12 plates per side. At the end of each stack of clutch plates is a spring washer that presses the plates together.

The real magic of a clutch-type limited slip, however, is how the clutches are pressed together harder under load. If you think carefully about the forces in a differential as the car accelerates in a straight line, you'll see that the driveshaft turns the ring gear, which is bolted to the differential housing.

For the torque to actually get to the wheels, though, the differential housing has to grab the spider gears--which are the four gears that sit sideways in the diff and connect the two rear axles--and swing them around the axis of the axle.

Once the diff is out and the cover and bearing caps (seen on the table to the left) are removed, the diff should pop out the back of the housing. It won't, though, so you'll have to pry it out.

In an open differential, this is accomplished by mounting the spider gears on shafts that pass through the differential housing. On a clutch-type limited slip, the spider gear shafts are mounted in V-blocks attached to the differential housing. When the diff turns the V-blocks and the V-blocks try to push the spider gear shaft, the shaft tries to fall into the V, pushing the blocks apart. The blocks, of course, push on the clutch packs, tightening the diff. The more torque you apply, the harder the clutch plates grab.

This is the fundamental difference between a clutch-type and a viscous limited slip. The stock viscous diff responds to a difference in wheel speeds, relying on the slipping plates to heat the fluid and tighten, which means it must react to wheelspin after it begins. The KAAZ responds to torque, so it reacts before the inside tire has a chance to spin.

There is one other feature of clutch-type limited slips worth noting. The V blocks can be machined with Vs on both sides of the spider gear shafts, so the clutch packs get loaded both under acceleration and engine braking; they can be machined to only work under acceleration or they can have Vs of different angles, so there's more effect under acceleration than under engine braking.

You should order new differntial bearings from Nissan before swapping the diff. Removing them from the old diff is difficult to do without damaging them, so your best bet is to simply press new ones onto the new diff. We couldn't get any, so we re-used ours.

The first configuration is called a two-way limited slip, the second a one-way. You've probably guessed by now that the third is a 1.5-way. Which is best is a matter of driver preference, but most consider a 1.5-way the best compromise for good steering feel and consistent rear-wheel grip. The KAAZ 350Z diff is currently only available as a two-way.

The 350Z differential comes out relatively easily. It can be removed without removing the rear anti-roll bar or exhaust, but it's a tight squeeze. Newport Exotic Cars, importer of the KAAZ, charges about $400 for installation. The KAAZ differential can be set up with different amounts of locking capability by swapping the order of the clutch plates.

The diff is shipped in what KAAZ calls the 100-percent locking configuration, which, if you continue to call the differential plates D and the axle plates A, means each side has the plates stacked DADADADADADA. Every time there is a D next to an A, there is an active friction surface helping you do powerslides. If you swap a few plates around and stack them DADAADDADAAD, you knock the number of active friction surfaces from 11 down to 8. KAAZ calls this configuration 60 percent locking. Yes, eight is 72 percent of 11, but we're sure KAAZ has its reasons.

The discs with tabs on the inside rotate with the axle, the tabs on the outside rotate with the diff. The grooves help keep gear oil on their friction surfaces.

Don't even think of using the 100 percent setting. The KAAZ diff is so strong, this setting is effectively solid. The rear wheels are trying so hard to go the same speed that steering feel goes to hell, turn-in is ruined, and the only smooth way around a corner is with both tires spinning. With 600 hp and racing slicks, this might be a good setting, but not with street tires.

With a few plates swapped around, the diff is seamless and nearly invisible. There are some clunking and popping noises in parking lot maneuvers when the diff is cold, but out on the road where it matters, all is smooth. Where the stock, tired, sheared viscous diff would enter corners feeling open and exit feeling like a limited slip (with an unpredictable transition somewhere in between), the KAAZ diff is much more consistent.

Little subtleties, like the shape of the rear spokes compared to the fronts, make the Nismo wheels look more right on the Z than your average aftermarket wheel. The 275/45-18 tire Nismo uses on these wheels looks even better in the wheel well.

It's clearly rowdier than the stock diff, making itself known much more often, but it also allows you to be more rowdy. Doughnuts, powerslides, and virtually any form of unbridled hooliganism are much easier now. We may try to tame some rowdiness by swapping a few more plates in the future, but we'll wait until we get the stink of gear oil out of our clothes first.

The KAAZ diff is not, unfortunately, the complete handling solution we had hoped it might be; it's but one piece of a larger puzzle. The next piece, we're guessing, is tires. As we learned in our six-hour Z thrash back in the October '02 issue, the Z's wheel wells will swallow a lot of tire. The fronts will easily accommodate 255s, and the rear can swallow 275s. With a carefully chosen offset, 265s and 285s are even possible. But while massive rear tires are tempting for visual effect, steamrollers will only exacerbate the understeer problem. We wanted at least 255s in the front, and were tempted to try 255s in the rear as well, but we wanted neutrality, not a tail-happy monster. For our first stab at tires, we chose a slight stagger, with 255/35-18s in front and 265/35-18s in the rear.

Tire choice took some soul-searching. Another set of gummy R-compound tires was tempting, but it already takes ludicrous speed to reach the Z's limits on the stock tires. Rather than pushing the limits to the stratosphere, we decided (at least this time) to pick tires for their behavior at the limit rather than ultimate grip.

The Toyo Proxes T1-S is far from the stickiest tire we could have chosen. They are, subjectively, probably on par with the stock tires for ultimate grip, but the T1-S has a soft edge. With its relatively soft sidewalls, the T1-S goes from grip to slip slowly and progressively, and large slip angles are possible while still maintaining control. Rather than moving the limit up, our goal with these tires is to make the limit more approachable.

Finding wheels to hold these tires turned out to be the most difficult task in the whole project. We're weight snobs, and couldn't stand to put any wheel on that didn't save us some pounds. We're also pretty picky about width and offset. We wanted 18x8-inch fronts and 18x9-inch rears, with something like a 32-mm offset up front, and maybe a little less offset in the rear. We found the selection surprisingly thin. There are several wheels coming out soon, however, that should match our goals perfectly, so don't be surprised if we change wheels every time you see the car.

The stock flywheel is a complex dual-mass piece with separate front and back halves separated by springs. Supposedly this can't be resurfaced, so if you're already replacing a clutch (shame on you if you burned it up this fast) a light flywheel may actually save you money over replacing the stock dual-mass.

This time we ended our search where we started. At Nissan. By the time you read this, Nissan dealers should have these Nismo wheels, followed by a rollout of a full line of suspension, exhaust, brake, and other go-fast goodies. The Nismo wheels measure 18x8.5 with a 25-mm offset in front, and 18x9.5 with a 30-mm offset in the rear. Nissan suggests a 275 tire to go with the 9.5-inch rear wheel.

The Nismo wheels are forged by Rays Engineering, a name well known for making things light and strong. The fronts weigh 18 pounds and the rears only 18.8.

The subtle design features, like the deep pitch of the rear wheel's spokes compared to the flat fronts, and the machined lip that adds a deep, rear-drive machismo to the wheel, makes the simple, five-spoke design look perfect. Five spokes look so right, it kinda makes you wonder why any other kind of wheel exists.

The JWT flywheel has to match the thickness of the stock flywheel, so the backside is hollowed out to save weight.

The softer sidewall of the T1-S gave us a better ride, even with a shorter sidewall than the stock tire. The downside to the T1-S comes in a loss of the Z's sharp turn-in, and with it a sense that some grip was lost. Skidpad testing says otherwise, where grip is actually the same 0.88g as before, with the same frustrating understeer. On the slalom, where turn-in and steering feel matter more, speeds actually dropped from 70.2 mph to 68.8. The tighter limited slip didn't help here.

Our final tweak for this month isn't directly related to handling balance, but rather to responsiveness and a bit of out-of-the-hole speed. In a quest for smooth, quiet drivetrains that are strong enough to take relentless abuse, Nissan has adopted dual-mass flywheels on all (both) its new engines.

The JWT flywheel, like most aluminum flywheels, features a replaceable steel friction surface.

Big, strong gearboxes like the one in the 350Z tend to rattle and groan like a UPS truck at low rpm. The noise comes from torsional vibrations originating from the surge of individual power strokes. A dual-mass flywheel separates the crankshaft and clutch sides of the flywheel by a set of springs. Using the power of math, Nissan engineers chose appropriate spring rates and flywheel mass to damp out these vibrations, making the gearbox smooth and quiet and eliminating complaints from those who would whine about funny noises.

Unfortunately, that math also shows Nissan engineers the Z flywheel has to be 25.5 pounds to damp things properly. That doesn't seem overly heavy when a quick zing of the throttle unleashes 274 lb-ft of torque, but when we replaced that 25 pounds with 13 pounds of Jim Wolf aluminum flywheel, we were amazed what we had been missing.

The crank position sensor reads the small cutouts on this metal ring on the back of the flywheel. To duplicate the ring with suitable precision, Jim Wolf Technology had to resort to a CNC waterjet cutting machine. Even CNC laser cutting caused spark scatter by a few tenths of a degree.

Free-revving in neutral is just silly now. The lighter flywheel frees horsepower once used for accelerating its 25 pounds. In high gears, the difference is negligible, but in first and second gear, where each mile per hour means many more revolutions of that 25-pound flywheel, the difference is surprising. Hard-driving fast shifts are now easier and smoother, but cruising around, revs actually drop so quickly between shifts that lazy gearwork makes you drive like a clutch monkey.

The gearbox does indeed make noises now. At idle and accelerating hard below 2000 rpm, you would think something was wrong if you didn't know better, but since you put the flywheel in, you understand the noises. Above 2000 rpm, the gearbox is silent. The noise shouldn't translate into accelerated wear, by the way, it just sounds like it does.

Despite the reduced inertia, the Z is still easy to launch. You can still lug it at 500 rpm away from a stop. You can still accidentally start in third gear. Other than the noise, there is no downside, whatsoever.

The fatter tires, lighter flywheel, and more determined limited slip added up to exactly the same quarter-mile time (14.0 seconds), but a higher trap speed (101.4 mph vs. 99.2). This suggests the lighter flywheel did free up some power, but the oversized Toyos offered less grip off the line than the stock tires. Indeed, the 0-to-60 time increased from 5.8 to 5.9 seconds, suggesting something was lost at the launch.

Our goal of a more approachable limit doesn't show in the numbers, but it does on the street; the car feels more neutral in high-speed corners. Still we're not sure the trade-off in steering feel from the soft-sidewall T1-S was worth it.

We have succeded in making our Z more responsive, more consistent, and a bit easier to drive at the limit, but we're still looking for a more involved rear, and, of course, some more power. It's not that we need more power, really, it's just that... well, if you read this far we don't have to explain the quest for power to you. Next time.

http://www.sportcompactcarweb.com/projectcars/0308scc_project_nissan_350z/