In our last installment of Project 300ZX Twin Turbo (September '99), we began detailing our plans for a new engine built to handle 600 to 700 hp. Unfortunately, progress on that engine is running slower than expected (which, ironically enough, is what we have come to expect). While we haven't had time to begin assembly of our monster engine, we have attacked a few projects that take less time. There are still plenty of parts to be installed and work to be done to the car in preparation of receiving the big-gun engine. In this installment, we do more suspension and chassis work to get the car prepared for the day our engine is ready to drop in.
Shocks
Even though our stock shocks (modified by Ground Control Inc.) have been providing excellent service, we decided that 600 hp and high triple-digit speeds would require more sophisticated dampers. Since we are anticipating racecar levels of power and speed, we started looking beyond the level of your typical aftermarket performance shock and put our efforts into finding a suitable racecar shock. Since we had good luck with the tuning flexibility provided by the double-adjustable shocks that we are using for Project 200SX SE-R, we felt this same adjustability was desirable for Project Z. It is perhaps even more important here, due to the car's power, raw speed and level of sophistication.
After perusing offerings from high-end racing shock companies like Leda, Penske, JFZ and Koni, we returned to the folks at Ground Control for help. Ground Control has recently introduced their awesome, state-of-the-art Advance Design racing shock absorber. We selected Ground Control for several reasons: The other brands of shocks that we checked out were pure racing, universal types originally intended for real, purpose-built racecars. This made adapting them to the Z's production-based street suspension a difficult and expensive task, which would require lots of machining, welding or both. The Ground Control shocks have a modular design that makes them easily adaptable to both racecars and modified production sedans. In fact, Ground Control already offers shocks for most popular offerings such as Honda, Acura and the Toyota Supra. Different types of end mounts and shock shafts can easily be screwed onto either end of the shock body to adapt the shock to most types of cars. If Ground Control does not currently make an application for your car, they probably can build one on a custom basis.
The shock body itself is designed to be easily lathe cut to the correct length for any application. Ground Control makes spherical bearings, polyurethane bushings and clevis ends for the shocks to make them easily adaptable to just about any sort of car.
In the relatively short time that they have been in production, Ground Control shocks have amassed quite a few victories. Four SCCA road racing and solo championships have been won on these shocks that have been in production for less than a year. This is an impressive accomplishment for this new design and a tribute to the design's basic soundness.
The other reason we selected the Ground Control shocks is that they feature digressive valving. Digressive valving is a revolutionary way of valving shocks that is entirely different than in previous shock designs. A digressive shock has stiff, progressive damping at low shaft speeds--typically shaft speeds of below 3 inches per second. This helps control body motion such as roll, squat, and dive just like a typical stiff racing shock. At shaft speeds higher than this--in the 3- to 10-inch per second range--the damping force stays more or less constant despite the rapid change in shaft velocity. This helps keep the suspension supple in response to bumps and irregularities in the pavement. This suppleness helps the car maintain traction by reducing hop in bumpy turns and provides a good ride, even at the high damping rates needed for body motion control.
Digressive valving is the opposite of how shocks have been traditionally valved. Usually shocks have more damping force at higher shaft velocities. This curve can be shaped and regulated to a degree by adding different valving stages. (Such shock valving consists of hydraulic fluid bypasses with check valves of differing rates and flow volumes vent hydraulic fluid inside the shock at different shaft velocities for better bump response.) Even with these tricks, a conventional shock's damping curve is much different than a digressive shock's curve. Conventional shocks tend to have more damping at higher shaft velocity despite multi-stage valving. Digressive shocks were first used with success in rally and off-road racing and have only recently made their way into the on-road racing area.
The Ground Control shocks have a general valving layout and a single-tube construction much like a conventional DeCarbon type shock such as Bilstein. A single-tube shock has the advantage of better cooling and the ability to run a larger piston. A larger piston displaces more oil, and greater oil flow through the valves gives better damping control. The piston is attached to the shock shaft and uses a flexible, Swedish-steel stack of thin, precision ground, shaped washers to control fluid flow. The washers cover the bleed orifices drilled into the piston for compression and rebound fluid flow. There is a washer stack on both sides of the piston to control fluid flow going either way, in compression or rebound. The washer stacks act like one-way check valves for the fluid.
As the incompressible fluid passing through the orifices in the piston hits the washers, they flex and bend out of the way to allow the fluid to pass. The stack of the washers is kind of like a leaf spring, with the profile of the stack and the thickness and diameter of the washers controlling how the washer stack flexes. This controlled flex regulates how fluid passes through the orifices in the piston. That is one of the main ways that Ground Control can precisely control the shape of the damping curve. Another ingenious trick that Ground Control uses to control the shape of the damping curve is to use shock pistons with varying concaveness in the surface that the washer stack seats against. This allows the washer stack to be set with differing preloads. The shape of the damping curve can be precisely tailored with great flexabilty by changing the diameter, thickness and contour of the washer valve stack, the size of the orifices drilled into the piston, and the concavity of the piston sealing surfaces.
The Ground Control shock can be adjusted for both compression and rebound damping by a slotted shaft running up the shock's main shaft which controls the size of the shock's major bleed orifice. In a very easy-to-reach, convenient way, the shock can be adjusted at the very top of the shaft by turning the two stacked knobs at the very top end of the shaft, a red knob is used for adjusting rebound and a blue knob controls compression. This beats the typical, hard to reach adjusters used by most racing shocks.
To help prevent bubble-forming cavitation, to increase the hydraulic fluid's boiling point and to give room for the shock shaft's displacement under compression, a floating piston separates the oil from a chamber full of high-pressure nitrogen, filled to about 200 psi. Separating the oil from the gas allows the valves to flow only in fluid, which improves damping action. In some types of conventional shocks, the oil is mixed with enough air to allow for the shock shaft's displacement. While the air is supposed to be in a separate chamber, it can sometimes make its way to into the piston's path and as the bubbles pass through the valves, they cause dead, undamped spots within the damping curve. In non-pressurized shocks, it is easy for the fluid to have localized boiling inside the shock under rapid shaft motion due to cavitation.
The nitrogen pressure can be easily adjusted through a Schrader valve at the bottom of the shock. Sometimes this can be a useful tool to increase the suspension's preload without changing the spring rate and/or ride height. The gas pressure may need to be increased to prevent foaming and boiling under severe conditions.
The shocks feature high-overlap Teflon bushings to reduce static seal friction, or "sticktion" as engineers call it, and to also assure a long service life. Low-tension Teflon oil seals with built-in scrapers also help ensure long service intervals. The shock body is machined from lightweight 6061 aircraft aluminum which is then hard anodized inside and out for corrosion and wear resistance. To prevent a harsh, chassis-unsettling jolt when bottoming out; the shocks have a trick microcellular urethane bumpstop. This soft material is shaped to provide a controlled progressive stop to the end of the wheel's travel. MCU bumpstops have made a huge difference on some of our other project cars and we think it's great that Ground Control has provided them as standard equipment here. We noted that the Ground Control shocks weigh half as much as the stock shocks. This can go quite a way in reducing unsprung weight. The shock body is threaded externally to accept Eibach ERS racing 2.5-inch coil-over main and tender springs. The spring hats and adjusting collar hardware are machined from 7075 aluminum and hard anodized for wear and corrosion resistance. The Ground Control Advance Design shocks are designed to be simple to rebuild and maintain. With a few tools, one can even revalve, rebuild and repair the shocks at home. If not, one can simply send them back to Ground Control for a quick rebuild.
Springs
Since the Z is a relatively heavy car, we used stiff 500 lb/in front springs with 300 lb/in rear springs. Even though these springs are over twice the stiffness of the stock springs; as we have found in the past, stiff spring rates are essential for keeping lowered cars off the bump stops for consistent handling. With a properly valved and adjusted shock, a car can still ride quite well even with stiff springs. We used Eibach progressive ERS tender springs for both the front and rear.
The front tenders are 200/250/300 in/lb progressive while the rear springs are 150/200/200 progressive. These springs feature flat, trapezoidal-wound wire for minimal coil-bind height. The tender springs help in three different ways. They act like a progressive pre-spring to help the first half-inch of travel with a lower, smooth-riding rate. This helps the wheels find traction in rough turns as well as offering an improved ride. They also help preload the springs so they will not become unseated or make rattling noises under droop. Finally they help the inside wheels drop and find traction under hard cornering. This helps prevent the inside drive tire from spinning as much on the exit of the corner.
Shock Mounts
To bolt the shocks in place with a minimum amount of soft rubber (rubber can cause a small amount of undamped travel), we obtained some Tien Pillow ball upper shock mounts from Speedlab.com. Speedlab specializes in importing exotic Japanese parts on a special order basis. All one has to do is look through the extensive list of parts featured in their on-line catalog and place an order. Usually your parts show up within two weeks. We were pleasantly surprised by the quickness and reasonable price of Speedlab's service. The Tien upper shock mounts feature an all-aluminum construction, mounting the shock shaft by means of a single spherical bearing instead of a big, squishy biscuit of soft rubber. This makes every bit of suspension motion go through the damper instead of having undamped travel due to the flexing the upper mount. On some cars this flex can be as much as one inch up and down--not good considering that this is uncontrolled wheel motion.
Unfortunately we discovered that the Tien shock mounts were designed to fit the stock Nissan shocks and would not work with the thicker shaft of the Advance Design shocks. Ground Control modified the Tien shock mounts to fit the Advance Design shocks by boring out the bearing bushings to accommodate the bigger shafts. For the front shocks, Ground Control opted to use a high-durometer urethane bushing in the lower mount. In the rear, Ground Control used their universal clevis-type mount, which fit perfectly with no fuss.
We thought it would be very hard to improve the handling of our 300ZX much further. We were wrong. The Advance Design shocks drastically changed the way the car felt. Before, the Z felt like it had gobs of grip in the corners but felt big and heavy, like a defensive end. The springs and shocks changed that feel, seemingly shrinking the car's bulk, making it feel light and nimble like it somehow magically lost 1,000 lbs. Project Z feels much more coupled during transient maneuvers and lean under cornering was dramatically reduced. The car did not seem to lean much before, but removing most of the lean made it more apparent in its absence. The car is better planted in rough corners too, as the shocks' digressive valving allows the wheels to find traction.
Project Z's ride is surprisingly good too, considering that this is a nearly full race setup. The solid Tein shock mounts transfer a bit more noise and vibration to the body structure, but other than this small annoyance, they actually improve the ride because all suspension movement is now damped.
Overall, the ride is only slightly firmer than the "Touring" setting on the stock suspension and defiantly softer than the "Sport" setting. That is until the suspension is leaned on and the stiff part of the damping curve comes into play. We left the shock settings on the full soft position with the exception of the front shock compression damping which we turned up a couple of clicks. We had wanted our street settings to be near full soft so we could have the option of going much stiffer on the track. Ground Control can custom valve your shocks to your own personal requirements.
Adjustable rear upper arms
In our last installment of project Z's suspension, we had complained that although the rear camber of the car is adjustable, the range of adjustment was not enough to correct negative camber to the degree that we wanted once the car was lowered. At our chosen ride height; we were stuck with 2.75 degrees of negative camber, even when our adjusters were turned to the full positive direction. Although some negative camber is a good thing to maximize cornering power, this much in a car with Multi-Link(TM) suspension is too much for optimal use of the rear tires.
As if they heard of our plight, Stillen came to the rescue with their new adjustable upper control arms. These arms are constructed of gold-cadmium-plated tubular steel, which is much stiffer than the stock stampings. The squishy rubber bushing is replaced by a spherical bearing that is adjustable fore and aft to pick up an additional 3 degrees of camber adjustment--just what the doctor ordered to improve out-of-the-hole traction and cornering.
HICAS Eliminator
While we had the suspension apart, we also installed Stillan's HICAS Eliminator. During the heady days before Japan's bubble economy burst, all the major Japanese auto manufacturers were locked in a death battle to produce the ultimate sports car. No expense was spared in developing new technology in the race to out-do the other car companies. Nissan, in a fit of Japanese-gadget fascination, came out with HICAS, an automatic, computer-controlled, four-wheel steering system.
Although this system makes the car faster in a slalom and quicker in some types of turns, it imparts a weird non-linear feel to the car at times. Sometimes the HICAS input to the rear wheels makes the car feel like it's about to spin, causing the driver to counter steer when it really isn't necessary.
Former champion racer Steve Millen, chief test driver par excellence and head honcho of Stillen, hates the way HICAS feels. We fully agree with him giving the old HICAS system the axe. The Stillen HICAS Eliminator is beautifully CNC machined from a solid billet of aircraft aluminum. It replaces the big, heavy hydraulic rear wheel steering ram, giving the rear tie rods a solid place to be mounted. The kit gets rid of about 20 lbs of mass by getting rid of the heavy ram. The kit also comes with a bypass line so that the car's hydraulic system can remain intact. Although some people, like engineering editor Dave Coleman, like HICAS and the way it feels, we personally like the way we can now feel and predict what the back of the car will do with it gone. What Steve Millen prefers is a good baseline for us.
Since our suspension was apart, we took our car back to Johnson's Alignment for yet another precision four-wheel alignment and corner weighting session. We reduced the front caster a little more to 9 degrees positive while setting the front camber at 1.75 degrees negative. Toe was set at 1/16-inch toe-in. In the rear, we set the camber at 1.5 degrees negative and the toe at 1/16 inch. The reduction in camber really helped our traction when moving forward. Now, even with 400 hp at the wheels, Project Z does not suffer from excessive wheelspin on dry, clean pavement. Full throttle results in only about 15-20 feet of spin before forward velocity catches up with the tires' traction.
On the negative side, the more we refine project Z's suspension, the slower the car feels as it becomes easier and easier to control. What used to feel like a fire-breathing monster is now actually starting to feel rather tame.
Rear Strut Tower Brace
As nice extra touch, Stillen provided us with one of their new rear strut tower braces. Even though the Z is fairly stiff for a T-top equipped hatchback, it still lacking a little in stiffness when compared to newer cars like the C-5 Corvette. The rear strut tower area is one of the places that a brace can do the most good for a hatchback car. The Stillen brace is a CNC-machined billet aluminum piece, which is a trademark of Stillen parts as of late. The brace required some cutting of the interior panels, a tricky job made a little easier by the use of a template that came with the brace.
The brace actually made a noticeable difference. The car felt a little more connected and the ride was improved a bit. Stiffening the chassis is a good way to make the suspension work a little more as bump force is fed into the suspension instead of being used to flex the chassis. The brace was well worth the effort. Due to the brace's forward position, it had little impact on rear hatch storage area and looked very nice back there.
Foam-Filling the Chassis
In any high-performance car, it is impossible to make the chassis too stiff. The stiffer the chassis, the higher its natural frequency, making the energy imparted to it by bumps less likely to excite the body's structure. A stiffer chassis enables the use of stiffer springs and shocks without hurting the ride. This is because a stiff, non-flexing chassis transfers more force into the suspension where it can be dissipated by the springs and shocks instead of transferring the force to the occupants. A stiff chassis is also more responsive to roll rate tuning for balancing understeer and oversteer. This is one of the reasons why automotive engineers are continually investigating ways to stiffen chassis without adding weight.
In a final bit of reengineering to stiffen the body, we injected the chassis with catalyzed rigid structural polyurethane foam. Structural foam, in the 2 lb per cubic foot density that we used, can stiffen chassis members up to 40 percent.
Higher densities of foam can increase stiffness by up to 300 percent. Since we cannot retool custom parts to redo the Z's body, we figured that this would be an excellent, low-cost way of greatly increasing chassis stiffness. Injecting foam is not a new technique for chassis stiffening. The Infiniti Q45 uses this sort of foam in some of its chassis members to increase stiffness, as do a few other premium cars. In fact, the foam we chose is the foam recommended to repair damaged Q45s.
To get the correct foam for our project, we contacted Art Goldman, Foamseal's automotive product manager and author of an SAE (Society of Automotive Engineers) paper on the use of structural foam for the stiffening of automotive unibody structures. We used Foamseal's two-component foam kit, p/n 11-22 to fill the main members of the chassis. Like we mentioned earlier, Foamseal is the supplier that I-CAR, a national certification group for quality auto repair, recommends for the repair of damaged, foam-filled chassis. The Foamseal kit uses a two-part catalyzed polyurethane foam, which quickly cures into rigid, waterproof, closed-cell foam. To prep the car, we carefully masked off all painted areas anywhere where the foam could drip. As this sort of foam is a thermosetting catalyzed plastic, we realized it could be icky if it spilled on paint or any part of the car's interior. This foam is nasty stuff. It is impervious to all known solvents and cleaners.
Rubber gloves must be worn. Get some of it on your hands and it will stay there for more than 3 weeks--don't ask how we know. Do not get this stuff on your paint. Wear old clothes; we ruined ours while learning how to handle the product. We injected the foam into the rocker panels and frame rails of Project Z through existing bolt and drain holes. When injected, the foam reacts like shaving cream and quickly expands to fill the empty space. You can judge how much foam to add by watching its expansion progress through some of the holes. Once injected, the foam expands and begins to cure in about a minute so you need to work fast and plan how you inject the foam before you start.
The life of the foam kit is limited to a few hours once the seal is broken. We filled all of the Z's unibody frame members using five foam kits. When foaming a chassis, you must remember the wires and other lines that pass through the chassis must be relocated or they will be entombed forever.
We were amazed at how this simple procedure improved the performance of the car. The chassis now almost feels like it has a roll cage. A sloped driveway can be driven up sideways with nary a creak. Even though the Z already has a pretty tight chassis, it feels more solid. The ride has improved and road noise has been reduced noticeably. We bet that the car will be even more responsive to chassis tuning measures in the future. If you are a slalom racer, a road racer, have a lowered car or even just want a smoother ride; foaming is a worthy, easy-to-do modification. Foamseal has foams in densities as high as 10 lbs per square foot if you desire to make things even stiffer.
Do not--I repeat--do not attempt to use cheap, hardware-store canned foam. This is not the same thing, and if injected into your chassis, will form a gummy mass that won't dry. Foamseal foam is a professional grade foam, which although it is a little unforgiving to cleanup mistakes, has superior mechanical properties and catalytic curing so it will dry even in an enclosed space.
These changes are continually improving and refining Project Z as we eagerly await our motor. We still have some more parts that we will install and review in upcoming issues. Stay tuned, the best is yet to come!