Clutch
We lost ourselves in Project EVO's newfound performance, enjoying its ample 323 lag-free whp quite frequently. Perhaps we were enjoying it a bit too much because our clutch started to slip. Frankly, we were surprised that the stock clutch held up so well against the abuse. We decided live with its pleasant-to-drive manners to the bitter end.
Since we have higher power goals on the way for Project EVO, we needed to get a clutch that would contain big power yet be friendly to drive as practical transportation. Since launching AWD vehicles can punish clutches in a multitude of evil ways, we needed to get something significantly tough. Usually race type clutches with a high torque capacity are unstreetable, on/off grabby light switches. Although able to take torque and heat, race type clutches typically wear quickly when slipped and driven under street conditions.
Race type clutches also tend to hit hard and thus put a lot of stress on the transmission, axles, transfer cases and the rest of the drivetrain. After doing a bit of research we turned to Nukabe Automotive, Cusco's official North American distributor to let us test Cusco's Super Twin, twin-disc clutch. The Cusco Super Twin has the reputation dichotomy for being able to take power and AWD punishment while still being streetable. A twin-disc clutch is just like the name sounds, it has two discs; the advantage is literally you have the torque handling of nearly two clutches in a small, compact package.
Usually twin-disc clutches use two smaller discs. The advantage is that when there are two clutch discs you can get away with smaller diameter discs and pressure plate, which reduces inertia considerably. This gives you quicker revs, faster shifts, easier heel-and-toe downshifting and better acceleration.
A disadvantage to a twin-disc is that since a twin-disc clutch has four wear surfaces, a twin-disc ,if it's slipped a lot as in ordinary driving, experiences accelerated wear and a rapid drop-off in clamp load from the pressure plate with subsequent slipping. This sensitivity to wear can make rebuilding a twin disc in a daily driver an annoyingly frequent occurrence.
We had our doubts as to the Super Twin's streetable and long-term durability as we have used plenty of race-type multi-disc clutches on other cars in the past. Race type dual discs are the king of grabby. They are nearly impossible to modulate with no engagement travel. They are light switches. Dual disc clutches are usually low inertia as well which is great for quick throttle response while racing but when combined with the grabbiness and short engagement, make for a clutch that is only suited for racing and nearly impossible to drive in stop-and-go traffic.
To make matters worse, when the clutch is disengaged, the floaters and plates in the clutch rattle and clank around making a noisy "chingging" sound. It sounds like something is seriously wrong with your tranny and even if you know what the noise is, it's super annoying. Twin-disc clutches usually have solid hub discs as well. Although these are good for low inertia and crisp shifting, they make the engagement even harsher and transmit all sorts of noises through the drivetrain into the car.
With all of this negative multi disc clutch experience, we were skeptical when the guys at Cusco recommended the Super Twin and said no problem when we expressed our past experiences and concerns. The Cusco clutch has many unique and ingenious features that tame the beast giving us the advantages of a twin-disc clutch with few of the disadvantages.
To address the issue of rapid clamp load drop-off with clutch disc wear, Cusco uses a special diaphragm spring in the pressure plate. Using a special coating, material and geometry, the Cusco spring can maintain a consistent clamp load even when the discs wear quite a bit. Cusco benchmarked their twin-disc against others, and found that their springs maintained the clamp load over nearly twice the wear distance.
To make the clutch have a longer engagement travel, allowing it to be smoothly driven off the line, the fulcrum of the pressure plate's diaphragm spring, located on top of the pressure ring, is a flat spring with give in it. This additional spring action gives more pedal travel to the stroke of the pressure ring as the clutch is being engaged. The fulcrum also works at a reduced leverage ratio, further increasing pedal engagement travel and reducing pedal effort needed to disengage the clutch. The result is a lot of room in the pedal travel, right as the clutch starts to grab giving an initial feel close to that of a factory clutch combined with a light, non leg busting pedal effort.
To cushion the drivetrain, make for a smoother engagement and to reduce drivetrain noise both of the clutch discs use a sprung hub. This is an unusual concession to streetabilty in a twin-disc. The discs' friction material is full metal, usually the best for burst resistance, torque capacity and heat resistance but the worst for niceness on the street. Cusco has perfected the material's characteristics so that the material has the good attributes of metal but still allows a fairly smooth engagement. This is a major feat as anyone who has daily driven a metal disc clutch on the street can attest too.
The floater plate between the two discs has some unique features as well. First, it is fairly thick and has a ventilated center to help pump cooling air into the heart of the clutch. Flat leaf springs help keep the floater actively centered and under tension when the clutch disengages, reducing the annoying chingging noise that all twin-discs are affected with. Twin-disc clutches tend to wear the pressure plate side disc faster as well as its gap tends to close first as the clutch is engaged. The centering springs keep the gap, and thus wear characteristics, consistent between the discs.
The pressure plate cover is machined out of billet aluminum giving it both lightness and stiffness compared to the stamped steel that most clutches use. The flywheel of the clutch assembly is forged steel with a lightweight thin cross section. The support pillars for the clutch are also steel which is an advantage. Many twin-discs have support columns made of aluminum for lightness. The trouble is that on a streetcar, the frequent use causes the drive tangs of the clutch floater plates to wear grooves into the pillars. The floaters get stuck in the grooves causing sticking and inconsistent clutch action. In this case the harder steel is the superior material.
All of these technical features make for an amazing clutch. It takes the punishment of AWD drag launches and will hold all of the power that the engine mods we will make in the future will produce. Although it is not perfectly silky smooth like a stock clutch, the Cusco twin-disc is still the smoothest engaging, least chattery and quietest twin-disc we have ever driven. Short of an ultra expensive exotic carbon/carbon clutch, we have never experienced a twin-disc so smooth and it's the smoothest easiest to drive all-metal disc clutch we have ever used, period.
| GREEN | 7-14PSI |
| YELLOW | 15-19PSI |
| BLUE | 20-23PSI |
| RED | 24-28PSI |
Compressor Bypass Valve
We had noticed that when we tried to get boost above 20 psi, the boost pressure would fluctuate and drop quickly. After doing a little research we learned the stock compressor bypass valve starts to blow open at around the 20psi point, dumping boost back into the compressor inlet.
Like all mass airflow (MAF) meter engines, the EVO's 4G63 works better with a re-circulating compressor bypass valve instead of an open-to-the-atmosphere blow-off valve. With a MAF-equipped engine, dumping air that has passed through the airflow meter will cause the car to momentarily run super rich resulting in stumbles and stalls. For better performance the bypassed air should be returned to the inlet track before the compressor but after the MAF.
Matrix International came to our rescue with one of its EVO drop-in Forge compressor bypass valves. Designed as a direct replacement for the factory bypass valve and manufactured from billet aluminum, the Forge EVO15 re-circulating valve is a well-made, high quality piece. In addition, the Forge valve is completely tunable to the desired boost level by swapping in one of the four supplied springs.
In addition to the springs the valve can be fine tuned by adjusting the preload with shims. Normally one shim would be used to add the pre-load needed to bridge the range between two of the colors (e.g. Yellow to Blue) however more than one shim would add enough pre-load to bring a particular spring to the next pressure rating (e.g. Green w/ two shims = Yellow). We ended up using the blue spring and one adjusting shim and noticed crisper throttle response, better spool recovery and a more stable boost when we ran above 20 psi.
Race Pipe
In anticipation of another tuning session running real leaded race gas, we installed Vishnu's race-only pipe to bypass the cat for tuning and when running on the track. The test pipe is a well-made 3-inch diameter stainless piece. Between the test pipe and the Forge bypass valve we noticed a surprising gain in throttle response and seat of the pants power. We are waiting to dyno these parts until the next installment when we do some tuning.
Suspension
Although in our first round of suspension tweaks to Project EVO, we had obtained a very good compromise between ride quality and handling, we wanted to step it up a notch for track events. Although our original Hyperdamper II setup worked exceedingly well on the street, it was a bit on the soft side for more hard-core track use and time trialing.
The Hyperdamper II has a 30 percent stiffer spring rate over stock. The front springs are rated at 5 kg/mm-280 in/lb up from the stock 3.8 kg/mm-215 in/lb. The rear springs are 6 kg/mm-335 in/lb up from the stock 4.8 kg/mm-270 in/lb. As you can see the stock EVO and even the Hyperdamper II had fairly mild spring rates that are more suited for brisk street driving than turning and burning for trophies on the track.
We replaced our Hyperdamper IIs with HKS' Japan market only, racing calibrated Hyperdamper Pro. The Hyperdamper Pro is found on many of Japan's top time attack cars. It was conceived and calibrated for track use with R-compound tires. HKS deemed it to stiff for general street use for North America but we had heard of this suspension's awesome reputation and begged HKS USA for a set.
The Hyperdamper Pro is a mono tube design. Which means the damper has a single tube and a large valve-containing piston in hydraulic oil under several hundred pounds of nitrogen pressure behind a floating seal. The mono tube design helps disperse heat better; the large piston displaces more fluid per inch of shaft motion allowing more sensitive and accurate damping. The large piston has more bearing area and can offer longer life. The high gas pressure increase the vapor pressure of the fluid, reducing cavitation in the shocks damping valves ensuring smooth fade free operation under all conditions. The Hyperdampers Pros feature 30-way external damping adjustment with the adjusters controlling primarily rebound with some affect on compression simultaneously.
The Hyperdamper Pro unit also has an inverted shaft design meaning the shock body is mounted upside down so the large body of the shock serves as the shaft while the smaller shock rod attaches to the housing internally. Inverted shocks are much stronger and rigid; the EVO with its WRC rally heritage comes stock with inverted shocks. The front struts have an adjustable pillow ball mount so camber can quickly and easily be adjusted, a must for track cars. The rear shocks also have a pillow ball upper mount. Pillow balls eliminated camber-reducing flex in the suspension as well as ensure every millimeter of suspension travel will be controlled by the damper instead of flexing rubber.
One of the coolest things about the Hyperdamper Pro is that the ride height can be adjusted independently of spring perch height. The lower suspension mounts can be threaded up and down so the car can be lowered without affecting spring preload or wheel travel. A low car can have full wheel stroke, a critical thing to ensure consistent balance under all sorts of cornering conditions. We raised the rear ride height an inch higher than HKS had set it and lowered the front of the car to make the wheel well gaps consistent knowing we had not affected our travel. The adjustable mounts also make corner balancing easier because you can affect the corner weight without changing the spring preload. This is a very great and useful feature.
The Hyperdamper Pro has an aggressive 12 kg/mm-670 in/lb spring rate in the front and a 10 kg/mm-560 in/lb rate in the rear. This is a whopping 315 percent stiffer than stock in the front and 210 percent stiffer in the rear. Although these spring rates seem spectacularly high, they are actually a little on the soft side for a racecar. The front springs are a straight rate and the rear springs have a straight rate main spring and a lower rate tender spring, which we suspect is mainly there to keep the short main spring from rattling under suspension extension.
The suspension makes our EVO handle like a racecar. Steering response is immediate, body lean is undetectable while cornering is incredible. The spring and damping rates are well matched. There is no hopping or bobbing and the suspension keeps the wheels planted at speed, even on rough or undulating pavement. We haven't needed to touch the damping adjustments yet.
The suspension also nearly rides like a racecar. Although the suspension is barely tolerable on the smooth, well-maintained roads of the OC where we live, you would have to be very hardcore to take this sort of ride on a daily basis and it might be unpractical in other parts of the country where cold winters really tear the road up.
Bump Steer Reduction KitAlthough the EVO has a well-designed suspension, when the car is lowered, some of the geometry in the rear gets slightly out of kilter. This has to do with the location of the toe links in the back. The toe links swing though a different arc than the rest of the suspension causing the rear wheels to steer themselves as the suspension strokes. This phenomenon is called bump steer. Bump steer can make the car feel twitchy in the back at the limit and under trailing throttle.
To reduce the rear bump steer we installed Whitelines bump steer correction kit. Whiteline is an Australian company that has an impressive array of suspension components for many Japanese cars such as Honda, Toyota, the WRX, Nissan S13/314 and Sentra SE-R. Whiteline also has many geometry correcting kits for cars that need it, an unusual detail for a suspension component manufacturer.
Whiteline's bumpsteer elimination kit for the EVO relocates the position of the inner pivot of the tie rod upward. The kit consists of a urethane bushing with an internal eccentric. This puts it at a more favorable angle so it does not allow the toe to change much as the suspension strokes. This simple and inexpensive part makes Project EVO much more predictable, especially in bumpy turns.
Once our revised suspension parts were installed we took Project EVO to West End Alignment to have Darren Nishimura work his magic. Darren set our front camber at 2.5 degrees negative and the front toe slightly out. The rear camber was set at 1.5 degrees negative with zero rear toe. Darren also set the corner weights for zero cross weight with 200 lbs in the drivers seat.
Different Wheels and Tires
We had bought some wheels and tires for another project but the trouble was they did not fit! On a whim we tried them on project EVO and they fit perfectly. Not wanting to waste a perfectly good set of wheels and tires we left them on the car for the time being.
The wheels are bronze anodized 18x8.5 Rays Gram Lite Fs with a 30mm offset. These are ultra light one-piece forged wheels tipping the scales at just less than 18 lbs. We like their simple thin- spoke styling. The tires are Michelin Pilot Sport 2, sized 245/35-18. The Pilot Sports are considered to be one of the premium ultra high-performance tires around. Their dry grip is on par with our BFG KDs and they are better in the wet weather we have been dealing with in Southern California lately. Perhaps the KDs had a tiny bit more grip at the limit on dry ground but the Michelins had a tad better transient response and definitely more wet grip.
Driving Project EVO is interesting. The EVO is now clearly track biased in its manor. The ride can be punishing but it gets better the harder the car is pushed. Fast rough turns are now gobbled up with loads of wheel travel; even at the cars low ride height. Before the car would wallow and hop, now it stays planted and the power can be put down hard.
Even though the Hyperdamper Pro suspension has a more frontward bias in the spring rates than the Hyperdamper II or even the stock suspension, the car has less understeer than before. Essentially the car is still neutral but the tendency is for the car to now four-wheel drift, leading with the rear wheels instead of the front. This is better for getting around a track quickly.
The old suspension would touch the front snubbers when pushed real hard in a turn. This would make max effort cornering end in grinding understeer, a safe mode for beginning drivers but not the fast way around. With the Hyperdampers ample travel and stiff spring rates, this does not happen and the front wheels seem to find traction better.
With the higher rates, we find that detectable body roll has disappeared. With the elimination of bumpsteer, the rear of the car is very easy to catch and hold in long beautiful slides. We also like the fact that the dampers and the sway bars are baseline good when set right in the middle settings. This is a good help for tuning the car for specific tracks and conditions. We have a lot of adjustment leeway to go either way. Now it's time to up the power ante again.