MotoIQ Project EVO VIII

Project EVO VIII part 4, Getting More Trackside Grip, Killing Some Slip

By Mike Kojima

After enjoying driving our much-improved EVO VIII for a few months we were again motivated into another round of mods by the need to fix a necessity, a slipping clutch.

To see the other segments of Project EVO

We were enjoying Project EVO’s newfound power, enjoying its ample 323 lag free whp quite frequently.  Perhaps we were enjoying it a bit too much because soon our clutch started to slip.  Frankly we were surprised that the stock clutch was holding up so well to the abuse and we had decided live with it's pleasant to drive manors to the bitter end.

Since we have some on the track 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 like no other cars, 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.

Cusco EVO Twin Disc Clutch
Flossing anodized billet aluminum, Cusco’s Super Twin Clutch is a trick piece

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 decided to 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 4 wear surfaces, a twin disc if it's slid 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 Twins streetablity and long term durability as we have owned 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 lightswitches.  Dual disc clutches are usually low inertia as well which is great for quick throttle response while racing but when combined with the grabbyness and short engagement, make for a clutch that is only suitable for racing and nearly impossible to drive in stop and go traffic.

Cusco twin disc clutch disc
Unlike most full race twin discs, the Cusco part has a spring hub to cushion shocks and to keep drivetrain vibrations from entering the car.

To make matters worse, when the clutch is disengaged, the floaters and plates in the clutch rattle and clank around making a noisy chinging when the clutch pedal is pushed in.  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.  Typically a twin disc clutch allows a rattling howl to enter the driver's cabin at between 2000-3000 rpm.  This is horribly annoying.  On an EVO, worse things happen.  The ECU can read clutch rattle as detonation and add the noise to its knock count.  Generally a knock count greater than three causes the ECU to pull timing which reduces performance.

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Thursday, February 11, 2010 5:00 AM
I've always maintained (insisted) that a twin disc clutch can be the "best of both worlds" when it comes to torque holding and drive-ability. Everyone with experience with one has always told me otherwise; that they were ultra grabby.

I see now why that is the case. It's all in the design. The design you chose for this Evo is the design I've always had in mind when I insisted twin discs could out perform aggressive single discs in all areas. Apparently everyone else with aspirations of street driving has been buying and companies have been building the wrong kind of twin disc for that use.

I'm very anxious to see how it turns out for you guys. A follow-up would be nice to see later on down the road.
Friday, February 12, 2010 6:42 AM
That BOV is going to less than great for part throttle scenarios while tracking the car. It will make the car buck, shudder, flutter, surge and all other forms of crappiness BOV related. I've been there with a Forge BOV. You should check out the APS twin vent, it uses a large brass piston which provides better mass dampening for the system and eliminates all those issues.
Mike Kojima
Mike Kojimalink
Friday, February 12, 2010 8:44 AM
Once we adjusted it according to the article, it worked fine and we did not notice any of those issues. We did have to fiddle with it though.
Friday, February 12, 2010 2:20 PM
Hello Mike,

Why do so many JDM coilover manufacturers use a stiffer front spring for the EVO (and also the new STI with the multilink rear)?

I've heard that it's for safety, or because their philosophy is more front camber and more rear swaybar, but they often spec the same rates for the older STI....which has struts in the rear and thus it's a much more rear biased in comparison with the same rates.

I was happy to see HKS did a 5k/6k for your previous suspension. And I'm sure the much stiffer rates for this suspension are an overall improvement in this case since understeer would be reduced now that you're off the bumpstops up front and have much less roll. But surely there would be a bigger improvement by going with a stiffer rear spring, especially for a focused track coilover like this.

There are suspension tuners and time attack winners out there proving that a stiffer rear is the way to go for the EVO and it jives with real suspension tuning science. Even Mitsubishi gave the car a stiffer rear!

What gives?

Mike Kojima
Mike Kojimalink
Friday, February 12, 2010 7:05 PM
I don't agree with the stiffer front. JDM companies do it even for FWD cars. My personal EVO has much stifer rear.
Monday, February 15, 2010 3:22 PM
good to know...thanks!
Tuesday, May 18, 2010 1:23 PM
Any differences between the Cusco twin and the Exedy twin besides the blue vs. purple plate?

Also, where the heck did you find a Vishnu test pipe? Shiv dropped this platform a few years ago like a leper with herpes and bolted for the 335.
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