What’s the Diff?

Update by Scott Lear to the Mini Cooper S Club Racer project car
Jun 8, 2010

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In a higher-horsepower front-wheel-drive car that’s constantly shifting its weight around a race track, an open differential is unforgivably bad.

One-wheel drive is a bummer, and a surprising number of cars are, essentially, one-wheel drive. As long as you’re not breaking traction it’s not a problem. However, in a higher-horsepower front-wheel-drive car that’s constantly shifting its weight around a race track, an open differential is unforgivably bad. Our 2005 MINI Cooper S, though armed with a suite of John Cooper Works performance upgrades and MINI Challenge racing goodies in as-delivered form, was hampered by its mechanically open differential.

As a result, powering the car out of tight corners was an exercise in throttle restraint. It was far too easy to get the supercharged MINI to light up the inside tire, and the car was slower than it should have been coming out of corners. MINI did offer a limited-slip differential as an option on Cooper S cars, but this stock unit, while better than nothing, leaves something on the table in a full-race situation on very sticky tires.

OS Giken had our solution. Their Super Lock line of clutch-type limited-slip differentials uses many small clutch plates to ensure smooth power distribution to both drive wheels in a slip condition—up to 100 percent lock. And because it’s a clutch design, the differential works even if one of the wheels is off the ground, unlike a standard Torsen design that becomes an open differential if one tire loses contact with the pavement. The Japanese manufacturer has expanded their lineup from brands like Toyota, Nissan and Honda to other popular track brands, including Porsche, the Chevy Corvette, BMW and, by extension, MINI.

Their Super Lock LSD for the MINI Cooper is not an inexpensive upgrade; prices from retailers are typically around $1700. However, the build quality is truly first rate. It’s almost a shame to have to seal this thing away from the world inside the transaxle case. The reviews we’ve heard from racers using OS Giken Super Lock diffs have all been glowing, and a similar OS Giken diff won our differential comparo in a Honda S2000 in our February 2009 issue. Better still, our friend Ian Stewart won the NASA Performance Touring C championships in 2009 in a similar MINI Cooper S equipped with this exact differential.

Since we were going to have to take the transmission apart anyway, we decided to go whole hog and upgrade the clutch at flywheel using OS Giken components. Again, this is a higher-end kit and the price reflects this: another $1700 or so for the combined single-plate clutch, lighter flywheel and pressure plate assembly. Since our MINI Cooper S still has a reasonable amount of inherent value as a newer-model car, we don’t feel these upgrades are out of line from a budget standpoint.

The tight packaging in the nose of the MINI means that transmission work can require a pretty comprehensive removal of components. In fact, the easiest way to do the job is to remove the entire nose of the car and pull the engine out completely. NASA TTC champion Ian Stewart’s MINI and Volvo shop, Maitland Importers, is just down the road from us in Orlando, Florida. Considering the shop’s MINI racing and maintenance expertise, we loaded our MINI on the trailer and headed west for Orlando. With time constraints this was a wise decision, as master technician Philip Cavagnaro had our MINI’s block on the ground in about an hour.

Since we had the whole shebang apart anyway, we took the opportunity to install, refresh and upgrade our entire suite of bushings and mounts with Powerflex components. From engine mount inserts to suspension bushings, this is the kind of job you’d regret not doing while everything was apart. It’s so much easier with the engine on the ground and a car on the lift. The Powerflex website has a great graphical breakdown of all the possible bushings for any given application, along with the prices. Expect to spend between $25 and $50 each for various sets and pairs depending on your needs.

In our winter downtime, we also installed Ugli Gripz on our Sparco steering wheel. Ugli Gripz is a do-it-yourself kit that allows you to put custom moldable grips on any steering wheel. First you apply silicone tape over the wheel to protect the stock material. Then you apply the grip material and mold it to the shape of your hands. Finally you apply a surface coating, which needs about two days to dry completely. The result is a rubbery, grippy feel that’s about as firm as high-density foam. Naturally, the overall diameter of the wheel grip is increased with the addition of material, so this is a great option for somebody looking to thicken up a wheel.

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nderwater PowerDork
6/9/10 11:15 a.m.

"...the easiest way to do the job is to remove the entire nose of the car and pull the engine out completely."

Yipes! It's statements like that which scare me away from buying a MINI.

kcmoken New Reader
6/9/10 3:59 p.m.

That scared me too. But the next statement (had the MINI's block on the ground in about an hour) would indicate maybe its not so bad. I'm still looking for the right one, and I guess if it had a OS Giken that would add considerable amount of points to what I would consider the "right" one.

Ian F
Ian F MegaDork
6/10/10 8:43 a.m.

I've pulled the transmission from a MINI to do a clutch job (and again to replace a transmission). Removing the engine is NOT required... nor would I say it would make the job faster or easier. Done per Bentley, you don't even have to drain the coolant.

Scott Lear
Scott Lear
6/14/10 3:29 p.m.

I have heard from several people that transmission access can be gained without removing the engine, but the removal certainly worked in our favor considering the Powerflex bushing/mount installation. The engine removal was astonishingly easy for our experienced technician, and gave us great access for photos, too. . If I'd been doing it in my own garage on jackstands, I'm sure I wouldn't have been as eager to take everything apart.

skullsroad New Reader
6/16/10 8:32 p.m.

I was planning on buying the Torsen for my SVTF but I don't like that it becomes an open diff once a wheel goes airborne. I would assume that cause a weird "catching" sensation once the tire hits the ground again. But darn it, that OS Giken piece is way over budget for me.

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