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Project 370Z - Improving Grip by Tuning a Cusco RS LSD!

By Mike Kojima

So far with our Project 370Z we have been focused on dialing in the chassis.  After some track time, it has become apparent that the rear differential is the next handicap in achieving faster lap times.  Although the Z has a viscous limited slip diff from the factory,  our testing has found it to be highly inadequate.  In fact it is hardly a limited slip diff at all, allowing a lot of inside rear wheel spin, even out of medium speed corners.  In some street tire testing at Buttonwillow our lap time was being limited to the low 2:06 range for configuration 13CW.

What we needed was a good mechanical limited slip, a Salisbury clutch type with good tuneabilty.  Most people don't understand that the ability to tune a limited slip is very critical for good lap times.  Most of the current clutch type differentials on the market are tuned for aggressive lock up to work well for drifting.  Although this is great for drifting, it is not the best thing for getting the best exit speed from a corner.  Most diffs on the market just want to squirt sideways on corner exit.

For greater flexibility in tuning our 370Z we chose a Cusco RS differential.  The RS has tremendous flexibility in its set up, meaning you can tune it for everything from mild street diff for a low power car on small tires like an FR-S to the demands of a 700 whp Formula D drift car and everything in between.  Check out the story and we will will walk you through some tricks in diff tuning and tell you more about the Cusco RS and why we picked it.

More about Project 370Z!

The Cusco RS is a Salisbury clutch type mechanical limited slip differential.  We will get more into the specifics of that in a bit.  The stock Nissan diff is a viscous coupling which gets its resistance to spinning just one tire from steel plates turning in a sealed drum of silicone goo about the consistency of snot.  Although Viscous LSD's are good for smoothness and quietness, they suck for putting the power down due to all of the slip they have by design (sort of like the torque converter in an automatic transmission).  The silicone snot also breaks down quickly and the diff loses what little effectiveness it had in a few track sessions.
The Cusco RS has a forged steel case.  The forging is much stronger than your typical cast case meaning that the case can be made thinner and lighter.  The case has generous windows to the interior allowing for good flow of lubricating oil to the clutch packs making the diff run smoother and cooler.  You can actually see the clutches here in through the circular windows.
You can get a good look at the case thinness here.  You can also see the cross shaft cams and pressure ring ramps through the side windows of the case.  We will be explaining this all in a bit.
We begin by removing the stock differential.  The weird looking stuff on the rear diff cover is an ARC heat sink.  These heat sinks supposedly give extra heat dissipation to whatever they are stuck on.  Seems kinda sketchy but ARC has well engineered parts that work well in general.
 Howard Watanabe of Technosquare removes the rear diff cover to disassemble the stock diff.

 

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Comments
180sx0
180sx0link
Sunday, March 24, 2013 9:38 PM
Great article.
Dan DeRosia
Dan DeRosialink
Monday, March 25, 2013 3:06 AM
Some part of me just filled in "If you haven't heard about WPC treatment yet..." on page 4 with "... then you obviously haven't been reading the site very long". Seriously though, had been looking at the Cusco diff and thinking about WPCing it, good to know I'm not the only one.
M-P
M-Plink
Monday, March 25, 2013 4:41 AM
Had a Cusco RS on my old twin turbo 350Z and used those exact same settings except with Motul PA90 diff oil. Worked very well for me. Next time I'll have to get the plates WPC treated as well.
Mike Kojima
Mike Kojimalink
Monday, March 25, 2013 6:53 AM
My own 300ZXTT has a Cusco RS and it chatters like crazy. I am going to do an article on tuning and WPC treating it to make it more daily driveable.
Jonathan
Jonathanlink
Monday, March 25, 2013 7:07 AM
What is the service interval on a cusco RS?

I am between a wpc cusco 1.5way and a quafe on my Mini cooper DD and i think their service interval is the going to be the deciding factor because removing the tranny a huge pain.
M-P
M-Plink
Monday, March 25, 2013 7:13 AM
Mike, mine was very chattery until I switched to the Motul PA90...that quieted it down a lot. Sometimes I did wish that I had set it to 60% lock instead of 80% lock for the street, but on the track it was beautiful.
sethulrich
sethulrichlink
Monday, March 25, 2013 7:20 AM
Never mind, in the actual assembly pic on page 5, it looks consistent. :-D
rawkus
rawkuslink
Monday, March 25, 2013 11:29 AM
Mike,
Is that 300ZX ever going to hit the track!?
Mike Kojima
Mike Kojimalink
Monday, March 25, 2013 3:43 PM
Johnathan,

Having run both, let me tell you that I really hate gear type LSD's in a FWD car. You will be much happier with a clutch type. You will probably want to set the initial torque low.

I think that the only place for a gear type diff might be in the front of an AWD car.
Ztaal3
Ztaal3link
Monday, March 25, 2013 4:58 PM
First, great read as always!
Second, I was wondering what setup you would recommend with this diff on a
Trackday g20, running r comp tires and around 150 hp?
Third, what will it cost to have some one doing a wpc treatment of the diff?

Thanks for all your hard work Mike!
Mike Kojima
Mike Kojimalink
Monday, March 25, 2013 5:22 PM
I am not sure what the factory default setting from Cusco an FF diff will have. Maybe nothing. You would have to take it apart and look at it. Cusco has a good guide that comes with the Diff.

Go to the WPC website to get a better idea of the pricing. It is perhaps the biggest single thing you can do for a clutch type diff. Gets rid of almost all the ratcheting.
warmmilk
warmmilklink
Tuesday, March 26, 2013 12:37 PM
how do you know that the rear diff is over heating during track sessions? are there certain symptoms in handling or something? or do you have a temp sensor on the diff?
Mike Kojima
Mike Kojimalink
Tuesday, March 26, 2013 2:33 PM
We use an infared pyrometer. You are in trouble at around 300 degrees. Usually the diff siezes and you are in for a wild ride. Have used temp labels in the past too.

thedriftbunny
thedriftbunnylink
Tuesday, March 26, 2013 10:59 PM
Mike, in photo 2 of page 3, it's my understanding that the full locking will be achieved when the cross shaft is forced to the left on the "gentler" slope and the half locking will be achieved when the cross shaft is forced to the right on the "steeper" slope. Or am I missing something?
Mike Kojima
Mike Kojimalink
Wednesday, March 27, 2013 2:55 PM
Notice that the direction of the cross shaft is different. The square end wedges harder.
Dan DeRosia
Dan DeRosialink
Thursday, March 28, 2013 8:29 AM
Here's one, and I don't expect anything more than a broad, general answer if that. To what extent do you retune suspension settings to take advantage/compensate for what the LSD is doing? Say, moving balance more towards oversteer than normal and increasing initial torque to have the diff keep things more in shape on turn in, for one example (I'm sure you can think of others). Or does that get to where you can have the suspension tuning and diff tuning chasing each other around in circles?
Mike Kojima
Mike Kojimalink
Thursday, March 28, 2013 10:12 AM
Yes you can totally tune the car around the LSD and vice versa. An example is a drift car. The diff is locked very tightly or completely yet it is critical for the car to be able to turn in sharply so the driver can initiate a drift easily. So on a drift car I do stuff like a wide front track, narrow rear track, toe out in front and a careful balance of scrub, caster and camber. This would make a car with a differently tuned diff darty. Or when going from an open diff to a mechanical LSD I might back off the rear roll stiffness, rear compression damping, add rear toe in and have a little more front toe out.
Dan DeRosia
Dan DeRosialink
Thursday, March 28, 2013 1:10 PM
Aha! Cool, that makes sense, thank you for that. Now to actually get something with a diff that's tuneable in ways other than preload. I tried cutting new ramps on the Mazda diff (which is probably the same unit as every other Salisbury type diff that came in a Japanese sports car) in the RX-7 on the mill, but it didn't work quite as well as I'd have liked.
EB Turbo
EB Turbolink
Thursday, March 28, 2013 6:47 PM
Why would switching to the cone springs yield higher breakaway torque? I guess it will depend on how many springs you are using in the RS configuration vs. The rate of the MZ cone. Adding the internal springs creates a higher preload on the plates and less load on the cross shaft. This makes for quicker lock timing and higer breakaway torque. Opposite for what you would want in this application. I would have thought using the MZ with slower lock timing and using less breakaway torque(60% or 40%) would habe been a better starting point. Do you measure the breakaway torque before installing the diffs? For comparison and to have an actual number to match to the % setting.

EB Turbo
Mike Kojima
Mike Kojimalink
Thursday, March 28, 2013 6:59 PM
EB, if you go to Cusco's web site you can see charts regarding the changes in breakaway and lock timing of different configurations. I was going to photoshop them here but got lazy.

A cone spring usually has a higher rate over a short distance making it well suited for this sort of use. The drawback is that it looses clamp force rapidly over short distances as well so the initial breakaway will decline quickly with clutch plate wear.

I think there is a chart on Cusco's web site showing this effect as well.

I like the coil springs a lot, one of the reasons why I like the Cusco and OS Gikken diffs.

EB Turbo
EB Turbolink
Thursday, March 28, 2013 7:15 PM
The internal springs in the Cusco's and OS Giken diffs work in different directions. More springs in a Cusco means faster lock timing and more lock. More springs in an OS me an slower lock timing and no change in initial torque. I prefer the settings of the OS Giken. However the OS has a lot less surface area on the clutch discs. This means less initial torque but it will also wear quicker. All of my diff tuning has been in FWD race applications. FWIW through a whole season of Grand Am ST racing the diffs (Cusco) showed very little signs of wear.

EB Turbo
Mike Kojima
Mike Kojimalink
Thursday, March 28, 2013 7:23 PM
Yes I know this and it is one of the reasons why both diffs are so tuneable. The OS has less surface but two more discs per side. The OS also has cone springs that counter the coil springs. The cone springs and the coil springs are available in 3 different rates and you can also vary the number of springs and stagger the weight of the springs for finer adjustment.
EB Turbo
EB Turbolink
Thursday, March 28, 2013 7:27 PM
You can also get thinner clutch plates and get different pressure rings.

EB Turbo
EB Turbo
EB Turbolink
Thursday, March 28, 2013 7:32 PM
Cusco parrs are really expensive compared to making the same setting changes with an OS.

EB Turbo
Mike Kojima
Mike Kojimalink
Thursday, March 28, 2013 9:21 PM
The thing about the Cusco is that you are most likely going to be able to get the diff to do what you want with what comes with it. The OS will probably need to change pressure ring springs and cone spring and I have found that OS is reluctant to sell the tuning parts to the public, even to me. For instance they would only sell me tuning parts because it was for "Dai Yoshihara". I think they want to control tuning only to authorized OS service centers or something like that. For grip driving on most cars I had to change cone springs and pressure ring springs most of the time and the same goes for drifting. I didnt know you could change disc thickness and pressure rings. Do they have pressure rings with different ramp angles?
EB Turbo
EB Turbolink
Thursday, March 28, 2013 10:14 PM
I've had the opposite experience. I started using Cuzco diffs. My shop had a box of about 7 assorted pressure ring sets. Once all of the good ones got used I contacted Cusco about new rings. I was qouted some $500+ for a set of rings plus a 2-4week wait time to get delivery from japan. We then switched to OS Giken. I was sent a diff very quickly. I also got parts next day as well. My rep had a lot of info on theory but not a lot of practical experience. With the data from the cuscos and the parts and notes from OS, I felt the OS was a lot better choice for my application. The program didn't last too mich longer so I don't have any long term data on the OS.

I believe all cusco pars are interchangeable. I will let you know what pressure rings I have and if you are interested in we may be able to work something out.

EB Turbo
Mike Kojima
Mike Kojimalink
Thursday, March 28, 2013 10:23 PM
I know Cusco has interchangeable pressure rings but I didn't know OS did. Cusco also has a pressure ring set where only the ramp windows interchange. I think that's pretty cool and infinitely tunable and probably expensive.
EB Turbo
EB Turbolink
Thursday, March 28, 2013 10:56 PM
I was talking about the cudco for different pressure rings. I don't think OS needs different rings besides 1, 1.5, and 2 way settings. The way you change the OS pressure ring springs you have a lot more lock timing adjustment than with the different cusco pressure rings. I have seen one of the adjustable diffs like you mentioned. I didn't get a chance to play with it as it went directly to the customer. They are really cool though.

EB Turbo
Jonathan
Jonathanlink
Friday, March 29, 2013 7:12 PM
What are the thoughts on the Cusco rs vs the os giken in a daily driver? The os is about double the cost for my mini. Everyone says the os giken is better and I suspect it is because they havent smoothed out the locking on the Cusco with a wpc treatment. That or the extra plates in the is giken offer finer tuning of the lockup. Any in put would be appreciated.
Mike Kojima
Mike Kojimalink
Friday, March 29, 2013 8:26 PM
That or they haven't set up the Cusco. I think the OS has a little better adjustabilty of the initial torque but the performance of the two diffs are pretty close to where if I had to pay for it, I would take the Cusco by a long shot. I would actually WPC the pressure rings and the side plate as well, we kinda forgot to do it. WPC makes a mechanical diff much much smoother and quieter. Once you have tried it you will swear by it.
Jonathan
Jonathanlink
Tuesday, April 02, 2013 4:59 AM
Turns out the cost difference is only $500 then WPC for the Cusco on top of that (the OS Giken is quite in the mini application at least) so it become a bit less of a no brainier.

How much initial torque would you recommend for about 200-200whp?
Mike Kojima
Mike Kojimalink
Tuesday, April 02, 2013 8:38 AM
The OS Giken diff like all mechanical diffs should be WPC treated as well. As far as break away torque, it depends on what you are doing with it. Drifting needs tight, for road racing less tight usually unless there is something about your car that requires otherwise. Bigger tires, sticker tires, more power sometimes but not always favor tighter diffs.

Suspensions that transfer more weight, tires, car configuration all change what you might want out of the diff.

Generally I run the tightest diff possible for drifting and the loosest that will not allow inside wheel spin for road racing. Initial torque affects turn in the most. It also affects mid turn on longer turns a bit. Clutches affect corner exit more.
kyoo
kyoolink
Friday, April 12, 2013 12:35 PM
I run the Cusco Type RS 1.5 way for my evo 9's rear diff - I didn't get it WPC'ed, unless that is the exact same thing as shot peening - which from a previous article seemed to say they're similar but still different.

The Cusco for me has worked as advertised - completely changed the driving characteristic of the car, and it will finally power oversteer out of tight corners - if I want, it can go completely sideways (street tires). Like people have mentioned though, I don't just get chatter with this diff - low speed in parking lots, gas stations, driveways etc. will result in BANG BANG BANG coming from the rear end.
Tim
Timlink
Thursday, April 18, 2013 3:34 PM
Did you guys replace the differential main bushing on the subframe as well? The black residue on the OEM differential cover indicates that your stock differential bushing is blown.

Are you sure that the OEM Nismo cover provides 1/2 quart more capacity? I don't think that's a true statement.
Mike Kojima
Mike Kojimalink
Thursday, April 18, 2013 4:30 PM
Yes it's true acording to Nissan Motorsports.
Footwork
Footworklink
Wednesday, January 14, 2015 1:05 PM
Great article and lots of good info. I have a question regarding setup for a Tomei T Trax Advance LSD. One issue is the diagrams in the manual. "to increase initial torque" it shows 1)the canceling of one set of clutch plates 2) removal of the cone spring and 3) the addition of "toothed spring" (which is not included with the LSD...) Am I correct to assume that removing the cone spring would reduce initial torque (provided it is equal thickness to the "toothed spring")?. Anyone on here have first hand experience setting up the T-trax advance for s13? Thanks!
kyoo
kyoolink
Friday, February 13, 2015 8:10 AM
I know the article is old, can anyone discuss what the optimal setup would be for an evo 9 on street tires for autox? I think previously I had it set at 12 springs, all plates active, and 1.5 way and I think the car understeered more on decel than I would have liked, maybe too much initial torque? I may try 8 springs & deactivating a pair of drive & driven plates (that would decrease lockup 10% correct?) I don't want to lose any on power characteristics, but would like to lose some off power characteristics..

I guess I'm a little confused as to under what circumstances a clutch diff would create less grip - under/oversteer, and when it would create more grip. I feel like it would always create more grip at the axle it's on, in this case creating more understeer, but obviously that isn't the case. Mike?
Mike Kojima
Mike Kojimalink
Friday, February 13, 2015 12:10 PM
I think you are on the right track
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