silver92b

iTrader: (1)

My '92 1.6 w/JRSC is developing a clutch slipping problem. The strange thing is the symptoms are worse when accelerating from 4th or 5th gear at highway speeds.... The car generally does not slip from standing (although lately it has begun to slip when hard WOT is applied in 1st gear). My blower does make up to 9~10 p.s.i. under WOT...

I have an EXEDY Stage 1 clutch kit (rated at 200 hp) ready to go in, but I want to make sure that I am addressing the underlying issue and not just changing parts. The existing clutch is a 1.8 Exedy stock replacement unit and only has about 20K~30K miles on it.

I'd appreciate any good advise about this issue before I go ahead with the clutch replacement. BTW, I have a 4.1 Torsen LSD ready to go in as well. I hope this will be a positive step to take. At least the speedo is more likely to show the correct speed .......

18psi

iTrader: (76)

Why is it strange that its slipping in higher gears? That's what its supposed to do since it sees more load there.
And 20-30k at 200 (your car probably has around that to the crank) and probably abused is not too bad IMO.

fooger03

iTrader: (2)

The underlying issue is a wimpy clutch.

To fix the underlying issue, replace it with a less wimpy clutch.

Yes, 18psi is correct - you should expect to see clutch slip at WOT in 4th/5th gear long before you'll actually notice it in first/second.

Seefo

iTrader: (5)

you want advice? Replace the clutch (and flywheel while you are down there, if it hasn't been replaced recently).

gearhead_318

iTrader: (1)

You would think your clutch would start to slip in lower gears, but it doesn't. Don't know why, probably voodoo.

Plenty of good clutches out there that will easily hold the powa.

matthewdesigns

iTrader: (8)

To be expected. I can go WOT in my car 1-2, but 3+ and it slips like a fat b1tch on a banana peel. Get that clutch in there.

18psi

iTrader: (76)

Once I went past 200 mine slipped in 3-5th any time I put any significant load on it.
I finally take it out expecting it to be toast but it looked damn near brand new. Just couldn't handle the torqz. And that is unacceptable because torqz = funz.

the stage2 should be ok

silver92b

iTrader: (1)

Thanks to everyone for your input. It looks like I came to the right place to post this question. Most Miata ppl without forced induction never have even heard of this problem. I suspected all along that the clutch is not strong enough for the application but I have received advise that suggested that the upgrade to 1.8 clutch would be enough for my car. I guess they were wrong.....

I guess this is going in there before too long....

matthewdesigns

iTrader: (8)

IDK what kind of power the JRSC will put out, but if your new Exedy clutch is only rated at 200hp then you might be borderline with it. I picked up an FM1 and a 1.8 flywheel, which is rated at 318tq...the stock bottom end/trans/rear will grenade before it gives up.

silver92b

iTrader: (1)

I was afraid of that... I've been advised not to go with a stronger clutch as it would be unnecessary. This is an old dyno reading. It probably makes no more power than that today. However, I think I rather have the clutch fail than the LSD and/or the bottom end.....

hornetball

iTrader: (4)

Recently did an ACT Stage 1. Really like it, nice feeling clutch and easy to drive. Got it from Renown for $275 shipped, which I thought was decent. No more slippage. Rated for 200 ft-lbs.

Remember that the clutch has no idea what gear you're in. It has engine torque on one end, and the car's resistance to that torque (through the transmission) on the other. In the higher gears, you remain in the peak torque area for a much longer period of time. Therefore, it is only logical that you'll notice slipping in the high gears first when you start to overpower the clutch.

If the Exedy is rated for 200ft-lbs, then you should be fine with it. Is it rated in ft-lbs or HP?

matthewdesigns

iTrader: (8)

I guess if you are not going to crank it up any more then it ought to be fine.

silver92b

iTrader: (1)

The description where I actually bought it from Ebay says it is rated at 210 ft.Lb. http://tinyurl.com/7yhup62

Part # Torque Capacity---- Disc OD ---- Disc Spline
10805 ------210 ft./lbs.------215mm ------- 22

silver92b

iTrader: (1)

I might go with the MSPNP in the future, but I don't know if the HP/TQ increase would be so high as to overpower that clutch rating...

fooger03

iTrader: (2)

"Voodoo"

LOL - good one!!

Let's work through some of the detail here:

As we all know, clutches usually "wear out" a little bit before they actually start to slip on a regular basis, if your clutch looks new when you pull it out, consider that the "wear" of the clutch plate has actually made it thinner, this means that the spring that actually puts pressure on the clutch is under less flex when it is actually clamping down on the clutch material, and springs that are under less flex will always apply less pressure -> translation = a worn clutch will hold less torque than a new clutch - You'll probably only ever notice this change though if you're very near the torque holding capabilities of your clutch in the first place - most users will have a clutch with capabilities well in excess of available crank torque - they'll start to notice clutch failure when the friction disk has worn out.

So lets go into the gears portion of today's lesson:

You will first notice your clutch begin to slip when your engine is producing peak torque - since cold air is naturally denser than warm air, it is generally expected that your car will produce a somewhat higher peak torque in the winter months - I theorize that clutch failures are therefore much more common in winter months than in summer months - case in point: the OP.

Now let's examine the difference in behavior of peak torque between gears:
For science's sake, lets make an assumption: The peak torque on our science car occurs at 5500 RPM.

As your clutch begins to fail, it doesn't fail in 5 FT-LB steps, it fails very slowly (until the point that it actually fails, but more on that in a moment) so your first clutch slip is going to happen at exactly 5500 RPM - the clutch will hold just fine at 5499 RPM, and it will hold just fine again at 5501 RPM. Chances are, the first few times your clutch slips, you won't even notice it, it will slip very briefly and very slowly, maybe a few degrees of rotation at first, and eventually by entire rotations.

Again, for science's sake, I'm going to briefly ignore the difference between static and kinetic friction - which is really a bigger deal than I'm making it out to be - we'll explore later.

As the clutch slowly begins to fail, the RPM range at which it fails will slowly grow - soon enough, the clutch can't hold 100% of power from 5490 to 5510 RPM - a range of about 20 RPMs.

Consider how long you spend in the failure range of 5490-5510 RPM in first gear.

Now consider how long your engine operates within that same RPM range in 4th or 5th gear. Not only do you have to stay within that range longer purely because of gear ratio, but wind resistance will also hold you into that RPM range for a longer period of time.

So, lets translate those extended times to differences in RPMs
While in the failure range, your engine will be accelerating faster than your tranny input shaft, this means that the longer you are in the failure range, the faster your engine will spin relative to the input shaft. In first gear, because you are only in the failure range briefly, your engine may only be spinning at 10 RPMs more than the input shaft by the time you exit the failure range - what is the chance that you are going to notice those 2RPMs? Now consider what you're doing in 5th gear. By the time you exit the failure range in 5th gear, your engine may be doing well over 15 rpms faster - again, you probably won't notice that difference, but consider that the failure range is only 20 rpms - your engine has increased 20 rpms, but your car has only increased speed as if by 5 rpms. Now lets re-introduce the difference between kinetic and static friction into this equation...

Static friction means that "when your clutch is holding, it's more likely to continue holding"
Kinetic friction means "when your clutch starts slipping, it's more likely to continue slipping"

Once you exit the small failure zone in first gear, your clutch is still slipping, though because you've passed the failure zone so quickly, we'll say the difference in rotational speed is 2 RPMs, the difference will continue to increase after you have left the failure zone until your engine torque falls off to a point where it no longer exceeds the kinetic friction - at this point, your engine RPMs will stop decreasing briefly until your car "catches up".

See how that friction thing works? Static friction in - Kinetic friction out - so even though your clutch may hold 200 ft-lb of torque *before* it starts slipping, it might only hold 180 ft-lb of torque *during* the actual slip - once it recovers from the slip, the holding potential will again be 200 ft-lb.

Now lets again investigate what hapens in 5th gear: Your car has increased speed by an equivalent of 5 RPMS, but your engine has increased rotational speed by fully 20 RPMs, at this point, your engine torque falls below the static friction threshold of 200 RPMS, but just like in the 1st gear example, your engine will continue to accelerate relative to the car until the RPM where it produces less than the kinetic friction of 180 ft/lb, at which point the speed difference between the engine and the car will begin to equalize.

So to summarize, consider these facts:
During both 1st and 5th gear, the clutch will begin slipping once the engine has strengthened to 200 ft/lb of torque, and it will continue to accelerate until the engine has weakened to 180ft/lb. At 180 ft/lb of torque, the engine will stop accelerating until the car catches up.

In 1st gear, the time spent between 200 ft/lb on the up swing of the torque curve, and 180ft/lb on the down swing might be as little as a half-second, and your engine may only increase speed by 50 RPM during that half-second

In 5th gear, the time spent between 200 ft/lb on the up swing of the torque curve, and 180ft/lb on the down swing might be closer to ten seconds, does this mean that your engine should only jump 500 RPM? not exactly.

Once the clutch starts to slip, it begins building heat VERY QUICKLY. In first gear, since it's spinning slowly and re-catches quickly, there's not a whole lot of heat generated - but in fifth gear, since it begins slipping and quickly escalates to high speeds, it gets super hot - this heat further reduces the torque holding capability of the clutch, so that in 5th gear, you're likely to go straight to red-line soon after the clutch starts slipping, while in first gear, you still don't notice the slip.

Need to save up money to replace your slipping clutch?
Then don't test your clutch beyond what you need in order to convince yourself that it is slipping. The very act of slipping will quickly ruin what is left of the friction material. Baby the car until you have the time and money to swap in the new parts.

hornetball

iTrader: (4)

Assuming the general relationship between HP and Torque on your car doesn't change, you would have to get to ~225HP before you need to worry about that clutch. There's no way you'll get to that HP with a JRSC.

Seefo

iTrader: (5)

you are afraid a stronger clutch is going to blow the differential?

I am pretty sure those VLSDs blow if you even touch them wrong. get a 94+ diff and sleep soundly at night.

silver92b

iTrader: (1)

I'm not afraid that the stronger clutch will blow the diff. I was only responding to the poster who said that his 318 ft/lb clutch would hold after his bottom end and diff granade. As for the diff, while my son had it in Tacoma the open 1.6 stock diff already failed once. The 6" diffs are known to fail. I'm replacing the open diff as well with a 4.1 Torsen LSD.

Doppelgänger

iTrader: (12)

If you're going to be in there, might as well get a flywheel too and enjoy a happier revv'ing engine I had a FM happy meal for years and loved it (with 250+rwhp) and driveability was that of a MSM. Last year I put in a Competition Clutch 6-puck ceramic (I don't do those "stage" things) with the FM flywheel and ACT pressure plate. Yeah...now it's an on/off switch. If you have the budget, sell the Exedy and get a FM happy meal.

silver92b

iTrader: (1)

Your ride sounds like a neat machine! I'd like to see it sometime. I have had those "real" racing clutches before in my EVO VIII. It's true they'll hold the power and all, but I did not like the "off-on" action. It was a PITA to drive on the streets. I prefer a more "civilized" clutch . My car has 110 less whp than yours so it probably can get by with half as strong a clutch anyway. I thought briefly about a lightened flywheel too, but I decided I didn't want to sacrifice the drive-ability for the quicker revving effect. I'm sticking with a 1.8 flywheel reground.
I wish I could have the 300whp engine with all the trimmings and the trick suspension, etc. Unfortunately, reality consists of a series of compromises to make things work within one's constraints. Between the clutch, LSD and labor I'm already spending a ton (for me) of cash. I think I'll just have to be happy with my little 150~whp '92 roadster