Technical Manual Trans Advice- the Importance of Matching Your Clutch To Your Application...

A clutch rating is just a quick/dirty way for the typical aftermarket clutch buying customer to feel assured that the clutch they buy isn't going to slip against the torque their engine is capable of. From the perspective of the aftermarket clutch manufacturer, if the clutch breaks something else downstream it's not their problem, at least the customer won't come back complaining about a weak clutch.

The T5 5spd serves as a good example. The T5 was an OE transmission for both GM and Ford, and the 2.95 gearset versions for both versions are nearly identical as far as case/gear strength. But the GM 2.95 V8 version has a reputation as being weak while it's pretty common to see Ford 2.95 V8 version running 10's on the dragstrip with slicks. The difference is the clutches that are commonly used with each version. GM T5 guys almost always go to the aftermarket and end up with around 2800-2900lbs of clamp from a 10.5" diaphragm, while the go-to clutch for the Ford T5 guys is the Ford Motorsport "King Cobra" 10.5" diaphragm which has 2124lbs of clamp.

Problem is the typical aftermarket clutch customer doesn't realize the ideal clutch is one that WILL slip. Not a clutch that slips against the maximum HP/TQ number that the engine is capable of, the clutch isn't going to see that static number at any point while the car is working its way thru the gears. The target should instead be a clutch that slips for a half second or so after a WOT shift into high gear.

The Racepak graph below is of a dragstrip pass with an engine that puts out around 425ftlbs max WOT steady state. I added some averaged binary torque numbers to the lower part of the graph to reflect the calculated torque that the engine applied to the transmission's input shaft during the pass. Note how this 425ftlb engine put out way more than 425ftlbs when it is losing rpm, and much less than 425ftlbs when it is gaining rpm. At no time during this pass was this 425ftlb engine actually sending 425ftlbs to the transmission's input shaft, because at no time during this pass was the engine operating at a constant rpm. The engine was either losing or gaining rpm at every point after launch while it worked its way thru the gears. Note that during the climb in 1st gear, less than half of that engine's potential torque output was actually reaching the transmission's input shaft!
...you might ask- WHERE DID ALL THAT MISSING TORQUE GO???...



To wrap your head around this, it helps to think of the engine's rotating assy as a torque storage device. Everything spinning ahead of the transmission's input shaft is all basically one big energy storing flywheel. The reality is that some of this engine's torque was being absorbed by its rotating assy as it gained rpm, and then that absorbed/stored torque was returned as rpm was drawn out of that engine's rotating assy against WOT.

From there, it's important to understand that the rate that the clutch draws engine rpm down is what controls how that absorbed/stored torque gets applied to the transmission's input shaft. Using nice round numbers to make it easy to grasp the inverse relationship, let's say a rotating assy gains 2000rpm in 1 second while absorbing 200ftlbs of torque during the engine's climb to the 1/2 shift point...
...If the clutch then draws out the same 2000rpm over the same 1 second time period after the shift, 200ftlbs gets added back to the input shaft torque for 1 sec.
...If the clutch then draws out 2000rpm over 0.5sec (half the time), 400ftlbs of torque (double the torque) gets added to input shaft torque for that 0.50sec.
...If the clutch then draws out 2000rpm over 0.25sec, 800ftlbs of torque gets added to input shaft torque for that 0.25sec.
All three above examples of discharge rate release the same quantity of energy. Give the car 200ftlb "boost" for 1sec (200 x 1 = 200), vs a 400ftlb boost for .5sec (400 x .5 = 200), vs an 800ftlb boost for .25sec (800 x .25 = 200), it's all basically the same amount of boost available from 2000rpm's worth of returning energy.

When you go beyond daily driving and move to the track, drawing that stored energy out too fast can cause other problems downstream. The initial problems are likely to be either traction or finding weak links in your drivetrain. If the tires go up in smoke, the typical solution is to buy better tires. If the problem is wheelhop, new bushings/shocks/chassis components. If the driveshaft or u-joints fail, bigger/stronger parts. If the transmission breaks, stronger trans. Break rear gears/diff/axles, spend another pile of money on a rear upgrade. But even after you get all those things sorted out, you will find your combination is still not performing to its potential, as now the engine bogs. But when the root cause of the cascading problems is an overkill clutch that draws too much torque from the rotating assy, addressing that problem first could save you from making a lot of un-necessary upgrades.

Given that a 2000rpm discharge releases the same basic quantity of energy regardless of how fast you lose the rpm, you have to ask yourself how big of a torque spike can your drivetrain/chassis efficiently handle? Are the tires going to be shocked into excessive wheelspin and waste a large portion of the returned energy? Is it going to break something? Would it be better to use a clutch that draws 400ftlbs of stored energy over 0.50sec rather than one that draws 800ftlbs over 0.25sec?

The clutch's stored energy draw rate is every bit as important to the performance of a stick shift car as choosing the right converter for an automatic car. When you just bolt in a typical non-adjustable clutch, you are pretty much locked into whatever rate it might pull your engine down against WOT. There is no advantage to buying a clutch that pulls 800ftlbs out of a 425ftlb engine's rotating assy. Don't buy a clutch with plans to "grow into it", it is far better to pick one that's the best match for your engine and use. Not a situation where too much is just right. Excess torque capacity not only increases the clutch's potential to inflict damage on your drivetrain, but it will also slow you down at the dragstrip.

All that said, I make devices that can momentarily soften the hit of any clutch. This enables your existing clutch to initially hit like a much softer unit without giving up any of it's holding power. A diaphragm clutch controlled by one of my Hitmaster units recently took home the $10k winner's share in the stick shift class at the '22 World Cup Finals!!!

Grant

 

My head is spinning.

 

Solid stuff. More then I wanted to digest, but I'm full up, ready for a nap now.

 

I never considered the impact a "stiff" clutch had, but it certainly makes sense that it would be harder on the components behind it when used aggressively.

 

There was a thread on here maybe 5 years ago? a guy designed his own clutch "slipper" setup. Had actually track tested it. Anybody else remember that?
Was that you @Weedburner

 

Interesting concept that I had not read or considered previously.

 

That was likely me, probably a thread about my original Hillbilly Clutch Slipper that I made 13 years ago from a $15 hardware store hydraulic storm door closer. A more current "under the radar" version of that has been thru the inspection barn at the NHRA's US Nats, even came out on top in A/S on radials!! It was externally controlling the hit of a Black Magic Nexgen Long style clutch that was set up with minimal centrifugal assist to act more like a diaphragm unit.

The same style 'tamer is currently being used by all the competitors in NMRA's dead hooking, wheels up, 302ci sealed crate engine Coyote Stock class. I've seen a 9.49 @ 141 with a 1.327 60' slip, they all use a diaphragm clutch with it's hit externally controlled by one of my 'tamers. The only thing that keeps those guys from launching even higher is a factory ECU that shuts the throttle blades down if the engine goes over 7800.

It's a hard fact of stick shift drag racing that your clutch needs to slip more during launch than it does after the shifts. Adjustable centrifugal assist clutches are popular and can give you less clutch clamp pressure during launch than after the shifts, but that additional slip during launch is achieved by compromising your launch rpm potential. When you ditch the centrifugal assist feature, you can crank up the static pressure and then control the launch hit externally with something like my 'tamer. It gives you the ability to store more energy prior to the start by raising launch rpm, and then put that additional stored energy to work without spinning the tires or breaking parts to improve your ET after the clocks start running. Wish I had thought of it 40 years ago, if I had there would probably be a lot less racecars today with automatics

Here's a link that explains some advantages of external clutch hit control over the adjustable centrifugal assist units... https://grannys.tripod.com/clutchtuning3.html

Grant

 

Very Interesting, back in the day 60's I broke everything in the driveline often using aftermarket clutch/pressure plate or modified pressure plate pressure always with a scattershield after one explosion ( lucky to have my feet) I will say my parts house for axles, rear ends, etc. did come from the junkyard speed shop, low budget street racer kid I was. Is this technology used today in modern performance cars with manual trans ? I have never talked to an owner of a newer super car that drag races some that has had this type failure and I don't believe any of the new super cars have a scattershield, I'm aware the traction control is a big factor with controlling the high HP/Torque today.

 

Grant and all y'all,

I'm about to install a Chevy 261ci from a 2 ton truck in my 1/2 ton pick-up. My engine came with an 11" clutch, would a smaller clutch be advisable for a non-racing, never heavy hauling truck? Or, doesn't make an appreciable difference, use what I have?

Thanks,
Cosmo

 

"Traction control" is a hi-tech way to say power reduction. If they called it what it is, many would look for other solutions.

Grant

 

For a driver it may not make much difference, but if you have the pickup's original clutch and it will fit the new engine, I would use that.

Grant

 

One of the problems of upgrading/strengthening components is you just move the weak point.

 

I'd say use what you got. When it comes to clutches ya have to think glue. How it adheres, how it releases it's grip.
Does it grip like a stickie, or like contact cement? How does it release. Keep in mind, some of what you "feel" someone else designed in for leverage and lever action.
The clutch in that 2 ton was going to grip and slip everything from a empty hauler to a load of grain, or something heavier. I'm guessing it's going to "feel" like a quick grab but one with some feel if you decide to slip it a bit. I'd use it.

 

Thank you for the feedback!

Using your brakes also creates heat and shortens brake life, but as long as you do not overheat them they will last a good long while. Same with a clutch. We are talking extending slip time a few tenths of a second, which can make a huge difference in terms of impact on the drivetrain.

If you are not slipping something in a drag race scenario though, you are slow! While a properly slipping clutch does "waste" some power that gets absorbed into the clutch assy as heat, that slipping actually makes it possible to net an overall power production gain. That's because slipping allows the engine to make more power strokes in a given time frame, and that power production gain can more than offset the loss of heat energy absorbed by the clutch. Also because the car is gaining speed while the clutch is slipping, engine rpm does not get pulled down as far after the shift as the ratio change predicts, further raising average rpm. It is possible for the clutch to slip too much and squander your power production gains, the trick is finding the sweet spot.

Grant

 

Not any kind of drag racer, but don't the top fuel dragsters have adjustable clutches that allow them to tailor the amount and timing of clutch grip to track and lane conditions?