At 09:21 AM 7/19/04 +0200, Sigbert Weinberger wrote:
>"being in the process to modify the clutch of my Twin Cam to an MGB unit I'm wondering how much horsepower such a standard clutch will handle. Thinking it is designed for the 90 hp of an MGB, the Twin Cam engine will hopefully have an output of 110 hp plus. Should I go for an uprated unit?"

Forget about horsepower, and think torque, torque, torque.

Clutch capacity is rated with two numbers, output torque and continuous thermal capacity (capacity to dissipate heat when it slips). If you turn it twice as fast when it is engaged it can handle twice as much horsepower. If you turn the input shaft twice as fast when it is slipping it will generate twice as much heat. So you upgrade a clutch to handle increased output torque, or increased heat dissipation (not necessarily for increased engine power).

Peak torque comes when you engage the clutch with engine running at high speed. At high speed the flywheel has a lot of energy, and that energy has to be dissipated as the engine slows down. The torque will depend on how tight the clutch grabs, and how much resistance you have at the propshaft when the tires start to spin, and which gear ratio is selected in the gearbox. The horsepower throughput will be a product of that torque and the speed of the output side of the clutch. Any excess input power would be dissipated as heat in the clutch when the clutch slips.

When the clutch engages, one of three things happens.

a.) Engine is not running fast enough or with enough torque when you engage the clutch, and it kills the engine. This is not good. The solution is to increase throttle setting and/or engine speed, and also to engage clutch gradually to allow it to slip some until ground speed increases and (maybe) engine speed decreases. When ground speed matches engine speed the clutch stops slipping, and you're driving.

b.) For lower gears, multiply the clutch torque capacity by the gear reduction ratio in the gearbox and rear axle to find the output torque at the rear wheels. If the resulting torque is enough to overcome the friction between the tire and the pavement, the clutch will grab instantly and the tires will spin. In this case torque is limited at the wheels by the grip of the tires. If the engine is running faster to start with, the flywheel will be dropping more energy as it slows down, and the tires will spin a little longer. If the engine can develop enough torque at full throttle to spin the tires in the lower gears, the tires may continue to spin until ground speed increases to match engine speed. Any excess energy not being used to accelerate the car will be dissipated as heat of friction at the tires (clutch not slipping).

In this case if you install larger tires or stickier rubber, the tires may get a better grip to provide more friction and more torque at the wheels (and quicker acceleration). If the tires can still spin, the increased torque accelerates the car faster. When it takes less time for the car to get up to speed, the tires will spin for less time, as more energy goes into acceleration and less energy goes off as heat. So the stickier tires can give you better acceleration and less slippage (less heating and wear in the tires). Through all of this the clutch has no problem at all because it is firmly engaged and not slipping.

c.) For higher gears, when the clutch does not have enough torque to spin the tires, the clutch will slip to dissipate energy as the engine slows down (same as the brakes dumping energy to slow the car). As the car is moving, the power throughput is a function of the clutch torque and the clutch output shaft speed. Any excess power is dissipated as heat from friction on the clutch disk as it slips, and the clutch gets hot. If the clutch is intended to slip continuously (hopefully not in your car) it will have a certain continuous thermal dissipation capacity. As long as that limit is not exceeded it can continue to slip in that fashion indefinitely, until the friction surfaces wear out. When the clutch is dissipating more energy than its rated thermal capacity (all cars do this), then it continues to heat up as long as it is slipping, and the temperature increases until something melts or burns (usually the friction lining). Hopefully your clutch will never slip that much to get that hot.

Here's the rub. If the engine continuous power output from throttle only is greater than the clutch power throughput, then the engine will not slow down, and the clutch will continue to slip until the ground speed matches engine speed. This is not good. If the clutch torque capacity is higher than the engine torque output at full throttle, then the engine will slow down. When the flywheel speed slows to match the speed of the clutch output shaft the clutch stops slipping, as all excess energy has been dissipated. This is a required condition for your car, so you don't continue to slip the clutch and burn it out. In short, the clutch torque capacity has to be somewhat higher than the engine full throttle torque output.

As you use higher gears in the transmission the clutch will see higher torque, as feedback from the tires through the gearbox. This means the clutch absolutely has to slip some while engaging in the higher gears. If the tires could not slip, and the clutch could not slip, then you would get instant deceleration of the flywheel, which would break the crankshaft from internal engine inertia. We know that doesn't happen.

The clutch has two functions. First is the common function of disengaging to allow engine and propshaft to turn at different (not synchronized) speed during shifting. Second is the function of intentional slippage while transmitting torque to dissipate energy and bring the engine flywheel and gearbox input shaft back into synchronized speed. Any clutch, regardless of size or design, will handle that first function properly.

The concern then is to match the clutch design to perform the second function in an acceptable (or desirable) manner. This function can vary depending on the end use, and may in fact be a matter of personal preference. For street use you may like a "soft" clutch that slips more and engages gently for a smooth start, no jerk, and does not spin the tires or kill the engine. For competition use where the clutch may be regularly used in a more severe manner, you may want a stronger clutch to engage quicker so as to minimize wear on the clutch. As noted before, the torque capacity of the clutch must be minimally somewhat higher than the engine output torque. The question then is, how much higher is good?

The first goal here may be to have enough clutch torque to be able to spin the tires in 1st without slipping. Most street cars can do that, so if you rev the engine and pop the clutch in 1st gear it will spin the tires (at least a little) and give you a good launch without burning up the clutch. From there on through various shifting functions, as long as you don't abuse it too much that clutch will not slip much and may last a very long time.

A race car with close ratio gearbox may effectively have no 1st gear, and final drive ratio may be selected to run engine near top speed (peak power) on the fastest sections of the track. Given a short enough track and high enough final drive ratio, the top gear overall may be about like 3rd gear in a street car. I fondly refer to this practice as having three 2nd gears and one 3rd gear. A stock clutch might not be able to spin the wheels in the lowest gear, and a race engine may have very little torque below 2000 rpm. The stock clutch may have to slip continuously on startup until the car reaches at least 10 mph ground speed. In this case the clutch may need to be quite a lot stronger to provide enough torque to spin the tires in 1st gear. So you might want (or need) to install a competition clutch for increased clutch torque.

When upgrading the engine power, you don't necessarily need to worry much about clutch capacity just because of increased engine power. When you increase engine power without increasing displacement, most of the improvement comes by improved breathing to increase torque at higher engine speed. So you get more net power at high speed, but maybe not much more peak torque. And if you use a lightened flywheel, you may even see less torque for the clutch to handle. The greater concern for the clutch is related to the overall gear reduction ratio (especially in low gear), and the amount of grip you can get from the tires. Taller gearing for low gear may beg for a little stronger clutch. Stickier tires for competition use may demand a stronger clutch.

Another reason for upgrading the clutch is for competition use where you might make a regular habit of power shifting. That means standing on the throttle for best acceleration while the clutch is still slipping immediately after a shift, and in particular maintaining high engine speed during the shift to retain maximum energy in the flywheel. In this case the clutch will often be slipping significantly during shifting to dissipate the flywheel energy and some additional engine energy at full throttle. This could overheat the clutch, or it could cause unacceptably rapid wear on the clutch disk. In this case installing a stronger clutch can reduce the slippage (while the engine is slowing quickly to match propshaft speed), transmitting more energy to the wheels, dumping less heat into the clutch, and reducing clutch wear. The end result is desirable, getting quicker lap times and reducing maintenance cost. While this is a noble goal for a race car, for a street machine it may give undesirable high pedal pressure, and also harsh clutch take up which may either kill the engine with a sudden jerk or spin the tires on startup when you don't need it.

This stuff is all a bit of a compromise. You may want a soft clutch for street use and a stronger clutch for competition. When you use the car for both street and competition, it quickly dawns on you that the clutch is not adjustable, and you have to decide which side you will compromise to favor the other. If you are determined to do your very best at competition, then you may settle for a stiff leg and having to finesse the harsh clutch on the street. If the competition is occasional and mostly for fun, maybe not so serious, then you might favor the stock clutch and just try not to beat it to death during competition.

After my expending all this hot air on explaining how things work, I suppose you would like a more definitive answer to your original question, hopefully based on some real world experience. Okay, here goes.

In February 1957 at car number 16226 the MGA clutch was upgraded with slightly stronger springs. This was 14 months before the Twin Cam was introduced (in April 1958). Someone might eventually prove me wrong on this next point, but I believe the Twin Cam used the same clutch pressure plate as other MGA at that time. The newer pressure plate with stronger springs superseded the earlier part, so all replacement parts since February 1957 should have the stronger springs. The softer springs are seldom seem, only found in original early pressure plates. The later MGA stock clutch is generally acceptable for use with the stock Twin Cam engine and bias ply tires. Aye, but there's the rub, those last few words about tires.

Since I autocross my MGA a lot, and seriously, I have some experience with various clutches. Modern tires are of course much grippier than the original bias ply tires ever could be, so some upgrade of the clutch could be appropriate. When I started using race tires for autocross I was for a while replacing the clutch disk about once per year (using the original MGA pressure plate). I figured that to be unacceptable, and desired to do something about it.

Being constitutionally cheap, I first took a small and relatively inexpensive step to install the MGB diaphragm type clutch pressure plate, also requiring use of the MGB release bearing, release arm, and front cover from early MGB gearbox. As a matter of convenience and least expense, acquiring a good used early MGB flywheel was cheaper and easier than paying for machining the MGA flywheel for the matching alignment pins (3 for the B where it was 2 for the A). This also knocked 8 pounds out of the flywheel without machining, which was a nice side benefit.

First lesson learned was that the MGB clutch could spin the race tires with a power shift from 1st to 2nd gear. This gave a nice acceleration kick in the back from flywheel energy with the power 1-2 shift, just had to be a little careful doing that in a corner not to break the tail loose. It also greatly reduced wear on the clutch disk, which was after all the original primary goal. Worn tires are much easier to change than a worn clutch disk.

Skipping a different very long story, my engine at one time had a bad oil leak at the rear of the crankshaft, which was causing abnormal (and barely functional) clutch slippage and very rapid wear on the clutch disk. This prompted me to install a competition pressure plate (bought from Victoria British). This required maybe 50% higher pedal pressure, but had a much stronger grip. Installed clean, this worked well for about a week before it got oil on it and also began slipping unacceptably. I then had the engine out three times in one month during development and installation of a rubber rear seal for the three main bearing engine. Having stopped the oil leak, everything was peachy again. I was using the stock clutch disk with the competition pressure plate, because the competition clutch disk had different splines (which would have required replacement of the gearbox input shaft).

Lessons learned here were, the competition clutch could spin the sticky race tires momentarily with a power shift into 3rd gear, and wear on the clutch disk was virtually nil. The combination of the sticky race tires and the totally non-slip clutch was regularly and more often putting more torque on the propshaft, and I ended up replacing one or the other u-joints about once a year. This was not a great sacrifice in exchange for good competition performance. My left leg got used to the higher pedal pressure with the first week of driving, no problem there, and anyone else's MGA or MGB then seemed to have soft clutch pedal. Pedal travel from disengaged to fully engaged was only about one inch, ranging from about 1/2" down to 1-1/2" down from top. I got used to popping it quickly using only the top half of pedal travel, so the increasing spring force for the lower half of pedal travel was not much of a factor (except when holding the pedal down for longer periods of time).

One day I borrowed another bloke's Toyota MR2 for a few autocross runs, and I promptly broke the guy's shift linkage. I was habitually using half pedal travel for the clutch, and ended up forcing the gears when his clutch was not fully disengaged. That experience (and helping the bloke repair his car) was a lesson well learned, and I thereafter became personally more adaptable when switching between various cars.

The short pedal travel with the competition clutch was a rather nifty feature for competition use, allowing for very quick shifts, but it did require a little more finesse for street use. An inexperienced guest driver in my car would commonly kill the engine a few times, then throttle it more and spin the tires on startup, and generally complain about the grabby clutch and heavy pedal force. The finesse required was to carefully feather the pedal to allow some clutch slippage on startup in which case it would act exactly like a normal stock clutch (after a little practice).

After three years of heavy and harsh usage of the competition clutch, the engine was out one day for some other reason. The friction lining on the clutch disk was still full thickness with virtually no detectable wear (except for a little surface buffing). But the rivets holding the lining material had worked loose, allowing the friction lining parts to wiggle around on the clutch disk. Using a real competition clutch disk may well alleviate this problem. Also the heavy steel release thrust ring on the pressure plate was a little loose, having worn the retaining eyelet a bit to allow that part to wiggle a little. So it was time to replace both the clutch disk and the pressure plate.

When the clutch lining comes apart in use it may double up the layers, and that will prevent the clutch from disengaging. Then I get to drive it home with no clutch, and pull the engine to install a new clutch disk. That did happen once before using the competition pressure plate, and once again later, but not during the three years while using the competition pressure plate. The last time it happened on the first autocross date after installing a new clutch disk. Since then I use only the common major brands of clutch disk, stay away from cheap aftermarket clutch disks (NAPA, Car Quest, etc), and no more problem with delamination.

Being cheap (and no longer plagued with the oil leak) I installed another stock MGA clutch disk and stock MGB pressure plate. The stock MGB clutch will spin the skinny street tires easily with power shift to 2nd gear, and just a little bit going into 3rd gear. It will also spin the sticky race tires quite well going into 2nd gear, but will never spin the race tires at all going into 3rd gear, no matter how aggressive I am with the power shift. The MGB pressure plate is nice for street use and mild competition. Luckily I don't use power shift into 3rd gear very often for autocross, so this is not a problem for me.

For competition road racing where power shift to 3rd and 4th may be common, the competition pressure plate must be a good thing to use to prevent slippage and excessive wear on the clutch disk. Otherwise you would have to "baby" the clutch some to prevent damage, which is in my book an unacceptable compromise for competition. For no holds barred racing the competition clutch disk is probably a good idea, as it may hold up better and be less likely to disintegrate from failure of the friction lining rivets. For serious street use and occasional mild competition, the stock MGB clutch works quite well, and the competition pressure plate may be more of a pain than it's worth.

My current warmed over street engine (skip the details) makes a just a smidge over 100 BHP. I run it fairly often to 7000 rpm, and it's almost unnatural to shift before 6000 because it breaths well. I have had this engine in the car for nearly 4 years and over 30,000 miles, driving quite aggressively with the stock MGB clutch including some serious autocrossing, and no problems at all.

So I can heartily recommend the stock MGB clutch for street use and mild or occasional competition use, and the lighter MGB flywheel is nice also. I don't think another 10-20 BHP in a stock or near stock Twin Cam engine at high speed would make much difference for the clutch. The important factor is the peak torque seen during a power shift, in which case the flywheel inertia is much more than the engine torque output. When flywheel inertia and clutch torque can spin the tires, you have enough clutch. The only reason to upgrade to a competition clutch is if you want stronger clutch torque to handle power shifts into the higher gears during regular competition use. And of course my all important factor, the MGB pressure plate is cheaper than the MGA pressure plate.

Hope this helps with your decision.

Barney Gaylord