|The MGA With An Attitude
CLUTCH FUNCTION - Practical Experience - CT-202
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 The newer pressure plate with stronger springs superseded the earlier part, so all replacement parts since February 1957 should have the stronger springs.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.