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MG MGB Technical - BHP Rating of MGB Clutch assembly?

Hi Guys,

Could someone tell me what the BHP rating is for the standard MGB clutch assembly please?

Pressure plate GCC118
Friction plate GCP116

Many Thanks
Mark
Mark Hester

Mark-
The Original Equipment Borg & Beck clutch should be capable of handling the power of a modified engine, but you may find that its lifespan is compromised more than you would desire. Of course, there are heavy-duty clutches available for the MGB. These are readily identifiable by the thicker coils of their take-up springs in the clutch driven plate that are wound at a shallower angle than those of the take-up springs found in the Original Equipment clutch. However, it should be noted that almost all of them were originally designed for use in Sherpa delivery vans. Yes, this transmission was in fact designed to also be used in delivery vans! That is why they last so long in our light little cars. These heavy-duty clutches make use of a more powerful diaphragm spring and hence will not only hold the clutch driven plate against the flywheel better in order to handle the increased power output of the engine, but consequently will also increase clutch pedal pressure and will accelerate wear of the carbon throw out (release) bearing. The increased resistance of the diaphragm spring makes the installation of a braided steel hydraulic hose (flexible pipe) a wise move in order to contain the increased hydraulic pressure required in order to actuate the clutch without the bulging of the rubber hydraulic hose of the Original Equipment hydraulic hose (flexible pipe). The flat springs in the cover can be thought of as a series of levers with one end attached to the pressure plate, pivoting in the middle on a large circlip that is attached to the cover, and the other end at the carbon clutch release bearing. Due to the MGB weighing less than the Sherpa delivery vans with their one-ton cargo capacity in which these clutches were intended to be employed, plus the take-up coil springs in the clutch driven plate being stiffer, some of these heavy-duty clutches tend to feel “grabby”, many engaging almost like an on/off switch.

There is a better alternative: simply replace the Original Equipment 8” clutch driven plate with the 8 ½” (215.9mm) clutch driven plate used in the Triumph TR7 (Roadster Factory Part # GCP253). Its splines are identical with those of the original MGB clutch; thus it will fit without modification. Having been designed to be used with a more powerful engine, its greater friction surface area will ensure all of the grip that you will need. Because of its compatibility with the Original Equipment pressure plate, it results in a smooth, light clutch possessing good “lockup” and reduced slippage under severe load conditions without putting increased stress on the clutch throw out (release) bearing and the crankshaft thrust bearings as a high performance clutch does. When used in Original Equipment specification engines they tend to last 120,000 miles, which is considerably better than the 80,000 mile life expectancy of the Original Equipment clutch.
Stephen Strange

Thanks Steve,

I'm trying to determine at what point I need to consider upgrading the clutch systems capacity.

i.e. What BHP output will cause the standard clutch system to slip.

I'm sure there must be some specifications out there somewhere.

The MGB friction plate GCP116 is also 8 1/2".
Are you saying the TR7 one has more friction material with the same outside diameter.

Would you have a photo of GCP253 as all I can find on the web is this:

http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=2487&SortOrder=12

Thanks
Mark

Mark Hester

I have a TR-7 disk on my B/GT and it fits and works perfectly.

The stock MGB clutch disk is about a 1/4" shy in diameter than the friction surface on the clutch plate (estimate from memory).

The TR-7 disk is slightly larger in diameter than that of the stock
MGB and utilizes the entire friction face area of the MGB clutch
plate.

With the TR-7 disk in place, the clutch feels as though it wants
to grab on (less slippage) during launches, but this is easily
compensated by moderating your foot action.
Daniel Wong

I bought and installed the Borg and Beck heavy duty clutch kit in the early '90s and it's still going strong. This was when you could still get a roller throwout bearing. I added the Doug Jackson kit that allows for the throwout bearing to clear the pressure plate completely when not in use. This extends the life of the throwout bearing to that of the clutch itself. The actuating pressure of the pressure plate is almost identical to the original unit and it easily handles the torque of my supercharged engine. What the exact numbers of its load capacity are I don't have access to. RAY
rjm RAY

Hi Guys,

I take onboard what everyone is saying on how easy it is to upgrade the capacity of the standard clutch system and I will probably do this anyway but my initial question still stands.

At what point will the standard MGB clutch start to slip.

120 BHP ?
140 BHP ?
160 BHP ?
180 BHP ?

Also an older clutch's performance will drop over time, so whilst it may deal with say 150 BHP from new I imagine over time as the springs will fatigued and it’s capacity will erode back to say 130 BHP. So some additional margin is required.

Does that sound correct?

I need to purchase a clutch system to get my crank balanced for a track day 1924cc twin cam engine so have nothing and need to buy everything.

Cheers
Mark


Mark Hester

Hi Mark,

My S/C BGT has circa 180 BHP and i have run it on a heavy duty standard type clutch with no slip whatsoever including track day...it also has a limited slip diff so this will load the clutch more due to increased traction.

Ian
I K Jackson

It is not BHP that makes the clutch slip, but torque.
High revving engines can have high BHP but get away with a small clutch.
Dirk/Antwerp
D. Van Ussel

When I started this thread I didn’t realize how difficult it would be to get a simple answer.
I’ve emailed www.borgandbeck.com which new redirects to firstline with no response.
I’ve also emailed www.apclutch.it which now supplies the standard clutch kits to people like Moss.
Nowhere does it appear that I can find what the torque capacity of the standard MGB clutch system is. Thus how do you know when you need to upgrade.

MGOC offer AP Racing clutch kit which they say is good for 176NM (approx 130 ft lbs) of torque but this appears to be a custom kit for themselves so I can verify their claims. For £150 this seems reasonable
http://www.mgocaccessories.co.uk/acatalog/MGOC_Accessories__Tuning_37.html

AP Racing themselves offer part number CP2000-3NB w which is capable of significantly more torque at 224Nm or 165 ft lbs. I’ve seen this on the web for about £180
http://www.apracing.com/info/products.asp?product=+MGB+Tourer+and+GT_1434_1426

Then there are the various options listed by Moss & Cambridge. Where the prices increase but no torque capacity is stated.

http://www.cambridgemotorsport.com/MGB-MGB-Clutch-And-Transmission

For someone attempting to build a performance engine I’d have to say the information you need is somewhat difficult to find.

Obviously I’ll a little more clued up not as I initially though it was a BHP not torque rating !!!

Hope this info helps someone out there.

Cheers
Mark






Mark Hester

It's torque from acceleration that puts pressure on the clutch, especially in top gear, not BHP. BHP is simply a measure of 'work done'. The torque will be almost zero with any BHP you care to name, if all the engine is doing is causing the vehicle to maintain a constant speed on the flat. If you are going downhill maintaining a constant speed the torque will probably be negative.
PaulH Solihull

I looked into this when I had my engine tuned. I decided to stick with the standard clutch and it has been fine for the 10000 miles or so I have done in the car. I got 93 BHP at the wheels on a rolling road. The extra tourque of the engine means you rarely need to slip the clutch.
Stan Best

Slip the clutch?
PaulH Solihull

Yes, as in slip the clutch to get moving. With high torque, you don't need to slip the clutch as much to keep the engine from stalling. I used to have a small diesel pick up truck, and with that you could just let the clutch out at an idle, and then hit the accelerator. The original clutch still acted like new at 185k miles, which was the last time I saw the truck. In this context, not slipping the clutch would be like using it as an on / off switch.

Charley
C R Huff

I've never needed more than about 1500rpm and the clutch is fully out by the time it is rolling at 5mph or less, thereafter using the torque, that has always been the beauty of the MGB.
PaulH Solihull



It turns out that the specially made for MGOC clutch kit - 176NM (approx 130 ft lbs) that I reffer to above is nothing more than the age old special tuning heavy duty clutch kit KT9679. At least I can not attribute a torque rating to this clutch kit as the manufacturers would not supply this.

Hope this helps someone
Mark
Mark Hester

This thread was discussed between 30/07/2010 and 01/09/2010

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