Engine and Transmission Tech

Hi all,

I’m rethinking my T5 combo for my 4.6L RV8 conversion, and I’m hoping to prompt a discussion on ideal clutch size for big engines in LBCs.

The transmissions for the engine swaps on this forum are often found in donor cars with 10” or 10.5” clutches (camaros and mustangs), but the cars tend to weigh 1.5x or 2x they typical MG or Triumph. Surfing the internet for clutch and pressure plate combo weights, it looks like an OEM 10 1/2” clutch kit can weigh 6-8lbs more than an OEM 9” clutch. (Comparing 24lb 1980s Camaro OEM kits to 16lb 1990s Chevy S10 OEM kits, all with clutch and pressure plate.). Higher performance aftermarket camaro and mustang clutch kits can weigh even more.

I suspect that 10.5” clutches are overkill for most LBC v8 applications, due to much lower total vehicle weight, less driveshaft, axle, and wheel weight, and narrower tires.

As others on this forum have said, too much clutch grip can place excess stress on the diff, trans, or engine, and would be wasted entirely if the tires break loose long before the clutch starts to slip. In this sense, excess clutch capacity equals unnecessary flywheel mass and unnecessary clutch pedal pressure. On the other side of the equation, an undersized clutch will slip before the tires will.

So my question for everyone is, in the typical LBC v8 application with 250-300 ftlbs of torque driving relatively narrow 215-or-less tires, how much clutch does the car actually need?

An aluminum flywheel can drop 10 lbs of rotating mass, but a similar gain could be achieved at less expense by switching to a smaller clutch. Doing both might work wonders.



Edited 1 time(s). Last edit at 11/27/2017 10:35AM by RDMG.

I know Carl, on this board uses a S10 pressure plate. 9 something diameter. You would have to redrill your flywheel to that pattern. You could use the 93-95 Camaro V6 clutch disk for the 26 spline Chevy input T5. Ford Turbo 4 clutch disk for 10 spline T5. I like the light billet steel flywheel, kind of inbetween the stock and aluminum flywheel. LSC built one for Scott's LS4.

My experience with big Rover power in little cars is you need the extra clamping force the bigger disc provides. A buddy of mine had Ted build him a custom clutch last Spring and he loves it. I'm going to be putting a 4.6 into a TR8 coupe real soon and the plan is to call Ted and have him put together the same clutch and an aluminum flywheel. Not a fan of the center force II clutch thats out there. Its heavy, real heavy. Seen several of them cause the pivot pin pop right thru the clutch arm. If you do go with one of those, make sure you reinforce the pivot on the clutch arm. I have no experience with the new Spec clutches in various stages available now, but I'd love for somebody to guinea pig them for me.

Clutch weight is a factor but so is clutch life, pedal pressure, and of course ultimate clamping pressure. Any choice is a compromise between between those factors and more. Even physical size plays a critical part and so does driving style. If you are inclined to let the clutch out slowly with plenty of throttle, you may need to consider a larger clutch than if you let it out quickly with minimal throttle.

Any clutch that you can physically fit in will work. How comfortably and for how long is the real question. If you tend to baby the clutch a 9" disc may be perfectly fine. But, any slippage under any condition will shorten the life of the disc and can in some cases shorten it quite dramatically.

Finding a disc with the correct spline count can also be quite a challenge. Some transmissions may severely limit your options.

But in general it seems that the 10-1/2" is adequate for all but the more extreme builds. We do have an 11" in the MG-Roadmaster but that's a 455 in a 2700lb car and it is expected to last pretty near the life of the engine in spite of a fair amount of abuse.

I run a 10-1/2" behind my 5.7L SBB and expect it will hold up pretty well. A 9-5/8" should be OK if you don't abuse it, and are not averse to maybe having to replace it at some point.

A lightened steel flywheel around 20lbs or less is a very good choice. I think it is quite reasonable to think you can get down to 18lbs with billet, maybe as low as 16. With aluminum on the street, a steel insert or ceramic hardface is a necessity. I'm not sure how durable the ceramic is.

Be sure to allow for wear. When fitting it all up it is a good idea to bolt the pressure plate on without the disc and check freeplay. This is the travel you need to allow full use of the clutch facings.

Jim

I’ve pulled the weight and torque specs for some of the possible clutch kits for GM and Ford-pattern T5s from the Phoenix website (take with a grain of salt), and referenced them with the engine torque and car weights for the cars they came from:

Third-Gen Camaro 305ci V8, 240lbft engine in 3200 lb car. 10.5” dia, 26 spline clutch:

OEM: 24lb 310lbft
Stg 1: 26lb 390lbft
Stg 2: 26lb 430lbft

Fox body mustang 302ci V8, 300lbft engine in 3300 lb car. 10.5” dia, 10 spline clutch:

OEM: 26lb 265lbft
Stg 1: 28lb 325lbft
Stg 2: 28 lb 390lbft

Fourth-Gen Camaro 3.8L v6, 225lbft engine in 3300 lb car. 9.75” dia, 26 spline clutch:

OEM: 18lb 225lbft
Stg 3: 19lb 325lbft

Mustang SVO/T-bird turbo I4, 240lbft in 3100 lb car. 9 1/8” dia, 10 spline clutch:

OEM: 17lb 250lbft
Stg 1: 18lb 295lbft
Stg 2: 18lb 355lbft

All the above cars weigh 1.5 times an LBC, so one can expect the max torque ratings to be conservative. Arguably, you could multiply all torque ratings above by 1.5 to get an accurate figure for an LBC application. F=ma.

If the 9.75” clutch from the 4th gen Camaro bolts up to the stock GM flywheel pattern for the 10.5” clutch (I think some have said only a few pressure plate holes needed to be elongated?), then I would argue that a stage 2 or 3 version of that clutch is the largest clutch any LBC would need. Save 7 to 10 lbs of rotating mass without buying aluminum flywheel or milling anything.

Clutch force is not only a product of the area of the friction surface. You need to take into consideration coefficient of friction of the surfaces, the clamping force and the fact that force applied farther around from the center has a greater effect than that closer to the center. Lots of race cars use 7.5" clutches. You can make a smaller disc work and save rotating mass, but at what expense? I'll say it again, my experience with big Rover powered cars is you want the bigger disc. You want to stick with a smaller disc, then plan on changing friction material and finding a cover with more clamping force. Thats the route I tookhb on my TR8 race cars, but wouldn't go that route on a street car. The car weights and power are a nice touch, but have little to do with it. All comes down to can you under acceleration use all of the power your engine is making and transmit that power to the wheels without spinning the tires. If you can, then your clutch is seeing the max amount of torque your engine can produce and it doesn't really matter if its pushing an ant or a building.



Edited 1 time(s). Last edit at 11/29/2017 06:28AM by tr8todd.

Hi Todd,

I hear you on your experience behind the wheel, and that counts for a lot more than my armchair “engineering.”

That said, I think vehicle weights and power have everything to do with it, and I’m hoping to understand what I’m missing?

The above clutches were spec-ed by engineers to provide enough friction and clamping power to propel a heavier car with wider tires, using the same or similar engine power.

From my armchair perspective, if you drop the weight by 50% or so and keep other factors (tire grip, rear gearing, engine power, weight distribution, suspension type, etc,) generally equal, then the total torque the clutches should be able to support should go up by about the same 50%. With that in mind, any of the above clutches should provide more than adequate friction and clamping power, and the lighter clutches provide a benefit of lower rotating mass, and lower polar inertia.

As a thought experiment, if you take a stock 3300 lb camaro and somehow cut 1100lbs out of it to reduce mass by 50%, you should be able to increase engine torque by 50% and the stock clutch will still behave similarly. Similar results if you drop a v8 into an LBC.

How about looking at other cars with similar weight, power and size, like maybe a Z4 or NSX, then comparing what their factory setup is to the GM stuff. That way you benefit from their engineering.

As Jim N. mentioned, I did use a S10 pressure plate along with a Mercedes clutch disk.

I originally bought a 10.5" clutch & PP for a Camaro. After having a friend shave some weight for my McCleod steel flywheel, I could not bring myself to bolt on all that unnecessary weight. The TR8 uses a smaller clutch. Since, I have a Buick 215, I did the same. Still getting it done after almost 17 years.

Work is work. Doesn't matter if the engine is pushing a small car fast or a large car less fast. The amount of work is the same. Larger clutches will last longer, be easier to slip and get the car rolling in first gear without producing excess heat, and will clamp better when work is being done. Race car, small is good. Street car, bigger is good.

Dan B, excellent idea! I found these clutch specs for a BMW Z4 and a TVR Griffith:

BMW Z4 s35i, turbo 3.0L engine, 295lbft engine in 3200lbs car, 9 7/16” clutch, 26 splines (pricey)

TVR Griffith 450/500, lots of RV8 torque in a 2300lb car, 9.5” dia, 23 splines. (Details from TVR Parts Ltd website).

Given the similarity of mechanicals and weight in the TVR, I’m impressed that it has a 9.5” clutch. Probably was engineered for the 4.0L version, then the engine grew around it. Given Jim B and Todd K’s observations above, I wonder how quickly it burned up in the 5.0L version.

I found this centerforce doc that collects clutch bolt patterns for several automakers, I thought it might be useful to have on reference:

[webinjected.blob.core.windows.net]

I’m thinking an uprated 9.75” clutch may be in my future. Maybe a 10.5” after that! Many thanks to all for the input.



Edited 1 time(s). Last edit at 11/29/2017 08:59PM by RDMG.

My TR6 had a TR4 clutch. Light, could push it out by hand, but less long-lived. However, it was very nice to drive, light.

Ran a Renault 5 GT Turbo for several years in the 80s, that clutch was heavy. In traffic it was a real pain. At the end of the day, you have to enjoy driving the car or it's all a waste of time.

When the clutch starts to slip, it shows up in top gear, right? You accelerate and you hear it slip. So vehicle weight is not the ultimate determinant, it's torque. Big engine big clutch. Keep it simple.

As to flywheel weight, this was an interesting thread
[forum.britishv8.org]

Ivor

The 9-1/2" clutch will work but your driving style will determine how long it will work. As Todd suggested, the real test of a clutch comes when the engine is at peak torque regardless of the gearing or vehicle weight. Slippage is usually most apparent in top gear but with OD transmissions it can occur earlier. By the time this is noticed the disc is cooked.

Conversely, most clutch wear usually occurs in 1st. So it should be clear to see that here is where vehicle weight and gearing comes into play. However, slippage in any gear at any point will cause accelerated wear of the disc, and the danger point is at the torque peak. If the tires are not slipping, full engine torque is being applied to the disc regardless of the gearing or weight. As long as the clutch rating is sufficient it will not slip and wear here but as the disc wears the capacity does go down making slippage more likely.

So again, it depends on how you drive. If you never use full throttle you have very little to worry about. If you never rev to the torque peak ditto. But if you do, and regularly, things might be a bit different. Also since the smaller clutch will wear faster under any conditions, capacity will fall off earlier, slippage will occur sooner, and replacement will be more frequent.

Simple enough. If you want to put off disc replacements, use the larger size. If a small improvement in revving ability means that much to you, go with the smaller one. You can always change later if it doesn't work out.

Jim

Two more for the comparison chart;
2016 BMW 330d weight 3400 lbs only 255 bhp but 413 ft/lbs torque at 2000 rpm 91/2 clutch
2004 Mercedes 4 ton van 270 ft/lbs torque 91/2 clutch
If a 91/2 clutch is man enough for these two then it should be ok in an MGB. Although the Mercedes van has no more torque than a 4.6 Rover v8 it has to get a 4 ton van moving and typically many more times a day than the average car owner will do. While the BMW has a healthy dollop of torque for a 11/2 ton car. I know it proves nothing but it would suggest that a 91/2 clutch of a good spec should last a long time.
Philip

Of course, we need a big enough clutch/PP to do the job, but IMO going bigger/heavier than needed is not good either. I am very happy with the lighter weight flywheel/clutch/pressure plate combo that I chose.

Puts me in mind of the guys that insist the 7.5" S10 axle is good enough. For awhile it was, and now reports are starting to come in about failures. When you move into the upper part of the curve that's going to happen. The remedy is to either use a less powerful engine, replace the parts more frequently, or upsize. Your choice to make. Driving style of course makes all the difference.

Jim

Colin Chapman school.