What are the benefits for bigger or smaller clutches?

 

centripetal forces

 

And the obvious - More surface area

 

GM`s horsepower engines come factory equipped with a light weight 13 lb, flywheel and a 10,5 diaphragm pressure plate and appropriate disc for there high winding 427`s. Smaller circle and they rev quick. The 11 in. heavier assembly is used on both BB and SB basic engines. There are physic issues that makes the large assembly's more beneficial than the lighter smaller units for some purposes. Under most racing conditions the lighter smaller units is hard to beat.

 

So what would be best/recommened for a 454, for street and the occasional sprint application.

 

On my 69 427ci 435hp I run a Brute force 10.5" 10 spline clutch set up it does a great job as far as race ablity lets just when I get my harley the front wheel has a hard time staying on the ground 50% of the time .I dont need to tell you how I drive a car do I

 

If you have enough torque multiplication in first gear (first gear ratio times rear end ratio is greater than 9), then a lighter/smaller diameter flywheel will be desirable for quicker acceleration because of its lower inertia. With lower inertia, less horsepower is required to accelerate the flywheel to the higher rpm - this leaves more horsepower available to accelerate the car.

I ran some numbers a couple of years ago and compared a 30 lb stock small block flywheel for a 10.5 inch clutch versus a 15 lb flywheel. From memory, with an acceleration rate of ~2,000 rpm per second, the lighter flywheel was worth about 10 horsepower. The benefits of comparing a flywheel for an 11 inch clutch to the 15 lb GM flywheel (10.5 inch clutch) for the 454 will be even greater.

On the other hand, if you have a close ratio Muncie with a 2.2 first gear ratio and a 3.35 or numerically lower rear end ratio, you will be a bit short on rear wheel torque for starting from a standstill and will benefit from the larger & heavier flywheel with the 11 inch clutch.

 

It's true there are physics involved which is why a smaller diameter clutch will be capable of holding great power. When I was talking to the tech guys at Spec Cluthes, I noticed that my chart of their clutches lists the 10.5" setups as stronger than the 11". For the life of me I couldn't figure out why so I asked them. The way it was explained to me made sense......
"Applying the same amount of pressure(clamping force) to a smaller surface, yeilds a greater holding power" That is why their 10.5" is rated higher than their 11"

Hope this helps.

Richard
Tech Support

 

WOW never thought of it that way.

 

Neither did I. I was the kid in science class, in the back of the class, sound asleep, so their expanation took me a minute to digest. It makes total sense. The other benefit of going smaller, is less weight, which most hot rodders, are always looking for.
I would always try to go with a 10.5" clutch for these reasons.


Richard
Tech Support

 

Does a 10.5 clutch require more effort on the pedal?

 

The effort in the pedal has more to do with the design -PP fingers and TB and the adjustments. I just put a 10.5 brute force ZOOM clutch in my 69 427/435 everyone said they were hard to press in and out.BS man it may be easier to push in the CF but this is no harder to press in than the hand clutch on a harley is to pull in.


The picture of the clutch you posted is just fine. However I would try to buy it locally just incase.

 

Be careful with what you say. Applying the same force [lbs] to a smaller surface is going to yield a higher pressure [psi]. 11" vs. 10.5" gives a difference in area of 9.8%, so the pressure on the 10.5" clutch is going to be 1.098 x the pressure on the 11" clutch.

Not trying to be a jerk, just clarifying.

 

Brian at SKSpeed (the guy who built my transmission) said all the following. The aluminum flywheel will rev up faster, but it has less inertia, so it will hurt gas mileage and hurt driveability (especially at idle). He said the 35 lb flywheel is best for any street application. If the Aluminum does not have kevlar, it will also wear faster. The clutch plate itself weighs nothing and makes no difference. I have 11", 10.5" (which is really 10.4"), 10 spline, and 26 spline). I currently have a 10.5" 26 spine. It will engage with 2600 lbs of force as will all the stock ones. Does your clutch ever slip out once it is in gear...or fall out of gear? Once it engages, it engages. 10.5" will last just as long and hold just as much power. This guy has been testing these for years.

 

A lighter flywheel won't hurt mileage, in fact it will help it. This is analogous to removing weight from a vehicle - less energy is needed to accelerate, and as a consequence, less energy is wasted in the form of heat when stopping.

 

It's true. I hope he meant that it will help gas mileage, because if he didn't you should look to talk to someone else next time...

 

Heavy flywheels control idle better. If your piece is idling ok at approximately 800-900 and runs great it was not worth mentioning. Also these are not John Deere tractors plowing fields at 100 rpms and a 100 lb flywheel. We Put 400+ HP and higher up against a auto trans with a 2-3 lb flex plate and run great. No idle problems. Gas mileage, dont know but that would be controlled by how hard you press on the gas. Plus anyone really care. Todays prices to pay for the quest for power.... .

 

I'd have to disagree with that logic.... To a point it may make sense because the PSI rating is higher...on the smaller diameter clutch. So what would happen if your clutch was 3 inches in diameter with the same pressure? Would you think it would still hold? Would it last?

Look at a Tilton 7" clutch. Notice that these clutches are multi - disc and run with flywheels of a smaller diameter. Why do you think that design works rather than a single 7" disc on a stock flywheel? You can rate it higher in PSI which it true ... but thats not the complete picture....

Paul
http://www.5speeds.com

 

There is a fault with this logic. With Brian means about that statement is cruising speed.... The inertia of a heavier wheel maintains cruising speed with less effort than a lighter wheel.. Acceleration isn't the only concern because just as fast as it accelerates...guess what? It decelerates. So you are feeding the gas more to maintain a cruising speed.

 

Yes, physics is important, but it has to be applied correctly. If one assumes that the coefficient of static friction is linear, the shear force at the contact surface will be a function of the normal force applied by the pressure plate regardless of the contact area; therefore, for pressure plates having the same normal force, the shear force will be unchanged.

However, and this is important, the 10.5 inch clutch has a smaller average radius of contact, AND, since the clutch's torque capacity is a function of the shear force times the average radius, the torque capacity of a smaller diameter clutch is less than that of a larger clutch. But, why is the 10.5 inch clutch rated for a higher torque? Read on...

Smaller diameter racing clutches, such as used by GM with the L88, have higher torque capacity because the static pressure of the L88 pressure plate (3,000 lb) is significantly greater than that of a standard performance pressure plate, which will be approximately 1,800 to 2,400 lb (for lighter pedal pressures). This is why the smaller clutch has a higher torque capacity (and a lot higher pedal force)! In the case of the really small competition clutches, the torque capacity is boosted still further by adding multiple disks and idler plates.

As regards to 5speeds comment about flawed logic, "So you are feeding the gas more [with a lighter flywheel] to maintain a cruising speed" is flawed because the power required to maintain constant cruising speed is independent of the size of the flywheel. If the mileage were better, we could reduce the size of the gas crisis by using 700 lb flywheels! NOT!