5.5 inch clutch, pass or fail?
 

I am thinking about clutches again, and have decided that I would like to pony up for a twin disc. 949 obviously has a 7.5" clutch, but I know there are 5.5" clutches available.

Can anyone tell me why this would be a horrible idea. I want to lighten the rotating assembly, but do not want to knife edge the crankshaft. I thought this was a good compromise, but still put the fear of God in my competitors when I blip the throttle.

Discuss.

We use 5.5" clutches regularly on race engines (mostly Tilton or AP) without issue. Bear in mind that none of them that I know of are designed with any normal street driving in mind and may not last too long if slipped much (like starts/stops).

A twin disc 5.5 Tilton OT-III clutch with sintered metallic discs (our normal choice for road racing) is rated for 500 lb-ft of torque which ought to be enough. The cerametallic and carbon discs are better for modulation/slipping according to Tilton so that may be what you'd want to run.

Bring a large checkbook...

The 7.25 is probably going to be significantly more streetable than the 5.5 setup.

There's not really any need to go to 5.5 for a street car when you consider the entire clutch and flywheel for the Tilton 7.25 twin disk weighs about 16-17 lbs. Mine revved like a motorcycle.

If youre not turbocharged, you probably dont need 7.25" twin plates. Just singles. And you can buy various force pressure plates to pick what gives you the right amount of headroom without ridiculous pedal pressure.

I'd go with the rallye style no matter what you do if you plan to put it in a street car. The extra mass will make a significant difference in clutch life.

Our twin is under 15 lbs complete - clutch, discs & flywheel. The singles are a shade under 12 lbs. Makes a 100% stock engine rev like a bike.

Also look into a SOHC protege crankshaft when building your motor. Its not forged, but its 10-12 lbs lighter depending on who you ask, and proven to 300hp. I've been spinning mine to 8k on occasion since 2006... that with the 7.25" setup would be the bomb.

I'm just curious as to why/how you would street drive a clutch like that.. It'll be shotgun starts every time you hit a light or stop sign. Maybe my perception is off here, I've been in a car with the lovely 949 twin disc, but something lighter than that even? not what I think of when I think streetable. It seems pretty awesome, otherwise

My car is not exactly a street car. Maybe an occasional romp.

I want to keep the forged crank. On nitrous I should make well over 300 hp. Shooting for 450.

On the motor, I am shooting for the lowest no compromise ETs with my budget. I figured that this was a step I had to take to meet my goals.

Considering what I have said would you give me the same advice? I am not ruling out the larger clutch just yet. I am a little concerned with staging issues with the 5.5". Are they that jumpy?

If you're talking about their engagement, it depends on which discs you have to an extent but they're all designed primarily for road racing/rallying with dog engagement boxes and so don't have much modulation. Starting off from a stop is something you don't normally optimize in those situations since the intent is to only do it once per race/stage.

If you want to experiment with expensive parts to find one that works, you can probably find a 5.5" combo. If you have a more limited budget, I'd buy Emilios 7.5" twin disc setup, it revs VERY fast and is a proven piece on Miatas.

Thank you Rennkafer. I think that is good advice. I am on a budget, and do not really want to risk having a high dollar clutch that will not really work well staging and launching.