Change Clutch for a 3.2 to 3.4L conversion?

[polizei]

Hey Guys,

I'm hopefully converting my '87 3.2 Carrera to a 3.4L street performance car. My transmission is a stock G50 with LSD. I replaced the clutch disc, pressure plate and throwout bearing roughly 5K miles ago. According to Paul Frere's book the max torque rating of my G50 is 221 ft-lbs.

From reviewing similarly setup 911s, mine should produce between 200-215 ft-lbs at peak. All torque projections are within range, but on the high side it's getting close. Should I be concerned about the clutch assembly wearing out prematurely? Or am I OK?

I plan to run 1 or 2 DE events per year. Otherwise, the car will be used on the street. It'll be a convenient time to upgrade the clutch with the engine out, but, at the same time, I'm trying to spend my dollars wisely!

Thanks,

[ttweed]

Originally Posted by polizei

Should I be concerned about the clutch assembly wearing out prematurely?

How fast the clutch wears is dependent on how you use (or abuse) it. If the one you have has only 5K miles on it, measures within spec and shows no damage on visual inspection, I wouldn't replace it in the overhaul. The mild increase in displacement and torque you are planning shouldn't make much difference in how fast it wears if you refrain from slipping it a lot on full-throttle "dragstrip" starts off the line or speed-shifting w/o throttle lift, etc. If you treat it reasonably gently and only use as much torque as necessary when it is released, it should wear the same as before. It's not like you're dropping in an LS3 w/ 400 ft. lbs.

I plan to run 1 or 2 DE events per year.

A DE is not a timed event, there is no prize money on the line, and if you resist the tendency for the "red-mist" to arise when chasing your friends around the track and maintain empathy for your equipment, you should be fine.

YMMV,
TT

[stlrj]

Clutch wear is all about throttle and slippage. If you use lots of throttle with lots of slippage when taking off, resulting in a burning smell, it might last a year. But if you use a minimum of throttle and slippage when taking off, it will last you a life time.

[KTL]

The G50 clutch is rated to handle much more torque than even a 3.4L can produce. I don't have the spec book handy to look up the exact number. Nonetheless i'm confident you'll be fine, subject to the caveats that the other guys mentioned- your technique!

[m110]

When replacing the G50 stock rubber clutch with a spring center unit it is actually a 930 part, thus I would think easily up to the task of a 20% increase a 3,4 affords. I am not an expert on this.

[KTL]

The 3.2 G50 240mm clutch has 7400-8200 N of contact pressure. This is actually a bit less than the 915 has with the 225mm clutch, which is 7800-8500 N of contact pressure. By comparison the turbo clutch, which must handle a lot more torque, is rated at 9500-10300 N of contact pressure. This info is from the 84-87 911 spec book.

Also note that the clutch disc that is a suitable replacement for the 3.2 G50 is a 930 disc from the 75-77 930. The later 78-89 930 discs are rubber-centered like the original 3.2 G50 disc.

[Jaskas]

Hi KTL!

Do you have the exact partnumber for the 930 disc? What pressureplate shoul one use for this solution? Stock g50 or the 75-77 Turbo ?

[KTL]

Well for a standard/factory installation, the stock G50 clutch is the one to use. The 75-77 clutch is a different height, due to the short bellhousing 930 trans. If you're asking about a clutch for your '87 with 3.6 installed, the stock clutch does not work.

The 75-77 930 disc is currently listed as this one

Pelican Parts - Product Information: 930-116-014-02-M38

[spuggy]

Originally Posted by KTL

The 3.2 G50 240mm clutch has 7400-8200 N of contact pressure. This is actually a bit less than the 915 has with the 225mm clutch, which is 7800-8500 N of contact pressure. By comparison the turbo clutch, which must handle a lot more torque, is rated at 9500-10300 N of contact pressure. This info is from the 84-87 911 spec book.

When comparing numbers, be sure you're comparing apples to apples (like Kevin is here).

When I was looking for a 915 clutch to hold 350 ft/lbs at the wheels, a transmission guy told me that Sachs specs have a 30-40% safety margin. But that many aftermarket suppliers tend to use a relatively small margin, so their numbers are much closer to what they will actually do - they thus can't be directly compared.

Given that a 915 Power clutch (stock clutch, just cherry-picked to be in top 10% of the batch) only gets marginal north of 300 ft/lbs at the flywheel, I wouldn't have much concern over a 3.4 N/A on a G50 clutch.

I think the factory 75-77 930 friction plate is regarded as adequate for street to 500 ft/lbs (and more), with a suitable pressure plate. Seems like overkill for N/A.