I'm afraid I'm going to seem like a snob, but that's not my intention. When reading about the ZF-6, I am a bit surprised when people state that the synchros tend to wear, or that the transmission needs to be sent to the ZF Doc for a rebuild. I understand that it is a very tough transmission, so I assume that the ones that wear out are being abused. Is that correct, or does the transmission have some weaknesses that arise even if it is treated carefully? I have been to Bertil Roos' road racing school six times, so I know it's possible to handle a transmission well while still driving fast (and yes, I have made my share of mistakes while downshifting). Of course, drag racing must be especially hard on a gearbox ...

 

You may have read this before, maybe not. This is from ZFDOC

QUOTE:
Larry,

ZF Industries put a very conservative capacity rating on both the 610Nm and 540Nm rated S6-40s.

The 540Nm (400 lbs ft) version will easily handle up to 600 lbs ft of torque if built correctly.

The 610Nm (450 lbs ft) version will easily handle up to 800 lbs ft of torque if built correctly.

The only setup that may break the 540Nm transmission is putting a 150 straight shot No2 system on a 450 rwhp naturally aspirated motor where the No2 is not progressively (computer transitioned) introduced but just slams the extra 150 hp into the system.

The weak point is how the 540Nm unit’s synchronizer clutch body is affixed to the 2nd speed gear.

It is spline-fit and crush-crimped into position where as the older 610Nm unit’s 2nd gear synchronizer clutch body is welded into place. All of the 610Nm synchronizer clutch bodies are welded into place.

The 1st, 2nd, 5th and 6th speed gears on the 540Nm type S6-40 are spline-fit/crush-crimped but only 2nd gear sees the torque leverage anywhere near that required to distort/unseat the crimp. When this happens, the clutch body ring ruptures into 5 or so pieces, most of which get punched through a 4 inch hole out the side of the case turning into a $3000 repair job.

As you can imagine, this is a very ugly type of failure.



If you don’t plan on adding a crude 150 straight shot No2 system to your motor then the 540Nm rated ZF S6-40 unit will work very well for all of your engine power to driveline management needs.



See Ya,



Bill Boudreau

ZR51 Performance

Cave Creek, AZ

(602) 319-6575



From: Bill Boudreau [mailto:billb@zfdoc.com]
Sent: Monday, June 14, 2004 9:17 PM
To: 'Larry Dickey'
Subject: RE: 4+3



Hi Larry,

Sorry it took so long to reply, been very busy lately which is a good thing.



GM is clearing out the last of their ZF S6-40 transmissions from inventory.



If you are at all serious about converting over to the ZF S6-40, I strongly recommend that you contact your local GM Parts

Department and purchase one of these transmissions a.s.a.p. P/N 12455189 (ZF S6-40 540Nm).

It has been 4 weeks since the GM’s clearance price sale began and from what I’ve heard, there are currently less than 70 of 400 units left in stock.

GM is selling these units to the dealer for an incredibly low price of $1800 new complete with shifter. That is $2800 less than what they normally sell for.

onverting your 4+3 to a ZF S6-40 is the best way to go.



Regards,

Bill Boudreau

ZR51 Performance

Cave Creek, AZ

(602) 319-6575

END QUOTE

Anyway if you are concerned consider the TKO-600, Keisler says they have a direct install for the C4 now.

 

The weakness is the clutch hydraulic system and not from abusing the trans during aggressive driving. If the hydraulic system stops functioning at it's peak level, the transmission compensates for it with its synchronizers. The driver is usually unaware that the synchros are working overtime and doesn't know anything is wrong until it is too late.

 

ZF Doc and Rockland Standard Gear told me this as well. If the hydraulics aren't working right you will destroy the synchros. Then again, that goes for ANY transmission. The gearbox itself it one tough unit, originally designed in Germany for heavy trucks.

 

Thanks for the interesting replies. A properly maintained clutch system should help the gearbox last a long time.

 

And to top it off, GM seems to have a bad batch of hydraulics on the market... don't that beat all?

A number of folks here are dealing with leaky or weak hydraulics - including me!!!

 

The GM problem with hydraulics entered into delaying my modified slave from DRM this summer. They informed me that the cylinders they were receiving new from GM had rust in them when they opened them up to modify them. Go figure!

 

My local GM dealer is replacing the bad slave I have but I had to come up with a copy of the inovoice from gmpartsdirect, odometer mileage and VIN number.