I thought the 6-spd was stronger over the ZF 5-spd ? The ZF is definitely light weight. 2)Have you thought about running a 5 speed over the 6 speed? You can save some weight and the transmission is more durable. Just a thought for when you change the rear end.
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You would think the 6 speed is stronger but the synchros are smaller to allow for the extra gear and it weighs about 25-30 pounds more. It is a common modification for people running a S54 in their race car to convert their 6 speed to a 5 speed when they buy a custom diff for this reason. This is why it is not uncommon to see some of the old top NASA GTS guys running a 5 speed. This is one of the reasons BimmerWorld list the conversion on their site.