View Single Post
midlife midlife is offline
Registered User
Join Date: Apr 2004
Location: Central CA
Posts: 568
The second gear synchro does indeed have a harder job to perform (and gets worn more quickly due to lots of downshifting action) because the idler gear is physically larger/heavier than 3rd and up. More weight = harder to synchronize. This is why it's good from a manufacturing standpoint to remove as much weight as possible from large diameter idler gears. (GT-brand 915 gears are right around 15% lighter than comparable "drilled" gears, because they are side-profiled for weight reduction.)
I'm no transmission expert. (I have just started dismantling a 915 though), but it seems to me that every shift has to syncronize the mass of the input shaft, the clutch, and every slider and gear fixed to the input shaft, regardless of the gear you are selecting. The output shaft is hard linked to the wheels, and at a fixed speed. Third and fourth gear are fixed at the output shaft, and hence are part of itís rotating mass. The rest of the gears are fixed at the input shaft, and act as one rotating mass.
That being said, when you select fourth gear, the fourth gear syncro has to accelerate all the input shaft mass including both halves of second gear.
Lightening the mass of second gear would help all syncros not just second. Lightening third and fourth should have no effect, (other than vehicle weight and overall inertia).

I think that Porsche maybe should have fixed all the heavy gears to the output shaft, and fixed third and fourth to the input.
'69 911 Targa w/ 3.2
Old 10-13-2005, 03:40 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #32 (permalink)