mark kibort

greg also said they looked worn as well. I also admitted and clarified i didnt have a lot of knowledge in the subject.. only what folks have told me.

by the way... why is it borg warner gear box synchros are so much better and why do they grip so much more firmly, like they lock!?


and i said:

GregBBRD

The thing that most people miss about this style synchro is that the faster the slider goes over the top of the synchro and engages the teeth on the gear, the less wear there is.

Anything that slows this process down increases the wear on the synchro and the slider.

And don't forget that the synchro and the slider also contact each other (and thus wear) when taking the transmission out of the existing gear!

Slow shifting to be "kind" to the gearbox, double clutching, and hesitating part way through the shift all add additional wear.

The 901 transmssions (65-71 911s), 914 transmissions (70-76), 915 transmssions (72-85 911s), 930 transmissions (76-89), and the Porsche designed 924, 931, and 937 transmissions all used this style synchro.

Dedicated track cars, because they generally get shifted very quickly, hardly ever wore this style synchro out.

My advice on shifting one of this style transmission:

Shift it quickly as possible (without "crunching") in one continuous motion. If, for any reason, the clutch does not release properly (causing severe wear both engaging and disengaging directions) shut the engine off and fix the problem, immediately! Never ever attempt a clutchless shift!

These transmissions, when proper, are a delight to drive. (And really bad to drive when they are not.) Just enough "feel" (to tell you that you are fullly in the proper gear) to make any gearhead smile. And, when proper (and the clutch releases), will shift flawlessly for a minimum of 75,000 miles.

I absolutely love to drive one of these transmissions!

James Bailey

Oh crap no more " speed shifts" without the clutch....what was I thinking.... In all fairness the later Borg Warner style syncros seem to tolerate abuse somewhat better. Porsche learned that many ham fisted Amercans could not help but abuse the equipment. .... no fixing STUPID !!

RennPartsDirect

GregBBRD

And as the mass of the gears continued to increase to handle increased torque, this design did not have enough frictional surface area.

Even the Borg Warner design synchros had limits of surface area as gears got bigger.

The "dual" synchro design on the 1st and 2nd gears of the 993/early 996 Cup transmissions is really elegant, but the cost of those pieces was crazy....the price on the three pieces required for one gear in the race transmissions (steel synchro, not brass like in the street car) was $2000. Three pieces that weigh ounces and fit in the palm of your hand = $2000! That's for EACH gear!

I'd guess I've bought 40-50 sets of those things, over the years....

Maybe that's one of the reason I don't blink at $1000 water pumps?

mark kibort

most people do not drive a stick right. even at the track, i see a lot of slow shifting. its timing thing that needs to be quick and precise.
the 928 with its larger rotating transmission mass, doesnt like quick shifting . you need to be able to time how long the RPM Of the engine takes to come down before you shift, as well as blipping downshifts. most dont reaize that the slight dragging, spins up or down the driveline. want proof... go 70mph and downshift into 2nd via a short stop in neutral... wait for a nice ugly grind.
now, do the same thing with a big RPP blip, it doesnt grind... whats the difference? its because the driveline is passively spun up during the shift? I tend to think so. Funny, i feel so guilty as i got a lot of grinds out of the holbert transmisson for the first 3 years of racing it, especiallly when it got hot. every since 2004, i can probably cound the times ive heard a grind sound, over thousands of shifts, maybe on one hand after learnign to perfect the "blip"on the 928. (which is a little differnet than other race cars like , mustangs bmws, 911s, corvettes, aston martns, and mazdas ive driven at the track )

yes, when they work, they are nice with lots of feel. yes, i also agree, the double clutch is not needed and the slow shifting can wear things out faster. However, i do believe with a equilibrium of force, the shifter can pull the transmission out of gear without any wear.
big blip downshifts, need to be done with precision as well

what is the difference of the S4 or 85 synchros. more surface area? grippy surface?

Yes, its hard to question 1000 water pumps when failures are so expensive,not to mention the labor to do it all again. (especially for those do it yourselfers that are saving on the 1000 dollar labor bill to have someone do the water pump and timing belt.

James Bailey

Right expensive is a relative term....often depends on who your relatives were

James Bailey

Ah so very slow shift is like slipping the clutch on a 5speed rapidly wearing out the friction surfaces.....

mark kibort

Like dragging the brakes vs quick stops.

Also, but you seem to discount, proper rev blips with a smooth path across neutral, to spin up the driveline.. that saves the synchros you have to think about the force to spin up the driveline rotating mass (including transmission).
3000rpm , going up to 6000rpm in fractions of a second. You dont do it right, and that entire burden is left for the synchros . The borg warners are much tougher, but still can get worn if you race the car without care here. however, street use? not anywhere near as bad .. thats why you rarely see S4s with bad sycnhros and most all pre 85s are grinders unless you are careful.

James Bailey

The brake analogy only works if you lock up the brakes.....just as clutch discs would last forever if you never operate the clutch. It is the slippage which causes wear.

mark kibort

good point.. i agree. I think it was the wear factor i was comparing.
actually, what you said is a good analogy.. locking up your tires using the brakes , slowly or fast. slowly wears the brakes more.

Snark Shark

What causes the resistance you feel as you're pushing it into gear? Is it a good technique to push gently and wait for the resistance to lighten, before firmly and quickly engaging the gear? Or does the very fact that you're feeling resistance mean the synchros are sliding?

GregBBRD

The synchros on the pre-'85 transmissions have to physically compress as the slider goes over the top of them. This ensures that as they wear, they will still contact the slider.

This is what you are feeling.

I'm not sure if they are made from some type of spring steel to be able to do this thousands of times, but it would make sense, if they were.