Shifting to lower gear and synchro wear

[Vaughan]

I did a search and could not find an answer to a basic gear shift question...

When downshifting(fourth/fifth to second) is there less wear and tear on the synchros by double clutching or running the gears through 4-3-2 with the clutch depressed and then blipping the throttle before relaeasing the clutch into second?

[stephen wilson]

If done even fairly well, double-clutching w/ heel & toe will take some load off the synchros. Also, if you match the RPM's , there is no need to "row" down thru the gears, just go straight to second. If you "skip-shift", you'll reduce the number of shifts on the gear skipped, lessening wear. Don't downshift directly to 1st though, too easy to get the RPM wrong, go to 2nd first. Besides, 1st is geared too low for almost anything but standing starts. The best reason to DC/HT: it's great fun!

[blue2000s]

The function of the syncro is to match the speed of the intermediate shaft and the transmission shaft. The larger the difference in speeds between them and the harder you push into gear, the more it will wear.

Rev matching with the clutch pressed does help to smooth out the shift, but when you press in the clutch, the engine is no longer attached to the intermediate shaft and in between shifts, neither is the axle. So the intermediate shaft is spinning slower than both and it takes the syncos and clutch to match them all up when you re-engage the clutch.

Double clutching forces the intermediate shaft and engine to match speeds, which are ideally matched to the axle by the throttle blip, in that scenario, there is no syncro wear at all ideally.

So blipping is good, but double clutching is even less wear on the syncros. It is better to go through the gears as you drop down in order to minimize the speed difference if you are just blipping. If you're double clutching, drop all at once.

http://auto.howstuffworks.com/transmission4.htm

[eightsandaces]

It's way cheaper to wear out rotors and brake pads than to put excessive strain on any of the synchros. Frankly, IMO, the shifting mechanism, especially the hydraulics needs a rethink. Porsche is far from the nicest shifting car I've ever driven but in overall performance still beats the competition.

[Lil bastard]

There are lots of times where downshifting is necessary - flowing in traffic, elevation changes, turning, etc. And in these times matching revs is a good way to minimize synchro wear.

Then there are times when downshifting is totally unecessary and just causes excess wear to the drivetrain, like approaching a stop.

If you're a good driver, you can anticipate 99% of your stops - there's a sign alerting you to a stop ahead, you see the stoplight or stop sign, etc. This is the wrong time to downshift. Stay in gear to allow the car to bleed off energy while applying the brake, then go into neutral and stay on the brakes to stop the car.

It doesn't sound as cool, or get the attention of the blonde at the bus stop, but you will end up burning $150 brakes instead of $600 clutch and/or $2000 synchro replacement.

Of course, in an emergency, do whatever you need to stop in the shortest distance possible - downshift, throw out a boat anchor, anything!

Otherwise, respect the machinery - use the brakes to stop and the tranny to go.

Your car, and your wallet, will thank you.

Cheers!

[Frank M]

A syncro will wear quicker when speed shifting. This is similar to hard braking wearing tires faster.
When there is enough wear the syncro has difficulty slowing down the gears to properly mesh, similar to a tire with little tread not grabbing well on a curve or hard braking at high g force.

When this starts to happen then the syncro grinds and makes its teeth as well as the teeth on the gear round off.
The answer to syncro long life is to shift normal including down shifting approaching a red light. One will find they will tire of the vehicle before the syncro will require replacement.

Shifting fast will cause quicker wear similar to hard driving wears tires.

I find premature syncro wear on vehicles that do not replace the lube with the factory required lube. OEM is best no matter what.
The syncros in modern transmissions are made of different materials and the lube is engineered to the specific application.

[Quickurt]

I downshift only to be in the correct gear to accelerate into, through and out of a turn.
In the supposed "good old days" we would downshift to save the brakes from excessive heat and/or wear, but those days are long gone.
LB hit that nail on the head.
With Boxster brakes, you're actually wasting precious braking concentration on screwing with downshifts and probably screwing up your stopping/slowing distance.
If braking properly, you don't have time to row through more than one gear anyway. Stand on that center pedal, double clutch/heel toe down into 5th gear (4th in a base model) and you're already down to third gear speed. Before you can get it into another gear, you're nearly stopped.
You're better off spending that precious brain matter calculation time estimating the gear you will need to be in for the corner you're approaching and then making that one shift - smoooooooooooooooooooooooooooooooothly - and being in that gear BEFORE you' re done braking.
I have a set braking point for the turn in to my subdivision entrance road. From 70 mph I brake at the beginning of the previous driveway - 75 ft from the beginning of our entrance road curve in. I stand on the brakes, downshift from 5th to 2nd while gently turning in (loving the Porsche anti-lock system) and end up in the driveway, on the proper side of the road, at our 20 mph subdivision speed limit. The entire process takes maybe 3 seconds.

[Vaughan]

Originally Posted by Quickurt

I have a set braking point for the turn in to my subdivision entrance road. From 70 mph I brake at the beginning of the previous driveway - 75 ft from the beginning of our entrance road curve in. I stand on the brakes, downshift from 5th to 2nd while gently turning in (loving the Porsche anti-lock system) and end up in the driveway, on the proper side of the road, at our 20 mph subdivision speed limit. The entire process takes maybe 3 seconds.

That's what I'm talkin about.