I am so happy with my car in general I hate to comment about the negatives, but I thought I'd include recent problems so others can be on the lookout for fleet-wide problems.

My steering pump sounded like it was starting to cavitate so I checked the fluid level. It was significantly low so I took the car to the dealer after topping it off. This is the first time this has been seen at Tom Jumper in Atlanta and they do a lot of 'vette business so it is probably a unique problem. The reservoir itself was faulty so the part is on order.

I also asked them to fix an annoyance with the parking brake handle. The release button is canted slightly so it sticks now and then, preventing the brake from setting until you pop the handle to release the button. And we all know how important it is to set the parking brake on an MN6! Part is on order.

The last problem is a little more significant. I've noticed three times now when I'm slowing so rapidly I need to shift from 5th or 6th to 2nd that the gears grind if I go directly into 2nd. I think I have a sticking synchro but I don't know enough about transmission to really know. This happened the third time as I was leaving Jumper so I called back and they will look into it when I return for the steering reservoir repair. Any comments from the experts?

John

 

I Believe The Owner's Manuel States That You Should Only Go Down One Gear At A Time To Keep From Damaging The Transmission. I Find This Hard To Do Since I Have Gotten In The Habit Over The Years With Many Other Cars.

 

I Believe That The Owner's Manuel States That You Should Only Downshift One Gear At A Time To Keep From Damaging The Transmission. I Also Find This Had To Remember Because Of Habit.

 

Briar,

You are correct, I checked out the manual and it does warn about skipping more than one gear. Thanks for the info.

John

 

You're overriding the 2nd gear cone clutch by asking for too large a mainshaft speed change when downshifting directly from 6th to 2nd without double clutching. When you override the cone clutch, the dog teeth clash, causing the grinding sound.

Synchronous transmissions have constant mesh gears. What that means is that you never engage or disengage gear teeth. The gear pairs are always meshed. Instead, you engage and disengage dog teeth on the synchonizers. This locks and unlocks the particular mesh from the driving and driven shafts. (When unlocked the gear pairs just freewheel on their shafts.)

Now to keep the synchronizer teeth from grinding, they have tapered brass rings, called cone clutches, separating them. When you move the shift lever, the cones come into contact before the dog teeth. The friction between the cones bring the gears to the same speed as the shaft before allowing the teeth to engage. This is what makes a synchronous transmission synchronous. But if you demand too much speed change too fast, or the brass cones are too worn down, the clutch action is overriden, and the dog teeth are allowed to touch before they are speed matched. That's the grind.

To prevent this, and to extend the time before the brass rings wear down too much, you want the driving and driven shafts to be close to the proper speeds before trying to complete a shift. One way to do that is to work your way down (or up) one gear at a time. The other way is to speed match by double clutching. The latter is the only way to make a fast safe downshift several gear positions at a time.

People who drive large trucks are well aware of this requirement. People who've only driven light transmissions with little rotating mass often aren't aware this is how it all works. When they beefed up the Vette transmission to take 400 ft-lbs of torque, they approached the limit where you need to be conscious of this speed matching requirement of synchromesh transmissions.

 

shopdog,

That was an impressive explaination. I'm almost embarrased to ask, but I won't ever learn if don't, so here goes. I've been driving manuals for years now and have heard the term double clutching many times, but I have never heard much less learned how a double clutch is preformed or what difference it makes when shifting. Please educate those of us that arn't familiar with it.

 

Great explanation shopdog. Just like Wanta6, I too have read the term double clutching, and not really been told exactly what that means. I always assumed it meant depressing the clutch, shifting to neutral, letting the clutch all the way out, then depressing it again. Perhaps you can clear that up for us once and for all.

Keith

 

Ok, then I'm not the only one. But I guessed at your explanation too. But then I prefer my auto.

But when I can afford multiple Generations of Vettes, I will probably get a manual or 2 for fun as well. So I would still like to know.

 

Double Clutching

This should provide a good explanation of double clutching.

 

Interesting link. I have never heard double clutching on the downshift called double de-clutching, though.

 

Once again, the answers are all here if you are willing to ask. Thanks to everyone for the great explanations.

John

 

I think it is a term the writer made up. But the procedures are correct. The idea is to get a near speed match so the cone clutches don't have to work very hard to get the input and output shafts to the proper speeds. In the old crash box days, you had to get the speed match exact to prevent a grind. With a synchro box, you only need to get them reasonably close.

 

Downshifting. Depress clutch, take car out of gear. Release clutch, jazz throttle to get the input shaft (and engine) up to the proper RPM for the current road speed *in the gear you intend to enter*. Depress clutch, shift.

Upshifting. Depress clutch, take car out of gear, wait a moment for the input shaft to slow, shift. No double clutching required, though a quick double clutch, without jazzing the throttle, will slow the input shaft quicker than just waiting for it to coast down.

You need to memorize RPM-road speeds for your car in each gear so you'll know what the RPM needs to be for any shifting situation. After a while this becomes reflexive. You won't have to look at the tach, engine sound will be guide enough. You don't think about it, you just do it. I learned to drive on a 1947 2 ton Ford truck. If it ever had synchros, they were worn out before I got it. Double clutching quickly became second nature.

 

"Why double declutch?

It makes downshifting possible if you're traveling at a high speed and need to downshift from 5th to 2nd and then to 1st in order to stop fast and prevent an accident on a slick road or if you are having a brake failure. Yes, you may lose your tranny by doing this, but I'd rather bust a couple of gears than kill either myself or someone else."

This is really poor, dated information. Modern brakes are the most potent control on the car and the clutch should never be used for deceleration. Find a good (recent) book on road racing and high-speed driving/riding. BTW, when is the last time you had a brake failure?

 

I feel more enlightened now. Thanks to all that schooled the rest of us.

 

I'm still not certain why it is necessary to depress the clutch twice for one gear change. Rather than lifting your foot off of the gas pedal and depressing the clutch and then releasing the clutch and depressing it again while blipping the gas pedal, why not simply blip the gas pedal while it is depressed the first time? It only takes about a 1/10 of a sec for the engine to rev in neutral to get to the appropriate rpm for the downshift, so why not make that happen the first time the clutch is depressed?

 

Because by blipping in neutral with the clutch released, you spin up the transmission as well, so that the drive and driven parts are roughly at the same rpm.

 

Rock-it-Man - Thanks.

 

Shopdog, all i can say is WOW; you should be building corvettes in BG maybe they can make you a consultant in the design of the C7.

 

Great discussion here............ We all can learn!