I have a manual 6-speed but have no clue what this means. Anyone?

 

This is a technique usually used in the "old days" when gearboxes didn't have synchronisers.

Let's say you're cruising along at 60 mph in fourth gear and you want to drop to third gear. And to make this easier to understand, let's just say that the car turns 2000 RPM at 60 mph in 4th gear and 3000 RPM at 60 mph in 3rd gear.

With your Corvette obviously you would ....

1 ) push clutch pedal down (disengaging clutch)
2 ) slide stick from 4th to 3rd
3 ) release clutch pedal (engaging clutch)

Doing this you'll notice two things ... one is if you didn't give the engine some throttle to increase engine RPM to around 3000 before step 3, then you probably gota nasty jerk as you did step 3 ... and the other is that as you did step 2 there was some resistance before you could slide from neutral into 3rd.

Double clutching would look like this ....

1 ) push in clutch pedal
2 ) move gear lever from 4th to NEUTRAL
3 ) release clutch pedal
4 ) blip throttle to rev engine to bring RPM to 3000
5 ) push in clutch pedal
6 ) move gear lever from NEUTRAL to 3rd
7 ) release clutch pedal (while blipping throttle to 3000 RPM)

When done correctly (and quickly) it ..

a) sounds very cool ....
b) makes shifting easier, you'll find pushing the lever in step 6 will be much easier than it was without the double clutch
c) causes less wear on the gearbox synchros.

What you're doing is bringing the two gearsets that you want to match to the same speed before you engage them. In todays geaarbox their are synchronizers that "synch" the gearset speeds, but they do cause a momentary hesitation as they do their job .... double clutching will get rid of that.

 

To add to what was written, the idea is to spin up the input shaft to the speed it will need to be at for a lower gear. You should never need it. The only exceptions I can think of is driving a historic, and maybe in an endurance sports car (lower class non-sequential) to help with the tranny temps. Even modern crashboxes don't need it so, like I say, it is really unlikely to be needed.

Oh, and P.S. In a Corvette, the car will come out of ear with no clutch so you can actually do it alot more slickly than mentioned. You can skip a few of the steps and just blip the throttle.

 

Ahhh .... I'll add this ....

If you get really good at double clutching, you'll start to realize that the ONLY time you actually need to use the clutch pedal is when starting from a stop. If you have a good feel for engine speed versus road speed in various gears you can simply shift using the engine and stick shift.

Works like this .... 4th to 3rd as in previous example ....

1) throttle the engine so the car is not accelerating (very light throttle)
2) slide gearshift from 4th to neutral (don't touch the clutch pedal)
3) blip throttle to raise RPM to 3000
4) with engine at 3000 RPM slide from neutral to 3rd (no clutch)
5) go have fun !!

If you want an example that is less stressful to you (and your gearbox) try the following ...

Let's say you're cruising at 45 mph on the road and a traffic light turns red in front of you. Regardless of what gear you're in, don't touch the clutch pedal. Come off the throttle and apply GENTLE pressure to the shifter to move into NEUTRAL. When the car has slowed to about the engine's idle speed in that gear you will feel the shift lever smoothly slide into neutral.

Now come to a stop, and drive away from the green light as you always would.

 

Very informative response. Thanks!

 

if ya ordered the zs33/6 package there would be a second clutch pedal on pass. side . racing only youre co driver can assist you when ya got tired i love mine i may put them in all my cars call youre dealer ya can have it added for 100. $ hence double clutch

 

Black Z06, that is without a doubt the most simple and articulate explaination to that question that I have ever heard. I understand more how it works from your two posts than in the handfull of so called "proffesionally written" tech articles out there. Thanks a bunch.

 

....and then BAAM, crap time for a rebuild

 

? Explain please...

 

ive done this a bit, but i thought it was very bad for the syncs?

 

Depends .....

If you're matching the gear speeds correctly, then there is less wear on the synchros than regular shifting (no double clutch).

If you aren't matching the gear speeds, then, absolutely yes, you are not only putting MUCH more stress on the synchros but you can damage the gears themselves (to the point of Kerblammmo)!!

I wasn't recommending this technique (with one exception), only trying to use it to point out how engine versus gear versus road speed are inter-related.

The one exception is the technique of coming out of gear with no clutch action as you approach a light or other stop. By not using the clutch, just gentle pressure on the lever to slip out of gear, you accomplish a neat thing ..... The car will be in gear as you slow so if the light changes to green and you have clear road ahead stop the pressure to the gear shift and simply step back on the gas. By definition the engine will be somewhere above 1K RPM (as that is about where the lever will slip to neutral) and with our torquey engines you just drive on.

This will help in those situations where, say, you've slowed from 45 to 35 and have dropped the gearbox into neutral too early. If the light turns green ... what gear do you select and what engine RPM do you select to get going again ?? This technique will make for smoother driving, especially around town.

Now on the track .... well .... ..... Drive it like you stole it !!

 

A few years ago I had a '99 Camaro SS, put in a Mcleod clutch. Shortly after the install I had some problems, lost the hydraulic slave cylinder completely (ruptured diaphragm). As a result, I had to drive it (in DC rush hour traffic) w/ NO clutch whatsoever. I used the techniques described earlier in this thread, avoided coming to a complete stop whenever possible, and was able to get by for a few days. When I did have to come to a stop, I just turned off the engine, then started the car in gear.

It wasn't easy, but it worked!

 

There has been a few times when I wasin the right seat instructing students, I sure wish I had a BRAKE PEDDLE


Watch this video. Mostly Heel-n-toe dowshifting, but has a few double clutchs too.
Click here to see Video

 

Excellent dissertation. Also remember that synchromesh transmissions are nothing new. The first synchronized transmission system was introduced by Cadillac in 1929.

I used to fly for Andy Evans, whose team won Sebring a few years ago in Ferrari 333 SPs. I asked those drivers(he had about 5 drivers) about double clutching. They did it regularly, a necessity in endurance racing where longevity is crucial.

The simple truth is that in order to down shift SMOOTHLY, a double clutch is necessary. So called "blipping" the throttle is the sure sign of an amateur.

 

This isn't my experience with synchronized manual transmissions. On the C6 that grabby clutch and high torque makes it a little tougher, but on most manual transmission cars I 've owned I can downshift from top gear sequentially to second gear smoothly enough that you only hear the increased engine revs, you don't feel the clutch engage. And all I'm doing is "blipping", but I am an amateur.

 

Even if you match the revs I would still think it would damage your transmission since it has helical gears. I've only heard of clutchless shifting when the transmission has non-helical gears (i.e. nascar). How does this work?

 

Huh?

 

Huh ???????

The book "Going Faster" (Bentley Publishers) used by Skip Barber Racing School

http://www.bentleypublishers.com/pro...dba&subject=23

specifically uses the term "blip" in describing a double clutch .... page 96 in my edition .... "The only difference is that when you stop in neutral you give the motor (and the input shaft) a blip."

Alan Johnson in "Driving in Competion" (Bond/Parkhurst Publications) - out of print but Amazon has copies at:

http://www.amazon.com/exec/obidos/ASIN/0878800085

says on page 58 "It's faster than it sounds. With practice it becomes very fast. In-shift-out, gas, in-shift-out, gas."

In "Speed Secrets" (Motorbooks) which can be found at

http://www.amazon.com/Speed-Secrets-...997518?ie=UTF8

on page 24 author Ross Bentley describes a double clutch while braking as "move the shifter into neutral, release the clutch, rev the engine (blipping the throttle using the heel and toe method), depress the clutch again ......"

I guess all these "amateurs" are just wrong. How do you raise engine RPM without blipping the throttle?? Commune with the PCM by Vulcan mind meld?

 

I do this in my Corvette on occasion to demonstrate how you can downshift (or even up shift) a gearbox without the clutch. When you do it right (match engine speed to road speed for that gear) the synchros will mesh correctly and the gear will be selected with no problem at all. When you do it wrong the UGLY grinding noise from the Tremec gearbox leaves no doubt that ya screwed up !!!

as I said in an earlier post ...

"If you aren't matching the gear speeds, then, absolutely yes, you are not only putting MUCH more stress on the synchros but you can damage the gears themselves (to the point of Kerblammmo)!!"

It can be done, but it is not something I'm recommending ... just using it to explain "how it works".

---- >> Added thought after first response for Theels12588

The way I understand it, without the synchronizers, we would almost always run the risk of damage shifting without the clutch. The synchros "align" the gear teeth before they mesh. If you aren't using the clutch but get the road/engine/gear speed pretty close the synchros will do their job ... but if you are off some, the synchros will not be able to align the gears before they try to mesh ... hence BAD noises !!

 

Not to mention, the 333SP uses a sequential transmission (like a motorcyle.) There is no neutral in between gears so there is no such thing as double clutching this car. Someone is confused.

If I had to take a guess, I would say he is confused with racers preference to use the brakes to slow the car and amateurs use of excessive engine braking.