Thoughts On Hard Shifting And A Possible Cure?

[Frankster]

Just bought my 96 Hennessey Viper SN 15. Love it!

Here's the problem........ I've experienced the same hard shifts at higher RPM that seems to be such a common thread in this forum.

There's already an upgraded shifter installed so I think that's only part of the answer. Also, I think I know the cure but want to run it by the more experienced Viper owners and mechanics available in this forum.

Consider..................

The Borg-Warner T-56 uses Dextron II-E. At best guess this is maybe a 10 weight fluid. There are two shafts in the T-56 with 6 or 7 gears on each of the two shafts. Add up the bearing surface of about 12 gears running on 10 weight oil and we could be looking at a viscosity VS bearing surface issue causing the hard shifting.

My thinking is that even with 10 weight fluid the viscosity resistance adds up with so many gears spinning around at higher RPM. There's also a lot of inertial mass the synchros have to overcome.

For example, when I try shifting from 3 to 4 at high RPM it does not want to go into gear. But as the RPM drops down while I hold pressure on the shifter it will eventually go into gear. It seems the gearing has to catch up to speed to syncronize and go into gear. The 10 weight viscosity Dextron II-E is slowing down the works.

Could the 10 weight trans fluid through 12 gears add up to hard shifting at high RPM due to viscosity resistance? My guess is the answer is "YES".

I'm thinking of using a high quality, ultra thin, synthetic oil to see if it will cure the hard shifting problem. I'm just not sure of all the implications in doing that.

Reasoned comments are encouraged.......................

 

[Bobpantax]

Check your engine and tranny mounts. If it is only happening at higher RPM when shifting hard it may be an indication of mount damage. Since the car is twelve years old, if it has the original mounts, it would not be unusual for something like this to occur.

 

[FATHERFORD]

Tranny/motor mounts is something to look into. Mine hare bad and it's hard to shift in higher rpm's, it was even causing me to mis-shift.

 

[Tom F&L GoR]

At higher RPMs the difference between the synchro ring and gear is greater, so it takes longer for them to come to the same speed and allow you to engage. In an old 833 4-speed, I roughed up the bronze cone surfaces and it engaged much more quickly, until, of course, it wore smooth again.

Consequently, a thinner fluid should provide more friction on the cone surfaces to allow quicker engagement, so you are on track there. I don't think that the amount of bearing surface comes into play, simply the inertia of the metal spinning parts. Typical auto transmission fluid is 8 cSt, or an SAE 20.

With a thinner fluid you will probably hear "neutral gear rattle" or more. One successful fix has been to modify the clutch pedal ratio to get more clutch departure. Any slight drag on the clutch during shifting will make shifting more difficult; changing the ratio is like pushing the pedal down farther without having to push it farther...

 

[dave6666]

No need to repeat all the good info above, but the DS mount on my near stock '01 after 28K miles. Looked good until I got it on the bench and could really play with it...

You must be registered for see images

 

[Frankster]

Thanks for the response, guys. Looks like I may have a crunched syncro or two....that according to a knowledgeable T-56 guy. He also says a thinner, synthetic oil may make it work better.

Here's a link to a neat idea....

Sequential Shifting your viper without the $15000 price tag........

Paddle Shifters, Sequential and Electronic - ASaP

 

[carl B]

Could your clutch be dragging??

Carl

 

[Frankster]

Carl
More than likely it's a combination of things making my shifts difficult. I suspect the following.............

1. The tranny oil could be thinner to reduce viscosity friction.
2. I may have worn syncros.
3. Cockpit proplems - Inexperience on my part in using high performance shifting technique to include physical manipulation of the clutch and shifter.
4. A broken mount of some sort.
5. Some combination of all of the above.

Since I want my Viper to be more of a road machine I'm looking at going with a sequential shifter. I'd like Formula 1 type shifting but don't see anything available for a reasonable price.
Quaife make what appears to be a nice unit but at $16K it may not be worth it unless you're an all out racer. Here's a link to the transmission. AUTOTECH - Quaife Torque Biasing Differentials, Close Ratio Gearkits, Racing Transmissions, Custom Driveline Components

I wonder how the Viper would do with one of these

 

[Jack B]

At higher RPMs the difference between the synchro ring and gear is greater, so it takes longer for them to come to the same speed and allow you to engage. In an old 833 4-speed, I roughed up the bronze cone surfaces and it engaged much more quickly, until, of course, it wore smooth again.

Consequently, a thinner fluid should provide more friction on the cone surfaces to allow quicker engagement, so you are on track there. I don't think that the amount of bearing surface comes into play, simply the inertia of the metal spinning parts. Typical auto transmission fluid is 8 cSt, or an SAE 20.

With a thinner fluid you will probably hear "neutral gear rattle" or more. One successful fix has been to modify the clutch pedal ratio to get more clutch departure. Any slight drag on the clutch during shifting will make shifting more difficult; changing the ratio is like pushing the pedal down farther without having to push it farther...

Tom:

let me give another slant. I just did some very interesting research. I mapped the clutch slave cylinder and clutch travel. I also sliced the slave in half and took some interesting pictures. I will post these very soon.

What I was leading to, no matter how you change the clutch stroke, the slave is only going to travel so far. The slave only travels 1/2" when you depress the clutch.The at-rest position of the slave will vary depending on the variables in clutch system. As an example, when I took mine apart, the slave ram was 1/8" away from coming out of the inner bore, hence, losing all the oil. That starting position is dependent on:

1. The pressure plate finger position, in other words the distance forward of the transmission. This is dependent on the design of the pressure plate/disc assembly.

2. The clutch disc thickness. As the disk wears the fingers move towards the transmission.

3. The length of the throw-out bearing.

I will post all the findings and we can discourse some more. Let me throw this into the fray. I weighed three different clutch disks. They were:

a. 4.25 lbs
b. 8.1 lbs
c. 6.0 lbs

Which one do you think will shift better?

 

[Tom F&L GoR]

Jack - look forward to your findings.

I see the release bearing has a spring to keep it in constant contact with the fingers, so when you post, please comment on this relative to the starting point of slave cylinder travel.

Being a sucker, I will guess the lightest pressure plate and clutch disc combination should shift fastest since it has the least inertia. But that means I'll be wrong....

 

[Jack B]

Jack - look forward to your findings.

I see the release bearing has a spring to keep it in constant contact with the fingers, so when you post, please comment on this relative to the starting point of slave cylinder travel.

Being a sucker, I will guess the lightest pressure plate and clutch disc combination should shift fastest since it has the least inertia. But that means I'll be wrong....

Tom:

you are right, but, I haven't heard anyone comment before on the difference in weight for different disks, of the three I gave weights on, none were the Luk or stock disk. If someone has a stock disk, post the weight. Its just common sense, a disk with a large moment of inertia is going to keep the input shaft rotating, thus, making the shift harder. It should be no different than the clutch partially releasing.

About your comment on the spring holding the throwout bearing against the fingers, that is obvious, in addition, the follower in the slave should also be in contact with the bearing. Unless I am missing something there is nothing to push the follower off of the bearing. It is easy to prove, when you remove one, merely look at its position, since the hydraulics is now a closed system the follower will remain in the working (at rest) position.

For those that don't know, the throwout bearing is easily removed from the slave. Find the ground off spine on the slave, at the top (under the bearing shell) of that spine is a locking tang. If the spine is at 12:00, squeeze the plastic bearing shell at 9:00 and 3:00 and the bearing will release. The manual says the bearing is not replaceable, but, NAPA lists a replacement.