Handling wheel lock when changing down a gear.
 

I have a TH 400 auto box with change kit and standard torque converter which works great excepting at high speed when on the track and changing down from 3rd to 2nd gear.

Approaching a corner under heavy braking and then manually slotting it into 2nd gear at a safe rev the rear wheels lock for a fraction of a second giving a distinct chirp from the tyres and unsettling the car ever so slightly. Other than waiting to change when one comes off the brakes, which could be risky in it's own right as I am generally in the turn by then, do any of you suffer from similar problems and how do you mitigate them?

At the moment I just live with the problem but maybe one of you has a solution other than the obvious one of putting in a manual box.

You are experiencing compression braking and everything is working as expected - nothing is wrong with your car. You need to master the (difficult in a Cobra) technique of heel-toe downshifting. The maneuver is to, while braking, reach across to the accelerator and blip the throttle (to increase engine RPM) while simultaneously pressing the clutch and downshifting. Blip the throttle to increase RPM and then release the clutch. This gives the proper RPM matching of the engine to the new gear.

Search youtube for some videos.

It's a very difficult maneuver on a Cobra. The planar offset of the brake pedal to the accelerator is quite high making it harder to reach from one pedal to the other.

Edit: Ah, automatic transmission to boot. Not sure what the proper method would be, but it's the same problem.

Tony.......he has an automatic..............

I saw that (and probably edited during the posting window). Not sure how to RPM match an automatic, but wouldn't this manual downshifting practice be discouraged in any automatic? I wouldn't even think of going down to 2 from 3 while holding it at the red line. Not sure they are designed to do that.

i guess really you don't.......i've done it too like he has to corner without loosing speed but with an open rear. with a traction lock or locker it could make me loose my cookies.

not much to do to an auto for tracking......just live with it

You can try still blipping the throttle just as the downshift is taking place. I used to do it on my Camaro (I was saddled with an automatic) and it worked just fine. Takes a little practice though.
Larry

Can you just wait to shift right at the point when you have finished braking and are getting back on the throttle?

I have done it with an automatic but it took a lot of practice on straight line running never in a Cobra.Would think if racing were going to be a regular thing would add the the third pedal.Even then would set the pedals up to be better able to do the footwork.

Snake2998,

Interesting that "real" race cars, paddle-shifted and not, are catching up with our use of automatics :LOL:

I've been piloting an automatic on tracks such as Sebring, Daytona etc in my Cobra because I'm way too linear to do heel and toe and still learn the line...

My approach is to simply put it in Drive and drive.

My engine has enough power to provide tire motivation at all speeds so the idea of shifting it isn't an issue.

I have a C-6 built by Rossler in Girard, OH specifically for this kind of abuse; it works well on the street too. He does some real good GM trans buildups too.

Some other considerations have been cooling and heat generation. The cooling is taken care of by assuring plenty of oil flow in both the trans and the converter, and a large oil cooler. I confess it took four transmissions and three converters to get the right setup :CRY:

My torque converter is very low stall, basically stock, at around 1600 rpm. If you have a small block you may need a looser converter, but that may increase the heat load.

In any case, you need enough torque at the back tires to allow you to pick up the exit speed from a corner, but no more. If you have to feather the throttle exiting turns you may not want to downshift at all.

It's a pretty simple thing to try: leave it in Drive, run the course and see how it performs. If it doesn't increase lap times, you're good to go. If it does increase lap times maybe you need a little looser converter or a different rear axle ratio.

If the tires chirp after you do your turn-in - while you're not going straight ahead, you're asking for real problems. That sound means you have momentarily lost traction with the back tires; if you're cornering at anywhere near 90-100%, a spin is the usual result :eek:

Best of luck figuring it out!

Tom

Thanks for the feedback all - I will try the throttle blip and see how that goes.

As most of the track is second gear work with my setup (<100 MPH) with only the two main straights where I reach around 130 MPH calling for third gear, leaving it in drive doesn't work as it tends to seek third a bit on the slower parts of the track.

The other reason I manually change down is that the original unvented brakes would tend to overheat after 7 or so laps sothought a bit of compression braking might help - have now fitted large vented discs with Willwoods and combined with a new aluminium hub have saved 5 lbs per side (those jag bits are very heavy) so maybe my overheating problems are over. Haven't tried the new setup yet as am busy converting to fuel injection with a homemade system with a crossram manifold.

Should be up and running in about 2 months time.

Several years ago I had the good fortune to be invited to the GM test facility near Mesa, AZ. The best part was when I was given one of their Corvettes to drive on the autocross course on their very large skid pad. For a good driver, it was about a two minute course. .... The Corvette I was given was equipped with an automatic transmission. My instructor told me to select Drive and make full use of the car's anti-lock brakes. After each pass on the autocross course, I'd hit the high speed track for a mile or so to cool the brakes and then do it again. During the hour or so I was driving, I never noticed the transmission doing its job. It always seemed to be there in the gear I needed when I pressed the gas. But I wasn't using it for braking.

I agree with the advice to put it in Drive instead of manually shifting. If the brakes need help, solve that problem separately.

If you want to use the shifter and if you have your heart set on keeping the auto, you can spec a valve body for the T400 to soften the 3 to 2 downshift. It can be softened to the point of having zero engine braking at all, so someone like Hutch or Jakes Performance might be able to advise you of the best compromise.