Notchy 4th to 3rd downshift under intense load, hurts my laptimes Options

[nebreitling]

SO, this is probably a premature question until i replace my shift rod bushings... but in talking to Randal (who runs a rennshift), he mentioned he had occasionally had a similar issue:

when my car gets loaded down under threshold braking, making a 4-3 downshift becomes nearly impossible. the shift action 'wants' to go into 5th. forcing it over to the left (much farther than is necessary under normal conditions) will usually make it pop in with a bit of a grind.

engine mounts (rubber) are good, tranny mounts (911-engine) are good, OEM shifter in good shape, firewall bushing okay, transmission-end shift-rod bushings a bit worn, but not at all decrepit.

I hate going over the cyclone and grabbing 5th instead of 3rd... any suggestions?

[Brad Roberts]

This doesnt cure the problem...but.. dont downshift there. Use a tad less brake and let the car catch up on the downside. Keep your foot planted. This is very similar to what you have already learned with turn 9 by not downshifting into that turn.

It will bog for just a second as it catches up going down the hill. You know how to carry the speed. I keep getting phone calls about your Go karting.


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[lapuwali]

Just a potentially useless bit of info:

In my car, I have NO firewall bushing (broke the special "early conversion" bushing trying to install it), yet the shifter is just great. Like better than the what I thought was nearly perfect shifting tail-shifter I took out. I strongly suspect the order of the bushings in the feel category are: the cup on the gearbox shift lever itself, the coupler "bushings", the brass/bronze bushing for the bolt at the shift lever clevis (and the clevis itself), the bushing where the rod goes into the shifter console, and the firewall bushing. I'm beginning to think the firewall bushing is only there to reduce wear on the shift rod. I now have a piece of slit rubber tubing covering the shift rod where it comes out of the firewall, mostly to prevent clunking noises, but it has no effect on shifter feel.

I also loathe the Weltmeister short shifters.

[Brad Roberts]

This is a left hand turn also.. the shifter wants to stay in the 4-5 plain. I had the same issue in a fast AutoX right hander when trying to upshift into 3rd mid corner. The shifter wants to go into 1st (it did one time from the top of second) that REALLY rotated the car...LOL still had a decent lap after that tibit of excitement.

Ever ride with Steve Nieslony? He lets the clutch out during the downshift to use less brakes and to rotate the car in *some* corners. Works well in a LSD car.


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[Demick]

I think what Brad is trying to say, is that if the car is loaded up in a turn, the body of the car is twisting, which changes the position of the shifter with respect to the tranny. This can make you prone to hitting 1st when turning one way, and 5th when turning the other way.

I've had this happen a few times at AutoX. Now, I try to shift if at all possible in a straight line rather than in a turn. Happens to me when I am upshifting from 2nd to 3rd.

Demick

[nebreitling]

yup, it happens in turn 5 (left) and turn 11 (left), sometimes in 14 (right). it definitely happens under left-turn loads, but i feel it even under straight-line threshold braking.

with a shorter gearbox, i wouldn't even use 3rd at Thunderhill.

[Brad Roberts]

Demick nailed it. Some of it has to do with chassis flex and some of it has to do with G force on you and the shifter.

Took me forever to nail turn 5. I dont even know if I can still do it. It took a VERY early turn in and less brake while the car is loaded against the hill. Blind faith, knowing that the turn is still there.


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[john rogers]

I've seen this happen on early 911 vintage cars too although not too often. I would guess it is the flex of the body causing this as already noted, do you have a cage? I have a cage in my race car but not tied into the suspension points. The front mounts are a poly material and 911 sport mounts on the rear. I have a Rennshift, stock firewall bushing, aircraft type coupler and modified rear bushing to tighten it up. We took a lot of pains to make sure the shifter was absolutely centered with the trans in the 2nd -3rd gear shift plane. I have two hard shifts, 5th to 3rd at the CA Speedway as I do not try to go into 4th any longer as it wastes time and from 3rd to 1st at the races in Mexico for the U-turns at a course end where I don't try to get 2nd at all. I think the Rennshift makes these shifts much better than a stock shifter and so far I have been lucky enough not to miss any.

[nebreitling]

got a 1/2 cage. main hoop w/ diagnal down, petty bar across the chassis to right front wheel-well, and side bars -- the right of which is triangulated to the petty. body is pretty stiff now, but not tied to suspension...

i'll consider the rennshifter, but i've got an eye on some hoosier A's for the hillclimb....

[john rogers]

If you think about it, a great working shifter and linkage will make you faster than a set of new tires I'd bet??!!

[Brad Roberts]

Ha ha! I used to think that until I saw Randy Pobst run our 993 stuck in 5th gear around Thunderhill only 4 seconds slower than his times with all 6 gears..LOL

He is a machine.



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[solex]

I don't autox but have had the same problem with my car on the highway during hard acceleration. I replaced every bushing related to the shift linkage and upgraded to the 911 sport transmission mounts.

This help the shifting considerably (particularly 2nd) but I still have the "sticky" shift rod problem when going into 4th and 5th just as you stated.

This weekend I spent some extra time adjusting the shifter, had tools in hand so I could stop and perform some minor adjustments. I belive I now have the shifting as perfect as it can be, but still have some occasional problems with 4th and 5th.