Kickdown

[phn]

Hi, apologies for all my questions whilst I search for my perfect C5!

When test driving an Automatic C5 recently, I noticed that when I gave it some throttle (from approx 50mph) so as to engage kickdown, the downshift felt quite hard compared to other high performance cars I have driven - actually the rear end of the car gave a bit of a "wiggle" as the gear engaged prior to the car rocketing away! (But then the roads were damp and it was cold).

I was just wondering if it was normal for a C5 to have a fairly hard shift when kickdown engages? For info non-kickdown auto shifts were lovely and completely smooth.

Many thanks!

 

[BobbyV8]

The gearbox isn't the greatest and on kickdown you may have gone from 3rd to 1st which can be quite harsh. If you drive it hard you will get used to the wiggle on the rear end but it's nothing to worry about

 

[phn]

The gearbox isn't the greatest and on kickdown you may have gone from 3rd to 1st which can be quite harsh. If you drive it hard you will get used to the wiggle on the rear end but it's nothing to worry about

Thanks, glad to hear it's just a Corvette C5 auto box characteristic and not a sign of something wrong.
I guess being a 4 speed it would make quite a big jump if it drops it down a couple of cogs

 

[teamzr1]

Being the A4 tranny has a controller that has a calibration of many settings and tables that GM set
there can be other reasons of harsher decel such as

1. Someone had the controller tuned, and they changed the many tables as to when to upshift as to MPH, RPMs, torque, decel, etc
2. Changing to other wheel, tire, rear end gears and not tuning for that or changed the torque converter as when it locks, unlocks or stall RPMs

If buying used, best to query owner as to any mods that were done and if tuned or not
Could know somewhat in the blind by lets say going at 40 MPH and using a GPS, what speed it reports
if not matching than you know changes were made and now requires tuning the controller to get the A4 to properly up or downshift

 

[Stingray]

Kickdown in a C5 tends to be ferocious and can definitely de-stabilise the back end. It used to catch me when driving in 4th and I wanted to go to 3rd for some acceleration. Typically the car would change all the way down to second and then I'd be forced to lift off because the back end wasn't behaving itself. Messy.

My solution to "drive around" this characteristic was to make a manual downshift to 3rd before the acceleration. If the car stays in 3rd when you hit the throttle that's fine. If it still decides to shift to 2nd then at least you're starting from a better position - jumping down one gear and not two at once.

I reckon any of these high torque Corvettes could throw you backwards through a hedge at the drop of a hat if you're not paying attention. No amount of clever electronics will save the day once the laws of physics have decided to get involved!

 

[phn]

My solution to "drive around" this characteristic was to make a manual downshift to 3rd before the acceleration. If the car stays in 3rd when you hit the throttle that's fine. If it still decides to shift to 2nd then at least you're starting from a better position - jumping down one gear and not two at once.

Thanks - that's really great advice which I will definitely follow.

Any ideas approx how many MPH the Corvette can do in gears 1, 2 & 3? I wouldn't want to overrev the engine by shifting down too much!

 

[teamzr1]

Keep in mind for a C5 that the controller with the calibrations controls how the engine and tranny functions and what it commands

GM uses what is called Adaptive Strategy Learning so as for the A4 tranny it adapts to your drive style and as it learns it then adjust for your drivestyle
So for example if you always drive slow, engine never goes over like 2,000 RPMs the controller assumes your shift speeds
But than you change your drivestyle the controller is used to the old style and still commands for that so C5 does not respond well to the newer style

Best is to mix up the drivestyle such as using higher RPMs, use manual shifting, etc, will force the controller to learn different drivestyles

For the A4 there is like 40 different tables with values and commands to the A4 as to what all the values are

When I do custom tuning, I look at the owner's drivestyle and make changes to those values to more suit the driver and how they drive

As one example is just one A4 table as for normal driving, meaning low RPMs and MPH as what the controller commands of shifting depending on the angle of the gas pedal (TPS)
You can see in this table as when the TPS says the angle is at what speed is than what MPH it will up or downshift

So as example you see in table, if TPS is 12 %, it will shift from 2nd to 3rd gear at 23 MPH and downshifting from 3rd to 2nd
gear at 18 MPH so that is a 5 MPH difference from downshifts

When I tune the controller, I change those values to better suit how the C5 owner prefers the speeds of when to make the shifts


Taking those values and making a graph you can see better when controller commands at what MPH each gear for up and downshifting

 

[phn]

Keep in mind for a C5 that the controller with the calibrations controls how the engine and tranny functions and what it commands

GM uses what is called Adaptive Strategy Learning so as for the A4 tranny it adapts to your drive style and as it learns it then adjust for your drivestyle
So for example if you always drive slow, engine never goes over like 2,000 RPMs the controller assumes your shift speeds
But than you change your drivestyle the controller is used to the old style and still commands for that so C5 does not respond well to the newer style

Best is to mix up the drivestyle such as using higher RPMs, use manual shifting, etc, will force the controller to learn different drivestyles

For the A4 there is like 40 different tables with values and commands to the A4 as to what all the values are

When I do custom tuning, I look at the owner's drivestyle and make changes to those values to more suit the driver and how they drive

As one example is just one A4 table as for normal driving, meaning low RPMs and MPH as what the controller commands of shifting depending on the angle of the gas pedal (TPS)
You can see in this table as when the TPS says the angle is at what speed is than what MPH it will up or downshift

So as example you see in table, if TPS is 12 %, it will shift from 2nd to 3rd gear at 23 MPH and downshifting from 3rd to 2nd
gear at 18 MPH so that is a 5 MPH difference from downshifts

When I tune the controller, I change those values to better suit how the C5 owner prefers the speeds of when to make the shifts
View attachment 25242

Taking those values and making a graph you can see better when controller commands at what MPH each gear for up and downshifting

View attachment 25243

Wow - that is some depth of knowledge you have - thanks for the reply!

That's very interesting to understand how the auto transmission works and the speeds in gear, I never understood how auto transmissions determined the gears before so that is a great insight!

 

[teamzr1]

You're Welcome
With the electronics these Corvettes have and the controller using a calibration, there is a lot of ability
to how that can control such as an auto tranny
Gets even more complex with newer tranny designs with 8 or 10 speeds and also paddle shifters

The plus to that, it allows me to make changes to that tune to better how the car functions or allows changes
when doing mods to the engine or tranny

Take some time and so through the attached text file below with I exported out tune of the controller your C5 has