Mechanical vs Electronic manual shifting

[toddshotrods]

If I understand correctly, when you downshift using paddles on an electronically controlled automatic transmission the transmission kind of freewheels, until the rpm's match the selected gear. When this happens, how does the engine "come back online"? I would guess it's a smooth "re-entry" because of the torque converter. Or, am I dead wrong or slightly misguided on the whole electronic downshifting process?



If you have a street/strip or full manual valve body, and select a lower gear mechanically, via a conventional shifter, isn't the transmission forced to comply. I was told that with a full race valve body it's up to your brain to not tell the transmission to downshift into first at 90mph, because it will do whatever its told. If this is true, wouldn't converter slippage come into play in determining how abrupt or smooth the downshifts are? I know line pressure affects this too, just curious how much the converter factors in.

[Steve Chryssos]

It depends on transmission make with respect to overrun clutches. The Fords and Chryslers use a electronic overdrive, so when you downshift from fourth you have engine braking.

With the GM 4L60E/80E, it depends on the location of the mechanical gear selector. If the mechanical gear selector is set to fourth gear (D), then there is no engine braking on downshift (when using the paddle shifter). If the gear selector is set to 3rd gear, then there is engine braking, but you only have 3 gears available. You must use the mechanical gear lever to engage/disengage overdrive.

For my own car, I prefer to keep the selector in D (fourth gear) and rely on my brakes to set up for a corner. Essentially, the engine freewheels until I get back in the throttle. I find that I can create linear deceleration better with my foot than if I employed engine braking. Obviously, good brakes and big front springs play a role here. Consider it a minor compromise in the interest of powering out of corners in the right gear at the right time.

So the cornering sequence is as follows:
-Approach turn in a straight line.
-Decel to entry speed.
-Simultaneously tap downshift paddle to pick correct, lower exit gear.
-Blip throttle.
-Hit apex.
-Get back in throttle with engine at the correct, higher rpm without delay.

For a track day or auto-x, I might test both scenarios. If overdrive is not required and engine braking is not overbearing, I might choose to put the selector in 3rd gear. If I'm not hitting redline at the end of a straight in 3rd gear, overdrive is unnecessary. You have a choice.

Alternatively, If I leave the transmission in automatic mode, I am relying on the computer to select the proper gear AFTER I get back in the throttle. That decision process may take an excessively long period of time and/or result in the wrong gear. I get that question a lot: "It's a computer...It's smarter than I am....Why not just leave the thing in D and forget it?" No it's not. Computers are dumb. They get confused. The computer doesn't know the difference between an approaching a stop sign and a well executed corner.

With an old school manual valve body, the engine is forced to comply and bad things can happen (overrev). With the computer, you can set parameters to prevent accidental downshifts based on speed (i.e do NOT downshift into 1st gear above 20mph, 2nd gear: 35mph)
Likewise these aftermarket computers will downshift into 1st gear if you come to a full stop. With the manual valve body, if you forget to downshift, the car will leave in too high a gear--which is why manual valve body transmissions get a bum wrap. The drivers burn them up by driving in the wrong gear. Not a problem with the computer controlled stuff. And, of course, the computer gives you a choice between manual or automatic modes. The PCS computer actually lets you choose between Simple Manual, Full Manual, and Auto modes if you install a 3 pole toggle switch.
Converters definitely play a factor. Our initial experimentation suggested that a slightly loose converter was beneficial for manumatic use with engine braking. So we had our 2500 rpm converter adjusted to 3000 rpm. Now that I modified my driving style to compensate for the lack of engine braking, I want a tighter converter back.

[toddshotrods]

Wow, when you answer a question you really answer it! That was everything I needed to know and more, thanks a million!

Just out of curiousity, why do you blip the throttle just before the apex?

[Steve Chryssos]

Its just a little driving style/feedback trick to confirm that the transmission has downshifted.

Plus.
It.
Sounds.
Cool.

[toddshotrods]

...Plus.
It.
Sounds.
Cool.

Sounds like fun too. Thanks again.