To All C4 Members
I would appreciate any additional comments or data from members regarding this topic.
All early C4 manual transmission Corvettes have the Doug Nash 4+3 transmission, which automatically drops the 2nd, 3rd, and 4th shifted gear into Overdrive after accelleration has been completed, and Cruising has begun.
The purpose of this design, as I know of it, was to reduce engine RPM, and thus also increase fuel economy, in response to the new EPA mileage regulatioins put into effect during this time period. Later C4 manual transmission designs went to a 6 speed manual transmission to reduce engine rpm.
Regarding the 4 +3 manual transmission, there is a general trend for the 4 +3 Overdrive (Overdrive switch) to not work on continuing basis over time. If this happens, the 4+3 transmissioin becomes a basic 4 speed manual transmission with no overdrive.
My questions for your comments and data input are this:
If this occurs, what would be the general rpm consequences of having only the 4 speed, with no overdrive.
How much additional rpm would be experienced (2nd, 3rd, 4th gears) over the (2nd+Overdr, 3rd+Overdr, 4th+Overdr) rpms.
Will the additional rpm levels, be such, that it produces and increased wear or life of the transmission or engine?
What would be the expected reduction in mileage from the additional rpms.
What would be the main factors to start looking at, if the Overdrive starts to intermittantly not automatically actuate?
Is this a Computer programming glitch issue?
Is this a gear shift embedded Overdrive dissable switch issue?
Is this a mechanical binding issue in the overdrive itself?
Are their other issues to consider?
Again, and comments, experiences, repair cost history, information references, and data regarding this topic would be most helpful.
My understanding is that the 1984s and 1985s had most of the mechanical issues producing this effect, but the overdrive design was improved to eliminate/reduce these issues in the 1986, and 1987 models.
Bottom line, if Overdrive is not engaged, will the additional rpm created, produce any significant areas of concern.
Lastly, if needed, can the 4+3 be swapped out for the newer 6 speed transmission, and what would be the anticipated cost if done?
Thanks to all for your comments !
Best Regards,
Dan