Quote:
Originally Posted by tyl604
Brian - assume the working is still like my 81 300SD? No vac needed for upshift (in fact it is designed to bleed off vac at WOT). However needs vac for smooth downshift. Most of the posters do not seem to realize that loss of vac will not affect upshift.
Or did something change through the years?
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Vacuum affects both upshifts and downshifts by slowing the shift. Instead of shifting instantly, the vacuum delays the shift to...........maybe .75 second............so that the engine and driveline can match speeds without jarring the driveline. It's a delicate balance between clutch wear and driver satisfaction.
At WOT, vacuum is removed completely as the manufacturer realizes that the clutches will wear significantly if the shift is delayed under high power. Additionally, the driver is relatively unconcerned with the shift quality when he wants full power.