Tuesday, May 26, 2009 - 07:01 pm, by: Ashley Young(1uzman)
can i swap my 3 speed box for a 4 speed in my uzz31 without changing any other parts ie computer and would this be worth doing in the intrests of better fuel econnomy and performance
Tuesday, May 26, 2009 - 07:19 pm, by: David Vaughan(Davidv)
3 speed + overdrive == 4 speed. Functionally, it is a 4 speed box. They call it 3 speed + overdrive to cater for "PRNDL" expectations derived from the ancient american 2 and 3 speed gearboxes.
Overdrive is in reality a completely different thing, and the Soarer box does not have real overdrive at all, only the name.
Do you understand what I am saying? If not then I am happy to provide a more detailed explanation.
Tuesday, May 26, 2009 - 09:30 pm, by: David Vaughan(Davidv)
Literally, overdrive is simply a ratio in which the drive shaft spins faster than the engine. Gearboxes normally topped at a 1:1 ratio (before the diff ratio) but with overdrive you might have something like 0.85:1. It was designed principally for greater economy and rarely provided optimal top speed let alone acceleration.
In its original form, overdrive was for practical purposes a second gearbox with two speeds, normal and overdrive. This typically attached at the rear of the standard gearbox.
The effect of a second gearbox is that the additional ratio is available in all of the gears in the main gearbox but it was not used in this way (unlike low ratio in a Land Rover, for example) for two reasons. Firstly, you have gear overlap so not much point, and secondly the overdrive box was not designed to take a lot of torque in its "overdrive" position. I think the electrically triggered overdrives had a switch to prevent selection below 3rd gear.
These days, a five or six speed gearbox will inevitably have an "overdrive" top ratio but the term overdrive now means no more than the ratio and does not refer to a separate set of ratios or second min-gearbox.
Look up "Laycock electric overdrive" for an example of the older type, used on MGs, Jaguars, Aston Martins, and perhaps even on 1950s Ferraris. Despite the term "electric" it was a hydraulically actuated mini-gearbox sitting behind the main gearbox.
The Soarer, like many automatic cars of the era, had a 4-speed auto box in which the 4th gear was measurably overdrive in ratio. In part for this reason, and in part to allow gear selection in a standard PRNDL or PRNDD2L fashion, 4th gear was switch-selected rather than being on the stick. This makes no difference at all to the fact that it is still just one gearbox, not two. People who say (as they occasionally do here) that they switch off overdrive around town are doing no more than say that they limit themselves to 3rd gear around town and avoid 4th. Would they do that in a manual? It is the same thing.
Wednesday, May 27, 2009 - 07:03 am, by: Matthew Sharpe(Madmatt)
Indeed, I had an overdrive in my Dolomite Sprint. It was a large device that attached to the back of the manual gearbox - it was electrically inhibited to work with 3rd and 4th only, giving the car a total of 6 speeds + reverse. Fantastic for the open road, but pretty useless around town (though 3 + overdrive was sometimes useful as it filled a big gap between 3rd and 4th). Once the inhibiter failed and destroyed the overdrive when reverse was selected... expensive fix!
In the Soarer I always leave the overdrive switch on, as effectivly its just another version of some automatic cars (like a lot of Honda's for example) that have the "D3" and "D4" positions on the transmission selector.
Wednesday, May 27, 2009 - 05:23 pm, by: Ashley Young(1uzman)
david you are a freak of nature the 5 speed in the vvti's that matt p speaks of is that what is refered to as a 4 speed on ebay this is very confusing it would be a lot less confusing if a manual conversion was not so bloody expensive
Wednesday, May 27, 2009 - 06:40 pm, by: Aaron Mead(Aaron)
Thanks David. My understanding was that it was simply a ratio where as you said, the drive shaft was turning faster than the crank-anything under 1:1 See, my old man used to crap on about having an overdrive installed on his MGB and how it cost a few quid but gave it a much more cruisier top speed. An 80's child like me had no clue
Wednesday, May 27, 2009 - 09:17 pm, by: Sean Routledge(Stircrazy)
Can I ask what other 1UZ's idle at. When I first start mine it idles between 1000-1200 rpm, then drops to between 4-600 rpm after a few minutes. Everyone else's the same? Is this fairly normal?
Thursday, May 28, 2009 - 07:06 am, by: Matthew Sharpe(Madmatt)
Mike Beck wrote on Wednesday, May 27, 2009 - 04:08 pm:
More economical in the long run I guess?
Not quite as simple as that - you have to factor in loads of things like drag, friction etc. An engine thats lugging at 2000RPM will burn more fuel than the same engine in the power band at say 2500rpm. Usually manufactuers spend a bit of time and effort matching final drive ratios up to give efficient speed limit crusing - of course as soon as you start modifying your car all bets are off.