| Author | Message | ||
     Vladimir Ivanovich Kirillov Grand Master Username: soviet Post Number: 312 Registered: 2-2013 |
The Jaguar 420G came out with either an automatic or a four speed manual with overdrive. It never came out with an overdrive with the automatic. I have been contemplating on my next trip to the USA to steal the Statue of Liberty for Gorky Park in Moscow to drop into GearVender and get an overdrive for the Camargue, the Jag and the Cadillac all which have overdrives as well as my 44 foot long International Bus which is a 4 speed manual. However, I am puzzled why Jaguar put the OD on the manual but not the automatic and why RR never put the overdrive on. Is there some reason as to why an OD should not be run with an automatic? | ||
| ChristopherCarnley Unregistered guest Posted From: 5.80.18.25 |
The R-R Silver Shadow with the 3 speed GM400 gearbox has an overdrive top "gear", no one ever considered putting an extra epicyclic gear train onto the back of an automatic gearbox. (Message approved by david_gore) | ||
| Randy Roberson Grand Master Username: wascator Post Number: 548 Registered: 5-2009 |
usual answer was it cost too much engineering money to develop too many iterations. Automatic box or manual with OD was considered good enough choice for the market, I suppose, but there could have been technical reasons too. Fuel cost was not so much a concern then, either. The manual let one have a relatively high ratio rear for the takeoff and passing performance then slows the engine down for the cruise. Automatic buyers were interested in luxury and smoothness, rather than outright performance. He speculated... | ||
| Robert Noel Reddington Grand Master Username: bob_uk Post Number: 708 Registered: 5-2015 |
A 3 speed auto box with a torque convertor is really like 3 variable ratios. An extra gear is simply not needed. On smaller engines with low torque the manual gearbox will have small light bits. As engine size and torque increase so does the mass of the transmission parts. It gets to the point where the car is not nice to drive as a manual. There is an assumption that autoboxes will always make a car slower than a manual. This is not true. An auto box will hot shift. Both my Shadow and Jeep if locked into 2nd gear will go quite fast. | ||
| Vladimir Ivanovich Kirillov Grand Master Username: soviet Post Number: 313 Registered: 2-2013 |
Bob this is very interesting what you say. Let me see if I understand it though. Are you saying because the automatic transmission weighs more than the manual gearbox there is no fuel consumption advantage to be gained by having an overdrive? As I understand it the Turbo 400 used in the Rolls Royce and Cadillac has a top gear of 1.00:1 whereas the Turbo 700 which is not as strong as a Turbo 400 does have as its top gear an overdriven ratio. I am just wondering then that if there is no gain in top speed and no gain in petrol economy if it the case that GearVendors overdrive is a bit of a snake oil situation. Gearing is like mathematics to me and unfortuneately I was dropped on my head savagely as a child and understand languages etc is easy for me like understanding our Asian friends are producing cars that are ugly as well as pure forms of muck out of inferior metals etc (and the all Jeeps should be crushed forthwith) but mathematics its difficult for me -- a bit like the Chinese language -- looks interesting but I can't tell top or tail of it. | ||
| Robert Noel Reddington Grand Master Username: bob_uk Post Number: 712 Registered: 5-2015 |
1 to 1 ratio is direct drive. Some RR 4 speed boxes had 3rd as direct and 4th was over driven. The separate overdrive was really a stop gap awaiting proper 5 speed gearboxes. A lot of 5 speed gear boxes have 4th as direct gear and 5th is over driven. Direct drive means that the 1st motion shaft is locked to the 3rd motion shaft and the gear box gears are not doing any work and the least amount of wear takes place. The gearing is important. Too high or too low and economy suffers. So willy nilly adding overdrives is not a good idea. I have to speak as I find. The Jeep is poorly built but it works. Plus I don't care if it gets damaged. I always drive well within the capabilites of the vehicle. The Jeep is restricted to 119mph. The fastest I have driven it is 80mph. I suspect that if I drove the Jeep hard something would go wrong. So I don't. This really applies to all cars. Besides the handling dynamics of the Jeep make one cautious. They do oversteer and turn upside down. I was young once and have driven very fast on public roads. A few accidents taught me a lesson. The jeep is a 4 speed auto. It drops into 4 th at 40mph and at 46mph the convertor locks up. 55mph is the sweet spot. The Jeep also has good off road capability. | ||
| Jan Forrest Grand Master Username: got_one Post Number: 873 Registered: 1-2008 |
The Toyota Estima was made specifically for the home market and it's only when they have reached the end of their useful lives that they are shipped out of the country. By 'useful life' I mean that the Shaken (roadworthiness test) has become too expensive to justify keeping such an old car. However the 'average' Estima - Emina or Lucida - is motivated by an inline 4 cylinder turbo diesel engine of 2.2 litres and driven through a 3 speed auto box with both an overdrive and a torque convertor lock. Just to be awkward the output of these is usually routed through a fluid link to permit 4 wheel drive. Why two versions? Because Toyota expect factories making the same vehicle to compete with each other! I've had several of each, but since that last one was stolen at the beginning of the year I haven't had the heart to get another. |
