The Triumph Dolomite Club - Discussion Forum

I want share up a theory that someone put to me regarding transmission loses through the overdrive unit. That is, that since the OD is filled with a large volume of rotating fluid (rather like an Automatic gearbox) that a significant amount of rear-wheel horsepower is lost spinning the OD (like an Auto gearbox)?

If this is accurate, would more r/w horsepower be available by using a (gulp) toyota 5spd gearbox?

Thoughts?

The answer is "probably", I think someone did a little research and found that the O/D does bog the car down a bit.

If you are looking at a 5 speed, the LT77 as fitted to the later TR7 would be an easier fit as it has the correct bellhousing.

Because the overdrive is a step up unit you lose 20% of torque when you engage the overdrive. The unit runs in the transmission oil, so it is only partially submerged in the oil, but will suffer some drag. Also the epicyclic gear train in the overdrive is quite efficient so gear train losses will be at a minimum. It is probably the reduction in torque that gives the impression of a power loss.

A 5 speed gearbox may have slightly less loses than an O/D gearbox, but you are giving away a gear ratio.

A Sprint box offers you 6 gears, 5 of them less than or equal to 1 to 1. A 5 speed box only usually offers only 4 gears less to or equal to 1 to 1, unless you spend a lot of money getting a 5 speed close ratio gear set.

We ALWAYs use the overdrive in our race cars, and love the opportunity to flat change between 3-4 or 4+, which tends to frustrate someone who is running door handle to handle with who has to use the clutch.

With the wide ratio gearset you have a 6 speed close ratio and and no need to change diff ratios for long staights.

Mark

I love the overdrive unit and would not change for a 5 speed
I have mine running in second gear as well its had a little work done to it when it was out
7 SPEED