• Hey Guest!
    British Car Forum has been supporting enthusiasts for over 25 years by providing a great place to share our love for British cars. You can support our efforts by upgrading your membership for less than the dues of most car clubs. There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Upgraded members don't see this banner, nor will you see the Google ads that appear on the site.)
Tips
Tips

Questions about Overdriver

Share this page

Webb Sledge

Webb Sledge

Jedi Warrior
Offline
LAYCOCK DE NORMANVILLE! That's fun to say, isn't it?

But back to my questions. First, can anyone give me a rough idea of how much it costs to add an OD to a Triumph? Also, do the OD increase the top speed of the car? I can't see how it wouldn't, by cruising at the same speed but lower RPMs.
 
[ QUOTE ]
Also, do the OD increase the top speed of the car? I can't see how it wouldn't, by cruising at the same speed but lower RPMs.

[/ QUOTE ]
The top speed of a car is a balance between available horsepower & car wind resistance/drag. This balance is achieved when the rpm at which the engine has maximum horsepower coincides with the speed where drag matches the same horsepower.

Stated another way, top speed depends on the overall gearing, engine rpm vs horsepower characteristics, & car drag. It is maximum when they are all matched.

If a car is under geared, the horsepower will peak at a certain rpm & then decrease again as rpm continues to rise. when drag has increased to the point where power at the corresponding rpm is equal to the drag no more speed is available.

If a car is overgeared, as is usually the case with OD, the car will reach the drag limiting speed before the engine reaches maximum horsepower rpm & speed will be limited to what the engine can put out at that corresponding drag speed.

The bottom line is that absolute maximum speed will usually be higher in direct drive than in OD.
D
 
Webb:
[quoteThe top speed of a car is a balance between available horsepower & car wind resistance/drag.

[/ QUOTE ]

That's why occasionally in a magazine road test you will see the top speed listed as "xxx MPH (Drag Limited.)"
Jeff
 
Dubya,
I just added overdrive to my 73 TR6. $1600+.
I had John Esposito at Quantum Mechanics in Connecticut do it for me. My transmission was already a recent rebuild and in new condition. He has to take it down anyway and change the shaft in it to accomodate the J-type overdrive addition. The J was what my car would have come with but you can add an A-type if you so leaned. I got tired of cruising down the highway at 3500 rpm. Nice to drop the engine speed: less noise and wear, better gas mileage.
There are other vendors out there but I only have experience with John. He has been helpful.

Bill
 
[ QUOTE ]
[ QUOTE ]
Also, do the OD increase the top speed of the car? I can't see how it wouldn't, by cruising at the same speed but lower RPMs.

[/ QUOTE ]
The top speed of a car is a balance between available horsepower & car wind resistance/drag. This balance is achieved when the rpm at which the engine has maximum horsepower coincides with the speed where drag matches the same horsepower.

Stated another way, top speed depends on the overall gearing, engine rpm vs horsepower characteristics, & car drag. It is maximum when they are all matched.

If a car is under geared, the horsepower will peak at a certain rpm & then decrease again as rpm continues to rise. when drag has increased to the point where power at the corresponding rpm is equal to the drag no more speed is available.

If a car is overgeared, as is usually the case with OD, the car will reach the drag limiting speed before the engine reaches maximum horsepower rpm & speed will be limited to what the engine can put out at that corresponding drag speed.

The bottom line is that absolute maximum speed will usually be higher in direct drive than in OD.
D

[/ QUOTE ]

So basically if you get more power out of the engine to overcome the drag factor, your top speed will be higher in say a TR3, 4 or 6?
 
[ QUOTE ]
So basically if you get more power out of the engine to overcome the drag factor, your top speed will be higher in say a TR3, 4 or 6?

[/ QUOTE ]

Or if you just reduce drag... pretty easy on pre-50000(?) TR3As where the windscreen comes right off.

I usually drive with just the aeroscreen, but I'm not about to explore what the top speed is in a car where most of the braking, suspension, & steering components date from the Eisenhower era.
 
Back
Top