[TR] TR4A - Driving w/Overdrive

[Mark Hooper]

Here is another overdrive note:

Standard piston engines have a "power band". That is an rpm range in which the
engine power output is considerably stronger than in lower or higher speeds.
When performing manoeuvres such as rapid passing, the usual approach is to
select the gear that will have the engine in the power band and then
accelerating past the car in front.

The reason one does not always leave the engine turning rapidly in a lower
gear to have the power available upon demand is the increased noise and the
much higher fuel consumption in that range.

So, with an overdrive, one can perform a nice and simple trick. When driving
behind a car that one wishes to pass, one can be in the normal o/d gear that
lets the engine run below the power band. Then, at the appropriate time, push
a bit harder on the throttle and switch off the overdrive. The engine will
immediately jump into the power band and the you (and presumably your TR with
it) will start to accelerate toward the car ahead. This would be a good time
to steer around it. Once past the car which was once ahead and is now behind,
just ease off the throttle and re-engage the overdrive.

The above gives one a very smooth and rapid way of passing without losing time
on clutching and re-accelerating.

With a TR6, for example, the power band starts just over 3000 rpm. With the
O/D active, highway speed is around 2500 rpm in 4th gear. Turning off the O/D
will cause the engine to jump to about 3200 rpm, which has a lot more pulling
power.

Mark Hooper
1972 TR6




-----Original Message-----
From: triumphs-bounces at autox.team.net [mailto:triumphs-bounces at autox.team.net]
On Behalf Of tfansher at comcast.net
Sent: September 21, 2011 12:36 PM
To: Dave1massey at cs.com
Cc: triumphs at autox.team.net
Subject: Re: [TR] TR4A - Driving w/Overdrive

I agree with Dave, but will add one comment.B  John Mac while driving into
Stratford on Avon with Ronnie Babbit, Fred Thomas and me made the comment
that
anytime you are over 2000 rpm's is a good time to shift into overdrive.B  I
do
that in my neighborhood speed limit 15 mph, great for 2nd overdrive.B  So
that's my "Sweet Spot".

Tom

60 TR3A w/OD

61 TR3A w/OD

73 Stag w OD

62 TR4 w/OD- wish I had time for the much needed rebuild








----- Original Message -----


From: Dave1massey at cs.com
To: triumphs at autox.team.net
Sent: Wednesday, September 21, 2011 8:11:52 AM
Subject: Re: [TR] TR4A - Driving w/Overdrive

In a message dated 9/20/2011 8:07:46 PM Central Daylight Time,
guy at genfiniti.com writes:
> When is it appropriate to engage/disengage the OD? B Is there some
> recommended
> "sweet spot" for engaging/disengaging?
> Can you shift gears up and down with the OD engaged? B Or do you need to
> disengage the OD before changing gears?
>
>
If you ask 10 list members you will get 12 opinions on how and when to use
the overdrive. B Here's mine: