On Thu, 07 Aug 2008 16:21:17 +0100, Willy Eckerslyke
wrote:

Dave wrote:

I really disagree. I can make a much more acurate decision about when
to change gear with a rev counter than with my ear.

May I humbly suggest that you might benefit from fitting a vacuum gauge?
I was quite surprised by the results when I fitted one to my Vitesse.
Seems I'd been in the habit of changing gear at far lower revs than I
should have been for greatest efficiency.

See your vac gauge and raise you +12psi (gauge). Proper engines have
BOOST gauges. Shows vac too but it's no fun unless it's high in the
boost.
--
Peter Hill
Spamtrap reply domain as per NNTP-Posting-Host in header
Can of worms - what every fisherman wants.
Can of worms - what every PC owner gets!

On Thu, 7 Aug 2008 19:35:35 +0100, "Mike G"
wrote:


"Dave" wrote in message
...
On 7 Aug, 14:17, "Mike G" wrote:
"Dave" wrote in message

...





On 7 Aug, 12:25, "Mike G" wrote:
"shazzbat" wrote in
message

...
Change gear when it feels/sounds right, which you'll pick
up
very quickly. The rev counter is a pretty toy.

Yup. It isn't that long ago to a time when tachometers were
only
fitted in 'sports' or more expensive cars as std equipment.
They have little practical use.
Mike.

I really disagree. I can make a much more acurate decision
about when
to change gear with a rev counter than with my ear. At the
bottom of
the rev range this probably translates to a lower incidence
of
head
gasket failures, at the top of the rev range it definatly
means
higher
performance.

At the top of the rev 'range' maybe. but peak revs on a tacho
is
seldom the right time to change gear for maximum performance.

In almost every car I have driven the maximum power is achived
just
before the rev limiter.

I seriously doubt that. IME max power can be as much as 1k revs
below peak revs. Max torque below that.

The only exceptions, other than the very few
that did not have a rev limiter, were the 2 rovers I have owned,
where
the power did decline as you approached the red line, to the
point
where there was no more acceleration possible in that gear. I
assumed
this was because they were fitted with some sort of "soft" rev
limiter
where fuel and / or air was gradually restricted as the revs
increased, rather than abruptly stoped at some certain point on
all
other cars.

I have heard that on old cars the size of the inlets were the
limiting
factor at high revs, and so power would drop. I have never had
anything like that. What sort of cars do you drive, and were is
their
peak power?

On my car, 528i BMW. Max revs can go to 6,500 for short periods,
about 6,250 sustained. Max power is delivered at 5,300, max
torque at 3,950.
Mike.

As far as lower revs are concerned, you shouldn't need a tacho
to
tell you when revs are too low.

Possibly. I did used to use very low revs, in the assumption it
was
good for the pocket, and now I may have gone too far the other
way and
am too hesitant to use the bottom of the rev range. I will not
use
full throtle well above the point that you can feel the engine
complain.

IME you can hear and feel when to change gear, up or down. I
can't say I've ever needed to refer to a tacho, even when
driving

I guess it is personal preference. I sort of learnt to drive in
tractors, where you have a tacho but no speedo, so I use the
tacho in
preference to the speedo. YMMV.

For max performance it's not max power rpm that dictates change point.
It's what the power is in the current gear and next gear at reduced
rpm. The idea is that they should be the same so you have to exceed
max power rpm.

Get a copy of Cartest 4.5, run the standing start.
http://www.cartestsoftware.com/cartest4.5/index.html
it's a free DOS version that runs on XP and lower

For me with 6.4K max power rpm and 7.2K redline Cartest 4.5 says
1st2nd is 7.2K, 2nd3rd is 6.9K, 3rd4th 6.7K rpm and 4th5th 6.6K
rpm. The 3rd4th and 4th5th changes are not available on UK roads as
6.7K in 3rd is 92mph.

For Diesels the important thing for decent acceleration is to not let
the revs drop below peak torque. Above peak torque rpm the difference
between the highest and lowest useable gear is less than 10%. Too high
a gear giving rpm below peak torque the acceleration is dramatically
reduced ~ 50% of that in lower gear. If you feel the surge as it
builds up to peak torque you are just making black smoke and getting
in everyone's way, change down and press the pedal!
--
Peter Hill
Spamtrap reply domain as per NNTP-Posting-Host in header
Can of worms - what every fisherman wants.
Can of worms - what every PC owner gets!