"Doki" wrote in message
...

"Marlon" wrote in message
...

Hi all,
having just swapped an automatic for a manual, I now have gears and
a tachometer as controllable variables in my driving.
So it occured to me that I've never really done that bloke thing of
knowing about rev ranges, when "best" to change gear (other than when it
feels "right"), and such like.
So.... for instance, given there's an obvious upper range to be
avoided (as indicated by red lines), is there actually a practical lower
limit (obviously without stalling)? Is there a "sweet spot" in between?
Is it possible to determine at what amount of revs a particular
engine is theoretically "best" at - and would "best" be defined in terms
of fuel economy or some other operating measure?
I'd imagine that any such measures would vary according to the
vehicle (& engine size).
Is it obvious where I'm coming from, or should I just go get on with
enjoying driving without worrying about such stuff

In general, try and stay above 1500 revs. You can get away with lower revs
if you're trundling at low speed and have a relatively large engine, but
labouring the engine is a bad thing. Labouring is indicated by funny
noises...

Labouring is when the engine does not respond immediately to an increase in
throttle angle.

Tim.

Mike G wrote:


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.

This is only the case if the inlet/exhaust restrictions are the limiting
factor, otherwise the redline / rev limit are chosen based on the
maximum piston acceleration - the bearing loadings due to piston
acceleration increase as the square of the engine RPM. If you want to
increase power without increasing capacity, you improve intake and
exhaust and increase rev-limit, at the expense of increased bearing
loadings and reduced reliability.

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.
The BMW 2.8 engine deliberately has a restrictive inlet - reputedly to
preventing approaching the 3.0 lump for peak power output. A common
tweak is to swap it for the much less resitrictive inlet from the 2.5.

The point is that it isn't a good example of a 'normal' engine - many do
indeed produce max power very close to max rpm/redline

On 7 Aug, 20:52, "Tim.." wrote:
Labouring is when the engine does not respond immediately to an increase in
throttle angle.

Tim

So does that mean that you were harming the engine before you
increased the throtle, even though it was not making any funny noises
or anything? Or is the damage only likely to occur at those revs once
you use more throtle?

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 find them useful - 1.3 Astras rev to 8000rpm!!!

Easy to do if they rev there and you just got off a bike with a 10,000rpm
red line

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

Hillman/Chrysler/Talbot 1600

red line 6000rpm

High torque max power 69bhp at 4800rpm - will not red line
Normal 80bhp and something like 5800rpm
Tiger (saloon) or Ti (hatch) 100bhp at 6100rpm (in the red!)

My tuned 1600 came off cam at 7200rpm

Didn't worry, a 66.7mm stroke should be safe

Martin wrote:
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 find them useful - 1.3 Astras rev to 8000rpm!!!

Easy to do if they rev there and you just got off a bike with a
10,000rpm red line

I've seen 7500rpm on the tacho of a 1.1 Renault 4 a few times!

Mike P

Mike P wrote:
Martin wrote:
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 find them useful - 1.3 Astras rev to 8000rpm!!!

Easy to do if they rev there and you just got off a bike with a
10,000rpm red line

I've seen 7500rpm on the tacho of a 1.1 Renault 4 a few times!

Mike P

That should be " a tacho dangling under the dashboard of my mates Renault 4
when I was a student". It'd indicate overr 45mph in 1st if you kept your toe
down. It never, ever broke in two years of this treatment either!

Mike P

Easy to do if they rev there and you just got off a bike with a
10,000rpm red line

I've seen 7500rpm on the tacho of a 1.1 Renault 4 a few times!

I have over revved a few cars when getting off a bike, too easy - drove a
1600 Sierra which has a very low red line even though it is short stroke, I
had just got off a 12,500 red line like, 7000rpm is reachable!

On Fri, 8 Aug 2008 07:02:05 -0700 (PDT), Dave
wrote:

On 7 Aug, 20:52, "Tim.." wrote:
Labouring is when the engine does not respond immediately to an increase in
throttle angle.

Tim

So does that mean that you were harming the engine before you
increased the throtle, even though it was not making any funny noises
or anything? Or is the damage only likely to occur at those revs once
you use more throtle?

It's peak cylinder pressure that does the damage. Part throttle means
the cylinder doesn't fill completely or in a Diesel only a small
amount of fuel is injected so the contition that causes damage isn't
present. Damage is actually unlikely unless sustained and lack of
response will tell you stop but high fuel consumption is penalty.
--
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 Wed, 6 Aug 2008 07:19:47 -0700 (PDT), Dave
wrote:

- Engine damage occurs over about 0.5k over the red line. Unless
really critical, do not use the revs above this. Most modern cars
have rev limiters that prevent going much over the red line anyway.

It's not immediate de-arrangement (well yes it is with OHV pushrod
motors) but the initiation of small cracks in pistons and rods that
grow with every rev. It takes time at high rpm/load for those to grow
to the point at which engine integrity is compromised.

Nissan 1800cc CA18DET has a 7,200 rpm redline for UK market, the N/A
version the CA18DE in Japan has a 7,700 rpm redline. Bottom end is
identical. I don't know what they ran it to in Japanese F3 but it
should have been over 8K.

1972 Honda 350 K4 twin had a 9,500 rpm redline. Production race kit
was a pair of exhaust pipes, pair of carbs, pair of high comp pistons,
a cam, valve springs and a 14,000 rpm tachometer. They run to over
11,000 rpm in classic racing.

1974 Honda CB125S has a 9,500 redline on a 12,000 rpm tachometer. Over
18,000 miles in 18 months I saw 12,000 on 3 occasions (in top!). A car
magazine did their utmost to kill one. Eventually they put it on the
stand, then held the throttle wide open with no load, the rod
ventilated the crankcase at 17,000 rpm.

Obviously exploring the red zone means the warranty is void and
expected life is one race season ~ 2000 miles. Annual rebuild requires
new pistons and rods.
--
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!