recommended rpm

Can one do damage driving at low rpm's? My wife likes to drive our 86
740 turbo at sub 2000 rpm in OD, I suspect using the turbo to
compensate. It bugs me, but can it really do harm?

It's a second hand car, so I don't have a manual.

 

It should carbon up, use more fuel and possibly overheat. Oil won't pump
with enough pressure for the work being asked of the motor, so it won't last
as long. But it's hard to prove all that to someone who doesn't understand
and God protects fools, women and drunks.

 

I usually try to keep it under 2000 RPM cruising, though that's not
always possible. It's best to just drive it by feel, if it's lugging or
vibrating excessively then downshift to raise the RPM.

 

So long as your engine isn't pinging or rattling. If so you should be
using a better grade of gas. Cars with an ecm should make the fuel
richer to prevent detonation. What does it hurt?

 

It does go into the yellow zone on the turbo dial, which worries me
because I've heard overuse of the turbo can wreck the engine. The
previous owner says his teenage kids blew two motors! But what's the
difference between a teenaged boy driving it hard, and my wife
low-rpming it up the hills with the turbo in yellow?

 

Boy Racers (of whatever age) tend to rev the engine high and
long, and this is harder on turbos than having to operate at low
speeds and higher pressures. Do these engines even increase the
turbo pressure that much under low-speed load...?
--







http://freevision.org/michael/index.html

 

How could reving high be hard on the turbo if the turbo dial is showing
a lower reading for the same speed? I assume the turbo dial is
pressure, but I don't know.

The particular circumstance is driving on the highway around 90km per
hour, in OD, around 1800 rpm, then not gearing down for hills, and the
turbo going way into the yellow until back on the flat.

When I drive, I gear down for hills, the engine revs are higher
(2500ish) but the turbo reading is much lower, well below the yellow.

 

Using the turbo a lot will consume more fuel and cause more wear on the
turbo, but you really have to try in order to blow a Volvo motor. I
don't know what the teenagers could have been doing other than possibly
constant redlining the RPM or messing with the boost. I drive mine
fairly hard and have yet to damage even a high mileage motor.

Generally you want to keep the needle in the black unless you need the
power, boost won't hurt it but it won't help the economy any since the
ECU richens the mixture under boost.

 

The turbo pressure will increase to the full 8.5 PSI at any speed if you
give it enough gas. The pressure is always whatever the guage shows
regardless of RPM, though the guage is not calibrated. The difference is
that the turbo will be spinning faster to maintain any given pressure at
a higher RPM as more air will be flowing through the motor.

 

Yes if it is going that high I imagine it could hurt the engine.
Especially on that year I am not sure there is anything to enrichen the
mixture to protect the engine. I don't know. Using premium gas will help
to protect the motor. If she scraps the motor, you will really have
something on her for a long time!

 

For a given pressure, higher revs result in more heat and more wear
on the turbo. I doubt it glows dull red when run under load at low
speeds like it can at high speeds.

As noted elsewhere, if it isn't pinging, she probably isn't damaging
it.
You may want to see if you can adjust the wastegate to limit the boost.
--







http://freevision.org/michael/index.html

 

FWIW, the 740 series water cooled turbos are remarkably robust. I couldn't
convince my wife (who was the primary driver of our '85 765T for more than a
decade) not to rev the engine when she started it or to let the turbo cool
down when pulling off the freeway into a gas station. I could hear the turbo
spinning down as she got out of the car. At nearly 240K miles it is still
going, and the engine has never been apart beyond timing belt replacements.

Your wife is more valuable than the car, so relax and enjoy the ride.

Mike

 

Because the boost guage is showing just that, pressure. If the engine is
revving higher the turbo has to spin faster to maintain the same boost.
Really though either scenario will not hurt the turbo, it should last a
long time regardless of how it's driven as long as it's well maintained
and doesn't get extreme abuse.

 

Ultimately, turbo or not, anytime you pull to much hp at too low of an
rpm, out of an engine, the likelyhood of detonation increases. (Also the
temperature rise of the intake air aggrevates this situation.) That will
damage an engine. Modern cars enrichen the mixture to decrease this from
happening. I don't know what sort of hills your driving on. If you hear
pinging or a rattling sound during this event you are damaging the
engine. The exhaust gas temperature also goes way up and can cook older
turbochargers. I don't know what is in your car. I do suspect though,
she is asking a bit too much to be in overdrive going uphill. Especially
some hills I have in mind.

 

Maybe she needs this: http://www.viatrack.ca/



Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

void _-void-_ in the obvious place

 

In general high revs wear the top end of the engine (rings, cams and valves)
whereas low revs wear the bottom end of the engine (bearings). As long as
the engine isn't lugging it should be ok although personally I tend to keep
the revs between 2000-3000 for general driving (S70 T5).