grape said:
I am still in break-in period...the manual suggest to keep rpm not too high,
and I shift at almost 2000RPM per gear, since I don't wanna hurt my new car
btw, keeping rpm higher than 2000 is not doing good on gas mileage...since
nowadays the gas price ain't too pleasant.
Hi,
Can't say I'm an expert on what range to run you car in, but I've
learned a coupla things in the first million or so miles I've put on
smaller (under 2 liter) 4 cyl engines that might help... at least I can
say they "work for me!"
First thing I look for is the specs on the engine: I want torque and
horsepower peaks. I don't know where the numbers fall on the 2.5l, but
let's use my 1.8l for an example. Torque peak is in the neighborhood of
3200 rpm. Horsepower is around 5000 or so. This is similar to most other
fours I've owned, so it serves well enough for discussion.
Others can describe better than I what the differences between torque
and horsepower are, but let's oversimplify things (these are
generalizations, no need to nitpick any slight errors right here) and
say torque gives the ability to keep the load moving, horsepower gives
the ability to make it move faster. Once torque reaches its peak number,
it generally stays fairly flat over a wide range of engine speeds before
dropping, while horsepower continuously climbs until its peak, then
usually stays at that level only for a short time before starting to
drop off, sometimes dramatically. So while my engine redlines at 6500
rpm, it may actually be making less power at that point than it does at
5500.
Now, what's this mean for driving? Basically, for most purposes, I find
I get the best OVERALL performance (combining acceleration, passing
ability, fuel economy, engine life) by keeping my driving within the rpm
range defined at the lower end by the torque peak and the upper end by
the horsepower peak. So, for my engine, that's saying I should keep it
between 3000 and 5000 rpm.
Below the torque peak, the engine will work harder to do the same job,
down to the point you're lugging the engine, which CAN do harm. With a
four, you're less likely to harm it by winding it up than by lugging it
down (assuming you're NOT lead footing it. You'll get better lubrication
keeping oil flow up, better cooling by keeping coolant flow up, better
fuel economy by letting the engine breathe properly, etc.)
Above the horsepower peak, you're probably not gaining much (there ARE
reasons to hit redline, but IME most people don't REALLY know what they
are or use them properly.)
Now we look at shifting points. As a general rule, manual transmissions
on the cars I drive are set up with a span of somewhere around 1000 rpm
between gears (some are wider, some closer, but let's stick with this
number for now.) I want to keep at or above the torque peak in each
gear, so a beginning shift point is relatively easy to figure: take
where you want to be in the next gear, and add 1000 (watch your rev
counter to learn the actual shiftpoints to use in your car.) In other
words, I want to start in 1st, and still be at 3000 rpm after the shift
to 2nd, so I take 1st out to 4000, then shift. I continue that thru the
gears. For downshifting, if I'm starting to lose rpms below my target of
3000 rpm in whatever gear, I'll downshift. So if the car's not pulling
well (as in I have to do more than just cruising) at 3000 in 5th, I'll
drop to 4th, etc. For example, if I'm cruising along on flat ground in
5th at 3000, then start up a grade, when I feel the engine starts to
take more and more throttle to hold that engine speed, I'll downshift.
One should NEVER let the revs drop to where the engine starts to buck.
About now the "higher rpms mean lower gas mileage" folks start to wave
their hands in the air. This is where the guys who program the
automatics get to put their two cents in, and they usually screw it up
IMO. I'll agree, there's no sense in running excessive engine speeds,
but there's a range where the engine works best, and many automatics are
programmed to run below that range more than is good. Which they can do
smoothly, since there's slippage in the system as others have described.
But your manual has already told you what's not good when it started
jerking.
Last point for now is when do we wind the engine up further? Using my
engine, normal shift points when I'm not trying to get somewhere in a
hurry are around 4000 rpm, which keeps me at 3000 or so in each
successive gear. That's fine for normal use. But there are times I need
more: passing, merging onto the freeway, driving up in the mountains.
This is where experience and "feel" for a particular car, engine and
gearbox come in, but I'll sacrifice a tiny bit of fuel economy for the
control of knowing exactly what the car's gonna do next in these
situations. For example, when merging into 70 mph freeway traffic, I
know the car CAN go 70 in either 3rd, 4th or 5th. But in 5th, I'm down
around 3000 and the engine isn't coming up on power, so I can cruise,
but not accelerate easily. At 4000 in 4th, I've got power coming on, and
can continue to accelerate when that guy yakking on his cell phone in
the big SUV is coming up on me a bit fast and doesn't seem to notice I'm
in front of him. So I'll plan my upshifts to keep the engine up on the
power band with some reserve until I'm in a position to upshift
safely--which moves my 3rd-4th shift to 5000 and so forth. This is where
that "extra" range between the horsepower peak and redline can be
properly utilized once in a while.
There's WAY more to all this than what I've explained, and I apologize
if I haven't done a good job, but I think you can see what we're trying
to achieve. For a REALLY graphic demo of the "theory" see if you can
hitch a ride with a big rig driver and watch how he uses those 10 or
more gears for a while. BTW, once you've gotten your engine broken in,
you'll probably find you will actually get better fuel economy using
"my" method than by simply trying to keep rpms as low as possible! And
you'll probably also be amazed at how much your engine can do without
ever even seeing WOT (wide open throttle!)
Have fun!
Rick
