Best time to shift?....

hello,


I've been wondering this for quite a while. I've got a 99 SL1, manual
trans, and was wondering: when is the best time to shift? Especially
concerning gas mileage, is it better to shift around 2,000 rpm's if
accelerating slowly? How about the best shift points to save on
engine/trans wear? Is there any harm that can be done from shifting
early, like around 1,700 - 2000 rpm? what do you think?

just wondering......

 

I would try to stay in the range where there is plenty of torque. You
reach 90% torque at 1100rpm, and it peaks out at 100% torque at
2400rpm, and decreases after that.

These figures are for the 1995 single cam, but your 1999 should be very
similar.

John Cowart

 

IMHO the S1 engine is one of the best examples of a small gas engine
optimized for high torque output. It does not need to run fast to make
torque which is the force which turns the wheels. When operated at high
RPM, any engine will make more power only because power is a function of
torque and time or speed, it will actually make less force.

Piston speed is the real killer of reciprocating engines and high piston
speeds will cause higher piston, ring and cylinder wear and also very
increased oil and fuel consumption.

In my experience the S1 engine does not really like high loads like hill
climbing below about 1700 rpm (depending on weight and grade). At light
weight when on the level or when going slightly downhill the S1 will be
comfortable and make smooth torque at lower engine speeds. Your engine will
tell you if the RPM is too slow by decreasing speed (even with full
throttle) and not smooth production of power. It is really a function of
power available vs. power required. In general if your engine speed is
increasing then the power available is greater than the power required.
Excessive low speed operation at high load is often called 'lugging' and can
cause damaging loads to the engine lower end. Engine roughness and lugging
at too low rpm should be avoided.

Since power required increases with speed, the proper minimum shift point
also increases with each higher gear. Always start in first gear but shift
to second as soon as the vehicle is rolling. Shift to third as soon as the
vehicle is accelerating. I can usually get into third gear before crossing
an average intersection after starting first in line from a green light.
The important thing is the rpm AFTER shifting, it should be slightly higher
in each higher gear. This technique is called 'progressive shifting' and
was developed with the introduction of high torque diesel engines in heavy
trucks.
http://en.wikipedia.org/wiki/Progressive_shifting
http://www.roadstaronline.com/2005/07/068a0507.asp
more at Google search "progressive shifting"
http://www.google.ca/search?hl=en&q="progressive+shifting"&meta=

Use the lowest engine speed possible depending on load and grade. Listen
and feel what your engine is telling you. You can accelerate at very low
engine speed when going downhill. When climbing hills allow the engine to
slow to 2400 rpm as this is the speed where the engine is making peak
torque. It is also the speed of best volumetric efficiency and best fuel
economy/unit of power. On a steep grade you want the engine speed to be
about 25-2600 rpm AFTER downshift.

It takes a while to become comfortable with the techniques required to get
maximum efficiency and performance from a modern high torque engine. When
computerized fuel injection was introduced in the 1985 Corvette it took a
while to convince the racers that they would get lower lap times by
operating their engines at a lower rpm where they were producing more
torque.

Happy landings,

 

O just short shift and forget about all those other gears if your
worried about a couple cents worth of gas.

 

The little shift light on the dashboard has been optimized for mileage.
That's where they shift in the EPA tests. Not performance or fun!

In general the shift points for best mileage will result in the lowest
total number of revolutions for the engine and for least
engine/transmission wear.

In general the shift light will tell you to shift earlier than most
people would like to shift. (People would tend to "wind it out"). As
others have commented, the SL1's engine has a lot of torque at the low
end and there's little to no benefit from winding it out.

I've driven a SL1 for 15 years now and if anything how I abuse it is
stomping down on the accelerator from a dead start. Probably better for
the car in general if acceleration is more gradual.

Tim.

 

How does this change with the DOHC S2 engines? Should the shift points be
higher due to the higher rpm that max. torque is reached at? Is that just
for power or for fuel economy as well?

JB

 

I have little personal knowledge of the Saturn DOHC engines, but have
studied their specs and have read the reports of many owners in this NG.

I would like to see the graphs of torque, HP and BSFC? (fuel consumption/HP)
for both Saturn 1 & 2 cam engines. I have never requested them from Saturn
and have not found them on the web. If anyone has a URL, please post it
here. Without complete graphs we are limited to the published specs.
Usually, mfgs post and advertise the highest peak torque and HP amounts and
the RPM where these occur.

Saturn reports 114 ft/lbs @ 2400 RPM for both the S1 & S2, they also report
122 ft/lbs @ 4800 RPM for the S2. This is a 7% increase in torque for a
doubling of engine speed. A twin cam 4 valve engine will usually have
better breathing ability which may result in the best volumetric efficiency
occurring at a higher engine speed, the speed of best torque is usually also
the speed of best volumetric efficiency. Compared with the 1 cam/2valve
engine, we would expect the 2 cam/4 valve engine to maintain higher
volumetric efficiency as the engine speed increases. After doing the math
this will result in the S2 having more HP than the S1 and for the peak HP to
occur at a higher speed than the S1. The lighter valves and lack of rocker
arms will also raise the red line engine speed. The S1 has a HP peak of
100HP @ 5000 RPM, the better breathing of the S2 results in a peak HP of 124
HP @ 5600 RPM.

When you are racing you may find the best performance by keeping the engine
close to its power peak and since the S2 reports peak torque @ 4800 RPM and
peak HP at 5600 I would expect the best performance to be found by operating
close to the 4800-5600 RPM range.

However, I would also expect engine wear (particularly ring and timing
chain) to be ~4x as great at this speed as compared with operation at 2400
RPM. I suspect that this is the cause of the many reports of Saturn oil
burning and timing chain problems. I would also expect the engine to
operate hotter and burn 2x as much fuel. High speed operation is also
harder on the transmission and clutch and will also increase wear on
engine driven accessories like coolant and power steering pumps and
alternators and idler pulleys and serpentine belt and tensioner. This
kind of 'spirited' driving also usually results in decreased tire and brake
life.

The S1 is also 70 lbs lighter and has a much higher final drive ratio which
lowers engine speed at cruise. The S1 cruises 65 mph (110 kph) @ 2400 RPM
which IMHO is the ideal and also great engineering. I do not know what the
S2 RPM would be at this speed (one of the S2 drivers here could post the
answer) but based on math (without consideration of tire size), I am going
to estimate 2900 RPM. This higher cruise RPM means higher wear and fuel
consumption /mi.

Readers here may note that I greatly prefer the S1 and am not a big fan of
the S2.

The most important measure of an engine is the amount of torque it can
produce. The easiest way for a designer to increase torque is to increase
displacement. I am old enough to think that 'the only substitute for cubic
inches is cubic money.' Superchargers and turbochargers are often used to
increase effective displacement and volumetric efficiency, but also often
reduce engine life and increase maintenance costs. The most cost effective
way to increase performance is to decrease weight and increase displacement.
The easiest way to increase engine life and decrease maintenance is to
decrease engine speed and load. A modern Corvette cruises 60 mph at ~1700
RPM and IMHO gets very good fuel economy.

Just my .02, YMMV

 

Best Time to shift: Simple! Use the Light, Luke!
But it depends on what you are doing.
I have an SC2 and I usually shift between 2 or 3 thousand.
I also try to drive like I have an egg glued on my gas pedal and I'm
trying not to break it! I use my car for work - and I try to get as
good milage as I can. If you drive like this your engine will probably
last a lot longer.
But shifting at lower than 2000 is probably "lugging" your engine -
which is actually bad for it.

On the other hand. If I'm pulling out into traffic I'll occasionally
wind it up until it screams. The redline for the SC2 is around 6,500
rpm. It really starts to create loads of power above 4000 rpm and
that's where having 4 valves per cylinder really starts to have an
advantage.

Someone sated that the single cam engine weighs 70lbs less than the
twin cam.... I cannot see how that could be. Maybe 20 or 30 lbs at
the most but the cam only weighs about 10 lbs and valves, springs, and
rocker arms just dont wiegh all that much. The bottom end on these
engines is just the same I think.