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
