BSFC increases as you throttle back an engine. Higher BSFC = more fuel used per hp generated. The lowest BSFC is WOT at the peak torque RPM.
I think you meant that effeciency drops?

The greater % you throttle back an engine the worse the increase in BSFC, which is why big engines are bad for economy. It's not the extra displacement per se, it is because you have to restrict the engine so much to only make a very low HP number that it is producing power pretty ineffeciently at that point because of all the throttling losses and the resulting low cylinder pressures. This is why GM and Chrysler are using cylinder deactivation - cut out half the cylinders and now you can open the throttle alot more and produce the required power more effeciently.

I have often wondered, if you have a car with a very low CD, would it be effecient to have two engines? Shut off the 500hp one on the highway and run a 35hp unit to cruise on. When the driver hits the gas, the big motor fires up and you go! Hybrids have really developed the technology to quickly and seamlessly start and stop an engine so this might be possible. A 35hp engine running at near WOT would be quite effecient...

Exactly what I meant. :o

Regarding a drop in drag coefficient, like anything else, there are plenty of reasons why manufacturers don't bother with it. There are probably more, but they include
-Aerodynamic instability at high speeds
-Building great EV gliders
-Small increase in efficiency
-Interbrand pressure
-Lower cost to maintain
-Etc...
To take advantage of the increase in glider efficiency for gasoline engines, they need to drop in a taller OD gear, or use a CVT. The problem with a taller OD gear in today's automatic cars is the balance between having the tranny kickdown, or stay in gear with more throttle for better efficiency. Toyota/Lexus went to an 8 speed automatic with some kind of intuitive learning so the car could get better mileage, but most Corollas aren't going to get an 8 sped auto tranny. Even the Prius' CVT doesn't spin at the optimal rpm for fuel efficiency at ~55mph, since it still has to rev up in order to have what most consider to be acceptable passing power.

BSFC maps are generally pretty simple. NA diesel have a pretty even distribution of ovals.
https://www-personal.engin.umich.edu/...s/image016.jpg
As do TDI's. with the large drop at the bottom being associated with the turbo dropping out of it's efficiency band (turboed gasoline engine probably has the same deal).
https://home.earthlink.net/~rps1976/I...ion%20Map.jpeg
A DOHC NA gasser looks like this.
https://www.v6performance.net/gallery...0805_Fig11.gif
With the difference between the dotted and solid lines probably attributed to better control of ignition and possibly better fuel injection control. A VVT-whatever engine would have even larger BSFC ovals with the peak BSFC pushed a bit to the left... And an Atkinson cycle gas engine has near diesel BSFC ovals. I think a SOHC may have the peak higher, or at the same place depending on the cam (i.e. it may not let in as much air as early, pushing peak BSFC farther down), and carbed engines can probably have some odd maps, since they don't meter fuel linearly, or as precisely as a fuel injected car would, unless I'm at the right rpm.

2001 Saturn SC1 EPA ratings: 28mpg city / 40mpg highway / 33mpg combined
2001 Saturn SC2 EPA ratings: 27mpg city / 38mpg highway / 31mpg combined

However get a load of this:
I wonder what fuel economy improvement someone would see if the SC2 transmission was swapped with a SC1 transmission? Perhaps the SC2 does have better aerodynamics and more efficient engine, but it's hampered with a shorter top gear, thus decreasing its FE? I'd like to find the actual final drive ratio differences between the SC2 and SC1.

Peakster -

Yes, the S?2 drivetrains have what is called an "MP3" transmission, while the S?1 drivetrains have an MP2 transmission.

I think the only question is whether or not the ECU/PCM would get upset. Otherwise they should just swap right in.

This is a saturnfans.com quesiton. It must have been asked before. Maybe I will ask it again.

CarloSW2

Are you talking about a manual or automatic transmission? If it's a manual, I can't see any reason that the ECU would know the difference as long as you have the right signal for the speedometer.

For an automatic, it would depend on how much GM integrated the powertrain and engine controll modules. If the trans controller is seperate from the engine controller, that would likely need to be swapped along with the trans to get the right shift points and then communication issues might come up.

He is talking about the manual trans. In saturn s-series cars you often have to swap trans and the matched pcm together. I'm not sure on the particulars I ran into similar issues while looking into cruise control. Apparently the pcm monitors VSS and engine RPMS to make sure there's no funny business.

Peakster -

I found my 1997 Saturn brochure (another victory for packrats!). Here is the Cd info :

Attachment 342

I am going to post all the vitals in a later post.

CarloSW2

Interesting. Yet another source that says that the SC1 has higher drag than the SC2.

Eureka! The SC2 has a more aggressive spoiler
 

I think I found the main difference between the SC1 and SC2:

SC1:
Attachment 378

SC2:
Attachment 376

My question is why is it that the SC2's spoiler (which appears to separate the rear air flow higher than the SC1) is more beneficial for Cd?

One reason I suspect is that the 'kick' in the SC2 spoiler does a better job at eliminating/reducing vortecies at the rear departure by allowing more side air to tumble underneath.