With regard to WOT I nave not checked and so can't comment. If the Manifold absolute pressure is close to atmospheric then that is WOT.

The accelerator pedal moves a fancy rheostat or, more likely, a digital encoder that tells the computer controller what the commanded torque is. The computer then adjusts the throttle to achieve that response.

There is a complex interaction between the ICE and the alternator & traction motor. Note that the only difference between the alternator & the traction motor is the size of the synchronous machine. They can each do exactly what the other does.

The beauty of this design is that the computer causes each unit to do what is needed so that fuel economy is optimized for the specific situation, moment by moment. It does this by adjusting throttle position, excitation, and frequency to the traction motor & alternator.

Remember that the computer is also leaning the mixture with altitude. A rule of thumb that pilots use is 2% reduction in fuel consumption per thousand feet of altitude when leaning the engine.

It is very clear with my FEH that fuel economy improves with altitude.

Regarding altitude vs pressure: 14.7 psia at sea level. You lose roughly an inch of mercury pressure per thousand feet of altitude or about 1/2 psia per thousand feet. (the math: 29.92"Hg = 14.7 psia or 30"Hg ~15 psia)

Back to the original question: don't mess with the design of the intake manifold. It has the long branches is does for a good reason: tuning of the airflow.

 

I forgot to include that engine performance falls off with altitude.

So the improvement in fuel mileage isn't free. Note that the only way to raise power in a piston engine once at WOT is to raise RPM.

Aircraft engine performance reaches limits on the order of 15,000 to 20,000 feet. Turbo chargers raise this into the 30s but in the end even they have limits and engine performance will fall off after reaching a critical altitude.

 

I forgot to mention that you can read atmospheric pressure in your car any time you are in EV mode. Since there is no air intake in EV mode, the Manifold Absolute Pressure is always at ambient.

My car reads 12.4 psia at 4700 ft. when in EV mode.
So Denver would be more like 12.0 psia

psia = absolute pressure

I see 11.8 - 11.9 psia almost every time my foot is on the go pedal.
Even when driving "gently" and with low RPMs.

 

There will always be some pressure drop across the throttle assembly resulting from the flow of air (when the engine is running else you see ambient pressure). So the numbers work out.

Attached is a cutaway photo of the intake manifold. This is a complex component of the engine. I believe it is tuned for the cycling of the vales. As I mentioned the long tubes for each cylinder are set up to ensure that each cylinder operates independently of its adjacent cylinders.

A far simpler, but similar design rationale, was the old Chrysler Slant 6 engine of the 60s & 70s.

 

Bang-Bang servo system....

It appears to me that these HSD systems are programmed to operate in one of two more FE regions and no inbetween unless absolutely necessary. At low RPM the frictional losses are at a minimum but pumping losses are at the extreme. At or near WOT the pumping losses become quite low. So there are TWO most FE regions, run the engine with low frictional losses but with pumping losses fairly high when the engine load, COMBINED engine loading, is low, but at or near WOT but low RPM (Absent actual lugging) when the loading warrants.

The interesting aspect of this is that the control system can allow the battery set to be mostly depleted before moving from one "region" to the other, extending the time period for low engine RPM "closed" throttle operation, and then go "hog wild" with recharge rate, moving the engine into the low pumping loss region during the battery recharge "cycle".

 

Aircraft engine performance is not a good example because as an aircraft engine's performance declines with alttitude so does the drag from airflow over the wings. So things somewhat remain in balance.

With a roadgoing vehicle the gain from loss of drag with altitute is simply not enough to overcome the loss in FE.

 

The air density, so air resistance on the windshield et. al. is 18% less in Denver. 25% less at most of the ski areas. And low density air has no effect on the battery pack and electric motors. That may be your "hidden" advantage.

So, your throttle is 18% or 25% more open all the time and at WOT more of the time, so less pumping losses are seen overall.

[Cut out a part I didn't have proof for.]

 

Without reading the posts by wwest, which I won't read, I infer from gpsman1's post that this guy misused my rule of thumb for aircraft: 2% improvement in fuel consumption per thousand feet.

That rule of thumb encompasses two elements:
- engine performance
- aerodynamic drag

Since an aircraft is likely cleaner aerodynamically (for instance it has no "underbody" drag), direct use of that rule with a car is improper.

The portion that applies (& I have no way of dividing between the two) to engine performance is based on leaning with altitude with the reduction in engine performance with altitude in an engine with carburetors, dual magnetos & dual spark plugs per cylinder.

Compared to the computer controlled, fuel injected, precisely timed engines we are talking about, the aircraft engines I refer to are similar to the engines used by Neanderthals. I meant only an illustration of the potential.

For example: mine gets 37 MPG reliably (ie demonstrated over an extended period and multiple fuel tanks) at or above 8,000 above sea level. Bring it down to sea level and this drops to 28.5.

 

Bill- Where do you live that you spend tank after tank at or above 8,000 ft?
I can only think of a handful of towns above this level.

I spend a lot of time above there, but it's just for day trips, and portions of tanks. ( Colorado Mtns ) But yes, I get my highest MPG in the mountains.

Which seems counter-intuitive with all the steep hills, but steep hills are no disadvantage with this car!

Even when my car was new and I was new at driving it ( the first year ) I was getting consistently over 50 MPG round trips to the Colorado High Country and back to the Denver area using roads with 55 MPH or lower speed limits. I find it more difficult to exceed 50 MPG on flat territory at lower elevations. Doing "pulse & glide" ( that's gasoline powered pulse, with EV glide ) under 40 MPH I can exceed 50 MPG, but it takes a lot of effort. ( and mild weather )

 

Time in & near Yellowstone NP.

With regard to hill climbing, don't forget that with the increase in fuel mileage goes a reduction in engine power output. The computer controller continually matches fuel to available oxygen (as inferred from atmospheric pressure & temperature).

Also the function driving aerodynamic resistance is a cubic function and it starts going up fast above 40-50 mph.

I don't do things like pulse & glide but I do disengage the speed control when climbing a hill and allow the engine rpm to remain about where it was or a little above level cruise and see some gain with that.

I have learned the feather technique for the gas pedal to extend the ICE off times.