[Talos Woten]

Originally Posted by Daox

I doubt there is much difference at all.

There is a huge difference in air pressure around the wheels induced by the rotation:
https://www.researchgate.net/figure/...fig1_309416293
In a free flow wheel, the highest pressure zone is near the top while the lowest is near the bottom (right behind the surface contact). As we cowl the wheel more (with wells, skirts, etc.) the highest pressure zone migrates toward the front of the wheel (right in front of surface contact). Basically, the friction of the wheel is "lifting" air from behind the wheel and slinging it around to the front in the direction of rotation.

From an airflow perspective, the wheels are the dirtiest part of the car. Not only does the rotation of the wheels directly induce vortices, but the top of the wheel is traveling twice the cruising speed against the airflow. It's a big fat mess that only gets worse the faster we go.

Originally Posted by Daox

I do not believe so. It just has a MAF, not a MAP sensor.

(sigh) I was hoping it might be a derived quantity or somesuch. That would certainly make my life a lot easier.

Originally Posted by Daox

It is possible that warm air may be somewhat beneficial for your fuel economy on a Prius, but I am skeptical. On a non-hybrid I would certainly recommend it (and I run one on my Mirage), but the Prius is a bit different. As soon as the Prius' rpm are above 1200, the engine is pretty optimally loaded. RPM is the only differing factor now, and the RPM is changed by vehicle speed and engine load. Lets take a look at a 1nz-fxe brake specific fuel consumption chart to explain this.



This map shows that the Prius' engine is most efficient when operated between 2000-4000 rpm. If you're already in this range on while cruising you're not going to get a ton better. Hotter air should theoretically move the sweet spot on the map to lower rpms if that is what you want while colder air should move it to higher rpms.

Ahhh... this is very educational. Thank you. Thus far what I've noticed is that my cruising RPMs at 65 mph is somewhere in the 1600-1800 range, on reasonably flat roads.

Originally Posted by Daox

Ignition timing is pulled at higher rpms on most engines to protect it. This is all done automatically via the ECU. I'm not sure if that is the case on the Prius' engine because of its atkinson-ish cycle it uses. Its likely the intake charge never really gets hot enough to require this, which would be another perk of the design albeit at the expense of maximum power output.

I don't believe a ram air intake will do anything for fuel economy. There isn't enough pressure build up at the speeds we travel at, and combine that with the fact it may only be useful during acceleration and you have such a limited case scenario that its just not worth putting time into versus other mods.

Righto. Actually, that helps clinch the decision for me that this is a low priority project, so I'll put it on the back burner.

By the way, something else that I looked into was trying to change the fuel map somehow, either by getting a performance chip or fooling the ECU somehow. That was to see whether I could make the car going into lean burn. It turns out... Toyota already uses lean burn a lot, just not at cruising. When the load falls below a certain threshhold (say a slight incline), the engine will typically measure 16:1. Then at no load (coasting down an incline), the engine revs at 900-1000ish and goes into an 18.1:1 burn mode. Then at even less load, the engine shuts off and it uses electricity.

I've also noticed that the car only gets less than 1/5th the aero gain uphill. So when version 3.0 was fully aeroed and got 50%+ mpg, I would get less than a +10% increase going up a serious hill. The lion's share of the gain was going down inclines, and in particular, how quickly it would move between the three regimes above.