AeroRanger (brother to AeroJeep) boat-tail

[winkosmosis]

Originally Posted by KamperBob

Usually thinner air at elevation leads to richer mixture and less efficient. The rule I recall is a normally aspirated engine loses 2% max power per 1000' elevation. So 6500' should pay a 13% handicap compared to sea level by all rights. Then again, maybe you know something others don't...

Cheers
KB

The engine adjusts gas according to how much air is going in though... not volume but actual mass... right? That's the basis for hot air intakes.

When I had a Grand Cherokee I got much higher gas mileage driving around Montana and Wyoming than Texas.

[cons]

Interesting. Maybe that explains why my city mpg up here is terrible, but at hwy speeds air density is more important than engine efficiency.
I don't know, but I was really skeptical about that first 60 mpg run, and coming so close again on the same run seems like there is some accuracy to it.
And if that mirror was folded in the whole time, if it wasn't windy at all, and I didn't run the fan but wore clothes fresh from the freezer, I think I could do even better.
Lastly, I've been using mid-grade fuel and that seems to have helped some. Has anybody else noticed a difference?

Now I'm excited to do a full belly pan, boat tails behind each tire, a major plexiglass sheet to extend from the top of the cab in a straight line, resting on the hood, and extending out to below bumper level (only for road trips), LED lights, alternator delete, AC delete. Then it could be 70 plus range. That stuff will be a ways away though.

[cons]

Glad to hear that. That makes me feel a little more grounded in this 60 mpg thing because it still seems unbelievable and when I drive to MA in the fall, I feel confident my mpgs will be going down. (Then I'll just have to add some more things to get them back up though!) This is also a higher valley of the Rockies. Most of MT is around 3,000 feet.

Originally Posted by winkosmosis

The engine adjusts gas according to how much air is going in though... not volume but actual mass... right? That's the basis for hot air intakes.

When I had a Grand Cherokee I got much higher gas mileage driving around Montana and Wyoming than Texas.

[cons]

"One thing you should know is that I'm at high altitude (~7000 ft average).
That definitely plays a role as it effectively turns my 3.5L engine into a
2.8L (there's about 21% less air up here...less air means less fuel) and it means less wind resistance." From 5speed5 today

If true, me being at 6,500 feet turns my 2.3 into a 1.85L engine, helping to explain to me why these numbers seem a little too high.
I've heard you can't even throw a curve-ball at high altitude because the threads don't get grip on the air. I've asked others out here who say they get way better mileage up here too.

SO, these numbers seem way high. I don't want fellow ecomodders thinking I'm full of BS. I would be a skeptic hearing about a truck doing 60 mpg.
But remember, I think altitude plays a big role, and these 60 mpg are under pretty ideal conditions on only HWY (roundtrips uphill and downhill, upwind and downwind).

It might not be that out there for a small truck with a small engine.

[KamperBob]

That last explanation rings true. Mostly I hear about altitude derating of (NA-IC) engines in the context of RVing. Scenario 1 is the rig's ability to crest a 10,000' pass with effectively one piston on the bench. Scenario 2 is the generator's ability to run same appliances at elevation as sea level. I wonder if more efficient operation at altitude applies mainly to overpowered vehicles as opposed to say a frugal car with a tight nut behind the wheel.

Cheers
KB

[usergone]

Originally Posted by KamperBob

Usually thinner air at elevation leads to richer mixture and less efficient. The rule I recall is a normally aspirated engine loses 2% max power per 1000' elevation. So 6500' should pay a 13% handicap compared to sea level by all rights. Then again, maybe you know something others don't...

Cheers
KB

Originally Posted by winkosmosis

The engine adjusts gas according to how much air is going in though... not volume but actual mass... right? That's the basis for hot air intakes.

When I had a Grand Cherokee I got much higher gas mileage driving around Montana and Wyoming than Texas.

The way I understand it is that the closed loop mode is called closed loop because it uses sensors to get feedback on the A/F ratio, among other things. That is the main purpose of the oxygen sensor. It lets the ECU know how close to stoich it is running, so it can correct the amount of fuel injected. (sorry, I had to explain)


Thus, if you take an engine up high, with low air pressure, there will be less oxygen and the O2 sensor will pick up on it, then the ECU will just put in less fuel. This argument only holds true for fuel injection (and electronic at that) because a carburetor doesn't adjust itself

[5speed5]

Yes, altitude is your friend if you're an ecomodder/hypermiler and you have either a fuel-injected engine or an engine with a carb that is jetted for the altitude. Of course, you have to use it wisely. I imagine there are some people that just step on it harder to make up for the lowered power and they probably use more fuel.

[COcyclist]

cons, nice work on your aero mods. I live over a mile above sea level and I do believe it helps with highway mpg but you are doing something right too with your mods. Bicycle racers go to Mexico City for their high altitude velodrome (The hour record for bicycles is the record for the longest distance cycled in one hour on a bicycle. It is one of the most prestigious in cycling. Hour-record attempts are made in a velodrome, frequently at high altitude for the aerodynamic benefit of thinner air.- from Wikipedia) You seem to be getting some good results from your full boat-tail even with the square corners. Ingenious design with the hinge for tailgate access. I hope you can talk your buddy with the TDI Golf to start a build thread too. The photos in your web album are interesting.

[cons]

Thanks Cocyclist for that aero info. Squared is just way easier for me. I'm not a very talented builder of things.
I'm getting on Matt's case to finish his boat tail. If he doesn't do a thread, I'll do one for him. He's searching for the right material to put on it. The aluminum flashing didn't work out, he's a perfectionist, and there's not much to pick from in Jackson. He might just do a cheap, short-term solution like cardboard until he gets to San Fran in a month.

[COcyclist]

Can you salvage some large used coroplast political signs? I used a 4x8 sign for my belly pan.