Project: Geo Metro boat tail prototype - 15% MPG improvement @ 90 kph / 56 mph

[Nerys]

not so sure about that. with a 15% improvement I don't see a small flutter in Throttle Position having a 15% impact.

ie if we were talking about 2 or 3 % sure any small variation TP and Wind would have a statistically relevant impact.

but at 15% we are talking BIG change here not little change.

I don't even think a 10mph change in wind would have more than a few % impact on the results.

[3-Wheeler]

Originally Posted by Nerys

...ie if we were talking about 2 or 3 % sure any small variation TP and Wind would have a statistically relevant impact...

Hi Nerys,

For what it's worth...

On the Insight, the TPS is not as sensitive as the MPH indicator.

I have not kept a real close eye on this, but have noticed in warmer weather, and with the TPS at 17, the mph can vary I'd guess about 3 mph.

That is to say, watching the MPH indicator gives a better hold on speed than watching the TPS reading on the ScanGauge.

My tach is not nearly as sensitive as the MPH reading.

______________________________________________

I suppose one could put an externally mounted variable pot on the throttle body shaft and make that work in conjunction with an ohm meter reading...

Jim.

[Sgt_K]

Your car just showed up on the AOL scrolling news reel. Good job, maybe more people will start moding.

[gary paudler]

PVC is evil stuff, but the scrim-reinforced vinyl that is used for truck tarps is very durable and available in black and can be shiny. It is easy enough to sew or can be RF or hot air welded. There is clear vinyl that could be welded or sewn-in as a rear window. It could be supported by an inflatable cone sticking out horizontally from the back of your car with no rigid elements making it easy to stow for low-speed city driving. You could use the exhaust pipe as a compressor for inflation. Urethane-coated nylon or polyester would be a good alternative fabric.

[MetroMPG]

Originally Posted by PaleMelanesian

Metro - have you considered watching rpm instead of mph? There's a lot more resolution available there. 4 digits for rpm instead of 2 for mph.

Tach is a good idea, but I was monitoring the analog (original equipment) speedometer during the runs. An analog gauge gives better feedback than a numeric display - it's easier to see trending with a needle on a dial face.

[some_other_dave]

Originally Posted by gary paudler

You could use the exhaust pipe as a compressor for inflation.

Careful about that: Exhaust is hot, and it contains a pretty good amount of water. Neither one of which is desirable for inflating something made of plastic.

Anecdote: I have heard stories of someone in my car club who used the exhaust from his car to inflate a balloon (used to hold a sign aloft). It started out OK, but after a while the water in the exhaust started condensing out and overloaded the balloon. I don't remember if the water went back down the exhaust pipe and plugged up the motor, or if it just leaked out of the balloon, but either way it didn't work so well.

-soD

[Christ]

The heat might not be such an issue... most cars, I can hold my hand in front of the exhaust pipe for as long as you want to run it and it just doesn't get that hot.

Older cars, on the other hand, with carbs and no cat... those things got warm.

[aerohead]

Originally Posted by Cd

So why is it that the angle of the underside of that 'tail is greater than the top ?

( The 'tail on the EV-1 that is. )

Don Schroeder,who tested my CRX at Chrysler was killed on this track when he lost control of a RennTech V-12 Mercedes.
When A.J.Foyt was going for the record in the long-tail Olds Aerotech he claimed that is was very difficult to keep the car in its lane,And that was with tunnel-ram ground-effects downforce underbelly.
I went to Ft. Stockton in 1997 and they said it was very windy there and it really impacts the number of days when people can test.
It's only my opinion, that the tail cone on the EV-1 allowed the low base pressure of the wake to underride the tail and use it as a moment arm to improve traction over the rear axle.
The "six-pack" of extra batteries on the floor of the passenger side would have added downforce at the front.
Just a guess.

[aerohead]

Originally Posted by Frank Lee

I think it's safe to say some of these angles we take for granted can be cheated somewhat (made steeper) if other elements are in place. As evidence I offer the way the upper rear of the EV1 sits under the aero template. With the good Cd it has, I'd think the flow back there is good.

Frank,I agree. 22-degrees seems to be the magic limit if the C-pillars can feed some air towards the backlight.Chrysler was able to pull this off with the Daytona 500.
I tried to make the template a little liberal.Both development models for Kamm and Jaray have L/D ratios LESS than 2.5:1 and as you can see in the K-car windtunnel photo,the flow separates over the last foot of the roof as it falls way too steeply.
I do believe the EV-1 has separated flow.Check out Gallery-12 in the FLOW-IMAGES and see what you think.

[aerohead]

Originally Posted by NeilBlanchard

Hi,

I think if we consider the 3D flow, then the angles can be steeper than the 2D template. If the air closes in from the sides, it may be filling in where the flow down from the top cannot?

Neil,sorry,I'm trying to catch up.
I agree that angles can be steeper.Frank had just mentioned it and I used the example of Chrysler's Daytona 500 with the C-Pillar buttresses feeding air at the 22-degree backlight.
22-degrees is the angle Mair arrived at to be the absolute max for attached flow we could use,if everything upstream was clean and energetic.