The Holes!

[compdrag]

I haven't filled up yet by the UG says 36+ MPG it was .3 on the low side last fill up. Still doing the Cal on it, but it looks promising since my 370 mile fill up was 32mpg and I'm at 1/4 tank and at 350 miles. So looks like I got some good numbers out this tank. I did 60 MPH and let cruise control do all the work. I got very lucky on the return trip, and I5 was very nice to me today. I hit no traffic Jams going through Seattle or Tacoma. The trip up doesn't count because it was 5 am but coming through at lunch time on the trip home was a win!

[compdrag]

Ok well the results are in from the highway tank. 35.95 MPG which I think is very good considering I have a couple days of work on the tank too. Which is just cold running the car the mile to work a couple times a day. I think something is playing a factor here I don't know.

[Frank Lee]

Your testing contains a truckload of variables thus making any results suspect if not void. Join the club.

[AntiochOG]

I'd be interested to see more acceleration testing in an a-b-a type experiment. There are just too many variables to consider when comparing tank to tank. In this graph of fill-ups in one of our cars it is super common to see 10+ mpg fluctuations from tank to tank.

[paconassa]

https://www.youtube.com/watch?v=FTHVg0nID3s

[aerohead]

Originally Posted by Vman455

I found myself thinking about this the other day, and I'm not sure that the conclusion that airflow through the holes would help fill the wake is correct. The Subaru figures indicate a low-pressure area behind the spare tire well which causes turbulence. If holes are added in the bumper, for the air to begin flowing it would need to be higher pressure than the low-pressure wake behind the bumper. Do we know this is the case? Seems to me the first thing to do here would be run a manometer to in front of a stock bumper cover and behind it and see what the pressure differential is, if there is one, and thus if air would actually flow into the car's wake or not.

We know from diffuser research that the air cannot follow more than a 4-degree up-slope angle before drag increases beyond that of the 2.8-degree-to-4-degree diffuser,due to the flow separation and that's predicated upon a 'surface' to flow over.
If you cut a hole in the bumper,the air could actually flow forwards from the higher pressure wake,through the hole,to the lower pressured,more 'kinetic' turbulent air in front of the bumper.
Remember,the air in front of the bumper is in train behind all the underbody bits and pieces.It's all turbulence and has zero dynamic 'ram' pressure.The bumper cannot possibly function as a parachute.
You WANT the turbulence sequestered in front of the bumper,creating a pool of 'dead' air underneath the car,with the active flow skipping off it below.
Only a belly pan,or a belly pan with diffuser can lower drag.
If you see something on a 'track' car,avoid it like the plague.

[aerohead]

Originally Posted by paconassa

I'm personally not comfortable with the capabilities of this particular CFD technology and view the accuracy of it's predictions with suspicion.

[paconassa]

Aerohead, care to explain that a bit more in detail?
With which cfd are you comfortable with?

[Vman455]

Originally Posted by paconassa

Aerohead, care to explain that a bit more in detail?
With which cfd are you comfortable with?

For starters, his model is entirely unrealistic. A completely solid/smooth underside, no suspension, brakes, spinning wheels, no cooling system--this is nice as a theoretical exercise, but real cars are a little more complex.

[Vman455]

Originally Posted by aerohead

If you cut a hole in the bumper,the air could actually flow forwards from the higher pressure wake,through the hole,to the lower pressured,more 'kinetic' turbulent air in front of the bumper.

This is what I suspect. I've actually just ordered a Magnehelic pressure gauge from Amazon.