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Porsche 944 Track Car

Porsche Track Car Part 25 – Wind Tunnel Testing

I am fortunate to live and work near Charlotte, NC.  This brings me some opportunities that might not be available in other parts of the country.  As most know, Charlotte is a center of racing activity, in particular NASCAR.  Think what you will about NASCAR, it is very successful and attracts a lot of sponsorship dollars.  To be competitive, most of this money gets plowed back into the sport to make the cars as competitive as possible.  One result is that there are no less than 3 wind tunnels in the Charlotte area.

Two of these are way out of my price range, charging thousands of dollars per hour.  The exception is the A2 Wind Tunnel.  This one is right next to one of the expensive ones. Gary Eaker built both.  One to service the high end race teams.  And one to serve the little guy. Thanks Gary!

I had known about this wind tunnel for several years but really hadn’t had an excuse to visit or use it.  But recently I found myself with my Porsche 944 track car loaded on the trailer, ready to head for a HPDE at Road Atlanta but not having to leave Charlotte till mid-afternoon.  Bingo!  Let’s take the car to the A2 wind tunnel and see what’s shaking.  I contacted them and made an appointment for Friday morning.  They charge by the hour so my stay would have to be brief.

As explained to me by General Manager (and my techie for the day) David Salazar, there are 2 general types of testing.  If a race team shows up with a car that runs under the rules of a recognized sanctioning body, where there may be money on the line, he is not allowed to make any suggestions.  He just runs the equipment and reports the results.  You also get your own private garage bay where you can prep your car and keep it from the prying eyes of the competition.  Be assured, David has a pretty good feel for what works and what doesn’t, but to be fair to those that have spent their development dollars he cannot share the results of other race teams.

I was more fortunate.  I am showing up with a $5000 track car.  He was able to share his knowledge of general testing approaches and areas for improvement with me.

To set the stage, my car is basically stock, at least as far as air flow is concerned.  The only thing about my car that would affect air flow is the fact that it is lowered about 1″ below stock.  But it has stock mirrors, stock front and rear bumpers, air dam, spoiler, etc.  First we pushed the car into the tunnel.  The tunnel has four pads on the floor surface which are adjusted to fit exactly under each of the 4 tires.  Think of these pads as the 4 corners of a giant H shaped balance beam. For proprietary reasons I wasn’t allow to look at the workings underneath the floor but David explained that each pad could measure the corner weights at the 4 wheels.  But more importantly, the pads could measure drag, lift, and side force.  Using the math presented on their webpage they can calculate the downforce (or lift) at the front and rear axles,  the drag force on the car, the drag coefficient, the horsepower required to push the car down the road, and other terms.

Here is a link to a video that I took as David was setting up the car.

Video link.

David explained that the wind speed of the tunnel is 85 mph.  At this speed, it is not safe for anyone to be in the tunnel when the fans are running.  We retired to the control room, where you can watch the car on video monitors.   David explained that he would capture all of the data from each run and put it into an Excel spreadsheet that I could take with me when I was done.  The neat thing about the spreadsheet is that it can extrapolate the raw data gathered at 85 mph to higher (and lower) wind speeds.  In my case, I selected for the data to be extrapolated to 120 mph, which is a speed I rarely obtain but certainly feasible in my car at the end of a long straight.

It takes about 3 minutes for the fans to spool up, stabilize, and then collect data.  We did an initial run just to make sure nothing flew off the car.  Great news.  No loose parts!

We looked at the data from the first run.

REQUIRED SPEED INPUT > > MPH = 120
DRAG DRAG RELATIVE DRAG  LBS Force at Speed
HP lb ∆HP ∆Drag (lbs) Total Front Rear Side
86.9 271.5 133.4 115.2 18.1 -2.5

Interesting.  At 120 mph, there is 271.5 pounds of drag force, requiring the the car to use 86.9 hp to maintain a constant speed.  More importantly, the car is producing 115.2 pounds of uplift at the front axle at this speed but only a modest 18.1 pounds of uplift at the rear axle.  Certainly, there is no downforce present in stock configuration (lowered 1″).  And what was the drag coefficient?  0.371.

David knew that if we introduced “rake” to the car, the uplift would go down but that the drag force would go up.  To directly see this effect, we put 2 1/2″ of boards under the rear wheels and made another run.  Here are the results.

93.3 291.4 6.4 19.9 69.6 45.7 23.8 -5.1

Let me interpret that for you.  291.4 pounds of drag force use 93.3 hp, which is an increase of 19.9 pounds and 6.4 hp.  The front lift went down to 45.7 pounds and the rear lift went up to 23.8 pounds.

So we reduced the front lift but at the expense of more hp required to push the car down the road.

Next we effectively lowered the car closer to the ground by letting air out of the front tires to lower the front end by 1/2″ and we reduced the lift at the rear to 1 1/4″.  Here are the results.

89.8 280.7 2.9 9.2 97.6 72.1 25.5 -2.8

The hp requirements went down by 2.9 hp.  But the front lift went up to 72.1 pounds and the rear lift went up to 25.5 pounds.  From experience, David knew that beneficial results would be obtained by taping up all the extraneous openings at the front of the car.  We kept the rake angle from the previous run but we put duct tape over all the openings and gaps except for the inlet to the radiator. We did this in stages but the final results were quite impressive.

84.8 265.1 63.7 27.4 36.3 -1.9

The horsepower requirements were down to 84.8 hp.  Remember, we started at 86.9 so the gain was about 2 hp.  Free for the price of some duct tape.  Front lift was down to 27.4 pounds.  Remember we started at 115 pounds.  Rear lift was up to 36.3 pounds. We started at 18 pounds.  In other words a much more balanced car from an aero sense.  Not bad for a few hours work.

I was quite pleased with the results of our session and the professionalism of David at the A2 wind tunnel.  If you are really serious about evaluating the effects of that new wing you have installed, this is really the only way to go.

Oh, sorry.  Porsche 944’s do not produce any downforce.  So don’t try and drive one on the roof of a tunnel like the Formula One cars can do 🙂

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Discussion

One thought on “Porsche Track Car Part 25 – Wind Tunnel Testing

  1. Nicely done. I admire your thoroughness and your willingness to share with others!

    Posted by Anonymous | October 2, 2012, 11:32 am

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