Poor Man's Aero Kit -- Initial Testing

Jack Olsen · 04-15-2005, 12:31 PM

This Wednesday, I tested my new front and side skirts, as well as my extended underbelly panel and diffuser, at Willow Springs.

I ran a total of 8 sessions in the day. At first, I used the front and side pieces, but not the diffuser. Then I added the diffuser. Finally, I removed all of the pieces for a control session.

The initial results didn't involve any kind of data logging other than lap times. At Willow Springs, lap times typically drop off in the afternoon as the track heats up. Temperatures on the track itself were between 70 and 83 degrees all day.

My first concern was with engine temps. The results surprised me. I could see no difference between 'naked' runs and ones where the under-the-engine pieces were in place. Apparently, the ventilation I added is doing its job. In normal street driving, I've been seeing Cylinder Head temps between 200 and 250 degrees. On the track, I got up to about 280-290, but the needle never made it to 300, even. And it reached the same high point with the engine cover as it did without. (Plus, the sender was calibrated with boiling water, and checked out as accurate.) My car appears to run rich, based on exhaust gas testing on the dyno. Still, I'm surprised that these readings are so low.

Engine oil temps were between 180 and 210 all day, which is normal for my car in these conditions.

With the belly plates in place , I did see elevated transaxle temperatures, even with the transaxle cooler running. At the hottest part of the day, I got a reading close to 250 degrees. If I keep using the belly pieces, I'm going to have to add some ducting to the transaxle cooler.

I also got a lot more heat in the cabin, even with the heater boxes were set in the pass-through (off) position. I assume this is because the route the heat is normally able to escape through was obstructed by the belly pieces. I also have an aluminum panel where the rear seats used to be, which was not screwed down tightly and was probably allowing a lot of the trapped heat into the cabin.

The results? At this point, I'd have to say... completely inconclusive. In the first two sessions, I ran with the front and side skirts in place, and the belly covering going back as far as the transaxle. My best time was a 1:35.59, which is about normal for the old street tires I'm currently running. I wasn't pushing the car very hard, and I'm sure I could have lined up a 1:34 without much problem.

I put the diffuser on (in addition to the front and side pieces) for the next three sessions. Track temps were slightly higher (up to 83 degrees) by this point. But my lap times were comparable. The best I did was a 1:34.95.

Still, this is where things get complicated, possibly. With the diffuser in place, I could swear that I felt quite a bit of push in the car through the high-speed sweepers (one is 90-95 mph, the other 130-135 mph). So even though my times were comparable to the laps with no diffuser, the car felt different. I even spun once in turn 9 after the car pushed more than I expected on the exit and put two wheels off the left side of the track.

My conclusion on the diffuser runs? It might be that eliminating the big rear wing or extending the front lip might meaningfully improve lap times by balancing out the downforce between front and rear. In other words, it could be that the diffuser is effective, even though my lap times didn't change significantly. It could simply be that the car needs to be adjusted to take advantage of the new equipment.

Then again, it might not. I'm cautious about trusting subjective driving impressions. I'd rather see data.

Finally, I took everything off for a 'control' run.

Now, it wasn't until these two final sessions that I found someone who was going surprisingly fast on the track, and the fact that I was chasing him (former POC Chief Driving Instructor Rick White) might mean my data is even less reliable. When you've got a rabbit to chase, things tend to quicken up.

On the other hand, the wind was picking up significantly. Cars were getting blown over as much as 15 feet on the exit from turn 8.

But my times in these final sessions were the best I ran all day. I had several laps under 1:35, with a best of 1:34.48.

So, I had roughly comparable times all day, but the best came at the end, which is when the track is usually about a second slower than the morning.

My conclusions? Nothing, really. I guess I can say that I need a more precise way to test these components. Ideally, I'd come up with a way to measure downforce in both the front and rear of the car on a long straight. Strain gauges might be the best way to do this. It might also be possible to measure ride height in some other ways. I'm looking into it.

In three or four weeks, I should have my DL-1 data logger. By that time, I hope to have some sort of method in place to measure downforce with these different configurations. That will be part 2 of this testing.

(Edited at 2:18 pm on April 15th, for clarity.)

Bill Verburg · Bill Verburg's Garage

Quote:

In normal street driving, I've been seeing temps between 200 and 250 degrees. On the track, I got up to about 280-290,

Is that oil or head temps?

Dantilla · 04-15-2005, 01:06 PM

Quote:

[i]

The results? At this point, I'd have to say... completely inconclusive.
[/B]

We need more guys like you that can be objective about upgrades.

Jack Olsen · 04-15-2005, 01:13 PM

Quote:

Originally posted by Bill Verburg
Is that oil or head temps?

Those are cylinder head temps at the #5 plug (with the plug that's accessed from below). I'll make that clearer in the original post.

Oil temps were between 180 and 210 all day. No change in oil temps with or without the engine cover, which -- like the CHT -- surprised me.

Bill Verburg · Bill Verburg's Garage

Good to know

jluetjen · jluetjen's Garage

Did you try running with some tufts taped to the body or oil drops? Photographing the car on the straightaway with tufts attached or taking pictures of the oil drops after a session can be very helpful in visualizing how the air is moving around the car (or rather how the car is moving through the air).

chrisp · 04-15-2005, 01:31 PM

What do you use to measure head temps with?

Yellowbird RS · 04-15-2005, 01:36 PM

Yes probably you need to use some tufts taped to your car and take a video from a car behind.
9ff underbally:

Tyson Schmidt · 04-15-2005, 02:01 PM

Jack, it's not surprising about the tranny temps and the heat in the cabin.

In the 964, they dump the heat from the exchangers out into the rear wheel wells when the heat is turned off and not entering the cabin.

In the 993, they dump it under the engine.

I think we could try to adapt the 964 heater valve to work with your car, and it should solve both the problems: heat coming in the cabin through the rear seat area, and higher trans temps with the undertray below the trans.

Ryce Stallings · 04-15-2005, 02:55 PM

I know very little about aero for racing but I am curious. The intent of your aero package is to block air going under the car and "extract" air from under the car to create downforce!

Is the increased frontal area slowing the car down? Did it feel like the car had more grip with the package in place? Do you think the big wing more than you need? Does it slow you down.

OK, I know nothing but really like the thread.

350HP930 · 04-15-2005, 03:01 PM

Well jack, I am glad to see the strain gauge idea is growing on you.

greglepore · 04-15-2005, 04:09 PM

Seems like a lot of hassle for the minimal, or unmeasurable, gains. All of the mods essentially are to create a low pressure area under the car (or prevent a high pressure area) and it looks like the baseline aero config you run might be good enough, particularly with the long hood and the bigass rear wing. Given that the stuff can get bent, torn and otherwise cause a black flag, I'd consider the KISS rule.

It would appear that on feel alone, the difuser works, but unless you can somehow add front downforce, it's not a good thing.

Tyson Schmidt · 04-15-2005, 04:29 PM

Seems to me that it will not make the car faster if it increases rear downforce on a previously balanced car.

Like Jack said, it'll probably be faster if the front downforce was increased a like amount, or the rear wing could be trimmed. Or the rear upright could be swapped for the normal 3.8 wing, reducing frontal area and drag, while the underbody and diffuser result in the same downforce that the upright had.

Jack Olsen · 04-15-2005, 05:00 PM

The rocket surgeon is on the right path, I think. If I could step down to a ducktail, but maintain as much downforce as the 3.8RS wing is giving me, it would be a net gian.

But I'd like to have better data.

efhughes3 · efhughes3's Garage

Hmmm, I wonder if the real advantage of Aero pieces is to be gained when they are designed along with the body. That is to say, a "package deal"?

Tspringer · 04-15-2005, 05:43 PM

Hmmmmm. I would agree that if you feel your getting push, then perhaps you could reduce the rear wing and thus the drag but net out the same downforce.

Perhaps you could also use some suspension tweeks to take advantage of the changed aero stuff as well. For example, loosen the front anti-roll bar to increase front grip?

One thing I do know for sure.... you having fun!

beepbeep · 04-15-2005, 05:55 PM

Hmm...

more rear downforce with diffuser -> front push -> take off rear wing -> same rear downforce but less drag -> better lap times

Tyson Schmidt · 04-15-2005, 07:06 PM

My thoughts exactly.

BTW Jack, the ducktail may be too much of a step backwards. Unless that diffuser is damn effective. Still it would be cool to try it out.

Tspringer is also right-on about tuning the suspension. You could either stiffen the rear bar, or loosen the front to make it more neutral at high speed, which will also help reduce low-speed understeer.

I really wish I was there to help you play around with things Jack. We should do an event where I don't run my car, and we just test and tune your car.

rich22 · 04-16-2005, 01:15 AM

What's this "poor man" nonsense?

For more front downforce, you should consider a splitter. There's a reason all the best dressed racecars have them.

oldE · 04-16-2005, 02:00 AM

Jack,
I don't think, in anyone's wildest dreams, you will get enough downforce from a 'duck', even with the diffuser, to balance out the push (Understeer) you reported in the high speed sections.
As was pointed out above, you either need more downforce on the front, or less on the rear at that speed. Without doing a re-design, you know how to decrease downforce at the rear, and you might decrease drag at the same time. Your rear wing adjustable, isn't it? Try reducing its angle of attack by 5 - 10 degrees. I don't think you are looking for big changes at this point. Good luck.
Les