Factory wing

[Blue Streak 21]

A bit off of the subject, but has anyone cut large holes in the bumer of the Panoz to reduce the parachute effect caused by the rear bumper cover? I've seen several Porsche's with their rear bumper swiss-cheesed to allow the air to flow out from the pocket behind the fuel cell. This is similar to cutting a few holes in the rear window to allow better flow out of the driver cabin. An alternative coud be to create a diffuser. Any thoughts on which one might be better, or more effective?

[panozracing]

i will be cutting holes. not sure where and how many yet but the hole saw and jig saw will be used.

[NZGTRA17]

Originally Posted by Blue Streak 21

A bit off of the subject, but has anyone cut large holes in the bumer of the Panoz to reduce the parachute effect caused by the rear bumper cover? I've seen several Porsche's with their rear bumper swiss-cheesed to allow the air to flow out from the pocket behind the fuel cell. This is similar to cutting a few holes in the rear window to allow better flow out of the driver cabin. An alternative coud be to create a diffuser. Any thoughts on which one might be better, or more effective?

Jerry, this was the first aero mod that I did to the car. After having the trunk lid half blow off while racing I used a hole saw to cut a row of holes across the back lower edge of the car as can be seen in the photo below.



I did this in combination with other changes but have had no more issues with the trunk lid. I have not had a chance to measure pressure in this area at speed.

Personally if I had the choice between this mod and a diffuser I would add a diffuser as it will certainly be more effective aerodynamically. Cutting the holes is more of a band aid in my view - but it works and costs virtually nothing. A pro diffuser will cost you around US$750 if made from carbon fibre.

Kel.

[gosborne]

Kel – I’ve been giving thought to a diffuser as well and, it seems to me that to gain maximum benefit of a diffuser (enough to warrant purchase and installation) we need to consider a simple under tray to smooth the airflow under the car. I don’t think this would be that difficult and wouldn’t need to go the extent of sealing wheel wells etc. but, probably needs to be considered as part of the package. Thoughts?

update:
After thinking about this further, unless the diffuser tray could extend far enough forward or an under tray far enough back to channel the airflow cleanly past the rear diff and rearwheel openings, I'm not sure the diffuser will give maximum benefit?? Think about a smooth underbody from the front splitter back to where the cockpit enclosure ends, (about where your battery is mounted.) which wouldn't be that hard to do. That smooth underside would speed up air flow under the car creating low pressure with at least some downforce as the result. But, once the airflow reaches the void toward the back of the car it would seem that it would get pretty distorted rather than flow smoothly past the diff and rear wheel wells etc. and thru a diffuser??? Not sure how much that would diminish the value of a diffuser or worse, create instability?

[Blue Streak 21]

Kel, thanks for the pictures. I like the modification you've made to the bumper. Good to hear that it has been effective. What do you think about cutting 6" diameter holes in the flat part of the bumper cover? I assume that the flat portion of the bumper would be the highest pressure region on the cover. I don't have any flow calculations to prove my assumption, just that air caught in the fuel cell region is turned on that surface. The turning of the air could cause a pressure peak. Why 6" holes? Well that's the biggest hole saw that I can find.

I agree with Gary. I've had similar thoughts on what would be needed to create low pressure under the car. Until you smooth out the undertray of the entire length of the car and seal off the volume around the fuel cell, I don't think you'll get the full intended results from a diffuser. I seen pictures of the extent that the aero guys go to for ALMS racers like the Ferrari, or Corvette. The under tray is a work of art. All I'm certain about is that I don't have that kind of budget.

So it's low budget hole saw time.

[Jim woodruff]

Originally Posted by Blue Streak 21

Kel, thanks for the pictures. I like the modification you've made to the bumper. Good to hear that it has been effective. What do you think about cutting 6" diameter holes in the flat part of the bumper cover? I assume that the flat portion of the bumper would be the highest pressure region on the cover. I don't have any flow calculations to prove my assumption, just that air caught in the fuel cell region is turned on that surface. The turning of the air could cause a pressure peak. Why 6" holes? Well that's the biggest hole saw that I can find.

I agree with Gary. I've had similar thoughts on what would be needed to create low pressure under the car. Until you smooth out the undertray of the entire length of the car and seal off the volume around the fuel cell, I don't think you'll get the full intended results from a diffuser. I seen pictures of the extent that the aero guys go to for ALMS racers like the Ferrari, or Corvette. The under tray is a work of art. All I'm certain about is that I don't have that kind of budget.

So it's low budget hole saw time.

I agree that the pressure under the trunk lid is great. On my sports racer
with the areo package I installed fiberglass louvers on the engine lid. On the Panoz I used a 4" plastic dryer vent from Lowe's on the rear lid very cheap and works very well. Cut one hole and you are done.

[gosborne]

Originally Posted by Blue Streak 21

I seen pictures of the extent that the aero guys go to for ALMS racers like the Ferrari, or Corvette. The under tray is a work of art. All I'm certain about is that I don't have that kind of budget.

The benefit these race cars have, in addition to budget and technical resources, is that most if not all run a transaxle arrangement and can get an under tray in place without worrying about dealing with a big ole floating (Ford) 9" rear end housing.

The unit below is an Emco Gears unit used by a lot of the Grand-Am series cars. A lot easier to run a tray under that unit and only worry about Axles/CV joints exiting at either side.

The question that remains for me is - is there any benefit to at least a partial undertray that stops short of the diff and fuel cell area????

[NZGTRA17]

Gary, Jerry,
I dont have a hoist in my garage so I am not well equipped at this stage to panel up the whole underside of the car. I was looking at trying a diffuser on its own for a start. I suspect that this will still produce worthwhile results but that panelling the underside would further enhance the diffusers efficiency.

Kel.

[gosborne]

Kel - I'd love to hear any results if you happen to go that route. I'm still struggling with how it would work. What I am struggling with is this:

To my understanding, the purpose of the diffuser is to slow the air (that has been accelerated under the car so that its higher speed can produce a lower pressure under the car and ultimately some downforce) as it exits the rear of the car and blend it in behind the car closer to the same speed and pressure it was before the car ran into it.

Having said that, it seems to me that if we don't have a means to substantively increase the speed of the air travelling under the car (via splitter and smooth undertray or the like) we have less of a need (maybe none??) for a diffuser to slow it down on exit?

I'd really like to get some definative info on this. Anybody have a bother-in-law who happens to be an aerodynamicist??? J