"Exhaust charged" diffuser - 986 Forum - The Community for Porsche Boxster & Cayman Owners

truegearhead · 03-08-2020, 03:05 PM

Last week I finished my "hood extractor" and splitter so with a little time on my hands and some left over material I decided to mock up a diffuser that has a Formula 1 style (before it was banned) exhaust dump in it. Its obviously very roughed in but you get the idea. The exhaust outlet area is going to be replaced with aluminum panel to protect the composite aluminum panel that the diffuser is made of. While the Boxster is already a pretty flat bottomed car I'm going to install an 1/8" aluminum composite skin under the full length of the car.

jaykay · 03-08-2020, 03:47 PM

Wow amazing!

Stroked & Blown · 03-08-2020, 07:49 PM

Love the aero experiments!
Consider going lower with the outside fences to prevent tire squirt. If you leave them just a bit long, they will self clearance and you can clean them up with a cartridge roll.

truegearhead · 03-09-2020, 05:59 AM

Quote:

Originally Posted by Stroked & Blown

Love the aero experiments!
Consider going lower with the outside fences to prevent tire squirt. If you leave them just a bit long, they will self clearance and you can clean them up with a cartridge roll.

Great points, I considered these. I pushed it back and made it as low as I could without needing to remove it when I load and unload the car from the trailer. I already have to remove the front bumper, hood and splitter. I think I'm going to make side walls that easily come off without getting under the car. That'll be version 2.0.

Racer Boy · 03-09-2020, 05:14 PM

If you look at pictures of the exhaust dump diffusers from F1, the exhaust is routed to exit into the airflow so that accelerated the airflow going through diffuser. The exhaust didn't just dump straight into the diffuser at a right angle. This is one of those things that is far more complex than it looks.

Have you read the autobiography "How to Build a Car", by Adrian Newey? It's an excellent read, with his interesting story, but there are also numerous diagrams explaining some of the technical aspects of his car designs. Since he was the first to come up with that innovation, he discusses it a bit (chapter 73), and there is even a diagram to illustrate the concept.

One of the novel things they did was to work with Renault to come up with a system so that even when the throttle was closed, the engine still blew a high-velocity flow of air into the diffuser. They found that without this system, the diffuser efficiency wasn't very good. Again, more to it than meets the eye.

If you want to get some good ideas for diffusers, take a look at the current crop of IMSA GTD or GTLM cars. They have giant diffusers (with no exhaust dumping into them). They are quite large, extending past the rear bodywork a fair bit, and they flair outwards once the diffuser gets past the rear wheels. You would probably have to come up with a large apparatus that you will have to take off the car to load onto the trailer, sorry to say. Take a look at the diffuser on the GTLM 911, it's a pretty complicated piece of work!

I really admire your willingness to try new stuff! I'm guessing that just making the bottom of the car a relatively flat surface from front to the rear of the car will reduce lift somewhat, along with lessening drag.

truegearhead · 03-09-2020, 06:25 PM

Quote:

Originally Posted by Racer Boy

If you look at pictures of the exhaust dump diffusers from F1, the exhaust is routed to exit into the airflow so that accelerated the airflow going through diffuser. The exhaust didn't just dump straight into the diffuser at a right angle. This is one of those things that is far more complex than it looks.

Have you read the autobiography "How to Build a Car", by Adrian Newey? It's an excellent read, with his interesting story, but there are also numerous diagrams explaining some of the technical aspects of his car designs. Since he was the first to come up with that innovation, he discusses it a bit (chapter 73), and there is even a diagram to illustrate the concept.

One of the novel things they did was to work with Renault to come up with a system so that even when the throttle was closed, the engine still blew a high-velocity flow of air into the diffuser. They found that without this system, the diffuser efficiency wasn't very good. Again, more to it than meets the eye.

If you want to get some good ideas for diffusers, take a look at the current crop of IMSA GTD or GTLM cars. They have giant diffusers (with no exhaust dumping into them). They are quite large, extending past the rear bodywork a fair bit, and they flair outwards once the diffuser gets past the rear wheels. You would probably have to come up with a large apparatus that you will have to take off the car to load onto the trailer, sorry to say. Take a look at the diffuser on the GTLM 911, it's a pretty complicated piece of work!

I really admire your willingness to try new stuff! I'm guessing that just making the bottom of the car a relatively flat surface from front to the rear of the car will reduce lift somewhat, along with lessening drag.

Well, what can I say. I agree with you. There is a reason I left a diffuser for last, they seem to be black magic and very hard to pull off. An exhaust charged diffuser (even if design properly) would only help at corner exit, with 230hp I certainly don’t care about that. This’ll be rev 1, we’ll see. Your points are 100% accurate though.

Racer Boy · 03-09-2020, 07:15 PM

It's amazing to look at the IMSA cars, and it quickly becomes apparent that those cars are far removed from what non-factory financed racers can come up with. I can't guess how many hours are spent in the Porsche wind tunnel to perfect that diffuser, and to make it work with all the other parts of the car. For instance, Newey talks a lot about how the diffuser and wing work together, even though it seems like they aren't very close to each other on the car. Aerodynamics aren't necessarily an intuitive thing when you get beyond the add-ons that we consider putting on our cars, like a lip spoiler and rear spoiler!

Again, I admire you thinking outside of the box! It's cool that you are diving into those fabrication projects. When I raced a first generation RX7, I fabricated a cowl-induction airbox filter assembly for it. I spent hours on it, making sure it fit just right, and using high density foam to seal it against the underside of the hood. I riveted it together, and sealed all the seams. It didn't look nearly as well finished as your handiwork, but it worked pretty well, giving me a few mph at the end of the straight. It's really gratifying to have a project like that work!

truegearhead · 03-10-2020, 05:10 AM

I'm taking Racerboys feedback and reworking the design. I've bought these long square tips and will cut them diagonally and fit them into the base of the diffuser. I'll have to push the diffuser back to accomplish this but it will allow me to have the exhaust enter the diffuser tangentially. This started as a "left over material" project and not I've overnighting parts from Japan sigh....:ah:

jaykay · 03-10-2020, 06:44 PM

Yes then you get the coanda effect happening which is what makes a blown diffuser work

Hasbro · 03-10-2020, 08:18 PM

I was considering something similar but having the exhaust exit from above the diffuser along with other airflow from the engine compartment (cooling) through the trunk. Wouldn't that also assist an increased velocity from the diffuser air and from the upper body flow?

truegearhead · 03-11-2020, 11:21 AM

Quote:

Originally Posted by Hasbro

I was considering something similar but having the exhaust exit from above the diffuser along with other airflow from the engine compartment (cooling) through the trunk. Wouldn't that also assist an increased velocity from the diffuser air and from the upper body flow?

Thats another way to do it. Another fun one would be to have the exhaust exit on the top of the trunk to power the rear wing. I think this would be easier to pull off.