My bolt-on 911 aero: it will never be pretty, but now it's much easier to pack

[Jack Olsen]

The video site Petrolicious contacted me a few weeks ago and said they wanted to do a video about my garage, which I was happy about. But they wanted to find out if there was anything I was currently working on that they could shoot to show me actually doing something in the place. Well, the little shop recently saw a Miata engine swap and a few small projects for my kids' school and my Jeep and Mercedes -- but the Porsche has been reliably reliable for so long that I feel like I've got to knock on some wood before I type another sentence.

(There -- did it.)

But there was one thing. I bolt on goofy aero pieces for track days. I put on a big rear wing and also a blunt front end with a front splitter. It's easy to put the wing on before I head out for a track day, but I can't do that for the low front splitter, which is just too low for urban driving, and the pieces take up a LOT of room in the car for my drive to the track. A passenger riding with that stuff is just about out of the question. And I've had some ideas about how I might be able to make some slight improvements in the front end design while also making the thing more compact.

Plus, it bottoms out sometimes at very high speed bumps on the track itself, and has been sharpened to a ragged knife's edge in front -- which means it's about time to make a new one even if I can't make it more compact.

Here's what it looks like when it's all put on the car. Not pretty -- but pretty effective:



So I told Petrolicious I would be making some ugly aero stuff.

Anyone curious about the function of a splitter? As the car speeds forward, air hits the front end. Some goes above the car, some goes below, and a lot shoots to either side as it wraps around the car. In doing so, it piles up like plowed snow, and a splitter does two things with this high-pressure air. Most basically, it 'splits' the airflow so some goes above the hood (and to the sides) and a separate segment goes under the chassis. But the high pressure air being compressed by the speeding car also puts downward pressure on the splitter's flat surface, which pushes the nose of the car down, creating relatively penalty-free downforce.

If you have an overhang from the car up above the splitter, then that high pressure air is also pushing up on that piece of the car, so a flat nose is preferable to one that leads up above -- which would happen if I simply bolted a piece of plywood to the underside of my bumper.

In any case, here's the stuff I've needed to fit in my car for the drive to the track. It's broken into a left-side and right-side piece because otherwise it wouldn't fit anywhere. But it's still big and frustrating stuff.



My first idea was that the vertical wall part of the design didn't need to be permanently attached to the horizontal part. The three-dimensionality of those two pieces being mated together was what made the thing so space-filling. So I used some rivet-type nuts to make it so I could just screw the vertical piece on after I bolted on the horizontal piece. A bonus was that I could now get a much clearer view of the bolts for the horizontal piece as they mated up with the inset nuts in the underside of my front bumper.

Here's just the horizontal piece, which is now much easier to bolt on:



I also added a vertical piece at the far ends of the splitter. Because a lot of the airflow is rushing sideways as it moves to the sides of the car, a vertical wall where it's unloading at the sides allows even more high-pressure air to be held up on those corners, for a little bit more downforce without the drag penalty of canards.

Here's the new two-piece setup all on the car for a test fitting:



I cut the aluminum up for the Petrolicious cameras and did some test fitting. Then I was able to test fitting it all inside the car. The good news was that the front vertical pieces now fit inside the trunk -- even with all of the stuff I normally carry up there for track days.

I also re-did my bolt-on rear diffuser. It puts two air guides down under the rear of the car and feeds high-pressure air into an expanding piece just behind my muffler where the air pressure allegedly lowers and sucks the car downward. Here's what it looks like when it's on the car:



Previously, the side pieces were connected to the horizontal top piece rigidly. Now I used piano hinges, which meant the whole package could also fold flat and fit inside the trunk.

Boom. Now that piece also fit inside the trunk.

Here's what I already carry in the car for a track day:



Here are the front verticals and redesigned diffuser packed in there:



The horizontal front splitter pieces were still too big for the trunk. But they could either ride on the passenger seat or (with a little fiddling) fit in the back seat area around the cage.

Front seat stowed:



Or in back:

[Jack Olsen]

But last night I looked at my left-over piano hinge from the rear diffuser and got another idea. I cut the front splitter pieces in two, front to rear, and then hinged them. Now they could be tucked into the front footwell, each folded in half.

Here's the hinge:



Here's what it looks like in place:



Easily fits in the front seat footwell:



No, wait, I did a little puzzle-playing and learned they could now fit inside the trunk as well.

All in, and the trunk still closes easily:



My goal was achieved. I could now drive to the track with a completely empty passenger seat and all my bolt-on stuff stowed inside the trunk. I'm going to test the new pieces at the track next week, and then (possibly) have Petrolicious shoot the stuff in use at Willow Springs next month.

What can you take away from this set of pictures and text? Probably nothing. The number of guys devoted to bolt-on aero pieces like mine are pretty small. It might be just me. But I enjoyed the process enough that I thought I'd post it here.

Thanks for reading -- and how 'bout those stone chips?

[SDFla7Nose]

Jack

This is simply inspiring, my one venture with homebuilt aero only ended up with a clunky plywood splitter and some stiff ABS, the fact that this is quickly and easily reversible is amazing, let alone that it all fits into the car with room to spare,

Keep up the good work

[EPiK]

amazing.. and I for one love the realness that is stone chips. bravo.. excellent creativity

[juanbenae]

Jackie oh! I had pancakes for dinner.

[DHE11]

Originally Posted by Jack Olsen

....The number of guys devoted to bolt-on aero pieces like mine are pretty small. It might be just me. But I enjoyed the process enough that I thought I'd post it here.

Thanks for reading -- and how 'bout those stone chips?

Hi Jack,
The latest modifications look to have worked out reallly well. Great packaging to get it all in the front.
I appreciate a bit of homemade aero, and was amazed how much of a difference my amatuer efforts made on high speed/high g tracks.
I never got to the rear diffuser on that car, but your latest version has insirpred me with some ideas for my new build...
Thanks for sharing.
Darien

[famoroso]

I LOVE it when a plan comes together. Super gratifying. Those A-ha! moments of elation, when your vision takes form, especially each time you closed the trunk lid and it fit / worked, must have been the best. Congrats.

[JonT]

Nice writeup Jack. Does the front splitter deform much at high speeds? Did you raise the lower piece up a bit because the previous setup was rubbing on high speed corners?

[Jack Olsen]

Thanks, everyone. Minor-league project, for sure. But still it was fun to work on.

Originally Posted by JonT

Nice writeup Jack. Does the front splitter deform much at high speeds? Did you raise the lower piece up a bit because the previous setup was rubbing on high speed corners?

It's pretty rigid with the aircraft cable connecting to the bumper up above. But the ride height comes down as the suspension compresses, and then bumps can cause contact. For the new version, I brought the splitter up about 3/8".

I also made the vertical guides for the diffuser deeper. I attached a camera back there awhile back to see what my ground clearance was like, and it looked like I could go closer to the ground. But it will also be 'self-correcting' as the asphalt sands down the aluminum.

This photo was snapped at medium speed. It'd be interesting to see what it looks like at the moments it does contact the track, but that's a tricky camera to set up.

[rwest]

Jack,

Always enjoy your adventures in eking out the most from the least!

Do you have any methods to compare effectiveness of any changes, or mostly feel and seat of the pants?

[fanaudical]

It's always fun to read about what's going on in your garage. Looking forward to the Petrolicious vid!

[Jack Olsen]

Originally Posted by rwest

Do you have any methods to compare effectiveness of any changes, or mostly feel and seat of the pants?

I did a fair amount of testing with the initial setup -- both with wool tufts, video, and also ride height data. I did a bunch of 100-mph straight-line tests with ride height, and then compared them to data from track runs. I also had a helpful aerodynamicist from the airline industry who ran CFD tests based on my particular car shape and my particular aero pieces.

But that was a few years back. Since then, it's mostly seat-of-the-pants and lap times. I think adding the outside vertical pieces to the spoiler will be something I'll be able to feel. I'll try running with and without them next time I'm out there. With the diffuser, I'd like to get actual ride height data -- but that's a little tricky, given that a diffuser's effectiveness in a straight line and its effectiveness in corners (where body roll is a factor) will be harder to isolate. I don't have the kind of time I used to have to sneak off into the desert for an afternoon of high-speed testing. We'll see.