Aero Testing

[bigjae46]

I’ve been planning a big cooling and aero related modification for awhile now. Before I did anything, I want to get some reference data to know whether I've achieved my objective of reducing drag, improving cooling and saving a couple of lbs.

The plan is to use a non-M manual radiator and a large oil cooler mounted on the back. Ideally I'd use a fullface oil cooler. Unfortunately, nothing off the shelf exists at this point. Fabricate a vent on the back of the radiator to vent out of the hood. The idea here is to minimize air coming into the engine bay to reduce drag, improve downforce and be able to use a flimsier hood which can be 2-3 lbs lighter. I currently have a bunch of gaps and openings so I'm guessing I have a big ol' high pressure region under the hood which is made worse with the front splitter.

The complication is having enough room on the backside of the radiator to make a vent that promotes flow attachment up and over the engine then out of the hood without screwing up the flow up the windshield which could affect flow across the rear wing.

Back to the radiator/oil cooler. I think most people go the wrong direction by adding thicker radiators, more radiators, and more fluid. One thing that has had a huge impact is reducing my coolant volume to about 1.75 gallons. I might have a radiator with a fullface oil cooler custom made, need to get off my butt and get pricing.

First, I want to take a pressure differential across the radiator. I installed an air probe before and after the radiator connected to a meter mounted on the dash.



Second, want to check for flow detachment across the hood and windshield. I’ll record this with a GoPro mounted above the windshield.

[heinzboehmer]

Hell yeah, looking forward to seeing the results.

[Slideways]

Third picture has me convinced this car is held together by gaffer tape lol

Jae is doing some race team aero sh!t by himself. Very impressive, looking forward to updates.

[bigjae46]

Some learnings - if you delete the cabin air filter housing, you need to seal up the hole. You get a lot of air being sucked through the opening into the engine bay. I think this reduced the differential pressure across the radiator, I have to watch the in-car which has the manometer readings.

Had some issues with the pressure probe tubes - need to secure them better. They would move and get jammed between the hood and fender which caused a pinch point.

When I towing the car to the track, all of the tufts were straight. On track, the tufts behind the side vent showed some turbulent air so there is some air extraction from the vent.

Here is the baseline with the cabin air filter opening covered.

***VIDEO with cabin air filter opening uncovered coming***

I signed up for a PCA event in Dec to do some more testing. Here's the plan

- I'm going to get better air pressure readings.
- Tape down my floppy hood and windshield cowl, cover the cabin air filter opening
- Cover the kidney grills
- Rip off all of the duct tape around the radiator ducting check the differential across the radiator
- Uncover the kidney grills
- Un seal the windshield cowl

Third picture has me convinced this car is held together by gaffer tape lol

Jae is doing some race team aero sh!t by himself. Very impressive, looking forward to updates.

Lol...thanks! Had to seal all of the gaps in the ducting. We'll see what comes of this.

[Estoril]

The trailing edge of the hood bounces around more than I would have guessed. Just me - I'd find that kind of distracting at speed.

Full props on the experiment and lessons learned.

[bigjae46]

The trailing edge of the hood bounces around more than I would have guessed. Just me - I'd find that kind of distracting at speed.

Full props on the experiment and lessons learned.

I made the carbon hood. I made it light...too light at 6lbs...and the underside is two small plates for the hinge mounting. As a result, it tends to flutter. I consider my hood to be more of an engine bay covering than an actual hood...lol

[bigjae46]

Here is the video with the cabin air opening uncovered. The hood tends to lift up at a slightly lower speed

[Darbshaw]

I have some plans to dig into cooling this winter. Saw your post on professional awesome Facebook page. Excited to see this is your car and a more detailed breakdown of your findings!

[bigjae46]

First, my biggest issue. Any ideas on how to better capture views of the strings on the car? Although my roof go pro flew off...I still need to find a better way. I definitely want to do some testing on splitter and down the sides of the car. Eventually, I want to put strings on the entire right side of the car. Might have to enlist photographer buddy?

A week ago, I closed off the kidney grills and cabin air opening. Added a gurney in front of the right hood vent. I also smoothed the transition from the hood to the windshield.

Top speed in Nov was 125mph, I was able to get to 130mph. The intake air temps were about 10 degrees lower than in Nov.

I uncovered the kidney grills in two afternoon sessions, I got worse water cooling but better oil cooling - mostly because the 2nd oil cooler was uncovered. Also, the strings on the right hood vent with the kidney grill open were standing up and the ends were bent over. It was wicked to see! But there is a longer area of turbulent flow behind the vent.

I noticed there was better flow attachment at the base of the windshield.

And the GT3 sounds waaaaaayyyyy better

[bigjae46]

In an attempt to get some data of flows on the front bumper, I bought what I call a "cock stick". It's the long suction cup arm that street racers use to record their street racing. I used a GoPro Session4 in a waterproof case.



It gets a lot of looks driving around the paddock and you have to be careful not to spear a wall or a pedestrian.



It worked great! My hood flutters at speed so the stabilization was working overtime. The video is usable and I can get the front corner and even the sides of the car. It is super distracting. I just did 2 laps and came in.

Summer sucks in TX so it will be a great time to collect more baseline data.

[CrookedCommie]

You have the n15 design hood vents? Are you finding that they evacuate air but the cabin air filter cover does not and should be covered?

I'm debating blocking off one or both of the bumper brake ducts so they don't dump air into the engine bay. Will not be using them for brakes anymore. Currently have one feeding directly into the stock intake and the other not sure what to use it for, so dumping air into the pass side engine bay.

[bigjae46]

You have the n15 design hood vents? Are you finding that they evacuate air but the cabin air filter cover does not and should be covered?

I'm debating blocking off one or both of the bumper brake ducts so they don't dump air into the engine bay. Will not be using them for brakes anymore. Currently have one feeding directly into the stock intake and the other not sure what to use it for, so dumping air into the pass side engine bay.

I haven't done enough testing to say conclusively. Yes, N15 vents and they work A LOT better with a gurney flap. I have a 3/4" tall flap right now, probably could be a little smaller - like 3/8 or 1/2. The gurneys pull out a lot more air which may cause more air to want to get sucked into the cabin air opening and through any gaps between the hood and windshield cowling. The air will flow to the low pressure region. My thinking is as you lower the pressure in the engine bay, it disrupts airflow somewhere else.

I have deleted the brake ducts. I cover them up with duct tape right now. Working on a cover at some point hopefully this year.

[heinzboehmer]

Are you finding that they evacuate air but the cabin air filter cover does not and should be covered?

I'm pretty sure BMW put that opening there because it's a high pressure region. That opening serves as an air intake on stock cars, not an exhaust.

Your car is basically full racecar, right? If so, I would experiment with blocking off that opening entirely. I do wonder if you could make some simple ducting to help cool the trans. Would probably get a BUNCH of lift out of it if not designed correctly though.

We need to get this dude to do a breakdown on an E46: https://youtube.com/@premieraerodynamics

[CrookedCommie]

Yes I'm thinking blocking it off and skip installing the middle hood vent. I'm wondering if I should also cap off the pass side brake duct instead of venting it to the header side engine area.

[heinzboehmer]

I'm wondering if I should also cap off the pass side brake duct instead of venting it to the header side engine area.

I would. Cap it off at the bumper for max aero gains.

I've been trying to figure out something to do just that for a street car, but I can't think of anything that doesn't look like you just stuck a plug into the bumper. Maybe CSL bumper is in my future.