Coweling and engine bay aero

The Convert

Captain
Jun 4, 2017
1,433
I know there have been numerous discussions on this in the past between those that chopped their coweling up, swapped to M3, or removed it all together. I can't find it, so I am putting this here. The portion of the video discussing the pressure zones and direction of airflow through the engine bay start around 7:00-7:30. Great watch overall and recommend it to everyone interested in modding.

@doublespaces please fix if not done correctly, but be gentle, it's my first time.



 
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KClemente

Specialist
Nov 26, 2019
78
Yeah one thing a lot of people don't think about is the high pressure air inside the bay vs the high pressure air on the lower part of the windshield. If you're ever serious about engine bay aero, the M4 GTS hood has the perfect vent placement for our cars.

https://www.e90post.com/forums/showthread.php?t=1306425

Basically, point 2-3 on his photos is what he states would be the best to vent air
 
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Sergeant
Jul 27, 2017
281
This thread is scaremongering shit by the op.

That video has nothing to do with engine aero, by cutting your cowl you actually increase flow through the engine bay and directing the air over the wind shield causing extra down force on the car, but it does increase drag.

7.00 to 7:30 talks about air getting under the car, get a front lip to combat that or dont as it will make stuff all difference to engine aero.

Most cars will never see over 145mph, god forbid we all will flip, crash and burn our cars.

Great video full of paid advertising where to get your products.
I think it's a very complex area.
At low speeds the heat from the engine bay will definitely flow out from the gap created by removing the cowling.
I've done it and I can see it when the car is hot and I'm stationary. For road driving which is often alot of starting, stopping and travelling at low speeds then I think removing the cowl is very beneficial. You've removed weight. You've removed material that can heat soak and you are stopping hot air getting trapped.

When on the move at 60mph plus then a pressure wave does does build at the front of the wind shield but this pressure wave is there regardless of if the cowl is installed or not. So with the cowl installed air cannot rise out of the engine bay upwards. With the cowl removed it probably can't either as the higher pressure there will either stop it or slow it. We'd need to measure the pressure created at the front of the car and within the engine bay. So alot of this warm air will probably still be forced downward with the suction created by the fast moving air passing under the car. Basically I think engine bay air flow out of the bay is probably largely unaffected by removal of the cowl when at speed.

Obviously we have a moving car getting cooler external air forced into the bay from the front due to it moving forward at speed and drawn into the bay from the engine and turbos creating a negative pressure. The engine pumps a fair volume of air. A good intercooler is essential in a car like this to benefit from that cooling air hitting the front of the car. Alcohol fuels also do a good job of cooling the charge.

I tried the strings method. I tapped lengths of string on my hood\bonnet to see where the air flow was at low and medium speeds, between 20 and 80 mph.
String between the hood and windshield was forced down into the engine bay at higher speeds.

The thing is the 335 was a poorly designed car when it came to aerodynamics. High speed tests showed the car producing lift at the front and the rear. A front air dam and rear spoiler are a must. A good spoiler can reduce overall drag by lessening the vacuum created behind the car.
I think it's also beneficial to remove all the materials included in the car purely to quieten the engine to the passengers in the cabin. It wasn't about performance. It was about comfort. Rarely do these two things function well together.
 
The cowl is a performance robbing part, a big IF the cowl helps aero but it robs our cars of cooling which is far worse in terms of track lap times.

The cowl is only there to keep noise and water out of the engine bay which still can be with a properly trimmed cowl.

With the undertray on there will be no front end lift as the air gets passed around the engine bay, over the windshield, around the gearbox and channels the air past the diff especially the m3 diff.

When I hear people talking 135 or 335 engine bay aero I just laugh my head off.

The car was speed limited from the factory for a reason as it wasn't designed to run high speeds.
 
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Sergeant
Jul 27, 2017
281
The cowl is a performance robbing part, a big IF the cowl helps aero but it robs our cars of cooling which is far worse as seen by people that track their cars.

The cowl is only there to keep noise and water out of the engine bay which still can be with a properly trimmed cowl.

With the undertray on there will be no front end lift as the air gets passed around the enngine bay, over the windshield, around the gearbox and channels the air past the diff especially the m3 diff.

When I hear people talking 135 or 335 engine bay aero I just laugh my head off.

The car was speed limited from the factory for a reason as it wasn't designed to run high speeds.
There is front end lift despite the trays. It was measured.

windtunnelm3.jpg
 
I never said there wasn't, but there is no proven test showing removing the cowl will cause extra lift.

Lets say the was a small lift, a big if.

What is more beneficial little extra down force or a much cooler engine where it doesn't hit limp mode quickly.

What would be better for lap times a cool running engine or getting an extra 1 to 2mph around the corner with a heat soaked engine with reduced power.
 
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I also like to see that with and without the cowl and a modified one.

All that shows is the air travelling over the bonnet and hitting the windscreen.

Proper testing in a wind tunnel is what's needed.

If someone is worried about aero that much run one at the track and take it off whilst running on the street.

But I rather enter the corner a little slower and punch out with more mid to top end with cooler running engine.
 
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Sergeant
Jul 27, 2017
281
...there will be no front end lift...
🤨

Well. I agree with you. The cowl is best removed. Any change in engine bay aero is likely to be little to non in my opinion but I can't be 100% on it.
I think they are probably correct in the video though that heat doesn't escape upwards when at high speeds but it looks like cold air is actually being forced in.
Likely it is forced down and out by the positive pressure from the front and the wind shield.
 

The Convert

Captain
Jun 4, 2017
1,433
I think this is a great video and an informative thread. Until @martymil got all up in it mind you. I linked it to a few friends who thought the video was interesting and they might play around with a few things on their 'race cah'.

I thought @martymil was too busy to post on the forums because he had to run his business? Which is amazing he can run a business considering how much time he spends running his mouth on the forums and shills for you know who. Unless of course someone asks about his casting experience and then he suddenly gets all quiet and you can hear the crickets. So strange.

I'd neg rep all those posts too.
That’s awesome that you have some friends that are going to do some testing. Do you have any info on their cars’ current states and what they’re planning to do? Super excited to see what they find.

Aero is one of the most interesting build aspects to me, possibly because it’s so complicated and I’m so undereducated on it. I thought the video was a great display of real world examples of what does and doesn’t work and why, as well as getting into some of the basic theoretical principles.
 

MrBlah

Private
Aug 31, 2019
31
as someone who does go 130+ on the track this is something that interests me as I'm considering hood vents to help with water temp limp mode on my N54 135i.. Not sure this thread helps me yet though.....

I also have a 6" splitter with air dams on the ends, so the air needs to go somewhere....
 
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Nosrok

Private
Jan 31, 2017
38
as someone who does go 130+ on the track this is something that interests me as I'm considering hood vents to help with water temp limp mode on my N54 135i.. Not sure this thread helps me yet though.....

I also have a 6" splitter with air dams on the ends, so the air needs to go somewhere....
don't know if you saw the link to the 1addicts page, that build is top tier.

i was hitting 135 on the 1/4 mile, it's a roll racing event 40 rolls as standard. I've upped the boost and played with the tune some so hoping to break 140 but as for aero mods i recently added an m4 style spoiler and a diffuser with an opening. Hoping it helps with drag a bit. My oem unit popped off on 1 side at the end of my last pull of the night. gave me a bit of startle as the car jiggled a little when it happened.

unfortunately the event has been cancelled the past few times they've tried to run it, South Florida has a rona problem.

I have the tune from before i did the aero mods so whenever the events start back up i'll run the 1st one pass using the old tune and see if it makes any difference. New wheel setup and LSD won't allow for a perfect comparison but it'll be good enough.
 
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MrBlah

Private
Aug 31, 2019
31
I've seen his build, very serious stuff, mine is far more simple, just enough to effectively get around the track quick without issue, I am an instructor and half of the fun for me is instructing so the car needs to just work. I dont do too many TT events
 
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The Convert

Captain
Jun 4, 2017
1,433
don't know if you saw the link to the 1addicts page, that build is top tier.

i was hitting 135 on the 1/4 mile, it's a roll racing event 40 rolls as standard. I've upped the boost and played with the tune some so hoping to break 140 but as for aero mods i recently added an m4 style spoiler and a diffuser with an opening. Hoping it helps with drag a bit. My oem unit popped off on 1 side at the end of my last pull of the night. gave me a bit of startle as the car jiggled a little when it happened.

unfortunately the event has been cancelled the past few times they've tried to run it, South Florida has a rona problem.

I have the tune from before i did the aero mods so whenever the events start back up i'll run the 1st one pass using the old tune and see if it makes any difference. New wheel setup and LSD won't allow for a perfect comparison but it'll be good enough.
as someone who does go 130+ on the track this is something that interests me as I'm considering hood vents to help with water temp limp mode on my N54 135i.. Not sure this thread helps me yet though.....

I also have a 6" splitter with air dams on the ends, so the air needs to go somewhere....
Please post any findings you guys end up with. Would love to see enough people digging into this to warrant an aero forum section!
 

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