Aerodynamics discussion

[jamal]

Well, it doesn't specifically "generate extra downforce to the spoiler," but in theory it keeps flow attached to the rear window and decreases drag. It would take actual testing to determine if that increased downforce.

Originally Posted by ayau

Wonder what pressure difference the Griffon 86 makes.

That actually looks pretty nice and probably doesn't hurt. I wouldn't expect huge numbers but it might be enough to turn a little bit of lift into a little bit of downforce.

[WingsofWar]

You know why the diffuser center channel curves upwards? because it doesn't need to be flat anymore. No reason to keep up air velocity after air has reached behind the axles. It doesn't do anything to the downforce or liminar profile on the vehicle by extending it. Instead, use the high pressure air stream made up of slower moving air molecules and divert them to the area that greatly suffers from wake and vacuum which is directly behind the trunk and centerline of the vehicle.

[jamal]

Originally Posted by WingsofWar

You know why the diffuser center channel curves upwards? because it doesn't need to be flat anymore. No reason to keep up air velocity after air has reached behind the axles. It doesn't do anything to the downforce or liminar profile on the vehicle by extending it.

That's not even close to accurate. Airflow out from under the car and it's interaction with the air coming off the rear deck/wing is a huge deal.

[AlvinT@Borla]

Originally Posted by Ro_Ja

Exactly. There's no baseline.

This is Alvin from Borla Performance in CA. I did not know about this discussion... Thanks to OrbitalElipses for cross-posting. The intent of our exhaust diffuser (77012) is an aesthetic accessory to our cat-back (140496), filling the void of the OE muffler, and returning OE-level aerodynamics.

I compiled this video footage yesterday with a quick edit today. I did not have the stock exhaust as a baseline. Perhaps one of you can make this video for us..

I look forward to reading and learning from all of you here. Let me know if you have any questions. Thanks in advance.

[WingsofWar]

Originally Posted by jamal

That's not even close to accurate. Airflow out from under the car and it's interaction with the air coming off the rear deck/wing is a huge deal.

...wat....ok i may not be a pro at this...but i think iv seen my share of data that says im on the right track..

quick examples of what i was thinking of...





The air current from rear spoiler and rear diffuser may interact, but at which point? How is there interaction after the airflow has detached from the vehicle a major deal? if the underbody surface is continuously flat after the rear axles how is this any benefit when you can just taper off when it reaches high pressure atmosphere.

[Dimman]

Originally Posted by LeeMaster

Corny question: Does anyone think poking holes on the fender trim would help remove hot air out of the engine bay faster?

Second question: I know this 'functional' roof spoiler has helped the Lancer Evo IX MR by generating extra downforce to the spoiler. I was wondering, how effective will this thing be on a factory BRZ spoiler or will we have to find a big aftermarket wing like the Tommy Kaira to make this roof spoiler effective?

Vortex generators are used to keep air attached long enough until it reaches the wing. So that the wing gets 'clean' (not turbulent) air. I think you will see them more on cars that have a more abrupt rear window drop off, plus the area that they affect is limited so wings are usually taller.

On a FR-Z, the effect won't be as much as on an Evo, I think.

Plus we have the roof sculpting, which could be doing something already. What? I have no clue.

[jamal]

Well, you want to make the area of flow separation behind the car as small as possible and do what you can to draw air out from under the car. So if you have a diffuser, it will create a low pressure area under the car. If you have a wing or spoiler down low off the trunk deck there will be a low pressure area below it which will help draw air out from the diffuser and create more downforce just from the underbody plus reduce drag. A smoother transition from the diffuser to that low pressure area would probably be helpful.

But, like I said, it just *looks* like it would work well.

[Dimman]

Originally Posted by WingsofWar

...wat....ok i may not be a pro at this...but i think iv seen my share of data that says im on the right track..

quick examples of what i was thinking of...





The air current from rear spoiler and rear diffuser may interact, but at which point? How is there interaction after the airflow has detached from the vehicle a major deal? if the underbody surface is continuously flat after the rear axles how is this any benefit when you can just taper off when it reaches high pressure atmosphere.

The rear spoiler/wing will be generating a big low pressure area behind the car, which can 'power' the diffuser, in a sense.

[AlvinT@Borla]

Borla Cat-Back™ + optional Exhaust Diffuser (combined weight 3-lbs less than factory cat-back)



Slight curve adds rigidity. Angled up ~10degrees to rear..

[WingsofWar]

Originally Posted by jamal

Well, you want to make the area of flow separation behind the car as small as possible and do what you can to draw air out from under the car. So if you have a diffuser, it will create a low pressure area under the car. If you have a wing or spoiler down low off the trunk deck there will be a low pressure area below it which will help draw air out from the diffuser and create more downforce just from the underbody plus reduce drag. A smoother transition from the diffuser to that low pressure area would probably be helpful.

But, like I said, it just *looks* like it would work well.

So basically your saying that there is a low pressure area behind the spoiler, and to make the low pressure area behind the spoiler to have more downforce, you try to feed low pressure (high velocity) air from the bottom channel of the diffuser of the car, to the top.

So in order to make this downforce by.....how? because the way i see it..the low pressure area is already making the most downforce, and feeding it any sort of air perpendicularity especially high speed air will make it no longer a low pressure zone, but normalized atmosphere.

but you talk about reducing drag behind the trunk area using a smooth transition from the diffuser..then...exactly how was I incorrect when i said, the reason why the center diff channel feeds upwards was to divert "to the area that greatly suffers from wake (turbulance) and vacuum (drag) which is directly behind the trunk and centerline of the vehicle"

sounds like we are talking about the same dam thing

[jamal]

Because of wing-body interaction. A wing that helps draw air out of the diffuser can make more total downforce with less drag than a just having a diffuser and a wing mounted up high working independently.

[Dimman]

Originally Posted by WingsofWar

So basically your saying that there is a low pressure area behind the spoiler, and to make the low pressure area behind the spoiler to have more downforce, you try to feed low pressure (high velocity) air from the bottom channel of the diffuser of the car, to the top.

So in order to make this downforce by.....how? because the way i see it..the low pressure area is already making the most downforce, and feeding it any sort of air perpendicularity especially high speed air will make it no longer a low pressure zone, but normalized atmosphere.

but you talk about reducing drag behind the trunk area using a smooth transition from the diffuser..then...exactly how was I incorrect when i said, the reason why the center diff channel feeds upwards was to divert "to the area that greatly suffers from wake (turbulance) and vacuum (drag) which is directly behind the trunk and centerline of the vehicle"

sounds like we are talking about the same dam thing

The air under the car will still be higher pressure than the area at the rear. The diffuser smoothly 'connects' them to transfer pressure from underneath (reducing lift) to the back (reducing drag).

It's important to not just think in terms of just high and low pressure, but of pressure differences.

[plucas]

Originally Posted by jamal

Well, you want to make the area of flow separation behind the car as small as possible and do what you can to draw air out from under the car. So if you have a diffuser, it will create a low pressure area under the car. If you have a wing or spoiler down low off the trunk deck there will be a low pressure area below it which will help draw air out from the diffuser and create more downforce just from the underbody plus reduce drag. A smoother transition from the diffuser to that low pressure area would probably be helpful.

But, like I said, it just *looks* like it would work well.

Originally Posted by jamal

Because of wing-body interaction. A wing that helps draw air out of the diffuser can make more total downforce with less drag than a just having a diffuser and a wing mounted up high working independently.

This is all true

[u/Josh]

Originally Posted by OrbitalEllipses

Well that settles that...

@ayau Check it Aran...

I wonder if you could aim the camera at the rear fender area to compare ride height at speed. This would indirectly measure lift, although you would need a very smooth road.

Just don't move the camera while you install the diffusor and you can make a simple side by side comparison of rear ride height(obviously you need to be going the same speed in each video and not accelerating). Then the difference in rear lift is ~ ride height difference * 2 * rear spring(wheel) rate.

Anyone with an aftermarket rear muffler who complains of excess rear lift could use this same testing technique to quantify the difference.

Thoughts?