Rear window Garnish - Air in or Air out

[MvM]

I have a question about the rear windows garnish on the NSX.

Is it used to let the hot air OUT from the engine bay or is it there to scoop air flowing over the car INTO the engine bay to cool it??

According to the Downforce website, their carbon rear windows garnish is raised to let more (cool) air IN.
http://downforce.biz/cart/product.php?productid=16202&cat=290&page=1

However, I am wondering, if that is so, where does all the hot air from the engine bay go?

I thought that the air coming into the engine bay through the right side intake was cooling the engine bay and would then flow OUT of the rear window garnish. IMHO, that would make more sense since the cool air would be coming into the engine bay at a low point. flow over and around the engine, picking up heat and then rise up and be sucked out of the engine bay by the under pressure air behin the engine hatch.
Would make more sense aerodynamically speaking as well in my view.

Also, would mounting the Downforce raised carbon version actually help lowering your engine bay temperatures? I like the looks of it, but it would be nice if it would also actually DO something

 

[A.S. Motorsport]

Good point but iirc last time i removed the garnish (or rear hatch glass trim as honda calls it) doesn't really do a lot on a street NSX. since it's just aluminum sheet metal underneath, i'd need to take a better look but iirc air doesn't really go anywhere.

btw, your fuel thank is directly underneath it.

It's attached with 7 screws so you can easily pull it out.

 

[rsevo6]

As we discussed yesterday the hot air going out through the window garnish makes the most sense.
In the MR2 Turbo (which has similar setup) I know for sure this is the case: cool air coming in the engine bay from under the car and through the side duct and exiting through the high louvered engine cover on top.

Will be important to know for sure on the NSX as I want more cooling in the engine bay and need to know which way in/out to send the airstream.

I expect in from under and side duct and out through the garnish.
On the 3.0 there there was an engine bay cooling fan behind the right side duct to take the cool air in, so logic dictates that this hot air exits through the upper garnish but hopefully someone knowledgeable either can confirm
or tell different with explanation

 

[A.S. Motorsport]

As we discussed yesterday the hot air going out through the window garnish makes the most sense.
In the MR2 Turbo (which has similar setup) I know for sure this is the case: cool air coming in the engine bay from under the car and through the side duct and exiting through the high louvered engine cover on top.

Will be important to know for sure on the NSX as I want more cooling in the engine bay and need to know which way in/out to send the airstream.

I expect in from under and side duct and out through the garnish.
On the 3.0 there there was an engine bay cooling fan behind the right side duct to take the cool air in, so logic dictates that this hot air exits through the upper garnish but hopefully someone knowledgeable either can confirm
or tell different with explanation

It indeed is the most logical way, Audi R8 is designed the same way it sucks are from below the engine and vents it straight up.
Ferrari and Lamborghini work the same way.

The upgrading (fan/motor) of the <97 setup would be best when extra engine cooling is needed.
iirc a Austrian member did make lower cooling ducts for his underbody setup.

 

[greenberet]

A few years ago I taped wool tufts to the window over the engine compartment and to the trunk lid to see where the airflow is laminar and where it’s turbulent. If I remember correctly, I saw wool tufts getting sucked into that vent. Thinking about it, if you want a vent to inhale air it should usually face forward and if you want it to exhale it should usually face rearward. And that vent faces forward. But it’s really easy to double check. Just tape a wool tuft onto the engine compartment hatch and see whether it gets sucked into the vent or blown away from it while you’re driving.

 

[ristner]

My unqualified guess is - it is needed when you stop the car and the engine bay has no wind from the speed of the car. The air around the engine, which now is getting hotter from the hot engine, need to be replaced. It want upwords to get away and the only way out is through the hole we are talking about - how about that explanation? My guess

 

[MvM]

A few years ago I taped wool tufts to the window over the engine compartment and to the trunk lid to see where the airflow is laminar and where it’s turbulent. If I remember correctly, I saw wool tufts getting sucked into that vent. Thinking about it, if you want a vent to inhale air it should usually face forward and if you want it to exhale it should usually face rearward. And that vent faces forward. But it’s really easy to double check. Just tape a wool tuft onto the engine compartment hatch and see whether it gets sucked into the vent or blown away from it while you’re driving.

In another thread you pointed me to the site of autoweek where they have all kinds of articles. One of them was on getting an idea of the airflow around your car doing the wool tufts trick.

I think I am going plan to do something like that for the whole car and have someone else taking pictures or film it with a camera.

 

[greenberet]

Cool! I got the idea of putting wool tufts on my NSX from this article in AutoSpeed as well. If you would take pictures of your NSX or even an HD film, it would be great to record that for posterity!

 

[swerve2007]

generally, the air above the car travels faster than underneath (conservation of momentum formula, making wings fly etc) due to its shape, creating a lower pressure above (unwanted destabilizing lift) which would be sucking the air from the engine compartment in our case. on the other hand, if greenberet's recollection is correct that the wool tufts were getting sucked into the engine bay, honda might have achieved quite a feat by creating aerodynamic suction under the car at a noticeable level without major design sacrifices like extreme groundefects etc.
i'd love to see the wind-tunnel flow assesment- that would explain a lot.

 

[RedWings]

It's attached with 7 screws so you can easily pull it out.

...and those 7 little fuckers love to drop into the engine, use a magnetic phillips screwdriver and save yourself some fun

 

[Huck]

If you think about it ---the engine compartment is not significantly different then the common front engine vehicles. In their case, you get air coming thru the radiator and from under the car. The actual cooling is done by the coolent circulating throughout the system--not so much by air flow directly against the motor.
I like the idea of some wool ----I would use knitting wool on my sail boats sails for flow and efficiency of sail trim. My guess is that on the NSX, the air coming over the top of the rear window---would draw the air out of the engine compartment under speed or it would be forced out by air flow from under the car coming into the engine compartment and backing up against the rear engine compartment firewall.
At idle, the hot air would rise and come out those rear garnish vents. The 3.0 engine compartment fan is probably more intended to flush out any vapors then to cool the engine. I can't recall if it is a push or pull but it really doesn't matter as long as it creates air flow similar to the side air vents.
I am going to tie some 6" lengths of wool onto the molding just to see what happens at idle, at speed, etc. and hopefully don't wreck the car while trying to see what is happening~~~~~~.

 

[troca]

My car has a full underbody seal and long rear diffuser (not the bumper type). My original under engine pan was a duplicate of the R pan with holes in it. I have an airflow study of the NSX (the Mugen JGT car) that shows flow over the rear glass onto the trunk lid and the garnish at the base is pointed into that flow. I was not happy with the "hole" engine pan and it didn't seem to be flowing air out of the engine compartment and into the air stream flowing under the car and out the diffuser. Last fall I redid the design of the under engine pan to have a full face of rearward facing louvers facing backwards as well as accomodate an aluminum oil pan with a finned bottom just sticking through the pan into the airstream. I also put a foam seal around the engine bay fan to seal the fan shroud against the body so the fan could not "short circuit" air it was trying to suck in from outside the car. Result is, I believe, that air is now being sucked out of the engine compartment (evidenced by some fluid drip markings showing airflow on the outside of the pan coming from the louvers and higher oil pressure, a sign of cooler oil) and the fact that the car seems happy at high speed (100 mph on a hot day) where as before when I first added the engine tray with the holes as in the R, I got an engine light a couple of times if I ran fast. Never happened before I added the tray with the holes. This led me to consider that the flat holes could be an inlet if the rest of the vents to the engine compartment weren't sealed enough to control air flow. At any rate I believe I now have some negative pressure (or at least lower than before) under the car and flow through the engine compartment is starting to be "directed" rather than haphazard. The car is very happy.

 

[Ponyboy]

Would love to see pics of your design...

 

[goldNSX]

In another thread you pointed me to the site of autoweek where they have all kinds of articles. One of them was on getting an idea of the airflow around your car doing the wool tufts trick.

I think I am going plan to do something like that for the whole car and have someone else taking pictures or film it with a camera.

Maarten, please do the wool tuft-test and end all the endless theoretically discussions here.

 

[d.n]

string taped down would work pretty well for this sort of thing

 

[RedWings]

better yet... anyone have a wind tunnel handy ?

I'll stop by :smile:

 

[Honcho]

Or we could just find the original wind tunnel data from Honda...or just ask one of the engineers. :biggrin:

 

[Gansan]

Just my opinion, but:

When the car is moving, the garnish sucks air in. The airflow direction pretty much dictates this. The air under the engine bay is relatively low pressure since it is coupled (connected) to the low pressure zone behind the car even if the airflow is dirty. This sucks air out of the engine bay, and more air enters to replace it. The air that enters the passenger side scoop also would exit under the car.

When the car is stopped, air comes out of the garnish because hot air rises and that's the highest vent in the engine bay.

 

[Huck]

I tied two pieces of knitting wool onto the garnish slots and draped them down onto the trunk ---about 12" long. Drove around at speeds to 70mph for about an hour. Forgot they were there and ran errands! When I did remember---I tried to look but you cannot see the garnish from the drivers seat. So I stopped and checked it out. Much to my surprizel the wool was down into the engine compartment BUT not sucked back through the garnish holes. It was between the molding and the rear glass on both pieces---left and right sides about 18" off the corners. This was a total surprize to me so I tried it a second time and got the exact same result. As they say "VERY INTERESTING"!--the opening was barely wide enough to allow the wool to fit through but both sides did the same thing in both test drives. So, it appears that the engine compartment allows the air to rush in backwards taking the wool with it. Never would have guessed that but then again, I never would have guessed that a pickup load of dead grass would blow backwards within the bed of your truck as you drive and it would build up behind the cab. So much for the aerodynamics and wind tunnel effects

 

[TrustJDM]

You can kind of get an idea of how air travels around the nsx-r in this video. perhaps you can assume the same w/the regular nsx to a very limited extent.

http://www.youtube.com/watch?v=2fKgGMlN1Ik

 

[stuntman]

The rear window garnish scoop is an air inlet scoop forcing air into the engine bay. It's been said the side fan is useless especially once the car is moving. It is not the same function or principle as an MR2, Audi R8, Ferraris, etc... which have many louvers and open mesh vents to vent air out of the engine bay. I would agree that when idling, hot air would vent out of the NSX garnish scoop but once you're rolling, the scoop catches air directs it into the engine bay.

Use smaller 4" tufts mounted in various areas around the garnish inlet. Use a go-pro camera and film the results. 12" tufts is probably too long.



Billy

 

[swerve2007]

small correction- the air is not forced-in through the garnish, it is sucked in due to pressure difference created right behind the bumper. generally the boundary layer is turbulent enaugh that a smooth flow into the small garnish openings is impossible, with the airflow going right over it.
cheers.

 

[greenberet]

I put three 4" wool tufts close to the trailing edge of my rear window today and went for a drive. When I stopped, two of the tufts were dangling into the openings of the rear window garnish and one had threaded itself into the gap between the rear window and the garnish. Air seems to flow into the garnish area when you're driving and not out of it, even with a rear undertray that feeds air into the engine compartment from below. MvM: if you take photos or a video it would be great to conclusively put this to rest.

small correction- the air is not forced-in through the garnish, it is sucked in ...

Side note - Billy is right, the air is forced through the garnish. There is no such thing as suction. :smile:

 

[troca]

The CFD study picture of the JGT car I have shows the pressure gradient along the surface of the car. Different pressures, from highest to lowest are shown as various color islands on the various surfaces and go from highest (red) to lowest (violet) in the order red, orange, yellow, green, blue, indigo, violet. The same old ROY G BIV we learned in high school science for light wave frequency. I don't know the speed used for the car in the picture and pressure gradients can move some with speed (speed = pressure) changes, but the picture shows yellow for the lower 1/3 of the rear windshield across the garnish and extending across the forward 1/2 of the trunk lid all the way to the outside of the fender tops. After that the deck lid turns green and then blue indicating indicating higher pressure (yellow) in the areas I mentioned than before or after the area defined. The thing I don't know is how fast the car was going or what the actual pressure in the yellow area was relative to the underside of the car. Yes, higher pressure always moves into lower pressure and if the area under the engine bay has lower pressure than the yellow area of the back window, garnish, and trunk air will move through the garnish into and through the engine bay. Don't know about the side vent, the JGT car shows green along the flat side of the car and only shows higher pressure where the wide body starts because the body forms a protrusion into the airstream after the door. This is pretty much a continuing flat area in the stock cars and I have heard that you would need a scoop that sticks out nearly 4 inches to really catch some air. so this could mean that air actually moves OUT the side vent at speed. At any rate I suspect that the pressure at the garnish isn't all that high relative to the underside of the car unless the underside is cleaned up and controlled and that is all I've been playing with. I think I have some pics of the underside all assembled last fall, if I can find them I'll post them. Otherwise I've been thinking about maybe a smoke bomb (like used for balancing A/C commercial ductwork) set off at the back of the roof might show something if another car filmed from behind in a quartering shot. Might be able to see smoke from over the top but also joining from under the car if flow is going through the engine compartment.

 

[NSX-SA]

My car has a full underbody seal and long rear diffuser (not the bumper type). My original under engine pan was a duplicate of the R pan with holes in it. I have an airflow study of the NSX (the Mugen JGT car) that shows flow over the rear glass onto the trunk lid and the garnish at the base is pointed into that flow. I was not happy with the "hole" engine pan and it didn't seem to be flowing air out of the engine compartment and into the air stream flowing under the car and out the diffuser. Last fall I redid the design of the under engine pan to have a full face of rearward facing louvers facing backwards as well as accomodate an aluminum oil pan with a finned bottom just sticking through the pan into the airstream. I also put a foam seal around the engine bay fan to seal the fan shroud against the body so the fan could not "short circuit" air it was trying to suck in from outside the car. Result is, I believe, that air is now being sucked out of the engine compartment (evidenced by some fluid drip markings showing airflow on the outside of the pan coming from the louvers and higher oil pressure, a sign of cooler oil) and the fact that the car seems happy at high speed (100 mph on a hot day) where as before when I first added the engine tray with the holes as in the R, I got an engine light a couple of times if I ran fast. Never happened before I added the tray with the holes. This led me to consider that the flat holes could be an inlet if the rest of the vents to the engine compartment weren't sealed enough to control air flow. At any rate I believe I now have some negative pressure (or at least lower than before) under the car and flow through the engine compartment is starting to be "directed" rather than haphazard. The car is very happy.

Do you have any pics of your undertray and rear diffuser?