Hood lift at high speeds

[grodenglaive]

The lift comes from the basic shape of the car - like every car, it's an upside-down airplane wing in profile. Front splitter/rear diffuser is a good way to reduce that effect by decreasing air pressure underneath.
A hood vent may help draw the high pressure air from the front (through the radiator) and fill in the low pressure area above the hood to reduce lift somewhat (ala Elise, ford GT, etc.) Assuming it is placed correctly.

[RedLeader]

I'll chime in here only in relation to the fender vent theory. We recently removed one of mine in an effort to provide it as a model for CAD engineering so we could prototype and create new vents, preferably functional ones. While, yes, the fender vent is reached through the engine bay, the available area to cut without getting in the way of the clips that hold the vent in place, is very small (maybe a 1"x3" slot possible) and not likely to help much. Obviously, in conjunction with other venting methods, this could have some effect, but I THINK its too small to affect much on its own.

Has anyone looked at taking the block-off plates at the rear of the hood off? One is clearly visible from the driver's seat, and it looks like just a piece of black plastic. That might allow air to vent out the back of the hood and up the windshield. If you did that, and cut into the fender to make functional vents, I think those two together might help the wobbly hood a little bit. Just a thought. I'd try it if the FRS vents weren't a PITA to remove and if they where vented themselves. Maybe that'll be this next weekend's project.

[OrbitalEllipses]

IIRC the back of the hood/windshield area is an area where you don't want a vent as air actually doesn't escape from there due to pressure differentials.

[chadstyle]

I can't say that I've experienced any hood lift at all at high speeds. I've been up to 130 mph on a few occasions and the car feels extremely planted and stable with no floating at all. It maybe the 1.5" lower ride height or it maybe the front lip and side skirts and diffuser but whatever it is, it works.

I have to admit, I purchased the lip kit more for the look than anything else. While installing it I thought "hmmm, this is really solidly mounted and could generate some down force at speed" but I don't know how fast I need to go before the aerodynamics come into play. 80mph? 100mph? 125mph? Who knows but I'd like more info from others who have installed lip kits and diffusers and have pushed the car hard...

[RYU]

I come back to this every so often to brush up on my aerodynamics. Enjoy!

I linked you somewhere in the middle of the article btw. Feel free to start at the beginning if you want.

http://world.honda.com/NSX/technology/t3.html

[Dave-ROR]

Originally Posted by GTB/ZR-1

Okay, I've now seen this myself now... Hitting ~115+ on Roebling's front straight this weekend--I noticed it happening over 105--it ripples. Disconcerting...

I've noticed the rippling thing a little bit before. When I read the thread title I assumed he meant 1/4" or so movement, which I definitely have not noticed.

BTW How was the car at RRR? 115? Bah you should hit 120-125 there

[Dave-ROR]

Originally Posted by RYU

I come back to this every so often to brush up on my aerodynamics. Enjoy!

I linked you somewhere in the middle of the article btw. Feel free to start at the beginning if you want.

http://world.honda.com/NSX/technology/t3.html

Good link and I so want a NSX-R

[mad_sb]

has anyone bothered to measure the air pressure in the engine comapartment at the speed people are seeing the hood lift?

A simple magnehelic pressure gauge is all that is needed.... Seems prudent before trying to design a solution to reduce positive pressure in the engine bay Since it may actually be lift causing the hood to flex rather than pressure in the engine bay.

Plus if it would provide a means of measuring how effective the designed solution is. And it turns out that the engine bay is not under positive pressure you could use the side vents to reduce pressure in the wheel well instead

[GTB/ZR-1]

Originally Posted by Dave-ROR

BTW How was the car at RRR? 115? Bah you should hit 120-125 there

~120, but not 125 (!) Cold & wet/drying conditions--didn't trust going 10/10ths @ 9. Also, haven't been on any roadcourses for almost 3 years--been concentrating on the cones. Only ran upper 1:20s...

[Dave-ROR]

Originally Posted by GTB/ZR-1

~120, but not 125 (!) Cold & wet/drying conditions--didn't trust going 10/10ths @ 9. Also, haven't been on any roadcourses for almost 3 years--been concentrating on the cones. Only ran upper 1:20s...

Ah I didn't realize it was wet up there too. In the race car I'm a wuss through a lot of RRR since the car is very unsettled.. I haven't taken the BRZ up there yet.

[RedLeader]

Originally Posted by mad_sb

you could use the side vents to reduce pressure in the wheel well instead

This would be VERY difficult as there's a frame member that sits between the fender vents and the wheel well. It'd be easier to vent the wheel well into the engine bay, then the fender vents out of the engine bay. Going direct from wheel well to fender vent would be nearly impossible.

Venting the wheel well to the engine bay would be super easy; just cut through the rear section of the metal wheel well and the plastic wheel well cover, then cover it with a screen of your choosing. Voila! But that would vent all that air under the hood, which would likely increase the hood wobbly-ness.


Sent from...somewhere?

[DBee]

Originally Posted by mad_sb

has anyone bothered to measure the air pressure in the engine comapartment at the speed people are seeing the hood lift?

+1

Opening the vent on the fender into the engine bay may not do much to lower under hood pressure because airflow around the fender vent insert is probably funky. Air pressure above the car and over a "flat plate" surface will be lower than under hood pressure due to air velocity over the car. Airflow over the side of the car, especially along a panel which is not flat and uniform in shape (the vent insert), is not always predictable and intuitive. If you have information on the boundary layer shape and other characteristics along the vent insert and know what the pressure difference is between the engine bay and the side of the car, then I say go for it.

Otherwise you're just cutting up a perfectly good car for something to showoff to your buddies without much benefit. Not saying it's not worth it, just that it will be an experiment. And this experiment may or may not fail.

[GTB/ZR-1]

Originally Posted by Dave-ROR

Ah I didn't realize it was wet up there too. In the race car I'm a wuss through a lot of RRR since the car is very unsettled.. I haven't taken the BRZ up there yet.

Yeah, the car lurched on me a few times--but was trying to drive it on the (close to) limit, as I was trying to inflate my attachments...
I actually went fastest in my last session, when things were almost dry & I was finally getting into the groove.

I forgot to mention there was a hellacious headwind, too. So, maybe you're right--going 10/10ths @ 9 & no headwind (maybe some tail) & 125 may be possible--but you'd probably need to brake after 2.

[Basti]

Originally Posted by RedLeader

I'll chime in here only in relation to the fender vent theory. We recently removed one of mine in an effort to provide it as a model for CAD engineering so we could prototype and create new vents, preferably functional ones. While, yes, the fender vent is reached through the engine bay, the available area to cut without getting in the way of the clips that hold the vent in place, is very small (maybe a 1"x3" slot possible) and not likely to help much. Obviously, in conjunction with other venting methods, this could have some effect, but I THINK its too small to affect much on its own.

If one wanted to make the garnishes functional the metal of the fender would need cutting in order to increase the opening but then the clip holes would be gone.
I'm currently more worried about that part as I'm not too fond of just glueing the vents to the fender.
Now imagine cutting all excess fender off on the inside and just letting little noses for the clips stand... one might then need to cut the garnish in the same way and hope that there will be enough space left between the visible outside of the garnish and the inner clips.
Even if that works (and, though it'll need some skill sounds very possible) this would not guarantee a good finished look. Very iffy.

However, if you guys CAD sth. like that up I'll be the first one interested.

Or maybe I'll visit a body shop for that...