The Penny Flutter test...aero theory.....

In the other threads with Skunk and BJ I said I would tape some pennies across the rear of my BB add-on repro hood scoop....

the scoop has the entire underside of it hollowed out from the OEM hood, so it's all hollow, somewhat similar to some hoods I seen at car shows....but not identical....

I will stick pix of the exact install on my site when wife gets back and processes them.....

The rear slot opening of the hood bulge is 1.75 tall and ~18" wide.....it is 5.5" from the rear edge of the hood, the car is a '72 and so has the stock wiper door and grill panel.....


at ANY speed up to 110 the pennies taped and suspended in the middle of the opening, just sat there totally vertically on average, just an occasional flutter back and forth just as if doing 20 mph in the hood, there is essentially NO AIR coming up from under the car giving any appreciable lift on that hood...none what so ever.....

pix will include a shot of the front of the car....and with the pennies as hung for the test, and the underside best as I can pix it.....

I can't understand why there is insignificant airflow through that scoop, and would have to be also about nil along the back edge of the hood also, where that gasket I don't have is supposed to be.....

:shocking::beer:

OK, took pix yesterday, and just put them on the site, 4 recent additions.....

ttt:devil:

I really like your hood, I have not seen one like that before. I checked out your site, that is quite the collection of electronics you have going there

zZArAZz said:

I really like your hood, I have not seen one like that before. I checked out your site, that is quite the collection of electronics you have going there

Click to expand...

Thanks, but the electronic gear shown on the site is my buddy since H school already some 45 years ago....I got him started on that shit about 35 years ago....his house looks like a antenna farm from outside, and a CIA watch station inside...:rofl:

Being as we was geeks in H school yet, he has this hobby, while he was Navy in the mid 60's....I was not military on account of bad skeleton.....but we keep in touch 2-4X/year.....just visited there last spring, when the pix were taken....

I turned my old ET career into car work, so my car is a geek heaven....or hell, depending on your outlook.....


:rofl::rofl: Hey, at least the wives know where we are.....

:tomato::hunter:

Looks to me like your scoop opening is too far away from the windshield to benefit from the high pressure generated at its base for intake. The opening is in neutral pressure zone. Plus with your TPI intake to the front the carb isn't sucking near there either. And your engine bay is so full of stuff that there's minimal exhausting, & it's not enhanced by under car pressure build up because of the under hood turbulance. I think it's only serving as a vent to let engine bay heat convect out when at rest....
Some yarn, a lighter medium, might give more insight..... especially if taped along the hood & windshield base as well as at the scoop opening.....(but what kind of flaming idjit would go and tape yarn all over his car????) What is it you are trying to determine?
Here's an idea! Duct tape a road flare on the front and take 'er out for another test drive for a smoke test! Hell, nobody in Florida would give it a second glance...They might wonder about a Corvette with pink steam coming from the radiatior, but nobody would say anything......


PS: Why do all three clocks have the same time? I would expect one at GMT, one local and one for a commonly communicated with location......

I did do a yarn test posted over 'there' about it, as that's where the interest was, and so the yard did say there was air going into the engine compartment,...at 100 mph....the air coming out of the back edge of the hood certainly was not obvious from any yarn test....telling me it was if anything a negative under the hood....so the air velocity is somewhere between not bothering to swing the pennies inward, and making the yarn flutter inward....that tells me it's not enough to worry over one way or another.....am I rong???

if the hood is in fact negative underneath on MY car that tells me there is some slight positive force from over the hood/front forcing air under the hood...and so what with essentially open headlight openings, no lights lowered , just two driving lights behind the grill...there is no uplift from headlights...

so since there is no apparent lift from hood bottom, just where is front end lift coming from on these cars??? it's not apparent on my car, mildly modded, but nothing wildly outta style .....what gives???

methinks it's way overblown, there is no lift of any serious nature....never was....

:banghead::drink:

A way to visualize the pressure areas is thus: Roll up a blanket to about 6" or so diameter and lay it at the base of the windshield. Place a loosely rolled up big sleeping bag (as high as the bumper at the front) in front of the car. Then drape a rope under slight tension from around the sleeping bag up over the roof, so it has some sag but not loose. The rope delineates the pressure zone. Above the rope is low pressure from the Bernouli effect of the air velocity moving across the body of the car. The sleeping bag in front is a high pressure area from the frontal area, and the blanket is a high pressure from the windshield. These force intake if it is allowed (grill & cowl induction). The triangles between the rope & the bag/blanket are neutral pressure areas of stagnant/turbulent air movement. (Now if you drape the rope on over the back, the areas behind the back window [of a flat window coupe] and behind the back end are low pressure & turbulent.)

The front end uplift comes from the air going under the car at the front, but because of the reduction of cubic area and undercar/engine bay turbulence denying a smooth flow a high pressure area builds up under the car. The gill vents relieve some of this high pressure from the engine bay, but the clusterfuck of undercar construction creates a great deal more. (This is where underpanning is used to smooth the airflow & so reduce uplift.)

I made a 4 1/2" front air dam for my car (see 'my pics', toward the end) as well as lowering the front with 550# springs. It made a noticable difference in front end float at higher speeds. It reduces the amount of air going under the car and so reduces the pressure build up.

Jphil, that is where I get my mental mystery from so the lift is always visualized as coming from under the car.....well it has to have a positive force under the engine type area to get any front end lift.....I can see the firewall and floorpans as maybe acting like a sled or surfboard, but that not front end lift but the whole damn car...

I cant understand any air under the front is somehow lifting that area without escaping one hell of a lot through that gigantic hole in the hood, no freeking way man....so how is air under the car lifting the car without flowing up and out??? I can't see that has happening.....the surface area of a oil pan and steering and A arms and the front thwart just not enough....the radiator angles back and down, so air hitting it it trying to skip up and over top, block by air damn/pipe wrap in my case....so if anything it's trying to force the front end DOWN, not up.....

I guess my huge questions come from the fact that I personally don't notice any front end lift at 150 mph, the car steers and handles normally to me...

that observation and the many comments about the f/e lift got me thinking so I want to understand.....I know about the pressure points and string you spoke about ......seen wind tunnel/smoke test pix....the thing is, I don't understand where any lift is supposed to come from on a shark....

:gurney:

OK, I'm new here, but I'll take a shot at this.
Your hood vent is way far forward of the L88 type vent on my car. Mine is open all the way at the back of the hood. This was designed to take advantage of the high presure area at the base of the windshield to push cold, fresh air into the vent for the engine. Your vent may not be far enough back to get this effect.
As or lift, and I'm certainly no aerodynamics expert, or even a rocket surgeon... But cars are shaped sort of like airplane wings. Lower pressure over the top creates lift without direct pressure from oncoming air pushing it up. There's not necessarily a "positive force" under the car, there could be a "negative force", ie. a low pressure area, over the car.
But hey, I only got a B in high school physics.........

Garys 68 said:

OK, I'm new here, but I'll take a shot at this.
Your hood vent is way far forward of the L88 type vent on my car. Mine is open all the way at the back of the hood. This was designed to take advantage of the high presure area at the base of the windshield to push cold, fresh air into the vent for the engine. Your vent may not be far enough back to get this effect.
As or lift, and I'm certainly no aerodynamics expert, or even a rocket surgeon... But cars are shaped sort of like airplane wings. Lower pressure over the top creates lift without direct pressure from oncoming air pushing it up. There's not necessarily a "positive force" under the car, there could be a "negative force", ie. a low pressure area, over the car.
But hey, I only got a B in high school physics.........

Click to expand...

I can understand it on the ass end, that makes total sense, with regard the airplane wing....I seen the streamers/yarn/smoke tunnel stuff years ago...

what I don't understand is where FRONT end lift comes from....much less that it's a supposed problem on a shark of all cars....I could understand it on a more 'normal' looking box car, front ends like a brick, no slope, nothing even remotely aero on it....

what got me going was all the comments about the front end being light at higher speeds in a shark....I only been 150 twice, with the top/windows down, and the car was all normal, seemed fine to me....nothing outta the ordinary except wind speed and phone poles flying past....CAR has more balls than I have, easy....so the conflict of opinions got me thinking...why???

that's all....me on a learning expedition, exploring the jungle wilds....

:bonkers::hunter::flash:

Well, the simplest explanation is usually right.
Front end lift is because............
1) There's some physical aerodynamic or weight distribution reason that causes the front to lift more. Who knows, maybe the spoiler on my 68 really does do something.
-or-
2) Both the front and the back lift the same, but the only thing you notice is that the car doesn't turn as well....so, there must be front end lift.

Garys 68 said:

Well, the simplest explanation is usually right.
Front end lift is because............
1) There's some physical aerodynamic or weight distribution reason that causes the front to lift more. Who knows, maybe the spoiler on my 68 really does do something.
-or-
2) Both the front and the back lift the same, but the only thing you notice is that the car doesn't turn as well....so, there must be front end lift.

Click to expand...

THAT is the interest, mine have been banzai straight line runs....no cornering....

:flash:

Garys 68 said:

Lower pressure over the top creates lift without direct pressure from oncoming air pushing it up. There's not necessarily a "positive force" under the car, there could be a "negative force", ie. a low pressure area, over the car.
But hey, I only got a B in high school physics.........

Click to expand...

No, you got it.
Put that in conjunction with trying to cram 10 pounds of shit--er, air--into the 5 pound sack of "under car" (filled with turbulence-creating projections which reduce any possibility of Bernoulie effect plus grill inducted air which has no place to escape except via the gill vents & undercar) and you get uplift. This undercar airflow smooths out to a degree and is being sucked out from under the car by the time it's passing the passenger compartment by the low pressure of the inside of the teardrop bubble of air pressure differential along the sides of the car. Which then in turn reduces the undercar high-pressure uplift effect at the back of the car. Vanes & panning to smoothe airflow in the back can really enhance overall under car low pressure as well as reduce rear end vortex turbulence & vacuum drag, thus promoting rear end stability as well.
Do not discount handling discrepancies from suspension geometry changes due to stance changes of the car due to these airflow effects--although our cars are tanks enough that that may be minimal until performance limits are reaching their limits. I do not know about that, I do not have any high speed driving experience other than midnite county roads. (And I need to stop that--my last ticket was pretty damn serious!)

There are books & books written about the subject. One very good one, that I have only barely perused, is "Vehicle Aerodynamics" published the Society of Automotive Engineers which is a 1 1/2" thick, 8 1/2" by 11" small print engineering textbook: very informative if you wish to persue the subject.

Our C3s are bricks, aerodynamically. Sexy bricks, yes, but bricks as far as aerodynamics is concerned. A Kia has a better aerodynamic shape than a C3 'Vette.
If your car handles well at 150MPH, Gene, then you have a good performing package, however (knowingly or not) you achieved it. I would be curious to know though--have you found this handling to stay true when subjected to road surface uneveness, cross winds or directional changes?

I have never hacked any corners at anything over say 50 mph....at most...and it's ok, keep in mind the tires are modern rubber, and there in are the largest improvement....which has nothing to do with float/lift at higher speeds....

the thing handles fairly well, I think, I really pleased with the outcomes over the years, as various projects/aspects were tackled....with one exception....

I had a set of larger VBP rear sway bar on the rear along with the 360 spring, it was WAY in hell tail assed light....way too much.....

the subtraction of the front bar down to something thinner....I think it's 7/8 on thee now with 460 springs....i'ts fine....but the rear was naked, and I put on a stock factory rear sway....right now I think it's fairly well balanced.....

but now I have unequal tires on there for years 255/50/17 front and 275/50/17 rear....

finally listening to all the guys on the forums made me try ditching my KYB shocks for the Bilsteins from VBP, and sure enough that bounseyness over the bridge went away....smooth as silk....ride is better by a fair amount, and surely they fixed the street I lived on about 6 months after the Bilsteins went on...figgers....:flash:

Just occurred to me, the side scoops, I really dunno the stock opening sizes....the side grills were long ago removed by previous owners....and the openings in the glass are 7"x3" on each side....thought just hit me that they maybe larger than stock....????

if so, that would go a long way to relieve lift?? no....it was referred to above, but I never thought of my scoops as being differant, just naked...