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.....

 

OK, I'm looking forward to those pictures. Kind of curious; why you decided to use pennies as opposed to tape, fabric, streamers, etc.

 

Pennies?

 

Yeh, just to keep the tape vertical, without it curling around in heat and changing shape enough to be useless something light weight that with it facing flat, would deflect really easy in any air movement....all they did was FLUTTER there was NO significant air flow one way or another outta that scoop, nothing, which told me there was no pressure differential under the hood, none, nothing worth mentioning....been say 100 lbs of lift, just for conversation, those pennies would have been layed out flat toward me....like being blown with shop vac....



OK, the pix are on the site, look under recent pix, I just added 4 of them taken yesterday just after the runs.....

 

Any underhood 'pressure' is vented through the gills behind the front wheels.

 

the reason the gills were added in the first place is to reduce underhood pressure , but it does not eliminate it completely.

In looking at your pics, the opening from your hood scoop is too far forward to take advantage of the low (or high depending on your perspective) pressure area at the cowl. On a regular L88 hood to opening is much closer to the windshield, and does not interfere with the wiper door allowing the "cowl induction effect" to occur , you can clearly see the impact of the that area at speeds as low as 60 mph with air flow into the scoop on my car.

your scoop and any other L88 for that matter, if sealed properly around the carb, will still allow cold air induction as it were....



cheers


tim

 

I hope I'm not misunderstanding your question. I have a cowl induction '73 and it is not a ram air device. It only becomes functional at WOT when the air cleaner is sealed against the hood. At WOT it sucks cold air in through the cowl vent and does nothing under normal driving conditions. Under normal driving conditions the snorkel sucks air in from the engine compartment. At WOT you can hear the sucking sound from the cowl.

My '73 has a solenoid activated cowl vent and is closed unless WOT and opens when the gas pedel is pushed on a plunger under the dash. Even if you have the cowl open all the time it won't suck in much cold air through the cowl unless the air cleaner is sealed against the hood.

 

The air currents are going to be more subtle than that. The pennies are too heavy. That is why you use yarn. I agree that the inlet isn't close enough to the base of the windshield to effectively use the high pressure area there but it can still be working. It depends on the local pressures in that area of the hood on whether it is venting or pulling air in.

 

It has been stated in another thread that sharks have a lot of front end lift at higher speeds, and so I was interested in IF that was true or not, and I have proven with this test that is is NOT true, not on my car anyway....btw, the opening behind the headlights free flowing, the rad is sealed pretty good, and has Dual Spals sealed in tight behind it....

so MY observations would be that IF a shark had even 100 lbs of lift at 110 mph, those pennies would be layed out flat toward me, they were not, they hung vertically like shown and just waved slightly in the backwash of air over top.....

so IMO, any statements of a shark being somehow aerodynamically unstable at high speeds due to front end lift are simply not true....or the effect would be noticed by 100 mph anyway.....

I know the side scoops vent under hood air, as I can feel the heat coming out.....coming off the headers.....



MY only other thought about front end lift would involve the headlights, acting as sails in the up OR down position that attack angle would act to lift the car right there at the nose, point of most force....

 

This is about pressure differentials, which may or may not be sufficient to give an indication of their presence and/or values with pennies. Also, it remains possible to have lift with or without some air flow (in either direction) at the cowl end of your hood, as such is not necessarily proof of overall pressure differentials above and beneath the entire body.

Happy New Year

 

I can understand lift coming from under the hood, not so much from the fender/liner area, but from under the hood....but in order to generate any significant lift that would necessitate a pressure build up, and if that is the case, there has to be a source of airflow that would act to build up said pressure, the open hood acts to relieve any pressures possibly building up....

but there is NO flow....I can't beleive the pennies are too heavy....so tmrw AM after all the idiots are off the road, I will tape some thread/yarn on the hood and try it again, just for the knowing....

100 mph blast against that hood is a whole lot of airflow, IF any, and so to generate a 100 lbs lift would surely make a huge airflow out the back of that opening....why NOT???

 

Aerodynamic lift has everything to do with airflow above and below the vehicle and nothing to do with internal engine bay pressures, be they negative or positive. The wing of an airplane does not create lift based on internal pressures. (maybe I am missing something here?) Not so sure I follow your objective with this test.

I am not able to view your pictures. Maybe you could include them in this thread? It would be interesting to see pictures of yarn verses pennies.

 

I'm not a full-fledged aerodynamicist, but I'll give it another shot...
Don't get caught out thinking only of air velocity in lieu of the air pressure differentials in play here, which vary significantly over the body. Air fundamentally tends to move from areas with higher pressure to areas with lower pressure (no differential, no flow). Since air is a compressible fluid with mass, its compression against itself before it escapes from the line of travel of encountered obstacles such as windshields creates a pressure increase over the area at its base relative to pressure distribution over the the body. Thus, irrespective of the lift or downforce picture as a whole, there is not necessarily a substantial differential of "positive pressure" (if any) within the opening at the rear of your hood vs. that in the area behind it. In addition to and due in part to the windshield's roll above, there may not be as much turbulence behind your power bulge for the creation of a "negative pressure" there as might be imagined.
Hope that helps.

 

Hey, what's the puppy's name?

 

The guy in the yellow shirt is my neighbor across the street, friend for 12 year, since I moved in here,...his dawg was nearly 15 when she died, and so his sisters/family got him this little cat/dawg it's name is SuzieQ......that damn thing is the ONLY dawg that feels like and LOOKS like a cat/kitten.....
except the ears on my buddy's Siberian....

Skunk, so you are saying there is a countervailing force of air compression between the windshield pressure bubble and the underhood bubble?? funny the two would equalize, at some speed I would think the pennies would indicate a airflow one way or another, but to my shock they never indicated jack....

I figger the probability of this hood being randomly mounted so to equalize pressures in this fashion as being like a lotto win....

I post tmrw after the yarn/string test....



BTW, Happy New Year guys.....

 

Someone has to be a aerodynamics or aeronautical engineer to explain this. I had allot more to say but I'll just wait until an expert on the subject chimes in

 

I know how a airplane wing works, my father worked at Martin Marietta decades ago....I can understand the lift in the rear from any car, even a rag top with the lid down....due to the overall camm effect and the pressures in the rear creating lift....

what I don't understand is the lift in the FRONT....guys complain all the time about this 'lift' and lighter steering feel and loss of control, etc.....want the truth, I think it's tires and maybe the way the suspensions set....not that my car is anything super or something, just makes me wonder about how much them old stock headlights are worth, I bet wether up or down they maybe as good as a parachute for lift ability.....they sure as hell not anything aerodynamic, and I don't have any complaints about speeds and handling...so there has to be SOME explanation that makes sense....

 

1999 Lemans 24 hour race.

Mercedes figured out what lift was!

Google or youtube

 

Don't forget when they are talking about light steering etc it is at 150mph or more. There is a big difference in 100mph and 150+mph. My Cuda was stable up to about 130mph then after that the standard steering felt like power steering

 

I wouldn't have used those particular terms, but more or less ...yes. The pressures are not likely at true equilibrium except for a brief period, if at all (and wouldn't really be all that miraculous an event), so it's more likely that the differential is just never large enough in either direction that the pennies would have actually indicated much of anything. The good ole tuft test should give you a much better idea of what's happening, and I/we look forward to seeing your results, however, do keep in mind that accurate, fine resolution hard data can only be obtained thru proper instrumentation.