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Old 09-10-2009, 02:11 AM
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Default C3 front end lift (aero) question.

I was going to post this in the tech section, but that area doesn't look like it gets much attention.
A fellow VM member (I'll let him identify himself if he wishes) mentioned that the lift at speed on his car is due to the bodywork area in front of the radiator. While I don't have any actual data myself, it seems reasonable that this area would contribute to the front end lift. Given that this area (actually, a volume) in front of the radiator is a diffuser, changing high velocity air into pressure (to increase efficiency of airflow through all the radiator fins), the slight pressure gain over a large surface area would create a "measurable" force of lift. The reason for the lift, in my estimation, is more than just the possibility of a negative pressure on top of the bodywork (the shiny part) ahead of the hood, but the fact that the diffuser volume is not symmetrical top and bottom. The top is sealed, while the bottom is partially open due to the "bottom feeder" slots (which would be at ambient pressure). Does this make sense?
If so, I'm inclined to do an A-B comparison in the future, blocking off these slots (while monitoring the temp gauge, of course LOL) and do some higher speed runs. I'm unclear at the moment how I'll actually measure the lift, or change in suspension travel, though.
Just kicking around some ideas.
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Old 09-10-2009, 02:53 AM
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If I am following you correctly, you are describing a high presssure area in front of the radiator and the underside of the hood and nose (directly in front of the radiator). Also a corresponding low pressure area on the other side of this?

I think that makes sense. Seems like you might want to barge the air going under the car with some going to the outside and some up into the radiator and close off some of the grill area. The top side might be more difficult to do something with. Look at pictures of the old Grand Sports. They had vents in the hood and some had a vertical strip at the front uf the hood that stood upright a couple of inches. Maybe that was a "fix" for what you describe.
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Old 09-10-2009, 04:40 AM
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I am convinced that our C3s beg and scream for vents on our hoods. The side vents aren't getting the job done. If we provide a place for underhood presurized air to escape thru a vent, more air in front of the radiator will get thru the radiator, reducing the pressure in front of the radiator. The pressure in front of my radiator is so great now with my license plate grill, that my headlights pop up at 140mph with the low vacuum of WOT. I have no data to back this up, just my hunches.
Bee Jay
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Old 09-10-2009, 05:09 AM
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I am convinced that our C3s beg and scream for vents on our hoods. The side vents aren't getting the job done. If we provide a place for underhood presurized air to escape thru a vent, more air in front of the radiator will get thru the radiator, reducing the pressure in front of the radiator. The pressure in front of my radiator is so great now with my license plate grill, that my headlights pop up at 140mph with the low vacuum of WOT. I have no data to back this up, just my hunches.
Bee Jay
I have no technical data to back this up either, but I can see the hot air (heat waves) come out of my BB hood vents that I cut open. So there is pressure being relieved AFTER the radiator through the hood. Its probably not enough of a pressure release to equalize pressure in the nose, but it has to help some.
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Old 09-10-2009, 12:06 PM
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Originally Posted by BBShark View Post
If I am following you correctly, you are describing a high presssure area in front of the radiator and the underside of the hood and nose (directly in front of the radiator). Correct. Also a corresponding low pressure area on the other side of this?
I think that makes sense. Seems like you might want to barge the air going under the car I'm not following what you mean here. with some going to the outside and some up into the radiator and close off some of the grill area. I don't believe the grill area is an issue, unless there is more air entering than is necessary for cooling. Then blocking off the grill would be good for drag reduction. The top side might be more difficult to do something with. Look at pictures of the old Grand Sports. They had vents in the hood and some had a vertical strip at the front uf the hood that stood upright a couple of inches. Maybe that was a "fix" for what you describe.
My main focus at the moment is the area ahead of the radiator. While I agree that it is a good idea to vent pressure and hot air out of the engine compartment for cooling purposes, the lower underhood pressures do lower the pressure that actually pushes the air/fuel mixture into the cylinders on engines with open air filter elements. But that's a topic for another thread.
My main interest is balancing the pressure "vectors" in the diffuser area ahead of the radiator. It seems to me (without actually looking at the car at the moment) that if sufficient air (for cooling) enters the actual grill area, while not allowing any to enter the bottom area (just ahead of the spoiler) by blocking off these slots with additional "floor" area, then this floor area in the diffuser will have a pressure vector in the downward direction, partially nulling out the upward pressure vector causing the front end lift.
I'm wondering if putting a manometer in the diffuser volume to measure the actual pressure, and then multiplying the pressure and the added floor area (covering the inlets) would give the amount of force pushing the front end down, partially negating the inherent forces pushing the front end up.
Ideally, (and this is just wishful thinking), in addition to the diffuser inlet shape change, it would be nice to have a thermostatically controlled air inlet/dump that would either restrict how much air was let into the diffuser, or was dumped out. This would keep the diffuser pressure from ever going higher than was absolutely necessary for proper radiator operation. This should reduce the lift that this area causes.
I believe a few old aircraft used a thermostatic system somewhat similar to this for their cooling issues and to reduce drag. (So obviously I'm not inventing anything new here.)
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Old 09-10-2009, 12:44 PM
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OP, Mike, was referring to me on that one, due to some thread elsewhere, everyone kept saying there was bad front end lift on these sharks....

now wether is was going 150 mph or not is NOT the issue here, I admit I was not, apparently, due to instrumentation errors, as I choose to believe Garmin, and fuck all the tach/speedo/tire diameter crapola I was using before....

so anyway, in that time period, I was doing a couple tests, my car is a '72 vert, with some front end modifications, nothing drastic, as removing headlights on any of these sharks MAY affect lift at speeds, honestly, I dunno about that, but certainly getting them stock operating headlights outta the way wether UP or DOWN, will assist airflow ....

I have a BB grafted on hood with the stock SB hood cut out underneath, so it's just one huge open area, I stuck pennies on tape in 3 spots on the rear opening, went 100 mph, the pennies just fluttered...no real indication of any great airflow....guys insisted there was.....so I repeated it with fuzzy yarn same 3 spots on the hood opening, and a couple over the rear hood/grill area....yarn just laying there.....

yarn hanging at speed did a bit of inward flutter towards the engine compartment, the one laying flat hardly moved at all.....I would have to say the flow from the windshield to the engine compartment area is not all that much, really....that yarn was not being blown all that hard, hell I could huff at it through a straw and get better reaction....still not enough to make a penny move inward when hung with 2" wide tape....

All this shit was done about a year ago, before my infamous 'paint' job.....

so having the hood hinges off I goofed and did not put the pins through the front edge...(I hinged my hood from the cowling, not the front due to up and over air induction....reverse hinged, for a vette) I fucked up royally, and so when pulling out on the commercial road, some 200' back was some traffic, I accelerate quickly, the the entire primered/prepped hood flew off the car with great speed and force, cleared my car entirely, as well as anyone behind......I picked it up and was greatly pissed.....

so that happening told ME that all the lift is in front of the radiator, I was only doing maybe 35 at most when it flew off.....obviously when it got vertical at 35 mph, that thing was like a spinnaker on a sailboat, oops....

I never thought there was all that much air resistance to the flow through the condenser and radiator, obviously there IS, LOTS of it....hence lifting that hood so good and proper at a mere 35 mph....

I never did any tests up front, as I can't see without a vid cam mounted, I don't own one....too much hassle anyway....


The above experiences tell me, there is no pressure in the engine compartment, those side gills pass almost no air, I would suppose if they did, there would be pressure under the hood, there is a mild vacuum under there even at good speeds....BTW, it is 13" to the hood opening edge from w/s base, and 8" to the back edge of the hood....

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Old 09-10-2009, 12:47 PM
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Quote:
Originally Posted by Bee Jay View Post
I am convinced that our C3s beg and scream for vents on our hoods. The side vents aren't getting the job done. If we provide a place for underhood presurized air to escape thru a vent, more air in front of the radiator will get thru the radiator, reducing the pressure in front of the radiator. The pressure in front of my radiator is so great now with my license plate grill, that my headlights pop up at 140mph with the low vacuum of WOT. I have no data to back this up, just my hunches.
Bee Jay
MY only comment is that if the vents are forward of the top rad edge, that the airflow through that rad assy will be greatly reduced, and high speed/freeway cooling will suffer badly....or your fans will be on a LOT, and maybe not enough to cool the engine....maybe a SB and the rad from HELL could do it...maybe....

vents behind that rad will do nothing.....

BTW, BJ, my vacuum headlights used to pop up a lot due to operational defects/design/parts? at lo speeds in traffic, sometimes.....even the WSW door did it, there is a cure for it, but another thread.....

Last edited by mrvette; 09-10-2009 at 12:50 PM..
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Old 09-10-2009, 01:28 PM
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Hey Gene,

If those side gills aren't passing much air, were does all the air thats coming in through the radiator go?? I suggest it must go down and out, some passing through the gills. I saw a video posted on the site somewhere of a guy doing the silly string (yarn) test and it showed the side vents working pretty well at 100mph. I only have my car to look at and it has an air dam in the front that sits pretty low to the ground that's used to direct the air to the radiator. It's a triangle shapped piece (pointy end front!) and it's made of the same rubber stuff the front and rear ends are so it's flexible. It's only mounted along the back and held by one bracket in the middle of the front (wonder how that'll hold at 140mph....). The point is that as the air hits it, wouldn't it produce down force before it's directed to the radiator, and if other cars have some arrangment to direct the air to the radiator wouldn't they also get the down force?? Maybe with some strengthening and modification, you could pick up down force there that would be multiplied as the speed increases?
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Old 09-10-2009, 02:53 PM
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Originally Posted by damoroso View Post
Hey Gene,

If those side gills aren't passing much air, were does all the air thats coming in through the radiator go?? I suggest it must go down and out, some passing through the gills. I saw a video posted on the site somewhere of a guy doing the silly string (yarn) test and it showed the side vents working pretty well at 100mph. I only have my car to look at and it has an air dam in the front that sits pretty low to the ground that's used to direct the air to the radiator. It's a triangle shapped piece (pointy end front!) and it's made of the same rubber stuff the front and rear ends are so it's flexible. It's only mounted along the back and held by one bracket in the middle of the front (wonder how that'll hold at 140mph....). The point is that as the air hits it, wouldn't it produce down force before it's directed to the radiator, and if other cars have some arrangment to direct the air to the radiator wouldn't they also get the down force?? Maybe with some strengthening and modification, you could pick up down force there that would be multiplied as the speed increases?
Well, maybe they do, it was just a guess on my part....can't see as it's a whole lot of pressure there or something would be coming OUT of the hood opening, not INTO it.....I have to assume the fans and rad all angled back would blast air down in front of the engine...., where it skips up again??

I still think any factory 'spoiler' is more the 'effects' than 'affects' if you get my drift....

PM me with your phone # I got time today, I just shot some black paint on the camper, so it dries in what sun we get....

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Old 09-10-2009, 05:19 PM
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Walk up to your car and pull up on one of the front fenders. It doesn't take much of a pull to lift the front end up 1". The air that comes into the front of the car, thru the grills or thru the bottom openings in front of the spoiler has a lot of area to come into the engine compartment, but only the side vents to go out. The rest is forced to the bottom of the engine compartment and out the back under the chassis, where our also infamous rear bumper parachute catches air again. The high pressure under the hood is what makes the front end lift at speed and give that light non-steerable feeling. My red Italian car does 171mph easy with only 300hp. But when you look at the bottom of the car, it's flat. Any intake for air, water cooling, and oil cooling, is more than matched with outlet vents. My entire engine cover is vents and in between the rear lights are more vents. Look at the C6R, that hood has more vents than I've ever seen on a stock based car since the Grand Sport Vettes of the 60s.
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