TrackAire

I'm trying to figure out what the air pressure is doing on the vertical sides of a cowl induction hood at my road race speeds (60 mph to 140 mph). I have an ACP Louvered Air Extraction hood that I'm installing on my C5 Z06 that has a slightly raised center section that starts near the radiator stack and ends at the windshield. I know that the windshield is a high pressure area, but I've not been able to find any documentation on what the vertical sloped sides of the hood (area over each valve cover) is doing. The ACP hood is vented near the radiator stack and has some vents over the wheelwell area. I'm wondering if I slot or drill five to seven holes (about 1.5" diameter) along the near vertical part of the hoods raised center section, will this let more hot air out at speed. I'm not concerned with form, just function. I still have my stock hood that I can install if I need to look pretty

My car is still a bottom feeder but I'm modifying the air dam to a much larger unit with forward reaching vertical sides to box in the air intake area so I can get as much air flow through the cooling stack as possible. My goal is maximum BTU extraction at track speeds for hot track days.

Does anybody have any documentation or air tunnel results of what the air is doing when going over the raised sides area of the cowl induction hood?

TIA,
George

l98tpi

I would suggest to make sure to seal off the area at the base of the windshield between the cowl and the hood. This will prevent the high pressure air from circulating under the hood and creating lift.

69427

I would consider drilling some small holes in the hood area of interest and hook up a vacuum gauge or a differential pressure gauge (such as a Magnehelic) to get some actual data/measurements of what's happening there. I've done that on my car when I've had aero or induction questions. The Magnehelic gauges are usually pretty reasonably priced.

TrackAire

That is actually a good idea. I'll look into whats involved on getting a gauge and sensor setup.

Time to educate myself on differential gauges.

Dubstep Shep

That's a fairly good idea. A simpler, less costly, though less accurate method is to hook up streamers to the areas in question and see what they're doing at speed. It has the advantage of showing the direction and some sort of relative velocity of the flow, though you won't be able to find out the pressure.

0Anthony @ LGMotorsports

At the base of the windshield you get some odd stuff going on, but typically speaking the air is going to try and get sucked in under the hood, so much as the air pressure under the hood is less than that above it, and it generally is.

Remember they sell COWL INDUCTION hoods. All of our TransAm race cars used this as inlets for the carb. Nascar still does use it for air intake for the engines as well.

We made a lot more downforce by sealing the area at the windshield as tightly as we could. This also made the louvers push more air through the hood as a result.

63Corvette

I feel sure that is true for your racecars of the C5.6,7 era, because y'all have tested your cars with instrumentation...............right?
However, I race a C2 coupe (63) and "we" all run with our hoods deliberately cracked open about 2". It "seems" to promote cooler engine temperatures, and "seems" to (if not downforce) at least negate some of the upforce or lift.........I know Lou is old enough to be familiar with this, SO how 'bout a comment from Lou..............
Thanks,

0Anthony @ LGMotorsports

Now I can tell you on the little I worked on the C2 GS product with Duntov Motorcompany that yes, front end lift on those cars was just horrible. I don't know if back then they were trying to do that with all of the radical style hoods or just try and feed the carbs with air. Either way the nose of those cars would seem to point itself to the sky.

Yes I am speaking of C4/5/6/7 cars however on keeping the hoods shut.

That being said...the air hitting the windshield is being forced to either go down through an opening in the cowl or up and over the car. Leaving a gap there (unless pressurized) is going to cause the air to enter.

TrackAire

Just to clarify, I'm not talking about the cowl area in front of the windshield, that area should be sealed off IMO. I'm talking about the area from the radiator back to about the middle of the motor.

Regarding the C2 issue, I can definitely see cracking the rear of the hood to let out air because as you stated above the incoming air pressure is so great that it overcomes the high pressure area in front of the windshield. I just watched the wind tunnel test on youtube of the C2, the air seems to flow very nicely around the windshield of the car...probably because as lateral surface area, the C2 windshield looks almost 12" narrower than a C-4 and newer Corvette.

Air pressure at the windshield can be goofy....In my H1 Hummer with its totally vertical windshield, at certain speeds in the rain you can actually get rain drops levitating 4 or 5 inches in front of the window and just hover there for 2 or 3 seconds.

0Anthony @ LGMotorsports

Depends on how the air is routed and the angle/height of the louvers letting the air out.