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I had an opportunity last year to be at Talladega during NASCAR's test runs and watched as drivers put their left hand up onto the window mesh as they came down the straights. Talking to one of the crew members later that night at the hotel, he said they got 1-3 mph increase from that alone and went on to say that several teams try and use mesh with smaller than allowed openings to improve their times. Several have been caught doing it seems as well.

 

Inserts to divert the wind would seem to be a good ideal for a race car but I doubt many people would be willing to put them on a street car. Same for adding anything that makes the car so low it could catch on a speed bump or a normal New England road. I don't think we could get the SDI car on our trailer without making the ramps longer so we didn't crush the side skirts. I'm thinking snap on-snap off...

 

As to air going into a cowl induction hood (as opposed to going out and over the windshield), I taped 5 colored pieces of thread under the cowl opening and then drove the car around the race track at speeds up to 115mph and not once did the threads ever go inwards.

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As to air going into a cowl induction hood (as opposed to going out and over the windshield), I taped 5 colored pieces of thread under the cowl opening and then drove the car around the race track at speeds up to 115mph and not once did the threads ever go inwards.

That's weird. Cary also had a strange experience testing a cowl hood. He said up to 50 mph the air went in, then after that he said the air kinda looked like it was going in and then back out and then back in again. Not sure what to make of that, but it still seems to me like the most efficient place to vent the hood would be right in the middle. I think we can say for sure that there is a high pressure area at the base of the windshield, so even if the pressure under the hood is higher and the air does come out, the air wouldn't evacuate as well with the vent in the high pressure area as it would in a low pressure area. And then there is Scottie's experience where he closed up the hole on his cowl hood on his RX7 and his cooling issues went away. Maybe that's a bad example though since it is a different car...

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I'm still reading and thinking - you guys can probably smell burning horseshit from where you sit.

 

Based on what we have seen so far, I'll close off the rear of the cowl bulge. There is far more evidence that the cowl opening is counterproductive to radiator flow than otherwise. I would be willing to bet that the air intake from an open rear cowl is always counter to air entering the underhood area through the radiator, but the net effect will vary according to speed. I think that these two sources of air intake will behave differently as speed varies, with the net flow through the underhood area varying with the vector sum of these inlet sources.This will explain:

up to 50 mph the air went in, then after that he said the air kinda looked like it was going in and then back out and then back in again.

i.e. Sometimes the flow from the front is greater than that from the rear, but they still oppose each other.

In the following case there is probabaly a stronger flow (bigger front inlet area?)from the front than the rear:

I taped 5 colored pieces of thread under the cowl opening and then drove the car around the race track at speeds up to 115mph and not once did the threads ever go inwards.

I agree that the hood-popping data strongly suggests that venting underhood air to the foreward/side is effective in reducing pressure.

If there is another wind tunnel session, I would really like to see a stock hood instrumented to give us a pressure map across its entire surface.

 

I will also make large openings through the inner fenders to the side vent area of my front fenders, in addition to getting the radiator sealing and ducting done as has been discussed - but it looks as if proper hood venting is a large piece of the solution.

 

I haven't yet considered drip rails and A pillars-

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The A-pillars need to be wider in the front. This will help keep airflow from diving into the open windows. This will generally clean up airflow across the sides of the car.

 

Replace the windshield with Lexan.... but instead of mounting it where the original windshield went... Make it wider... wide enough to widen the A-pillars past the drip rail edges.

 

You could heat-form the lexan to make it bend near the A-pillars and act as a fairing across the A-pillars.The original windshield gasket could be used as an O-ring to seal the interior of the car(instead of the conventional mounting method)...

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This is the redneck in me thinking here, feel free to throw beer cans at me:

 

If someone had a hood they were willing to sacrifice in the name of science, could you take a hood, uniformily cut holes in it, then cover those holes with something (redneck me is thinking duct tape), install on car, drive car, then observe where the material stretched and in which direction? I'm sure it might stretch one way, then the other depending on speed. That would be a problem I can think of right off, but perhaps that can be gotten around. How about small flaps that could be observed from the cockpit?

 

Just throwing some ideas out there.

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Yarn is the way to fly... It is pretty simple... 4" lightweight string/yarn, tape to hold yarn, if yarn flys out of hole then air is flowing out.. if yarn dives into hole the air is going IN, if yarn lays across hole then the hole might as well not be there.

 

covered holes will not act like the openings... you will only get a general idea of pressure differential where the tape puckers.

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This is the redneck in me thinking here, feel free to throw beer cans at me:

 

If someone had a hood they were willing to sacrifice in the name of science, could you take a hood, uniformily cut holes in it, then cover those holes with something (redneck me is thinking duct tape), install on car, drive car, then observe where the material stretched and in which direction? I'm sure it might stretch one way, then the other depending on speed. That would be a problem I can think of right off, but perhaps that can be gotten around. How about small flaps that could be observed from the cockpit?

 

Just throwing some ideas out there.

That is a damn good idea! Use the yarn like bj says...

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Yarn would be great if you could observe it at speed, either from in the car or beside it, but I'm thinking it would go back to it's original position once you slowed down. If you used something that would stretch under pressure, but hold the stretched shape when you brought the car to a stop, you could observe it easier. That's why I was thinking duct tape, since it's got enough stretchiness to move a bit, but not break. You'd end up with golf ball like dimples where pressure pushed down, and domes where it pushed up. Then there's the problem of breaking up the boundary layer, but I think the pressures above and below the hood are great enough to overcome that.

Hinged flaps could probably be seen from the cockpit, and I'm theorizing you could tell the amount of pressure difference by how high the flap floated (or didn't). Again, just rednecking it out here.

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covered holes will not act like the openings... you will only get a general idea of pressure differential where the tape puckers.

 

But that would get us in the ballpark about where vents/outlets would best be placed, wouldn't it? Then maybe go back and do more fine tuning with yarn?

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I don't think you will get a useful visual cue from the tape... unless it is a large area with really loose tape. You may find that as new holes are opened up it will change the flow of previous holes as you add them and open up more area... If you want to see flow then I would test HOLES not membranes.

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In my opinion, a proper cowl hood should be sealed on the underside with the only air entering allowed to be consumed by the engine intake (via sometype of plumbing). No air entering the cowl hood should mix with warm air entering the engine compartment from the grill or other.

 

Any testing of a cowl hood for aerodynamic purposes that is not properly sealed, and plumbed seems pointless.

 

In addition, if properly configured, any venting to the hood be situated so as not to preheat the cool air gathering at the base of the windshield to be consumed by the engine via the cowl hood. This is why you see evacuation ducts on Corvettes, and the Trans Ams on the sides. They evacuated all preheated air on the sides, insuring cool air at the base of the windshield.

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I'm late to this thread but I'm coming up on painting my car in a few weeks and need to figure out my options for vents / extractors. The evil genius in me is telling me to make 'H' cuts close to where the stock 280 vents are and pull the front flap up and push the rear down, fiberglass and bondo the sides and leave it at that. The resulting front 'lip' will create the low-pressure vacuum needed for extraction and the following indentation will smooth flow into the boundary layer. The idea is similar to MSA's urethane vent inserts, but those babies are pricey and I'm on a pretty tight budget.

 

...or I could just get a set of vents from Reaction research and cut / drill. I like the idea of making something myself, though.

 

Any ideas as to the proper placement or better execution of this crazy idea?

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I'm late to this thread but I'm coming up on painting my car in a few weeks and need to figure out my options for vents / extractors. The evil genius in me is telling me to make 'H' cuts close to where the stock 280 vents are and pull the front flap up and push the rear down, fiberglass and bondo the sides and leave it at that. The resulting front 'lip' will create the low-pressure vacuum needed for extraction and the following indentation will smooth flow into the boundary layer. The idea is similar to MSA's urethane vent inserts, but those babies are pricey and I'm on a pretty tight budget.

I think Owen did exactly this, but he did it with more of a JGTC flair and put several vents one in front of the other.

 

sjgtcday212.jpg

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auxilary was showing me Owen's car last night. I like it, it looks great and it's probably VERY effective, but it's a lot of work blending it in, and the vents themselves are pretty expensive...I managed to find the manufacturer(name escapes me).

 

I'll probably end up waiting until after the car is painted and whip something up with a friend that does fab work on aircraft (he build his own press forms and stamps aluminum). It'll be a cut and rivet, and I could anodize it or polish it, maybe get color-matched rattlecans and paint them.

 

I came across this last night and though it was a decent method of putting in a factory-looking vent without having

to do too much bodywork. Feedback on other forums is negative, though.

 

Consensus here on working vents seems to be that they should be positioned farther forward on the hood than the '76 and later factory vents. I suppose a tape-and-yarn yest is in order to find the right spot.

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I agree that the hood-popping data strongly suggests that venting underhood air to the foreward/side is effective in reducing pressure.

If there is another wind tunnel session, I would really like to see a stock hood instrumented to give us a pressure map across its entire surface.

 

You don't need a wind tunnel to do this. Use a simple water manometer and do the test yourself. Then drive the car at various speeds and have a passenger record the pressure results from inside the car.

 

Easy and cheap. Your local Home Depot or Lowes has everything you need.

 

Cary

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Oh, I'm sorry, I thought Owen did those himself with basically the same method you linked to, and did the rest of the fiberglass work to blend it in. He's pretty good with FG...

 

You know, I could be wrong. I found something that looked exactly like what he's got, but he could have done that himself.

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You don't need a wind tunnel to do this. Use a simple water manometer and do the test yourself. Then drive the car at various speeds and have a passenger record the pressure results from inside the car.

 

Easy and cheap. Your local Home Depot or Lowes has everything you need.

 

Cary

 

Right! - I'm gonna put 20 manometers connected to 20 holes in my hood and drive around to get a pressure contour map.

 

I don't think so.

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1. Nissan put a sealing lip on the rear edge of the hood opening ... How is the stock rear sealing lip of any benefit?

 

I'm guessing it has nothing to do with engine cooling or aero but rather to help keep hot engine bay air from going into the fresh air vent in the cowl box ... otherwise you may get full time heat coming out the interior vent ...

 

Cameron

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I have been sitting in the background for awhile, quietly observing the whole aero,cooling dilema. I`m somewhat concerned with some of the things that I`ve done but encouraged by others. here is the rundown of what i have accomplished so far.

 

1. shaved drip rails

2.shaved marker lights

3.shaved sail panel vents

4.3.5" cowl induction hood

5. whale tail

6.installed fiberglass MSA front and rear bumpers

7.cut out the inner fenders behind the strut towers

8.installed Z28 heat extractor vents in front fenders

9.headlight covers(yet to be installed)

 

I am in the process of flush mounting the windshield and hatch glass, and building a box to isolate the "cowl induction" intake to the carb/air cleaner.

 

Even before it was such a popular concern, I had planned to enclose the front of the car and channel all incoming air through the radiator. I never realized so little air was necessary to keep the engine cool.

 

Micheal and I have had several extensive conversations over the years regarding the aerodynamics "or lack there of" that these cars actually have,So it`s been in the back of my mind all through the paint and body process.

I`m really surprised that the wind tunnel results were not worse.

 

It seems that the whale tail turns out to be a good thing for which I am thankfull. It flows so well with the lines of the car and I`ve got alot of time in bodywork making it fit the way I wanted. It would really suck if it was "all for looks"

 

I guess it`s still up in the air as far as the shaved drip rails go. I would really love to see some CD numbers. Maybe in the future we could have a car tested with them shaved. Either way I LOVE the smooth look without them, and I have no intentions of welding them back on.

 

The flush mounted windshield will interest many of you. I`ll get some pics in my album soon so everyone can see how i did it. It`s way to complicated to go into now but it has been and will be very time consuming to finish. I will say it does look sweet.

 

I am planning to do a full belly pan for the car to channel the remaining air in the engine bay through the transmission tunnel to help cool the viscous posi diff. it will be modified behind the rear axle to let air escape smoothly under the rear of the car without catching inside the rear valance and bumper like a parachute.

 

The entire theme of my car is "smooth,sleek,and MEAN" It`s not fully assembled and still looks like it`s doing 150mph sitting still.It won`t be a sleeper in any way shape or form. especially when they see and hear the fenderwell headers and 3.5" sidepipes.

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dragging this thread from it slumber. i have taped on my hood and fenders several areas to make vents. the hood is a fg hood from 280zone. nice by the way. i plan on cutting 1/2" wide vents to get air out of the engine. also the hood has the snorkel that i think is more geared for the v8 guys but i am thinking that a hole in the front that goes straight out the back might draw out more air. kind of a flow through. the vents in the fenders aould look similar to these only i will cut out each one instead of the panel

 

http://www.raceace.com/products.html

 

or these

 

http://www.racerpartswholesale.com/product/765/Carbon_Fiber_Ducting_And_Sheets

 

here is a pic of my hood

 

IMG_4119.jpg

 

IMG_4108.jpg

 

IMG_4117.jpg

 

any comments?

 

let the bashing begin. ready, set, go

 

jimbo

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