hood vents: ideas...

[rags]

I don't claim to know anything about aerodynamics but I see a lot of pieces here that fit together but no one is seeing what I'm seeing. Taking bits anf pieces from what everyone here has said, I come up with the following.

 

If you want to vent the hood with hood vents, the vents have to be way forward of the trailing edge of the hood to stay out of the high pressure area created by the angle of the windshield in relation to the hood. Opening the hood and leaving it on the safty latch doesn't do much to vent the underhood air but it does do something to releave underhood temps. How does this happen if there is a high air pressure system at the base of the windshield? The answer is in the side openings of the hood. I can to this conclusion because a cowl induction hood causes temperature issues. The additional air that comes in caused by the high air pressure system at the base of the windshield hits the air that is coming through the radiator in effect slowing it down, and all of this air competes to exit under the car. With the hood popped, the radiator air has a way to escape out the sides of the hood and around the car.

What I would like to try, and will do so on my car, is to try to vent the underhood air to the sides of the car with someting like TitaniumZ has done at the TC cups. Also, once the front of the car is has been sealed and air can only enter the radiator or go over the hood, I'd like to try to help the air go around the windshield. Areas of concern would be the A posts at the drip rails and the windshield gasket.

 

I'm interested in what everyone thinks. Am I off my rocker? Not getting this at all?

 

Joe

[Challenger]

IM guessing the reason you open the sides of the hood is so you dont have the high pressure of in front of the windshield like with a cowl. The air would be moving faster along the edges of the hood then at the back of the hood creating lower pressure.

 

 

I'm interested in what everyone thinks. Am I off my rocker? Not getting this at all?

 

 

I give the same disclaimer.

[rags]

No comments? Come on Jon, BJ, Rick. What do you think?

 

Joe

[OlderThanMe]

I think to know where to vent a stock hood properly we should know where pressure starts building in front of the windshield. I think the middle of the hood where it is relatively flat would be the best place to suck air out through the hood.

That just seems logical...

The front of the hood has a good angle on it so it hits relatively clean air. The rear has pressure built up from the windshield.

[JMortensen]

I don't claim to know anything about aerodynamics but I see a lot of pieces here that fit together but no one is seeing what I'm seeing. Taking bits anf pieces from what everyone here has said, I come up with the following.

 

If you want to vent the hood with hood vents, the vents have to be way forward of the trailing edge of the hood to stay out of the high pressure area created by the angle of the windshield in relation to the hood. Opening the hood and leaving it on the safty latch doesn't do much to vent the underhood air but it does do something to releave underhood temps. How does this happen if there is a high air pressure system at the base of the windshield? The answer is in the side openings of the hood. I can to this conclusion because a cowl induction hood causes temperature issues. The additional air that comes in caused by the high air pressure system at the base of the windshield hits the air that is coming through the radiator in effect slowing it down, and all of this air competes to exit under the car. With the hood popped, the radiator air has a way to escape out the sides of the hood and around the car.

What I would like to try, and will do so on my car, is to try to vent the underhood air to the sides of the car with someting like TitaniumZ has done at the TC cups. Also, once the front of the car is has been sealed and air can only enter the radiator or go over the hood, I'd like to try to help the air go around the windshield. Areas of concern would be the A posts at the drip rails and the windshield gasket.

 

I'm interested in what everyone thinks. Am I off my rocker? Not getting this at all?

I thought that the first part of this sounded pretty similar to what I said in post #14, so I didn't feel a real need to respond. The rest is all conjecture. But if that's what you're looking for, here is some for you...

 

I'm not sure about the effectiveness of the TC area modifications when done alone, and I think their usefulness would be greatly affected by the shape of the airdam and the front fenders. Again looking to the NASCAR model which I believe is overlooked too much in general, if the front of the fenders protrude past the wheel openings by a fair amount this should create a vacuum in the wheel wells which would assist in drawing air out from under the hood and make the TC mods and fender vent mods more effective. If someone was using the MSA IMSA flares which are basically aerodynamically backwards in my opinion (because they expose the front of the tire and have a large protruding mud flap type shape in back) then I think the TC mods would be much less effective, if they were able to do anything at all.

 

I'm also not sure about the drip rails and windshield gasket. My hunch is that it would be better to have the windshield mounted flush with no gasket protruding into the airflow, and have it fastened with clips like a real race car. I have no idea where one gets the clips from though. I've done some quick searches on the net and haven't been able to find anything. On my car I've already cut off the drip rails in the process of fixing rust, so I think my goal is going to be to get the windshield net and quarter window flush with the outside of the car. I will also use the small mesh type window nets as opposed to the 1" thick nylon strap type, as my gut tells me that the small mesh type strung tightly will be better in terms of aero. I think a flush surface is better on the side of the car than having a bunch of different layers like the stock Z does, with the drip rails sticking out into the airflow and the quarter windows recessed back from the body line.

 

By contrast I bj seems to think that the drip rails have a beneficial effect, tending to force more air out and around the window openings, but I'm not sure that I agree with that. I think it would take more testing to know who is correct on that point, but I'm basing my opinion on the majority of new car designs where the drip rails are integrated into the roof instead of hanging out on the side of the car. But when you're racing with the windows open I can't say for sure who would be right.

[gnosez]

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.

[JMortensen]

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...

[speeder]

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-

[bjhines]

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)...

[scott19]

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.

[bjhines]

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.

[JMortensen]

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...

[scott19]

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.

[scott19]

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?

[bjhines]

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.

[primadonna z]

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.

[(goldfish)]

I also suspect that "proper" cowl induction hoods have the vent closer to the windsheild. i.e. the buldge extending past the normal end of the hood.

 

here are some pics I stole

 

 

[capt_furious]

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?

[JMortensen]

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.

 

[capt_furious]

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.