Heat extractor hood

[vette427-sbc]

Im looking to make a heat extractor hood and had a few aero design questions... I would like to utilize an L88 hood, and move the "hump" back a few inches to make for the air flow exit. My question is; since the hump is taller than the first ramp of the hood, will a high pressure area be created at my airflow exit and make the airflow area too turbulent for proper radiator airflow? I wouldnt be opposed to adding a small wicker to the front ramp to help create a vacuum if it was necessary.
Also wondering how big the exit vent should be? Too big and exiting airspeed might be too slow, and too small might just restrict flow too much?
Ideas or thoughts? Id like this to be as efficient as possible without making drastic changes to the overall design of the L88 hood

Design of a regular L88 hood for reference:
52359

original picture (note hood bulge location)
52360

And the modified hood/radiator:
Im hoping the airflow will act similar to the blue arrows and jump over the hump. The red arrow is what I am thinking might happen (high pressure area like a cowl hood) since the hump is taller than the ramp
https://static1.pt-content.com/images/pt/2011/12/heatextractor-1.jpg

[Bryce]

I would suggest adding a lip on that first rise in the hood, reference Gurney Flap for aero explanations.

Also I would keep the inlet to the radiator duct equal to the outlet.

Cool idea good luck.

[vette427-sbc]

Thanks Bryce... Im thinking I should have a gurney flap on there too (Ive been calling it a wicker).
As far as the exit duct, Ive been reading that it shouldnt be the same size as the inlet. The air flow is turned into low speed, high volume after going through the radiator. Im thinking that it would be more aero-friendly if the airspeed coming out of the extractor duct was not so drastically different from the airspeed going over the hood. I think the higher airspeed and smaller duct opening will help combat the high pressure zone I am hoping to avoid too. The only technical aero experience I have is the fact that I just finished the aero chapter in my Tune To Win book, so please correct me if my thinking is wrong.

:cheers:

[MIKE67]

If I remember correctly, the McLaren Can-Am cars had the outlets about equal to the Radiator area. The inlets were smaller than the radiator area. They also had a small lip at the front edge of the exit. The placement of the exit is in a low pressure area on Camaros and Mustangs. I am not so sure on the more aero Corvette or McLarens. There should be info somewhere about flow thru a radiator and areas and pressure drops.

[David Pozzi]

Have you seen this? http://www.halltechsystems.com/product_p/gt2.htm

[vette427-sbc]

I have not, thanks for the link... That one does not look too functional in terms of letting out air that has passed through the radiator. I am trying to have mine seal off the entire radiator so that all of the air coming through the rad exits out the hood.

[lvrpool32]

I was looking at doing something similar. You would have to go to a lift off hood and you would have to tilt the radiator forward, move the whole thing slightly down and maybe even forward as theres not a lot of room between the front of the water pump pully and the radiator, as the duct from the back of the radiator needs to be pretty wide and can't come up at too steep of an angle (I was thinking GT40 type hood vents as my model).Also you would have to remove a spreader bar if you have one.
In the end I just added louvres to the hood (either side of the L88 bulge, towards the back)....to get more air out from underneath.

Nick

[vette427-sbc]

The rad will definitely be tilted forward, and Im going to modify the hood hinges to pivot the hood closer to the front... Id rather not do a lift-off hood. Id also like to keep my spreader bar. I may try to find the aero tubes they use on race cars and use one of those since it will be in the middle of the airstream.

[The Stickman]

What Bryce says is good advice. BTW wicker is also correct. The thing that may come into effect is the pressure area at the base of the windshield. If the exit is to close it will end up acting like a cowl induction hood and bring air in or maybe stall it out.

[j-c-c]

Ok this is my first reply, I'm a little gun shy after all the daily beatings received over on moparts. Did a search here and this thread seems to be a good start for my question on hood extractors. I'm working on an early Dodge b body, with a fiberglass hood. In plan view the windshield has a lot of curvature. I've drank the koolaid on the high pressure area at the windshield base, but find it really hard to accept that that high pressure area exists across its width for a large portion, and in my case with a really curved winshield it would seem like only be in a narrow spot. I likely will do some yarn/go pro tests in the near future. I would like to cut some vents as close to the rear outside corners of the hood as possible, to 1. vent underhood at idle or low speeds for during street romps ( and I'm in So Fla), 2. to relieve any built pressure up at speed, and I am already running a decent custom front spoiler, the grille is already pretty restricted, and most cooling air enters thru cutout in front bumper. My concern is hood extractors may defeat all all negative and diserable underhood pressure. Of course this idea only works if the vents are in negative pressure location. Moving vents foward on hood is possible, but i like the thought that there is air movement across the the motor, even though minimal. this car is never going to see the track, and this maybe just a playtime experiment for another car that will see many track days.

Am i chasing my tail?

[Motorcitydak]

No, I would not say that you are chasing your tail. What car are you working on? Just because a car will not see track time does not mean that aero does not matter. Even at freeway speeds aero can come into play in a large way. Not necessarily for downforce but at least for aerodynamic efficiency. That would translate into better MPG for you if nothing less

Aerodynamically any old B body is not going to be perfect. You are correct in thinking that the middle of the bottom of the windshield has the highest pressure. The outer sides will have less but I do not know if it will be very low. It will certainly not be negative pressure. On a car with a squared off nose, the farther forward you go on the hood, the better chance you have for negative pressures.

Im my '68 Charger for example the factory gave me nice indents in the hood. I plan on cutting those out and recessing them as much as possible. It will not be an optimal design but at least will be a compromise of sorts between form and function

If you look at that picture of the Corvette, you will see that it actually picks up air from around the ground and from the design of the air channel, it is directed up at a very good angle. That will not only provide some additional down force (and wind resistance which is not so good) but will try to push the air coming over the hood to come up, reducing the air hitting the windshield.

If you can direct air not only out from under the hood, but also up you will improve quite a bit. If you have the air channeled already thru the bumper, can you box that off to force it all thru the radiator then up? I know that space is really not readily available behind the radiator but maybe something could be done?

The radiator is what cools the motor anyways, not some air flowing over it. If you do seal up your radiator, you may have issues from the exhaust but some exhaust coating or wrap can help with that. Also it would be a good thing if you could hook up your air intake for true cowl induction (not simply a cowl hood) but that usually comes at the expense of sealing off your HVAC inlet ducts that are in either corner under the cowl. Then you could allow just 2 3 inch ducts to run into the engine bay

To the OP: In the profile pic of that Corvette, the angle of the front ramp of that hood is pretty much equal to the angle of the front of the car. Air would go straight up from the leading edge the straight over that ramp, leaving a negative pressure area just behind that ramp where the opening of that hood is. I fear that if you do move it backward too far from that front angle, the whole opening will be in a negative pressure area and not much air would be under vacuum from the opening itself. Like you drew on the next picture, if you move the opening back, continue the ramp as mush as you can at the same angle to keep your negative pressure there

You would not have to do a lift off hood if you had some ducting that stayed with the radiator then just the hood with a hole in it and maybe some weather striping to seal the two together.

I have to ask tho, the L88 hood was obviously developed by guys that wanted excellent aero and heat extraction. What is wrong with using it exactly as they had designed?

[CarlC]

Check out the December issue of NASA Speed News. There's a great aero tech article using a Daytona on the salt flats as the example.

[vette427-sbc]

The original intention of the l88 hood was to be used as a cowl hood. The vents in the front were never opened. I have changed my plans a bit and I think I'm going to use my big block hood instead of the l88. The BB hood doesn't have a cowl so the hump is the same height as the ramp. Now I just need to talk myself into cutting up a nice original BB hood

[j-c-c]

When you say "original intent", do you mean yours, or the factory L88 hood designers?

On mine I cutting one of 4 fiberglass hoods I bought since I knew i I would have some apprehensionj cutting up a pristine hood.

Hoping not to hijack your thread, but I'm attaching my situation, red being likely heat extraction areas, blue being under hood fabricated cowling for engine intake .

[vette427-sbc]

Original intent of the designers i believe...
Keep posting! Keeps the discussion going

[j-c-c]

Be careful what you wish for lol

So to try and decide if my planned area for heat extractors is useful, i need to determine if the area above the possible vent location has a lower pressure then the area below it at typical speed. seems to me a shade tree plan to help determine this would be a partially liquid filled clear tube with one end open below the projected heat extractor point and the other open end just above it, and do a road test while having the tube in an obvious u, lower then the open ends, where it can be viewed while driving. Won't give any empirical answers but would show a pressure gradient I suspect, and save me filling holes in my fiberglass hood. Anybody tried a similar simple test and?

[j-c-c]

I just read the "CFD and C6R" thread and if my car was a c6r i would be wasting my time, hard to believe the widely separated twin high pressure areas at the base of the windshield. Back to the drawing board.

[James OLC]

We did some testing on the OLC using tuffs of yarn. The data was decent enough to show where and where not to put our hood vents. Easy to do and well worth the effort.

[David Pozzi]

I made a manometer. Get a clip board, tie or glue up to 4 aquarium hoses in u bends on it. I also put tiny aquarium shut off "ball" valves at the bottom of each hose. You can drive the car to test speed & just shut the valves off, trace the fluid levels onto the sheet of paper, & you have your readings.

Tuft testing shows flow direction & turbulence.

[astroracer]

If you have a positive pressure on the output side of the radiator, and you will because the inlet side will be feeding it high pressure air, I think any "turbulence induced" HP zone on the outlet side of the hood will not be sufficient enough to overcome the "forced air" high pressure zone exiting the hood. Seal your ducts up well enough so there are no leaks and you shouldn't have an issue with "back drafting". Not gonna happen, the sealed route through the radiator will have to exit through your hood vents no matter where you put them. A pair of electric fans would provide a positive output through the vents no matter what road speed the car is going.
Mark

[Mtrhd329]

It will certainly not be negative pressure. On a car with a squared off nose, the farther forward you go on the hood, the better chance you have for negative pressure.

so where would be an optimal position for a 73-87 c10 be? Close to the front of the hood or as far forward as possible on the hood while still being behind the radiator.

[sccacuda]

I'll throw in my $.02. I look at the car backwards, sort of. The hood area is a huge surface area that has air pressure pushing on it, downforce. The windshield then becomes a deck spoiler of sorts, so yes, high pressure air. The transition from hood to windshield will provide high or low pressure depending on it's angle (great place for Vortex generators if seeking high pressure and using more of the windshield for downforce). When you take away hood area for ducts, your losing front downforce potential, no way around that. So the duct needs to perform a function greater than the downforce forfeited, or alternative means of adding front down force needs to be introduced, i.e. splitter, dive planes, etc... Then the question becomes how to merge the air from the duct to the high pressure air on top of the hood. The wicker lips in front of the ducts seem to "kick" the air up, leaving a lower pressure area from the duct, allowing the airstream to merge more easily, and possibly create a vacuum effect on the duct. My only concern with having these too far back with a wicker is losing the downforce on the windshield. The air is kicked up and hits the windshield at the middle or top. If I had to chose a hood duct, I personally like the idea for the rear hood exits, exhausting in the low pressure area at the windshield base, as not to disrupt airflow or give up too much downforce. Exhausting from the sides of the car seems a better alternative, as it doesn't give up downforce or disrupt airflow, but probably not very practical. With all that said, I too am extracting from the hood!! I played with all sorts of ideas and designs, and in the end I chose a compromise of aesthetics and function. That should be the name of my car! I'm using the shaker bubble as the extractor vent, and leaving the front vents open to allow air "through" to create a venturi effect. Not sure if it will function that way, but that's the plan. As you can see, keeping the acres of hood downforce!

https://static1.pt-content.com/images/pt/2015/07/70cudafront_rodsters_REV4a_zpsxwhtjvhb-1.jpg (http://s1060.photobucket.com/user/340cuda/media/70cudafront_rodsters_REV4a_zpsxwhtjvhb.jpg.html)

[dontlifttoshift]

Where is there low pressure at the base of the windshield?

I have no science but.....isn't it likely that the pressure entering the front of the shaker will be higher than under the hood? That will not allow air to escape from under the hood at speed.

If you vent high(er) pressure air from under the hood to the top of the hood in an area of low(er)pressure with a properly designed lover, would there not be a net gain in down force?

[j-c-c]

Where is there low pressure at the base of the windshield?

I have no science but.....isn't it likely that the pressure entering the front of the shaker will be higher than under the hood? That will not allow air to escape from under the hood at speed.

If you vent high(er) pressure air from under the hood to the top of the hood in an area of low(er)pressure with a properly designed lover, would there not be a net gain in down force?

I agree pressure at base of windshield at speed is normally positive. I also think the spoiler analogy has merit, and a big cause of this high pressure area. I also raise an eyebrow on the comment that the hood, in its entirety is a DF generating surface, not normally on our type of cars from what I know. The easiest least effort for extractor location determination is driving 70+? mph with as many tuff yarns taped onto the area in question as you feel the effort warrants and get some idea whats happening at the surface and where, for starts. Use a go pro to spice it up.

[Mtrhd329]

In comparison to the radiator opening how much larger or smaller should my extractor vent be? I am thinking smaller will cause the air to speed up coming out of the engine bay and help join outside air at the same speed? Thoughts?

[medbali76]

I am thinking to put trackspec product ( http://trackspecmotorsports.com/shop-by-vehicle/camaro/first-generation-camaro-67-69/1st-generation-camaro-t1-hood-louvers.html ) on my 72 Nova hood. Any idea where would be the best place to install to get the best results?
My cowl went grille panel was blinded also to get a little smoother look so that is the other reason I want to install these vents.
Thanks