GrantB

I'm sick of working on the car with that stupid flip-forward hood. I'd like something very light and simple that eliminates the stock bracketry. Does anyone know who makes such a hood for a reasonable price? ACP's is just a wee bit expensive. I'm willing to go ventless if necissary (as I think I can add those myself without too much difficulty).

I may have something made locally by copying my stock hood (which I believe should be pretty cheap) if I can't find another solution.

The hood on Pfadt's car looks perfect (anyone know why they didn't louver it at all?):


Thanks for any help.

Independent1

Try ACP.

Here is link.

http://www.acproducts.us/corvetteracebod.htm

imp zog

For a quick solution, why not just use the stock hood? Remove all the bracketing and such and with a few holes drilled you can have a pinned hood. The stock hood is not very heavy, being composite plastic like the rest of the car, it's namely the firemat on it that adds weight (and there might be some structural metal in there too like the trunk). Not sure if it's okay to remove that mat or not, I won't guess as to whether or not the heat might cause melting... eww.

As for the louvers, the hood will have a lot of air pressure on it, so hood louvers don't necessarily draw air *out* of the engine compartment like you would think (Bernoulli principle?), they are actually likely to push air in thru the louvers instead. For example: stock cars still use carbs, which draw air in from a notch in the hood up by the windshield, and that works because the high pressure actually helps to push air into the air intake as opposed to starving it of air.

Functional hood louvers like on the ALMS 'Vettes are not for engine compartment heat extraction, those are wheel well vents designed to remove pressure in the wheel well area, for the purpose of decreasing lift. By louvering the engine compartment portion of the hood, you are likely to actually reduce your car's front end down force by reducing that high pressure. Our cars breath from the bottom/front, which flows thru the radiator, thru the engine compartment, and out the side louvers in front of the doors. If you are looking for better cooling, one of the best things I have ever done is run a shop vac over the front of the rad periodically, you wouldn't believe how much crap builds up in there. If you are tracking, you will want to install an oil cooler as well.

yakisoba

Many of the cars that have tipped their radiators forward (the C5 racecars did that) have vented hoods. But those are no longer bottom breathers, having been converted to front because of a splitter/undertray.

Aero is TRULY a can-o-worms. I am putting down the can opener.

kmagvette

I am not sure how much the hood contributes to downforce. I have a stock hood that I did not close properly. At around 90 mph it started lifting on the drivers side, passenger side was latched. At the very least there appears to be a low pressure area in the vicinity of the windshield.

Being the frugal (cheap bastard) that I am, I am thinking of removing the seal at the back of the hood to get additional heat extraction. Anyone try this? I will try a few runs with the seal on/off to see if there is any measurable reduction in temps.

Todd Z

Like imp zog stated, you can use your stock hood. I do have a silver hood sitting in my garage you can have for cheap. We can always make you one at the shop but it would be expensive and you stated you wanted to save cash. www.blackdogspeedshop.com

davidfarmer

the rear of the hood is actually a high pressure area....removing the seal will NOT help your problem.

I louvered my stock hood using JCWhitney louvers. Let me track them down, and I'll post. You'll have a LOT of money in ACP's hood, although it is very nice.

ryan0

ACP is baddas.. so light you can lift it off by yourself.

GrantB

My stock hood weighed right around 35 lbs, without the underhood silencer mat. I would guess that a thin CF piece would weigh around 8 lbs, saving a lot of weight with the brackets removed. The mat itself weighs nearly nothing. I have been running without it for 9000 miles and dozens of hot laps without issue.

What you're trying to do is take advantage of the fact that the air above the hood is moving faster than the air in the engine bay. This creates a difference in static pressure, allowing louvers to pull air from the hood. Well-designed louvers (which are sold by many racer supply stores) increase this pressure difference.

On my stock hood with the two engine bay vents opened up, there is zero positive pressure in the engine bay. I believe this is due to opening up those vents, and the downward design of the radiator. I measured this with a mamometer, with the other pressure reference in the cabin. As soon as I remember to pick up another NPT fitting, I'll measure the difference between static pressures above and below the hood. I'll post the results for anyone interested. I don't expect a louvered hood to help as much on a C5 as a car its radiator in the more standard position, but I do expect it to help a little.

Yes I know,. I've read Katz, Milliken and McBeath, so I have a basic understanding of race car aerodynamics. if I get a suitable hood I will open up the top of the wheel arches and vent the high pressure inside the wells. However, lower pressure inside the engine bay should reduce lift as well.

Is it possible to seperate the outer shell of the stock hood from the reinforcements and brackets? It looks like its epoxied together, so shouldn't be too difficult.

Todd, I am just not looking to pay ACP prices I'm sure they make a great product, but the price is approaching a tenth of the value of the car, which IMO is too much for a hood. I can get local people to make a thin CF copy of the stock hood for a ton less money.

argonaut

Help me understand the purpose of louvers on a hood better. In reading on corner-carvers a while back I came to understand that the area of the hood near the front of the car is a low pressure area, due to the high speed of the airflow over that part of the hood. The area at the junction of the hood and windshield is a high pressure area, caused by the abrupt slowdown of the airflow as it hits the windshield. So...if you place the louvers near the front of the hood, they actually do two things: pull hot air out of the engine bay (high pressure in the bay, low pressure on top of hood) and overall reduce lift, buy giving high pressure inside the bay a chance to escape.

In the post above, "Imp Zog" indicates that this is not the purpose of louvers.

Again - just trying to understand here, not trying to say who is right or wrong.

Opinions?

EDIT: as I was typing GrantB basically reinforced what I read on corner-carvers. Thanks.

davidfarmer

understand or not, louvers reduce lift and help cooling. When I did this to mine, you could literally see the hood stay down where it was bowing up before, and the engine bay stayed much cooler.

If weight is important, pay the money, but if not this is working for me

davidfarmer

btw, they are thin plastic, that's why I left the opening "bridged" a bit. Also, I just used the front 3 louvers, so the actual openings arent as far back as it looks from the front photo

davidfarmer

The Spark

A former engineer/manager on the Viper told me the hood louvers to vent the wheelwells added something like 10-15 mph on the top end.

They had to place the louvers and size them to reduce the possibility of sucking engine fumes into the cockpit on the convertible Vipers when the tops were down.

I removed the black shield inside the left fender opening on my C5. I left the right one in because of the PCM (?) is right below the opening. I didn't want water running onto it. One day I might build a shield and take the shield out to give more airflow.

GrantB

David,

Thanks for the pics. I noticed your hood was pinned on, did you keep the stock substrate and reinforcements, or did you pull all that off for weight?

It would be nice to have similar pictures of stock C5s or C6s, to see where the lowest pressure area is on the hood. Alternatively, I guess I could just copy ACT's.

I have both vents behind the wheel wells uncovered, and have never noticed the hood bowing at high speed (I believe my mamometer testing confirms there is not positive pressure in the engine bay). I also have some weatherstripping sealing the front of the hood, and depo headlights (which aren't sealed as much as they could be).

I hadn't realized that US Body was ~10 miles from my house. I think I'll pay them a visit tomorrow and look at what they have to offer.

C5-vette

http://forums.corvetteforum.com/c5-p...ge-400obo.html

Is this what you are looking for?

davidfarmer

in my World Challenge days, before they allowed the carbon hoods, I cut all of the reinforcemnt and glue out. However, you really can't get it much lighter, so I didn't bother in the hood shown.

All of my C5's have had the front edge of the hood (right behind the lights) bow out bigtime without pins. Now whether this was due to pressure from the nose, or simply leaking in around the headlights, I don't know, but it's probably both. I DO know however that the louvers let whatever pressure there is come out!

davidfarmer

here is the Corvette Fever wind tunnel video. http://www.youtube.com/watch?v=aduap_AS5j8


Not as cool as the GM fluid dynamics model, but it shows the reversal around the cowl.

imp zog

EXACTLY! Sorry folks, my attempt at advice above was *not* meant to say don't ever use hood louvers or start a debate; I meant that unless you have a wind tunnel or some other reliable method of testing your aero changes, you are rolling the dice and might not be getting what you think you are. You might be doing more harm than good from an aerodynamics standpoint.

Unfortunately there are many fabrication places that will sell their aero products as being "race tested" and such, but in reality they just sold some of their stuff to people who are going racin' and wanted their cars to look the part, because proper aero testing is very expensive and time consuming. Louvers too far back, you could be pushing air in; too far forward and you might be pulling air away from your intake rather than from the engine compartment... get the data on paper, and then believe it. Seacrest out!

GrantB

Thanks again David, thats a great video. Didn't know you'd run WC.

C5-vette, not exactly, but thanks anyways. The VIS hoods use the stock bracketry, and are more for street use. I believe the top layer is all CF though, so they might be lighter.

I really don't think its all that complicated. Yes I'd kill for wind tunnel time, but instead I bought a set of mamometers off eBay for ~$40. You can measure the pressure under the hood before and after louvers, and you can measure the pressure difference over certain areas of the hood. All that said, I think I'd probably just put the vents where ACP did.

Wind tunnel testing in and of itself won't tell you anything about where the pressure is higher or lower anyways. I think OEMs and race teams use a matrix of small, electronic pressure transducers for that (but don't quote me on that).