Pick the hood for looks.

Lets get a couple things on the table. One that Howard has covered. Firstly aluminum is a superior material for a hood when compared to carbon fiber. It dissipates heat better. Carbon fiber is like a blanket and holds heat in. Aluminum is just about as light as well. And its usually the color of your car!

Next. You will only gleam cooling benefits that make any difference when the car is parked after you have driven it hard or not. Meaning when you are moving down the road at any given speed the amount of wind moving through your cooling system should be more than adequate given you have a properly functioning cooling system and one matched to the mods that you have chosen. I chose to go as far as i could in this area without going to a vented hood. I.E 99 spec fans and a koyo n flow with all new cooling hoses and an underdrive pulley for the water pump with the pineapple air pump deletion pulley (which alone will likely make a bigger difference than an upgraded radiator given how much belt slippage is allowed by things like the greddy pulley kit).

Lastly and in my belief. You only truly get good benefits from a vented hood when it is designed for a v mount radiator and intercooler setup. Otherwise i dont think it truly makes much of a difference. But in that scenario it could be truly beneficial.

 

The alluminum materials properties as a heatsink notwithstanding, this has all pretty much been proven to not be true.

The vent in a vented hood has been proven to lower temperatures, both on the track (at speed), and when parked (heat soak). As anyone knows, the efficiency with which "wind" will move into your cooling system is affected by how quickly it will move out (be evacuated so more can move through). I suppose if you had an alluminum vented hood that would be the BEST solution, but please don't assert that the vent doesn't help.

Second, on the track (at speed), any air entering the nose openings must be evacuated. Where do you think that air goes? UNDER the car. Evacuating that air up and over the car instead should theoretically reduce lift. I don't know that this has been wind tunnel tested, but anecdotally I've heard more than one person confirm this.

Third, I disagree with the logic that in order to take advantage of a vented hood, you need a V-mount intercooler (presumably to "point" the exiting air at the vent). Think about that. One way or the other, the air will need to "find" or bend it's way from a horizontal entrance at the nose, to the evacuation point. Air is like water, if you pump it into a cavity, it will "find" the exit, particularly if it's under pressure. A v-mount may well be the ideal, but it's far from necessary to reap benefit.

Finally, as I pointed out on the previous page, comparisons to "other" cooling mods is a bit of a false dilemma. We're talking about a SYSTEM, and thus one mod isn't competing against another. They all help each other, and they all add up. You can bolt the largest radiator you can find under the hood, but if the air can't to it, or can't get out of it, it's not efficient, and you're using a big heavy piece of equipment when a smaller lighter one would do.

The big barriers to a vented hood are actually the cosmetic ones—whether you like the looks of one that works, can get a good one that fits well, the cost associated with getting it painted and matched (if it can), and how you feel about hood pins.

 

You can very well say that one cooling mod is "better" when put to the test of cost v.s. benefit. And i will say that a vented hood is definitely on the low end of that cost v.s. benefit scale. I'm well aware of the engineering oversights on this car. There are a couple. But one thing that they didnt spare on was the aero. I dont and will never claim to have a keen understanding of wind tunnel physics. But on thing is for sure mazda spent a lot of time in there and they didnt vent the hood. Instead they sealed the bottom of the radiator to the belly pan and slanted the radiator at a 45 degree angle. Why? I dont know but i bet if you go putting holes in your hood you will change how all that functions. And i personally cant guarantee that all the changes will be positive. A lot of peoples examples for how things make positive changes are less than scientific. Including the people and examples that prove that these hoods make positive changes even at speed. Is a change of a couple of degrees that were never in the critical worth effecting aerodynamics? I dont know. I dont know if it even will. One interesting find in this area though is that a lot of times people vent hoods or lift the rear it actually ends up pulling air through the vent or hole. Which increases lift. Hmm. There is a reason that the rear of the hood is sealed with a gasket and its not for rain. I'm just saying that these are the areas in which modifying these cars is a little more interesting. And like i said before my rant, do it for looks because a lot of the other details are conjecture.

 

Just following up on the above, in short, you're flat wrong.

If you read the Yamaguchi development book, Mazda expressly didn't add a vent or intake for the IC for aesthetic reasons (and likely rthe water/drainage reason) for a STREET car, and clearly they addressed this with their own Mazdaspeed aftermarket parts, including a vented hood that was was (I can't confirm 100%) wind tunnel tested. Maybe some aftermarket hoods don't do exactly what you think, but I don't think you can say that about every vented hood across the board.

I also recall notes on the development of the '99 nose that said Mazda added the bigger lip on the front, because air being ingested in the larger front opening was literally "spilling" back out and under the car. Why? Be cause it couldn't be extracted fast enough.

It's also been proven that there's a gasket at the BACK of the hood, because it's at the base of the windsheild and that creates a pressure area that will ingest rather than evacuate air. That's a different spot, so different results

I saw someone remark on another forum that virtually every racing GT car in ALMS, etc., has a big ol' air extraction vent in the middle of the hood... that combined with plenty of anecdotal evidence from people who've tracked the car that have seen temperature drops and reduced lift.

This isn't a difficult concept to grasp. A vented hood is the same thing as a larger nose opening. It helps get more air, more quickly, THOUGH a heat exchanger. What would happen to your cooling if you taped a piece of cardboard on the back of your IC? Wouldn't matter how big your inlet is if you can't get the air through and out.

So in short, maybe the benefits of SOME vented hoods are "conjecture", not all and they are most certainly not "for looks"... just providing you some handy background info, because this was covered like 10 years ago.

 

Bingo! Mr Hahn is on the money as usual. And I have subject hood and tracked the car, and had the oil spray out of my catchcan mount next to my stock mount radiator/IC through the KS vents straight on to my windshield so I know how the air flows, and IT IS A SYSTEM.

 

Has anyone addressed the vented hood for the autocross car. I like to autocross i have a nicely ducted v mount, but a stock hood.

I want the vented but the stock hood is much nicer than a fiberglass.

I have seen guys with 240s shim up there hoods to let air out the back like the feed hood. I was thinking of just doing this on auto cross days as a cheap alternative. what do you guys think?

 

I have seen lots of jdm cars shim their hoods up in the past...not so much now. Not sure if it was a fad or if it actually did something (prob a bit of both).

 

I have a scoot hood which is quite vented, it make a 5C temp difference when moving easy in the summer.

 

I have the Shine FEED hood and DID notice a difference in underhood temps. I do however, also have the Tweakit pulley and underdrive pullies as well. In my opinion the Tweakit pulley in combination with the FEED hood worked best.

As for shimming the rear of the hoods, it is done to INGEST more air. It is my understanding that the base of the windshield actually directs air into the engine bay. Look at the old muscle cars and you will see that the "COWL" induction hoods feed air from the REAR and not the front due to this vortex of air that is drawn in from the base of the windshield while the vehicle is at speed.

L8R

 

http://us1.webpublications.com.au/st...109741_3mg.jpg

see the above image. It shows pressure difference between the air moving over the hood and the air in the engine compartment at different points along the hood. This profile is pretty similar for the FD as well (and indeed for most cars). If you shim the rear of the hood, you will allow air from the high pressure area at the base of the windshield to go back into the engine bay and it will most likely exit out the bottom of the car at speed. Ideally, you would have vents in the hood toward the front where the pressure difference will force the air up through the hood, promoting airflow and theoretically not contributing any lift.

 

Ha i always was like what the hell on cowl induction hood. I would still think standing still heat would escape out the gap which would some what solve the lack of air flowing.

 

You're not spending a lot of time under motion at an autocross, but you may well be sitting in line, and the vent should help with heat soak.

 

This is more what i was getting at with a shimmed hood. You usually do a hard run and then its a dead stop for say 10 to 15 min.

 

Shimming the hood would definitely help while the car is sitting in between runs, but it just seems like a lot of work to install and remove shims for each event if you are driving the car to the event normally. Do you have a manual override switch to keep the fan on with the car off in between runs? I feel like just doing this would work better as well since forced convection is a much more effective process than natural convection.

 

Recently picked up a Shine FRP version of the FEED hood. Immediate 3 degree temperature drop over OEM hood and it certainly cools off quicker when parked as well.

 

Great input. I have been completely torn on the hood selection.

KS hood obviously moves more air, bit is definitely more aggressive. Which I don't like....

Feed hood still seems to flow, and looks better. If Shine comes up with the ducting for the Vmounts, then it will sway me completely...

And then there is the Mazdaspeed hood..... which I also like...

 

My Scoot hood makes a world of difference. Water temps ~82*C/185ish F at idle.

Pull up the car, turn off, immediately able to work on it pulling plugs or whatever is needed.