Carbon fiber hood
06-01-2012, 01:03 PM
#63
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Air flow as shown in those charts is going to change as holes are cut. Low and high pres. areas are going to change radically. Example is we all agree that the base of the windshield is a high pres. area, BUT that is with the hood sealed to the back of the eng. compartment. Remove the seal strip at the cowl and install hood risers http://www.miataroadster.com/miataro.../i-404640.aspx and I would suspect the air coming into the engine compartment by more then just the grille opening changes it to a lower pres area then the eng. compartment. Don't know for sure personally as I have never seen a wind tunnel result, but I do know that when we lived in Arkansas a few years back where the humidity is as high as the air temp., and believe me that is high, the spec miata guys are running hood risers for cooling purposes to let air flow through. It isn't probably as good as a vent located in the hood just behind the rad., but it still changes a high pres. area to a lower pres. area as the pres. from the eng. compartment is I suspect greater. Also back in the day of the rear facing cowl inducted hoods on the chevelles and other brands the intake was at the base of the windshield to take advantage of the high pres. area at the base of the windshield, BUT the hood was still sealed from the eng. comp. and the only hole in the hood for air was just a round opening over the air cleaner and it was sealed with foam weatherstripping so as not to let air pres. from under the hood effect the air flow in. Nascar uses the base of the winshield for air intake also but the snorkel on their air cleaner is sealed to the opening at the back of the hood and the hood sealed to the firewall.
As stated above Shanem's opening bottom looks to be at the transistion point of where the nose is vertical and the slope begins so there shouldn't be as much air deflection as if it was further up and even then if the opening was tall enough the air flow upward at the point of hitting the body would eventuallt turn flat as the forward air pres overtook it and deflected it into the opening, but as Bob stated a hood on the opening would catch even more. There are however other effects that would take place that you might not want if you are a serious timed track guy or racing in a class and that is decreasing down force. Nascar tapes up the upper grille opening in their noses for qualifying as they want the added down force and since thay are only out there for a couple laps aren't concerned with the lesser air flow for cooling. Also adding a hood over the opening would actually add up force if it was in the sloped area. So just going by what I have seen done for many years and what the spec guys were doing down south and not seeing a wind tunnel [which I would suspect they did as these were the national guys] result I would suspect opening the hood anywhere behind the rad and on back changes low and high pres areas greatly.So adding a relief vent behind the rad is very important to get the flow out and some areas are better then others but I think from what I have seen they all work.
As stated above Shanem's opening bottom looks to be at the transistion point of where the nose is vertical and the slope begins so there shouldn't be as much air deflection as if it was further up and even then if the opening was tall enough the air flow upward at the point of hitting the body would eventuallt turn flat as the forward air pres overtook it and deflected it into the opening, but as Bob stated a hood on the opening would catch even more. There are however other effects that would take place that you might not want if you are a serious timed track guy or racing in a class and that is decreasing down force. Nascar tapes up the upper grille opening in their noses for qualifying as they want the added down force and since thay are only out there for a couple laps aren't concerned with the lesser air flow for cooling. Also adding a hood over the opening would actually add up force if it was in the sloped area. So just going by what I have seen done for many years and what the spec guys were doing down south and not seeing a wind tunnel [which I would suspect they did as these were the national guys] result I would suspect opening the hood anywhere behind the rad and on back changes low and high pres areas greatly.So adding a relief vent behind the rad is very important to get the flow out and some areas are better then others but I think from what I have seen they all work.
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06-01-2012, 01:10 PM
#64
Air flow over the car changes as air sources change. The base of the windshield is as high pres area as has been stated by us all, but open up the seal of the hood to the cowl with say hood risers http://www.miataroadster.com/miataro.../i-404640.aspx, and release the pres. under the hood which is being fed from the front of the car which I suspect changes the base of the winshield from a high pres area to a LOWER pres area then UNDER the hood is thus letting air flow through.
It is all quite simple:
1. Force the air through the correct sized hole in the bumper skin to go through radiator core.
2. Eliminate all other intakes to engine bay except those directly ducted to cooling or engine intake. Full coverage front under tray is a huge benefit.
3. Know where the Miata engine bay exhausts waste heat and help it. For the record, that's:
a) Around the trans and out the trans tunnel.
b) Properly located hood vent. For the record that's a V shaped area from between the cam gears /rad and angling back towards hood hinges.
c) Through the gap between the subframe and frame rail and out through the wheel opening. Note: Factory MSM is almost completely covered from oil pan to mid point of trans. This improves cooling.
4. Reroute
5. Properly sized radiator
Note that I listed radiator last. Some of our endurance cars use OEM replacement plastic Koyo's. The rest if the mods are so effective that even without a vented hood, we have trouble getting enough temp on <75° days.
This image shows the opening sufficient for a 170whp endurance racing engine in 110° ambient, with a mesh grill that has 81% open area. The net area is about 35% of core area. What is not seen in this picture is the formed and fully sealed duct direct to the rad core we later added. The plenum is made of ABS plastic and riveted together. Just a simple rectangular box with divergent walls. No I don't have pics of the plenum, sorry.
As Andrew mentioned, NASCAR feeds 800whp engines with an opening barely larger than this. I almost feel sorry for the Miata track guys I see that have the front of their cars all hacked up with drag and lift inducing holes, misplaced vents, no reroute and still complain of cooling issues. Argh.
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06-01-2012, 02:25 PM
#66
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I would have thought the spec guys would have tufted or even wind tunneled this before they did it and why does Bill at miata roadster and others sell the risers? Guess I well have to put my stock hood back on and stack some washers and remove the cowl seal and tape some tufts on and see for myself.
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06-01-2012, 02:35 PM
#67
Some cars are setup to take advantage of the air coming in through with a missing hood seal - typically they'd remove just a portion of it and have that area ducted for an intake. Others just have no idea and think it looks race inspired. Others use it to cool the car when they're hard parked. It does let heat escape from the engine bay when not moving. Drifters use it because they spend their time sideways and sitting in the grid.
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06-01-2012, 03:47 PM
#68
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Hood risers would see air coming into the engine compartment from the base of the windshield and then exiting into the lower pressure area of the transmission tunnel. This would likely raise the pressure under the car reducing available downforce as well as reducing the deltaP between the front and rear of the radiator and other coolers. None of that is a good idea.
Many people performing a modification based upon faulty understanding does not increase the validity of the modification.
Many people performing a modification based upon faulty understanding does not increase the validity of the modification.
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06-01-2012, 04:07 PM
#70
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rccote to begin with that is not my hood, I bought a carbon fiber hood from chasers and the vents are up front right behind the rad. where the best spot is. If Shanem's hood opening is ducted to the rad. then he isn't just air cooling the motor as you put it. Go out and drive around and see how many cars have a lower and upper opening in front of the rad. and I'm sure some pretty smart engineers designed atleast some of those vehs. In fact there are some that have a opening in a very sloped section of the nose. I'm not saying anyone is wrong and anyone is right and I don't take anyones advise lightly. I just brought up some points as to what others are doing. Have you ever did a test for the air flow personally and would be interested to see some pics of the results. I plan on doing a tuft test myself to see the results.
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06-01-2012, 04:46 PM
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Hood risers would see air coming into the engine compartment from the base of the windshield and then exiting into the lower pressure area of the transmission tunnel. This would likely raise the pressure under the car reducing available downforce as well as reducing the deltaP between the front and rear of the radiator and other coolers. None of that is a good idea.
Many people performing a modification based upon faulty understanding does not increase the validity of the modification.
Many people performing a modification based upon faulty understanding does not increase the validity of the modification.
Hood risers are a total poser Idea reserved for the hard parkers.
I dare anybody to drive 100 mph without there hood being latched or pinned. I did that by mistake with my Erebuni hood with a properly placed hood duct and the hood stayed shut. A hood with no opening would have flew up and smacked the windshield.
Single inlet needs to be at the stock location. It does not need to be any larger than stock for a track car even at 3 to 400 hp. Stock exit is at trans tunnel. Best place to increase air flow for cooling through you’re heat exchangers and reduce both drag and lift is to put an opening in the hood as far forward as reasonable where the pressure on the external surface is lowest.
Bob
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06-01-2012, 06:14 PM
#73
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- which on a serious note can be/is just as bad - was no fun driving with 90% of my radiator covered up so get proper temps on cold track days, but the churbo loved it.
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06-02-2012, 09:23 AM
#75
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The Miata has a windshield with less rake angle likely making the area at the rear of the hood an even more pronounced area of high pressure than represented in this pic. That makes it worse for hood risers.
It appears that one might see beneficial flow characteristics without cutting the hood if the trailing edge of the headlight doors were raised 3/4 inch or so.
The bumper cover on the Miata needs to be sealed to prevent lateral movement of air between it and the hollow ABS bumper underneath (as I have done). If you remove the turn signal, you can stick your fist in the open void above the rigid bumper and under the bumper cover's top edge. That opening and others like it allow air to easily exit the nose past the headlight into the engine compartment without going through the heat exchangers. That is for the benefit of those who have never opened that up and made modifications to seal it. It is at least twelve square inches of leakage on either side.
It appears that one might see beneficial flow characteristics without cutting the hood if the trailing edge of the headlight doors were raised 3/4 inch or so.
The bumper cover on the Miata needs to be sealed to prevent lateral movement of air between it and the hollow ABS bumper underneath (as I have done). If you remove the turn signal, you can stick your fist in the open void above the rigid bumper and under the bumper cover's top edge. That opening and others like it allow air to easily exit the nose past the headlight into the engine compartment without going through the heat exchangers. That is for the benefit of those who have never opened that up and made modifications to seal it. It is at least twelve square inches of leakage on either side.
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