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Aero mods and cooling

Siestarider

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#1
enu.kz/repository/2010/AIAA-2010-4955.pdf

Link should lead to a 2010 paper titled Aerodynamic Drag and Flow Rate through Engine Compartments. It is the best analytical/empirical summary of the topic I have found.

It says the ideal method of exhausting engine heat is via a duct to the theoretical tail of the car, this would provide maximum aero efficiency for cooling.

In the real world as it applies to our cars, it says seal the heat exchangers tightly to the intake shrouds so that little or no air bypasses them to use all available air flow for cooling, and loose heat via low pressure areas below the car, preferably via the exhaust/downpipe location, secondly via wheel wells, and least desirable, just behind air dam. Desirability determined by how efficiently overall drag is reduced.

So on to some hard data.

Test car is my Fist in track trim, still have extended air dam on it, all air pressures recorded at 80 mph, I doubt many of us average more than that per lap wherever we track. Anyway, something had to be constant so I picked that speed. Air pressures are vs cabin pressure with windows up and A/C on, so reference is probably slightly above gauge.

The Pyle manometer I picked up does differential pressures, I did not find A/C on vs outside air to be more than a few hundreths positive sitting still.

Thus far my measurements using a digital manometer indicate our engine bay is -0.2-0.3 inches H2O at underside of hood (tested 8 locations), about -0.4 inches about half-way down engine compartment front, back and siides, and about -0.5" right behind air dam. I.e our engine compartment operates at a slight vacuum at 80 mph, said vacuum becoming stronger measured closer to ground.

I only took one set of measurements in front of radiator/condenser, +0.8". In other words, we have 1.2" H2O pressure drop across radiator/condenser and IC.

From what I have read, this is a pretty strong pressure gradient. And from experience most of the community knows we need all the cooling we can get.

So what? Well, I do not see a place to put hood vents that will accomplish much. I really like the WRC vents, but these data suggest they will pull cool air from over hood into rear of engine bay, not exhaust hot air up and back as I had assumed.

This might be a good idea if delta P across heat exchangers is not reduced, could be detrimental if it is reduced, or maybe we have plenty of flow through heat exchangers and can afford to loose some of it to waste more heat out downpipe area and reduce high pressure at windshield. Aero and cooling gains.

These data also suggest the OEM air intake is very well designed and is difficult to improve upon. But snorkel could be very good mod.

Also suggest there is a lot of subtlety to radiators, its all about heat transfer rate, tracking is the toughest on them, as we already know, but bigger will only be better if heat transfer rate is improved.

I have yet to find a way to evaluate pressure delta across radiator, and heat exchange efficiency vs that pressure delta. Seems like an important part of this line of inquiry.
 


Sourskittle

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#2
And now... You said stuff I don't understand. Now were even. Lol.
Does this mean washers under the hood hinges to space up the back of the hood and removing the weather strip is a bad idea or not ? Lol.
 


TyphoonFiST

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#3
I wonder if a Naca duct mod could be done on the hood. I did One in my Typhoon and it turned out awesone and definitely showed temp drops with a CAI. Here are pics with it completed....


Here's the box that it puts air into.....


Side shot....
 


Sourskittle

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#4
Also wonder if moving the battery and air intake on the driver side and ducting that air around the motor and out the DP area would help...
 


OP
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Siestarider

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Thread Starter #5
One thing is for sure, I will get a lot of use out of that manometer before I cut any more holes.

Now I doubt that ventilating my crash sensor bar did any good at all. Also doubt that opening up the grille more does much for cooling, but it definitely decreases aero efficiency.

One of the weaknesses of that technical paper is that it does not consider how any of the optimal waste heat ducting locations could be installed on a real car.
 


LT Berzerker

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#6
In for more data as you progress cause I'm just sponging up the info :)
 


OP
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Siestarider

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Thread Starter #8
SS: Yep, the cowl intake propping rear of hood open should decrease cooling through heat exchangers in nose if these aero guys are right.

I should have the parts to relocate my battery and WW bottle to spare tire well over holidays, then see what room is left.

Typhoon: Beautiful use of NACA into a sealed duct with a designed outflow to throttle.

LT and Frank: What forums like this are for, sharing. I know it helps me organize data before I post, so its good for whoever is sharing too.

I have learned so much from RAM, Razor, SS, etc. First turbo for me, its all fun.

So I think my next aero mod will be to extend undertray so that the most aero damaging outlet at bottom of car (the front-most) is closed, see what happens with air pressure data and cooling. Based on theory and data so far, it should improve both aero and cooling, forcing the exiting hot air to wheel wells and DP exits.

Have to miss my next track day Dec 19, could not get clearance from wife, PT or surgeon (in that order) to track. Thanks to Hooked on Driving for letting me carry my reservation over to next one. Those are good guys to run with, especially if you are a noob as I was a year ago.

Driving our cars at the limits is a learned skill, I am getting there, need more seat time.
 


LT Berzerker

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#9
And now... You said stuff I don't understand. Now were even. Lol.
Does this mean washers under the hood hinges to space up the back of the hood and removing the weather strip is a bad idea or not ? Lol.
SS, on that note with our hoods being propped the side gaps might be a concern.

Basically you raise the rear to create a gap and in the process lose the seal on the sides as well which could hurt vacuum at speed.

My curiosity is adding striping where the sides are lifted where a gap might be. Though, depends on how aggressive your lifting the rear of the hood.... At what point does the factory weather striping on the side stop doing its job?

And finally, adding what is basically a gurney flap to the trailing edge of the bonnet would help create low pressure; similar to what the WRC hood vents have on the front and inner edges :)

I've been waiting for a good thread like this for some time for functional aero mods :) outside of just a splitter...
 


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#10
[MENTION=1391]Siestarider[/MENTION] you should make a build thread with some pics of your mods. I'm curious :)

take care.
 


Sourskittle

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#11
Think a splitter would actually help cooling? Sorry if this is a dumb question. Not trying to be a smart ass. Just never really put much into cooling, aero or such things. More boost, more oct, more POWA !! Lol. Guys ask me questions about tires and I'm like, " yea... I use them with rims", lol.
 


Chuckable

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#12
One thing is for sure, I will get a lot of use out of that manometer before I cut any more holes.

Now I doubt that ventilating my crash sensor bar did any good at all. Also doubt that opening up the grille more does much for cooling, but it definitely decreases aero efficiency.

One of the weaknesses of that technical paper is that it does not consider how any of the optimal waste heat ducting locations could be installed on a real car.
I'm about to an install of my Mishimoto oil cooler and planned on adding some holes to the crash bar sensor as well as opening up the grille as well. Am I following you correctly that you do not believe these are worthwhile mods because of the pressure gradient? [???:)]
 


OP
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Siestarider

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Thread Starter #13
SS, on that note with our hoods being propped the side gaps might be a concern.

Basically you raise the rear to create a gap and in the process lose the seal on the sides as well which could hurt vacuum at speed.

My curiosity is adding striping where the sides are lifted where a gap might be. Though, depends on how aggressive your lifting the rear of the hood.... At what point does the factory weather striping on the side stop doing its job?

And finally, adding what is basically a gurney flap to the trailing edge of the bonnet would help create low pressure; similar to what the WRC hood vents have on the front and inner edges :)

I've been waiting for a good thread like this for some time for functional aero mods :) outside of just a splitter...
Unfortunately the hood and windshield meet at the high pressure point from which the cabin ventilation originates. My guess is wrong place for a flap.

I have followed the "gurney flap" for years, It could be the Brock/Hill flap, because Brock was designing and Hill driving when they first put the tiny vertical lip on the tail of a Daytona Cobra at Nurburg ring back in '65. But Gurney deserves credit, he drove Cobras too, and he was the first to adapt the "flap" to an aero wing.

Anyway, there are lots of good folks doing build threads, my main hobby here is getting a little quicker on track, and sharing ideas.
 


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Siestarider

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Thread Starter #14
I'm about to an install of my Mishimoto oil cooler and planned on adding some holes to the crash bar sensor as well as opening up the grille as well. Am I following you correctly that you do not believe these are worthwhile mods because of the pressure gradient? [???:)]
First, I did not post all the data because its hard to present in 3 dimensions. And its one set of samples from a machine I have never used before. Any number of things could be wrong. I did replicate measurements, but not all of them. Have to stop, open hood, relocate sensor tube, make sure it is turned so it does not get direct air flow into it every time, so maybe I actually kept 16 data points out of 24 attempts. Took couple hours.

That said, my take is that in stock form, assuming you seal all the edges well, it appears we have plenty of pressure gradient at speed to effectively operate heat exhangers up front without making more holes in front of or behind anything. I recall that NASCAR runs short track V8's making 850 hp with about one square foot of cowl opening, big tri-ovals with maybe 1/4 of that.

I.e. for best aero, make your openings in cowl as small as possible, your ducting tight and expanding behind the opening, and get as much negative pressure behind heat exchangers as you can.

I am actually stunned to measure 1.2" of pressure drop across condenser/radiator at 80 mph. If that is a real measurement, its just awesome original design. I better re-measure that one in particular.

Maybe my next test will include fog light facias, I opened mine up to create 2" round vents on inside of cowl for future brake cooling, may as well use manometer to check actual pressures in front and behind vents, try to understand whats happening.

Join the fun, get a manometer (I paid $60 for one) and take some data. Its always fun to see how things really behave vs guesses.
 


Chuckable

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#15
You, my friend, are awesome [like] I've been on all kinds of forums and rarely does someone take the time to do testing like this.
 


OP
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Siestarider

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Thread Starter #16
Think a splitter would actually help cooling? Sorry if this is a dumb question. Not trying to be a smart ass. Just never really put much into cooling, aero or such things. More boost, more oct, more POWA !! Lol. Guys ask me questions about tires and I'm like, " yea... I use them with rims", lol.
I am learning as I go. From what I have learned so far, a splitter is mainly used to add front downforce, and is generally associated with rear wings that also add downforce. Racers use these methods, but they add aero drag, and generally do not accomplish much at normal highway speeds, which are 70-80 around here.

Rumor has it that our cars are surprisingly stable at 138.5 mph bone stock. An informed guess is that it takes a while to reach terminal velocity. My highest recorded speed on any track so far is 118 mph. I have taken Bishops Bend at Sebring as fast as my car will go, do not feel like front end wanders or feels light.

So I am playing with my air dam first because we already have one, and I am not convinced that adding front downforce with a splitter will make my car better at track.

The coast down tests I ran indicate lowering air dam 2" (leaving 1" pavement clearance) improves aero about 5% between 90 mph and 40 mph. If this is true, my next session at PBIR should see higher top speed on back straight. Looks like my next chance to find out is Feb 14.

Sorry, lost my train, your question is about cooling. My answer is I don't know enough to have an opinion. Now I am the one lol.
 


Sourskittle

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#17
My best trap in the straight at sebring was 118mph too. Nothing like the amg SLS with carbon brakes going from 162mph to turn in speed was sick. The viper get to about 165mph before standing on the brakes there.
 


OP
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Siestarider

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Thread Starter #18
I gained the confidence to drift Turn 1 at Sebring, but not the confidence to drift Turn 17. Lol. yes, they are both blind turns at 90-100 mph, but 17 is double apex, has a visually distracting bridge over it at exactly the wrong place, any significant mistake is rewarded with a concrete wall, and just to top it off, 17 is even rougher than 1..
 


Sourskittle

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#19
My favorite of the 2 is 17 because the bump. Bump on a race track sounds like a "bad thing". And it is... But... To me, that's what makes it Sebring and not road america. Princess car can't handle a little challenge ? Maybe go back to ice skating, lol. It scary as crap in a viper though, lol. Your thinking.... "V10 inches from my feet, I'm nearly riding on the rear axle, and we just went from 165 to 70mph and were fixing to hit a bump under throttle !!". Ass = pucker.
 


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