Aerodynamics question(s) - Vintage Mustang Forums

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post #1 of 243 (permalink) Old 03-19-2015, 11:20 PM Thread Starter
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Aerodynamics question(s)

Here's what I'm dealing with: '66 coupe, trying to fine tune for road racing. Not anything competitive, just out to have fun & be safe. Still stuck with the C4, and with 3.25 gears can't get much above 110mph without seriously worrying about the engine (whole 'nuther issue). At this speed, the front end will lift - not enough to affect steering, but enough that I worry about it lifting more...and flipping (this has happened to new Stangs on this course, so the worry is justified).

Car currently has a '69 Cougar Eliminator hood scoop, with a hole cut 1" around air cleaner. The air cleaner itself sticks up about 1" above the hood, so the scoop also give hood clearance.

It also has about a 2* rake due to the Arning drop and standard eye leaf springs.

I was going to swap out the rear springs to mid-eyes, and lower the rear end to level (0 rake), but now I'm worried that this might make the lift issue worse. The way I see it, I have 4 options:
1) remove hood scoop for track days. Not sure if that's allowed even. - Free
2) Install chin spoiler - $100
3) Install mid eye springs to level car - $500
4) Do 2 (which ones?) or even all 3 of the above.

It's been suggested to get a cowl hood...but right now that's not in the budget.

Not sure what would give me the best bang for my buck, really. I'd hate to swap the springs only to fear driving the car . Thoughts/opinions are GREATLY appreciated!! I can post pics if wanted.


1966 Restomod/track car, 1973 Q-code Mach 1, 1997 F250 diesel, 2005 retired OSP Crown Vic, 2015 Focus ST-2
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post #2 of 243 (permalink) Old 03-19-2015, 11:55 PM
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chin spoiler
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post #3 of 243 (permalink) Old 03-20-2015, 12:11 AM
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what course are late model Mustangs back flipping on?

way back when, I buried the speedo on my 67 Mustang coupe , swapped in CJ, the front got very light and a "normal" freeway left turn took me from the left lane to right shoulder. I had a chin spoiler,Shelby Drop, 70 CJ front springs cut about 2 coils,stock rear springs and Koni Shocks.
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post #4 of 243 (permalink) Old 03-20-2015, 09:08 AM
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Biggest improvement in aerodynamcs is to cut holes in the fenders. Cut some NASA style HOLES IN THE FENDER back side even with the inner fender splash shield tappered forward in the upper side portion of the fender.

Cowl hoods can increase front end lift as air near the edge of the glass cowl push air into the engine compartment. Same with raising the back of the hood but raising the hood air trapped in the engine compartment can excape out the side gap along the fender.
Found raising the hood then duct tapoed between back of hood and cowel helped the most.
These changes made one of my mustangs increased economy, increased weight on front wheels at speed, almost completely eliminated hydroplaning in 2" and better of standing water.
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post #5 of 243 (permalink) Old 03-20-2015, 10:50 AM Thread Starter
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Originally Posted by cyclone03 View Post
what course are late model Mustangs back flipping on?
Pacific Raceways, turn 1. DO NOT LIFT. It's better now that they repaved it, but along with a slight jig right, it drops off at about a 45* angle. A good friend of mine rolled his '08 Shelby GT there...lifted slightly, weight transferred...voila. It's one of the first things any instructor will tell you about the track.


1966 Restomod/track car, 1973 Q-code Mach 1, 1997 F250 diesel, 2005 retired OSP Crown Vic, 2015 Focus ST-2
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post #6 of 243 (permalink) Old 03-20-2015, 10:53 AM Thread Starter
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Originally Posted by turbo2256b View Post
Biggest improvement in aerodynamcs is to cut holes in the fenders. Cut some NASA style HOLES IN THE FENDER back side even with the inner fender splash shield tappered forward in the upper side portion of the fender.

Cowl hoods can increase front end lift as air near the edge of the glass cowl push air into the engine compartment. Same with raising the back of the hood but raising the hood air trapped in the engine compartment can excape out the side gap along the fender.
Found raising the hood then duct tapoed between back of hood and cowel helped the most.
These changes made one of my mustangs increased economy, increased weight on front wheels at speed, almost completely eliminated hydroplaning in 2" and better of standing water.
Do you have any pics of these fender holes? Or links?

I'm trying to wrap my head around all this. Would the forward-facing scoop be redirecting air down under the car (causing the lift)? If I'm following your suggestion of sealing off the back of the cowl, would taking the scoop off actually make the lifting worse, ie more air escaping up?

I guess it's kinda guesswork, with each car different from the next. :-/


1966 Restomod/track car, 1973 Q-code Mach 1, 1997 F250 diesel, 2005 retired OSP Crown Vic, 2015 Focus ST-2
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post #7 of 243 (permalink) Old 03-20-2015, 11:24 AM
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hAVE TO TAKE SOME PICS as the one i DID IT TO pics are stuck on a burned up hard drive.
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post #8 of 243 (permalink) Old 03-20-2015, 12:17 PM
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I started getting into aerodynamics after reading an article based on the old big caddys. They did a study about how much weight was on the front wheels going down the road at 60 70 MPH. Back in the day one wold see them with the front a 1' or 2 higher in the front. Simply this translated into static front end wt around 2800 lbs at 60 70 wt on front wheels calculated to 500 600 lbs
After mods to the 68 the front ended up feeling much heavier

NASA style opening dipected. Arrows show basicly how the air flows around raised hood. were it shows air going back under the hood is were I ran some duct tape. Retangle shows a good area mabe to cut louvers have also thought about using boat hull vents some look a lot like the old hood vent looking one s on the cameros. The NASA duct did the most AS STATED THE BACK SIDE CLOSEST TO EVEN WITH THE INNER fender guard. This allowed me to do like 70 through a 10 mile strech of 2"s of standing water were I could only do 25 to 35 safely before the NASA duct.
Also found the front of the vehicle about an 1" lower than the front worked best also but by far the aboved mention mods did the most
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post #9 of 243 (permalink) Old 03-20-2015, 12:24 PM Thread Starter
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Hmmm. Not really willing to make cuts in the fenders and hood...I'd rather replace the hood, and I don't have the budget to do so at this time.

So if the rear of my car is 1-1/2" higher in the rear than the front, as measured at the ends of the pinch weld, I should leave it as-is? And just add the chin spoiler for more downforce? Or would that be TOO much downforce?

I wish I had a test day to try out variations. :-/


1966 Restomod/track car, 1973 Q-code Mach 1, 1997 F250 diesel, 2005 retired OSP Crown Vic, 2015 Focus ST-2
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post #10 of 243 (permalink) Old 03-20-2015, 12:31 PM
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Maintaining some rake is important from what I know, unless you have a ton of aero and that component is calculated in the spring rates and valving etc (read real race cars).

Getting air out from under the hood helps a ton, via venting routes. With a functional and sealed air scoop, you could duct the air through the radiator and out of the back of the hood, keeping pressures down under it, akin to what turbo has shown. Via vents in the hood or fenders, raising the rear of the hood slightly etc. The chin spoiler will add some downforce, but it also adds more air under the hood. Someone here with way more knowledge about it will chime in I am sure.

Google the string test I think, which involves cameras and a bunch of taped on stringers to show where and how air is traveling around your car at speed to see where you can best utilize it.

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post #11 of 243 (permalink) Old 03-20-2015, 12:36 PM
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Well I'm no aerodynamicist but I did stay at a holiday inn last night...

Chin spoiler keeps air from going under the car which creates lift/instability

Your hood scoop is likely in "dead" air, or the boundary layer on the hood
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post #12 of 243 (permalink) Old 03-20-2015, 12:37 PM
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One of things I thought about, front springs. With the stock, soft and long springs there is a lot of stored energy to lift the front. I was wondering if a shorter, stiffer spring would help since it won't have the same amount of energy and help keep the front down. Maybe turbo2256b can comment on that, he's a pretty sharp guy.

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post #13 of 243 (permalink) Old 03-20-2015, 12:46 PM
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Quote:
Originally Posted by Sparky66 View Post
Hmmm. Not really willing to make cuts in the fenders and hood...I'd rather replace the hood, and I don't have the budget to do so at this time.

So if the rear of my car is 1-1/2" higher in the rear than the front, as measured at the ends of the pinch weld, I should leave it as-is? And just add the chin spoiler for more downforce? Or would that be TOO much downforce?

I wish I had a test day to try out variations. :-/
If the hood is raised as mentioned then suggest running some duct tape across the rear opening were the cowel vent slits are as this is were aire is forced back under the hood. How close to the corners of the hood is needed varries somewhat depending on the vehicle. I did do string testing on this along the edges of the hood but on my stang was years ago and was totaled around 79 or 80.

Chin spoiler helps but also ads wind restance to some extent. What I DID WAS IN EFFET REDUCE FRONTAL AREA. The newer cars with the tires so close to the door NASCAR has found attemting to exit air in front of the tire instead of behind helps bust not as well as behind.
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post #14 of 243 (permalink) Old 03-20-2015, 12:52 PM
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Here are some of the marine vents I have thought about installing on hoods in the retangular area I MARKED.

ATTWOOD Venturi Vents | West Marine
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post #15 of 243 (permalink) Old 03-20-2015, 01:02 PM
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Quote:
Originally Posted by Huskinhano View Post
One of things I thought about, front springs. With the stock, soft and long springs there is a lot of stored energy to lift the front. I was wondering if a shorter, stiffer spring would help since it won't have the same amount of energy and help keep the front down. Maybe turbo2256b can comment on that, he's a pretty sharp guy.
I run 720 lb rate springs in the front of my 200 or 250 I 6s rear springs are 68 390 compitition springs with the trunk mtd battery that was an option.
Mods to rear springs rear hanger should be facing rearward at the bottom usally requires aftermarket rear hangers using the 2 closest holes. I then remove all clamps on he rear portion of the leafs (behind axle). Then clamp all leafs in front of the axle.
The 720 springs are cut down 69 70 ish front springs for a 429/460 torino. one coil removed.

This set up with 275 50s on the front and 295 50s on the rear nothing could catch me around a corner except mabe a unlimited go cart.
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