View Full Version : Need help with aerodynamic basics

04-02-2011, 08:43 PM
I'm doing a non-pro touring build on my 78 lincoln mkv. The car will be receiving a warmed over 460 punched up to 471, 100 shot of spray, has 3 speed auto, 2:47 gears and is in need serious aerodynamic help. Even with the original stock 460 engine at high speeds air pressures build up under the car and lifts the body upwards in an on and off motion followed by a strong up movement that scares you into letting off the throttle. It's scary and I probably shouldn't drive my car that fast I know. I'm not going to stop and I need some info about front air splitters, as in roughly what shape and where should I mount it (near front bumper or towards front tires, car has feet of front overhang), if at all? Would I be better off fabricating panels under the car to improve airflow and not running a splitter? Are pretty much all splitters really just cosmetic? Is smoothing out the underside of the car to minimize drag substantially better than bolting on a front splitter?

04-03-2011, 07:37 AM
you need to get the air out from under the hood first and foremost. fender vents and hood vents sound like they are needed pretty bad.

04-03-2011, 05:42 PM
If lowering the car is an option, that will help as well.

I'm working on fender vents and considering a hood extractor myself.

04-03-2011, 05:57 PM
Didn't realize fender vents can have such an impact. Luckily the car has cosmetics fender vents that can be made functional. What can I do to encourage air flow to the fender vents? Is any type of ducking advantageous, or is simply hacking into the inner side of the fender to open up the MkV's factory fender gills sufficient? I'll think I'll do the functional fender vents mod and a front air splitter (to deflect air not generate down-force) and feel what it does. In this car you can literally feel the poor under carriage aero effects. I really don't care about my drag coefficient as much as I want to mitigate the car's tendency to become an airfoil at speed. Thanks a bunch guys and I'll keep you posted on my progress once I begin.

Norm Peterson
04-04-2011, 03:28 AM
What are you running for front shocks and how old/how many miles are on them?

I'm not trying to discount the likelihood of unpleasantly strong aero lift forces here, just identify the resistance-to-motion side of the coin as possibly being too weak. More specifically, the rebound side of the shocks. When I take just a quarter turn out of the Konis on the Mustang to make its ride a little nicer for my wife, I can feel the car moving around on its suspension more.


04-07-2011, 05:52 PM
Give the car a decent rake by dropping the front end. That will create a small vacuum in the middle of the car pulling air out from the front of it. Block off just about the entire front of the car with duck tape, leave just enough air flow to keep the motor cool.Build a front splitter, make a piece drop straight down from just below the front bumper and make it wrap around the sides to in front of the front wheels. You can build that out of some pretty cheap stuff if need be, aluminum if you can get it formed or even that black plastic divider stuff that people use for gardens. Build side skirts to keep that negative pressure area under the car, that will keep out higher pressure air from the sides. Make the bottom the the car as smooth as possible, this includes under the engine. Once all that other stuff is done, build a rear diffuser and spoiler.

You can also put a small blade in front of the front tires to help with downforce. Tires are very un-aerodymanic so if you can keep air away from them you will use less power to go fast. Put the blade in front of em starting where the fender curves inward and give it an angle of 45-60 degrees. Ill see if i can find a pic to back up what Im trying to get across.

One of the reasons the car wanders around is because of the fact that a car is roughly shaped like an airplane wing which creates lift. That is from air flowing all the way over the passenger are of the vehicle. There are a few ways to deal with that, some are easier than others. You would chop the roof way down and that would help. One easy thing to do is just let the car eat the high pressure air that builds up at the base of the windshield by running cowl induction. Im not referring to bolting on a cowl induction hood and thinking its going to help. What you can do it build an airbox that ONLY draws air from the cowl. Seal up any HVAC holes under the cowl and let the motor breathe in the air straight thru the grill in the cowl. That will decrease the amount of air flowing over the top of the car and thus reducing drag as well as lift

It sounds like you have more of a problem with air under the front end so step one would just be to lower the front 2-3 inches and go from there. All these things will work together to increase downforce and thereby make the car better at high speeds.

04-08-2011, 07:16 AM
What kind of suspension mods have been done? I'm with Norm on this one, I'd look at your shocks and suspension before making a ton of body mods.

For example, your stock springs are likely very soft, in which case they are heavily preloaded, which contributes to lifting the chassis when even when aerodynamic forces aren't very strong. On my Monte Carlo, I had a huge rake (Nearly 5 inches from front to rear) in it with stiff rear springs, and still over 100mph the front end would lift and really float going down the road with the soft stock front springs.


04-08-2011, 01:08 PM
I agree, my car has nothing for suspension dampening. I imagine stiffer spring rates are in order and I'll have to see what I can do to solve that issue. There are no performance springs for the 70s Lincoln mark series or the t-bird platform mate. Due to over 30 years of sitting on those original springs I will agree that they are very preloaded and soft. Thanks for the input guys, I would never have guessed the suspension was the culprit.

04-09-2011, 10:56 AM
Even if nobody makes springs specifically for your car, I'm guessing it has either 5 or 5.5" OD coil springs up front, in which case you could just buy some aftermarket racing springs for it and cut or space them to get your desired height.


04-17-2011, 05:43 PM
I agree, my car has nothing for suspension dampening. I imagine stiffer spring rates are in order and I'll have to see what I can do to solve that issue. There are no performance springs for the 70s Lincoln mark series or the t-bird platform mate. Due to over 30 years of sitting on those original springs I will agree that they are very preloaded and soft. Thanks for the input guys, I would never have guessed the suspension was the culprit.

the 77-79 LTD2 was the "sporty" version of that chassis, and probably has some stiffer springs that will also drop the car down a bit. i know the 78 LTD2 "S" that i had was pretty nimble for a 4200+ pound tank.

04-17-2011, 10:35 PM
I had a 78 LTDII myself, I used to race my buddies MX-6 in that thing on country back roads. It was a body rolling nightmare that was surprisingly well balanced and able to hang with the Mx-6. The 77-79 mark V actually rides on a bigger platform believe it or not. It's carryover from the mark IV which ran the previous gen T-bird platform. I'll be looking to pick up some oem replacement springs and do some coil cutting or see if I can get lucky with a performance spring from another make of vehicle.

Norm Peterson
04-18-2011, 02:51 AM
Start with the Moog spring catalog. From the car year & model you can get their part number, and from the part number you can (in most cases) look up the dimensions, rate, and rated load in a separate table. Then you can look for other spring numbers with similar rated load but higher rate that'll physically fit.


04-23-2011, 03:37 AM
Thanks a bunch guys, as always your insight is helpful. I will be taking a good hard look at performance springs from various cars to pop in the boat and or cut down to size. This is a bit of a newb question as I have done no research on this topic, but are the diameters roughly the same for performance springs? As in will a little bit off on diameter from OEM matter a whole bunch if at all? They are just coil overs after all. I would imagine that as long as they don't pop out or my lincoln doesn't decide to make them it's ***** it'd be OK?

06-26-2011, 09:53 PM
Coils at different rates are available in a variety of diameters. The Moog catalog is a great reference piece, they tell a lot of detail about the springs. What you would want to do is look up the details on your stock springs and see if you can find something else in the catalog that shows the same diameter but with higher rate and a shorter height.

When looking for rear springs be mindful that very often they have a bit of a taper, being a wider diameter at one end than at the other. Make sure those match.

It is always best to avoid cutting springs if you can. Properly made springs have a top and bottom turn that are made to sit more flat in the pocket. If you cut that off and the spring is sitting cockeyed it will flex differently and act weird.

If you go to dramatically shorter springs make sure you switch to shocks with a throw that matches the new ride height. New springs are great but can be quickly rendered useless if you blow up a shock so the spring has nothing to control it.