Down-Force ? - FFCars.com : Factory Five Racing Discussion Forum

Shawnski · 09-10-2012, 02:34 PM

I have been contemplating while we finish the body, how to alleviate air pressure build up in the fender well. Since the Coupe is known to have less than desirable down-force on the nose at high speed, and since I am not interested in doing an air-dam splitter, I reasoned that releasing air pressure could help. While upper fender vents would be ideal, not wishing to spoil the exterior, I simply attached louvered vents unto the fender well splash panels.

The picture here shows the upper splash front passenger side on the left, and driver’s side rear on the right. The theory is air pressure is relieved through an opening and redirected via louvers into the engine compartment, which in turn is relieved through the side gills. Relief in the rear is the same concept via the crescent shaped cut-outs.

Not sure if this will even be effective since the louver faces down on the opposite side, but this method does not transfer unwanted road debris beyond the wheel well area.

CHOTIS BILL · 09-10-2012, 05:12 PM

I have found with aero stuff the only way to tell if it will work is to try it and see.

Bill Lomenick

Cone Basher · 09-10-2012, 11:44 PM

I think you are on the right track here for 100+MPH speeds. Back in the day when the Dodge and Plymouth superbirds ran in NASCAR with the long nose and a high wing on the back they had vents like these on top of the front fenders that they claimed were needed for tire clearence. Even though I was much younger back then I thought this was bogus. These cars got banned by Bill France because they were just to fast at high speed. Later as aero came into play the "experts" finally decided that this was to vent air from under the wheel and not for tire clearence. Smokey Yunick also had a similar concept on one of his cars where he had spring loaded "flaps" behind the front wheel that vented the high pressure into the interior of his car (Bernolli principle here). He claimed during inspection that these were there for "checking tire wear duing a race". Smoky was famous for 'bending the rule book" . One famous story I've seen was when he was going throught inspection they dropped the fuel tank out of his car and would not let him run because inspectors had found numerous other "violations". He got pissed and stated the car and drove off stating "make it one more". Seems like he had hidden reserve fuel inside the roll bar tubing. He also showed up with a full belly pan under a road race camero and that did not pass either. Neat guy, way ahead of his time in the Aero aspect of these eariler car configurations.
FYI
CB

mmarshall · 09-11-2012, 01:23 AM

I have a feeling those vents will be a little small for the amount of air coming in at speed. The air is moving by the bucketfull and those vents will be moving it by the cupfull. They do look nice though!

jkrueger · 09-11-2012, 02:36 AM

I have been thinking about the aerodynamics on the coupe as I have progressed through my build as well. I hadn't thought about the wheel wells though. I know DP cars use fender vents to relieve the pressure so anything you do has got to help.

I was going to put a belly pan on my car and I thought about putting two 3" (or bigger) NACA vents on the belly pan to pull air from underneath the nose ( to help reduce the air pressure there) into the engine compartment then out some hood louvers. I don't like breaking up the hood lines either, but for me it is function over form. I don't know if this would work at all either.

JC

tirod · 09-11-2012, 03:08 AM

I'm aware that dropping a dam and adding a splitter under the nose of the Coupe disturbs the looks - but it does work for a dedicated racer.

What is also used is a full width tray under the nose from front edge to the engine. It controls what air can get up under the nose. From the firewall area back, the underside of the chassis can taper into a larger exit under the cove, much like the GTM.

If there is no control of air entering under the nose, controlling it after that is still necessary, and works. Although not a definitive article on the subject, this link can get the ball rolling: http://www.autospeed.com/cms/A_2159/article.html

The biggest problem with air management is the lack of published articles explaining the basic priniciples. You can get all you want to know and more about cam timing, try to find some cut and dry good stuff to use for aero. Most of it is in competition cars, and they are a bit tight lipped about letting others understand their winning edge. As the link points out, you try some things, and then you know.

mmarshall · 09-11-2012, 05:07 AM

Tierod I think controlling the air before it gets under the car is much easier than getting rid of it once it is their, so your air dam / splitter idea is a good one.

I plan to eventually make a spliiter that moves slightly under air pressure. The faster you go the more it bends/moves toward the ground. As you slow to pull into a parking lot it would be tucked tight up against the nose.

CHOTIS BILL · 09-11-2012, 02:26 PM

One book that is pretty good on aerodynamics is “RACE CAR AERODYNAMICS” By Joseph Katz. Some things that usually help are adding rake to the car, keeping air from entering the front, and creating vortices along the sides of the car to keep air from entering from the sides. One thing I have found works very to add front down force are dive planes on the sides of the nose. Unfortunately most of these things change the appearance of the car. Sometimes just a small change like adding a small air dam on the bottom of the nose just behind where the nose ends will make a noticeable difference. If it were black and didn’t hang down very far it would not be very noticeable and would be worth a try.

Bill Lomenick

TimC · 09-11-2012, 05:44 PM

I seem to recall if the crescent shapes were cut out this allowed air to exit the rear of the coupe but so doing created lift. Leaving them plugged the air compresses into that space and creates a cushion effect preventing anymore air from entering that area forcing it out underneath the rear of the car.

Russ F · 09-15-2012, 03:08 AM

I've been trying to think of the things that may be causing differences in real or imagined front downforce between our FF cars and the originals which were, after all, regularly driven over well 150 mph, admittedly by some of the great drivers of the time. I can come up with a few: (1) the originals had full belly pans, (2) our FFR bodies have much more "droop snoot", although it's hard to imagine why that would be a lift-inducing factor, but it IS a difference, (3) the originals eventually ran REALLY large diameter rear tires (I estimate 28-29" dia.), givng the cars perhaps more forward rake than most FFR cars carry, which could help. Maybe the big difference is that the professional drivers of that day just put up with lots of front end float and got on with the job of going really fast.

CHOTIS BILL · 09-17-2012, 02:18 PM

A flat bottom car set up with some rake can produce quit a bit of down force. This can be increased by keeping air from entering the sides using small skirts or vortex generators. Just generating down force is only part of the story you also need to balance the down force to where the center of down force is approximately at the center of gravity of the car. If it is off to much you can get aero imbalance at high speed

Bill Lomenick

T70MK3B · 09-18-2012, 05:22 AM

I was waiting for a magic answer from anyone who has run a higher speed than me.
The biggest draw back I see for these cars is the height of the windshield as compared to the 5 correct cars with the short windshield. See the 3rd picture to the right.
There is nothing we can do to change that other than cut the body and find a shorter windshield.
Another is as posted prior not having a flat bottom.
Adding rake to the body does help as that is what we did while racing.
And still another is the nose lift from the rear spoiler, I wish I rivited mine on as I would have pulled it off for the high speed runs.
To sum it up the nose on my Coupe lifts about 2" while land speed racing. That may not be acceptable for some but it works for me.
Just remember it is your car and do what you feel will work for you.
Regards,
Mark
And yes, I subscribed to the thread as I am looking for more info to make mine go faster!!

jkrueger · 09-18-2012, 01:00 PM

Quote:

Originally Posted by T70MK3B

To sum it up the nose on my Coupe lifts about 2" while land speed racing. That may not be acceptable for some but it works for me.

Have you tried to run a higher lip on the radiator duct exit to produce more front down force?

JC

mmarshall · 09-18-2012, 03:10 PM

I would think a splitter with some dive plates would be the answer for front end lift. I plan to make one someday that is removeable so I only use it when necessary. If you remove the rear spoiler it will be very light in the rear at speed.

CHOTIS BILL · 09-18-2012, 10:01 PM

I used dive planes on several cars I made. It is strange that some cars I made needed extra front aero down force and some didn’t even though they were almost the same shape. I found just a fairly small dive plane made a big difference in front end grip and didn’t seem to hurt top speed any. The ones I used were formed to fit the body and tapered from about 1.5” in front and 2.5” in the rear and maybe 6-8’ long. I held them on with dzus buttons and could very the angle from about10 degrees to 20 degrees. I never needed more than 15 degrees it plant the front end. You might try making smaller rear spoilers of different heights to be able to balance the center of pressure. I think a flat vertical piece attached to the lower under side edge of the noise dropping down maybe 1.5-2” would make a substantial difference. Finding down force can be tricky because what works well on one car doesn’t work well on another. For instance I tried using flat splitter on the front like many others are using and it didn’t help at all. Unless you have a CFD program or use of a wind tunnel, trying out different ideas is the next best thing. One thing that might help is to spray a light film of WD-40 on the body and make a run. Dust will stick to the car and you can see where the air is going. It may show that the air is spilling off the front and the use of some side rails to keep the air on top of the hood might work wonders. Another very useful tool is a data system. It can show such things like if the front is lifting because low pressure above the hood or high pressure below the hood or is the front is being lifted by the back being forced down. For me finding what the problem is and how to fix it is one of the many fun parts.

Bill Lomenick

T70MK3B · 09-21-2012, 05:50 AM

Quote:

Originally Posted by jkrueger

Have you tried to run a higher lip on the radiator duct exit to produce more front down force?

JC

I may try that next summer if I try to break the 200 mark but will need to add another 150HP to get there.
It really amazes me as to how much power is needed to overcome the wind drag.

With the class I run in I am not allowed to run a splitter unless it is a stock part.
The nice part of land speed racing is we do not need to make any turns untill after slowing down so basically as long as the front tires have enough contact to keep the nose in front of the tail life is good.
Mark

Thomas W · 09-21-2012, 03:11 PM

Mark, how much rwhp are you making now to run that 183 mph in the 1.5 mile?
Thomas

tirod · 09-21-2012, 04:21 PM

I read a slightly different issue about the crescents - there wasn't high pressure in the rear wheel wells, the cove area was higher and air was getting sucked in. So the later cars didn't have them. Whichever, they didn't work.

The rear spoiler was the race teams effort at reducing rear lift, fashioned from a small panel of aluminum they flew with to the first race. From the angle, it's likely in stall. It's not what Pete Brock had in mind - he wanted an aerofoil, CS said No time for that silliness and it was left off.

The angle of attack is an issue - top speed doesn't need drag. Reading into the LSR and Bonneville forums, one of the issues is getting the aerodynamic pressure center located behind the center of gravity. That creates stability - the back end moves into alignment instead of wobbling with increasing magnitude. Having a high drag spoiler on the back does it, or having more surface area behind the center of gravity does it too. That's why so many streamliners and coupes have vertical tails on them - old tech intuitively arrived at back in the '30's, like the Mormon Meteor.

One way to optimize that for top end is to move the engine forward as much as possible - point being, that downforce needs on a short track car doing a lot of cornering isn't what LSR is all about. Straight line high speed stability and drag inducing downforce appendages are opposites.

Fundamentally, the problem with any car, much less the Coupe, is that the overall shape is an aerofoil - the top surface is longer, air is accelerated, and lift is generated. What we are trying to do is reverse that, in moderation. That means handling undercar air has to be part of the solution. That's where Magnahelic pressure gauges come into play - you can measure the actual difference in air pressure from ambient while the car is at speed. With a multiple hose and valve log set up, obtaining readings of different areas during a run - even at highway speeds - can be measured. Much of the reported improvements were in ounce per square inch increments. You take what you can get.

I'd like to say this is all outlined in an article from Car Owner's Building and Racing Aerodynamics magazine (

), the obvious point is you have to really dig around for information.

mmarshall · 09-22-2012, 11:37 PM

Here is some good info on aerodynamics I found a while back.

http://www.cassidy-online.com/porsch...r_copy_pro.pdf

Background

http://www.grupossc.com/pdf/testimon...sports-car.pdf

APR Performance - Adjustable Wings

http://www.aprperformance.com/index....sk=view&id=182

T70MK3B · 09-24-2012, 03:42 AM

Thomas,
I do not know the HP as I did not have the motor dynoed.

Tirod and Mike,
Lots of good info that you have presented.

The hard part is using that info in a way that looks stock to meet the rules for the class that I am competing in.
Regards,
Mark

CraigS · 10-10-2012, 02:05 PM

Mark-Picking up on an above comment about body rake and looking at your pics I wonder can you drop the front end down any? I should ask are you allowed to drop it? Looks to me it could go down about 3 inches.

T70MK3B · 10-11-2012, 04:21 AM

Quote:

Originally Posted by CraigS

Mark-Picking up on an above comment about body rake and looking at your pics I wonder can you drop the front end down any? I should ask are you allowed to drop it? Looks to me it could go down about 3 inches.

Craig,
We did drop the nose about 2" and it lifted all of that to be about level at speeds north of 175MPH or far enough down the track where the photographer was.
The down fall to lowering the nose is you need to put the tires up on blocks to open the hood or you will be missing some paint.

I would like to find a cowl induction hood scoop that looks close to the shape of the factory hood bump but about 4" or 5" tall to gain clearance for a taller intake and the nitrous plate......Yeah I know N2O is cheating to some but power is power and it takes a lot of it to go fast

Does anyone know if there is more carb clearance with the newer body?

Mark

tirod · 10-11-2012, 03:23 PM

I haven't heard of any extra clearance. The issue is that the early 289 development was mostly standard low profile manifolds, so the car was designed around that. Now we get semi-ram air gap manifolds and the air cleaner winds up above the hood line. That highlights just how small and low the car is.

There are some pics on the finished cars thread with Coupes with altered hood bulges. They get tall quickly, reminiscent of Pro Street drag cars. Might be why SA went to Webers on the race motors, because it helped hp and kept the profile lower. None of the pictures show any air cleaners, either.

As for the rear spoiler, PB didn't have it originally, as we know. Peter Brock and the Daytona Coupe

It's apparent PB was aware the Kamm tail needed help with controlling air coming off the back window into the void behind the tail. Filling it would relieve drag - or more literally, the vacuum being created by the air's own resistance to filling the space left by the departed body. Another way to do that is using an undertray at the rear to exhaust into the void, along with the engine. Likely, rules interpretations are involved.

Looking at the nose, the new kits do look dropped. The radius running from headlight to headlight seems to be sharper and lower. Kinda like the raised butt issue with the Mk3, it's not exactly original. And a lot more difficult to change.

T70MK3B · 10-12-2012, 04:42 AM

Tierod,
I have gone through a lot of the pictures on this forum but have not seen any hood buldges that look stock. There are a lot that are very tall and boxy but not the smooth lines like the original.
A new hood buldge might be a project for late spring time as the car is going into storage due to the fact that car show season has ended here in Maine.

Thank you for the link to the history of the Daytona Coupe.
Regards,
Mark

turbodon · 10-12-2012, 02:06 PM

I tried not to make it boxy.

351W based motor, 3/4" drop motor mounts, Vic Jr intake, 750 carb, 3.5" air cleaner and it just rubbed. at the back under hood. Hence bigger in front and tapers at rear.

turbodon · 10-12-2012, 02:08 PM

crash · 10-12-2012, 03:23 PM

What happened to the fuel injection you got from Karen, Don?

turbodon · 10-12-2012, 04:23 PM

I installed it. It runs great.

The Fast EFI has a victor Senior intake which is taller than Vic Jr. but the throttle body is shorter than a carb.

So now I have about 1/4" clearance on hood to air cleaner.

But when I built the Coupe it was carb based.

crash · 10-12-2012, 05:14 PM

Cool. I was hoping you didn't end up back with a carb for some reason. They are simple, but, as you know, there are SO many advantages to the EFI.

T70MK3B · 10-15-2012, 06:16 AM

Don,
I really like the shape of your hood scoop.
What did you use for a form to shap it over?
Regards,
Mark

Quote:

Originally Posted by turbodon

I tried not to make it boxy.

351W based motor, 3/4" drop motor mounts, Vic Jr intake, 750 carb, 3.5" air cleaner and it just rubbed. at the back under hood. Hence bigger in front and tapers at rear.