Just a few thoughts. As Redvette said, people spend millions in wind tunnels improving aerodynamics & we aren't able to replicate that. However, the C3 originated in the days when designers didn't pay as much attention to air flow underneath as they do now. Therefore it follows that some improvements should be able to be made.
A simple one is a factory mod. Fit the 80-82 front end on earlier models &, so I read, you've immediately got a 50% greater airflow through the rad coupled with a reduced drag coefficient & less lift.
This airflow into the engine bay has to exit somewhere & I really don't think that the side "gills" are anywhere near big enough for the job, so a lot of air is going to get dumped into the already turbulent air passing underneath. As somebody pointed out, you don't want to evacuate it at the base of the front screen as it's a high pressure area (another disadvantage is that if you've got fumes from the PCV system, oil leaks, etc, you're going to get it building up all over the screen). Maybe sealing the bottom of the engine bay as much as realistically practical & using baffles/wings to direct the air out through vents in the tops of the fenders will reduce the air underneath & maybe cause a bit more downforce? I've got no intention of cutting holes in those beautiful fenders (it'd be like cutting the eyes out of the Mona Lisa), but it's an idea (a bad one?).
When I got mine it had 225/70(?) tires on it. I swapped them out for a set of 255/60's not because they were wider, or looked better (honest :) ), but mainly because they had a slightly increased diameter & I was looking at ways of raising the gearing due to the amount of highway miles I do. The car sits noticeably higher with the BFG's on it & now I've got an OD tranny I'll look at getting some lower profile 15" tires when I need a new set. This simple change, along with those hideously, extortionate, they saw you coming, expensive front rubber spoilers being close to the ground should reduce the airflow under the car. I honestly think that the underside of the rear end is a bad turbulence & drag zone & some sort of pan fixed there will help, but trying to smooth the flow out further forwards looks like it could be a headache. If there's any simple fixes or minor improvements it'd be good to hear them (tubular A arms, etc?).
:cheers:

 

ZD,
You're in luck (I think!!). We were looking at a fan to cool the equipment we make & the problem was very similar to this. What we found was that if you seal up the engine bay so much that the air being forced through the rad can't escape easily then it will cause a rise in pressure. In our case the fan wouldn't be able to push enough air in. In the case of a car you'd be setting up a "barrier" due to the pressure build up & would probably be losing power trying to push this extra resistance along (unless your hood blew off!).
:cheers:

 

no need for a FULL tray under engine, measure the SQ. in" area of rad. now
measure the open area under engine bay..... there's a lot of room for improvement

 

Interesting discussion - BUT - unless you are going to be doing full-time HIGH-SPEED racing where's the payback? As stated earlier, big name racing teams spend millions getting it right. Of course they have a payback. Our cars were designed for the street and highway. I do have to agree with the post that if you remove the spare tire tubs, your back end would act like a big wind-drag..... even at freeway speeds.

 

I was thinking of the same thing, but with sidepipes I'd do it just under the floor pans. Frame rail to transmission and leave the transmission exposed. But another thing to consider is the use of dimples on the panels to increase downforce. I'd probably also put one behind the rear end to route air around the bumper cover. But air also builds up and creates it's own ramps. You end up with stationary air in places and the rest of the air floats over that air.

 

Vortec Generators are what is added to control/move the air around/ along a

surface to get it where you want it to go (yes small movements) they also work on top of car, but look stupid, most air shaped like a NACA dust only
reversed from being used as a air intake

 

Yup...
http://www.mulsannescorner.com/vortexlift.html

 

This is exactly what I am thinking...

Remove the rockerpanel do-dad and create a roll pan for the sides thats form fit to the edge of the fiberglass... cover the frame rails from the front where the slope starts (right behind the rear wheel) and caped at the back near the at the edge of the frame rails right in front of the rear tire. Then angle over from the front to the edge of the transmission tunnel then down the inner edge of each floor pan all the way back to about equal to the cut off on the outboard side.

Reason for this? Well if you do it correctly you have the same level from the front of the floorboard at the firewall all the way back to the rear suspension! A seal for the transmission crossmember and thats the only break on the inboard side...

Other ideas that I'm working on... What happens when you take a car thats setup about the same as our vettes, and steal the floorpan off of it? What race cars/supercars have the same sort of design on the underbelly where its sized about right?

At the moment I'm thinking smoothing it out... but once a basic platform is established I'll start looking into where you could mount a deflector at the rear to give us some more downforce... :cheers:

Kinda steap learning curve, but I'm catching onto the lingo... alot of this is just basic physics, so hopefully we can just use math as aposed to a wind tunnel... :cheers:

 

Engineers know basic physics/math, and wind tunnels are still necessary. Its very easy to wave your hands around and claim that one shape is better than another (like the popular articles do), but those arguments are extremely unreliable in real life. Half the time reality reality is the exact opposite of what you'd expect.

My 1986 Benz has a lower drag coefficient than a C5 vert, yet it looks like a box on wheels. Go figure.

Doing the math properly will work of course, but that requires a full nonlinear fluid calculation - requiring a heck of a lot of work and a supercomputer. Realistically, you still have to make some questionable assumptions, which is why wind tunnels still exist. Bernoulli's equation etc. does not apply in most real situations.

Smoothing out the underside "by eye" might be a slight improvement, but I wonder how you'll be able to test it?

 

Well... For one, I'll be able to take the running car to x speed, have a camera ready at one position... install the goodies and run the car at x speed with a camera ready at one position... and measure the difference in ride height to see what all down force is being created.

All I'm looking for is the results that made the racers say "this is worth looking into" back when they did just eyeball it an give it a try!

:cheers:

 

I don't think you'll be able to see much that way. Say you have 1500 lb of lift at 120 mph (its probably not this bad), and you cut it in half with the mods. Your ride height would change by about (800 lb)/(4*300 lb/in)=0.7 inches. What are the variations from bumps in the road? Good luck though. It might work.

When racers first started figuring this stuff out, they did high speed skidpad tests, checked if laptimes improved, or just asked the driver how it felt. Those are hard things to do unless you go to the track regularly ($$$). 60's racers also used big wings etc that gave much larger effects than you'll see. All the body refinements you read about today were essentially unknown back then (too hard to get right without computers and wind tunnels).

 

Yup... I'm just going off the hunch that what we have can be improved, and finding out how to improve it and do it cheaply is the only problem! :jester

I sure hope we dont have 1500lbs of lift, especially not with .50 of cd and rear tires that toe in and out when the go through the suspension travel! That'd mean we have a bomb of a car wouldnt it! :p:

 

i can picture a rear exit tray that looks like a Saleen MTang's rear end being
a possablity :thumbs: and if i had a hood to mock up like the GM Goodwrench
race cars vent air :party:

 

ZD,
One thing that's ****ling away at me about all this (apart from the massive amount of work to get an unknown result) is the car that literally took off at LeMans (I think it was LeMans?). If an underbody tray were to act like a huge wing in the right conditions then things could get nasty :(

 

Yup... I just think if in testing ya take it slow enough through the steps you should be able to feel if its starting to float... or if its starting to stick :cheers:

 

ZD you gonna go for 200 mph or maybe around 150 ?

 

Well, the idea is to get as much downforce at as low a speed as possible... because of the doubling effect of negative lift going hand in hand with increased speed I'm not so sure that its not wise to try to make the car produce downforce at 80 and then focus on the drag aspect.

Me personally, Unless the car has a full rollcage/tube frame with safety equipment I have no intentions of going 200... :cheers:

 

so plan on 120 max. ! do you know of a realistic 1/18 scale C3 model car?

 

I think that the LeMans problem happened as the car crested a hill, so there was already a reduced load on the tires contributing to the problem.

As with all race cars in that class, that one was designed for heavy downforce, but the problem was that aerodynamic properties can change dramatically depending on the angle that the body is pitching (from braking/acceleration or through turning). It wasn't uncommon in the early days for the front/rear lift distribution to completely switch under braking, which made the cars dangerously unstable. I guess things like that still happen sometimes.

Even though the pitching motions are much larger in street cars, I don't think this would be something to concern yourself with unless you start getting into big wings and (real) side skirts.

 

Reasonably realistic models do exist. Go to your local hobby shop.

The problems and merits of using a model for testing have been discussed at length in past aero threads if you're interested.