Been searching all day, limited info found. I’ve seen the article quantifying stock front and rear lift

i know in the past I had seen pressure graphics on the body but can’t find them

I need to figure out ideal placement of hood vents
Air Intake
get some ideas about the least visible spoiler, splitter and diffuser that might be effective
Potentially wing fitment
belly pans etc.

for the record, I think there is no excuse for allowing aftermarket aero in amateur racing, but my efforts to convince sanctioning bodies of this have failed and so I have to make some that is effective and removable

another reason it should be banned is that without a team of engineers, computer modeling, wind tunnel time and track testing devoted purely to aero, no one has much idea wtf they are doing, especially me. Without that, the best you can do is rely on smart people who have already done the work on the c4 body.

Hoping for input from such smart people. Any help is appreciated.

How low does the diffuser have to hang and how far rearward does it have to extend to be effective? Does it require a full flat body to work? Will it likely require a trans and diff cooler? How much downforce might such a flat bottom and diffuser make?

how high and far back does a wing need to be effective behind a c4 hatch?

how low and far forward would a splitter have to be to be effective? Will a flat spoiler from the front bumper down work well enough?

how important is it to vent the hood over the tires vs the engine heat? Where exactly will they work the best?

how effective would just opening the shark fills be?

can the air intake be effective in the ps front bumper well?

Many thanks for any and all input!

 

Theres even less info on aero on the c4 vert.

i imagine Cd is worse than the coupe, but by how much?

can i throw a hardtop onto get some of it back?

taking off the side mirrors is supposed to reduce drag a bunch on the c4.

like you, id love to know about the optimum placement of hood vents or if they are even needed (open up the shark gill vents)

 

To vent radiator airflow, or something else? If the former, then you're kind of limited to the space between the radiator and the engine. A gurney flap on the front edge of the vent will help create the low-pressure zone you want, if needed.

As provided by the factory, the stock bottom-feeder intake is probably pretty good if the air dam behind it is intact. But this negates the use of a splitter, so see below.

I sent you some info on spoilers from our mutual friend, the motorsports aero engineer. Wings are another matter entirely, and I don't have good info for you on them. A splitter and belly pan could potentially be one continuous surface under the car, feeding into a diffuser. The problem is that a splitter doesn't really work with the under-nose air intake for radiator and engine air. If your rules allow, then it's probably better to run an air dam straight down from the forward-most point of the nose. Then run your splitter forward of the bottom of that dam. A splitter works by having airflow dammed up behind it at the front of the nose, which creates high stagnation pressure, and that pressure works on the top surface of the splitter to push the nose down. The longer (more chord) and wider the splitter (i.e. the more surface area it has), the better in terms of effective downforce. I believe there is a point about 2x the height of the dam behind it where it quickly loses effectiveness. But if you have a 6" vertical dam at the back edge of the splitter, than up to 12" chord would be effective, and more chord than that would not add much more downforce. I think the new CAM rules allow 6" of protrusion outside the stock silhouette in plan view. To do this, you have to duct the air intake to this new front piece instead of keeping the bottom feeder stock design, of course.

So the big question is do your rules say anything about skirts or flat bottoms? If not, then the sky's the limit. You should skirt that floor from the front of the splitter to the end of the diffuser. The skirts ideally would touch the ground at static ride height, and the best designs (before they were banned in F1) were made to slide up and down so as to always keep a zero gap with the ground. A flat bottom going into a separate diffuser section is not the ideal way to made downforce with ground effects - it's just what all the rule sets started requiring when cars were able to make stupid downforce with venturi designs underneath (tunnels, IOW). You have some limitations on doing real tunnels just because of the shape of the C4's floor. But I would think that if you follow the natural shape of the C4's underside, you'd end up with a long/wide/high natural diffuser that would be quite effective. With skirts as described, you could make four-digit downforce in lbs at 100mph, I think. For this work, you won't want the splitter to be too low up front - you want to get airflow to go under the car. The biggest obstacles would probably be the suspension and exhaust. In this case, I'd actually consider routing the exhaust up into the hatch area from the rear axle line, and out to of the tail light holes. That would get it out of the way of your floor and diffuser, but this probably requires a fuel cell.

What's probably going to happen in short order is that you'd bring a car to OUSCI with this setup, and it would promptly get banned. Which is what sucks about rule sets that are "run what ya brung" and not well thought out. CAM is very much like this, just waiting to be exploited and then have lots of reactionary rules written to address the problem when someone demolishes the field. Also, it will be ugly as ****. A C4 is one of the most beautiful shapes ever put into production, and all the stuff I just described will ruin that. Hide it as much as possible with flat black paint...I guess.

The only real answer is the higher and further back, the more effective. The C4 greenhouse is probably pretty good compared to other production cars. So you can get away with less height and rearward positioning than on a lot of other cars. Keep in mind that the rear wing has leverage, using the rear axle line as a fulcrum. The downforce it generates will also generate some lift on the front wheels. The further back the wing is set, the more leverage it has on the front wheels. The wheelbase is 8', so if the wing is 2' behind the rear axle line and generates 400lbs at 100mph, then it will also reduce front downforce by 100lbs. If it's set back 4' behind the rear axle, then it will reduce front downforce by 200lbs. This is useful for balancing the grip, of course - it's not good or bad, just important to understand.

Overall there is no question that going all out as described above can get you a crap ton of downforce. The challenge is getting the center of pressure (CP) in the right place from front to rear. Usually racecars strive to get it just a few percentage points of the wheelbase rearward of the car's CG. So for a C4 with a 50/50 weight distribution, with the CG at 50% of the wheelbase, you might want a CP for total downforce at around 45%. This helps ensure that the faster you go (ergo the higher the downforce) the more rearward the grip becomes and the car remains stable, with a touch of understeer increasing with speed. If you have the CP ahead of the CG at all, then you increase oversteer with speed - a recipe for a bad day at the track! In almost every article I read (lots of Simon McBeath columns in Racecar Engineering, mainly), most sedans with add-on aero arrive at the wind tunnel with the CP too rearward. They basically have way more downforce from a rear wing than from any front splitters their rules allow, and the flat floors with rules-limited rear diffuser have their own CP too far rearward. So they usually end up working to increase front grip. You might not have this problem if unconstrained by rules on floor shape and skirts. But the only way to know is to build it all and then test it with load sensors on the shocks, or to run a reasonably accurate model in a good CFD software package. This goes directly to your point about time and money to do all of this right. In the ideal world, you'd build it, see where the CP is, and then use the rear wing angles and gurney and maybe front dive planes to adjust for good balance. I think testing and adjusting is the hard part after you've fabbed up all this stuff.

 

Dang matt, thats great! I’ll be digesting that, thanks!

merry christmas too!

 

One thing to add, Chris. There's a different calculation for autocross vs road race aero. For autocross work, where speeds are low and cornering/braking frequent, drag just doesn't matter. For road course work, drag may start to matter, even with your level of power. If you're able to change the rear wing, or at least adjust its angle, that might be very useful. Frankly, if the rules are truly wide open, then for autocross work I'd be looking at an insanely huge multi-element wing over the roof, or may one front (above leading edge of hood) and one rear. It would be ugly as sin, but you'd be looking at A-mod levels of downforce:weight. You'd want to remove all that and use one rear wing that was more trimmed out for road courses, I think. It depends on the rules, but I'm aware you have road course times as well as autocross and stop box events in OUSCI.

 

The gills help underhood air escape. Relocating battery and washer bottle will aid in increasing airflow. The windshield because of it's size and angle increases downforce as speed increases.

 

I am just curious what type of racing are you doing and what kind of speeds are being acheived,

 

for my my primary event, there is an autocross and a road course and you can’t change in between. Other than that mostly road course time trials and autocross events.

Probably do need a swappable autocross package, one for high grip courses, and one for courses with more big straights.

All more reasons to never allow aero. But that ship has sailed. Rest assured i’ll Be taking all that embarrassing ugly bullshit off the car for photos and general use.

 

it’s already ls swapped with all that crap gone. Will be opening up the gills too if that really helps.

 

the old race hood stood up, opened flaps, and tried to fly away like a terydactyl before I put vents in it. Literally damn near pulled the racelocks out of the fiberglass. So I know that location at least does something.

I’d rather know what I’m doing before venting this incredible carbon fiber hood.

 

If you enclose the rear around the diff with a diffuser it's probably best to install a pump and cooler for the diff. I removed the spare and fabricated an enclosure for the rear but I've noticed the diff spits out lube and looks to be overheating. This is on a fast road course. And I agree with you about the level of prep/cost for amateur events.

 

which carbon fiber hood ?

exotic muscle?

where is the optimum place for venting?
are gills enough, or do you think you need vents in the hood

 

Given that you have two low-speed events (autocross and speed stop) and one road course event, and there are no limitations on power, I'd focus on max downforce and not worry too much about drag. I am sure you have slightly more power than stock (LOL) and so you can still pull decent terminal velocities with lots of drag. The grip is more important.

Another thought: if you do go whole-hog on this and start making serious downforce, don't forget that you'll need to adjust your spring rates accordingly. The spring rates that work well for you now with no downforce will probably have you on the bump stops on the straights at tracks. You might even want to consider progressive springs, helper springs, or tuned bumpstops.

 

no not em. I’ll try to look back. The damn thing was $5k, but 17 lbs and uses stock front hinges. So light you dont need the support to keep it open. It really is a work of art.

Absolutely need hood vents - see my post above. I had them right over he tires and extendin inward to ven hot air as well.

 

can you post a photo ?

 

Sounds like the blackdog racing hood.

 

yep, i remember that. I notice it disappeared in the past few years from their site.

i was gonna throw them an offer on it since it hadnt sold.

that said, i wonder if this is it....if it fit well and they have molds to build another.

again , would love to see a picture

 

I’ll post pics soon. It was 5 freaking thousand dollars. But it is a fine fitting work of art.

on the stock hinges it stays open perfectly without the prop. Worth it if you have the mental deficiencies necessary to spend that kind of $$ on a c4 hood.