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vette427sbc 10-29-2018 06:28 PM

Internal Body Aerodynamics
Im getting closer to combining chassis projects with body projects... The first of these looks like it will be regarding coolant, oil and PS fluid coolers. Im curious if anyone has any data on how much (if any) engine compartment cooling is done by the fender gills? I had done some shade tree aero tests many years ago and the results showed no large amount of air was passing though these gills at speed (doesnt quite eliminate a cooling factor though) Also, I see that some of the 80's C3 race cars had the fender vents above the wheel wells... Any info on their effectiveness would also be appreciated :thumbs:
Id like to see if I can reduce high pressure air in the wheel wells by ducting them to the gills while having my coolers in line of this airflow. This would allow me to:
Keep the coolers hot air out of the radiator airflow
Shorter oil lines (less weight, less pressure drop, less area for failure)
Keep the coolers (weight) inside the wheelbase for better vehicle balance.

69427 10-30-2018 01:46 AM

Good topic. I'll enjoy the discussion.

Just my perspective/reasoning here. I like the look/function of the upper fender louvers, but I've put a self-imposed restriction on myself for most body mods (I like the stock/sleeper look), so I haven't gone to the vented fender option. What I have done is block off most of the front (3) grilles, and about half of the two underside air intakes for the radiator, in an effort to minimize how much air I need to manage or prevent going under the car. So far, the cooling system seems to still work just fine on 90+ degree days.
I've also added some aluminum sheet panels to the frame, between it and the lower header pipes/collectors. These two panels (l & r) just block off most of the air in the engine compartment from flowing under the footwells, and instead encourage the air to exit the side gills instead.
I also changed out the longer/heavier aluminum undertray from the PC spoiler back to the oil pan, to a slightly shorter/lighter lexan tray that extends from the PC spoiler to the front crossmember (less weight and better access to the front suspension). I then divert some of the front undercar air outward to the rotors for cooling.
I still want to vent the hood, but haven't gotten around to it yet.

rtj 10-30-2018 10:09 AM

In FL I opened up the fender louvres a lot, as the carb would vapor lock. Seemed to help a little. But on a 75 they are originally small. A small fan would help.

Best change I made with factory helt belt driven fan was a pusher fan in front of the rad.

Duntov sells the louvers for the fenders and claims they work. They may be running no inner fender.

No reason not to run a fan in the duct from the wheel well to the louvers.

Alternatively, You might be able to pull some higher pressure air from around the rad support.

phantomjock 10-30-2018 10:13 AM

Somewhere [I'll look but will be a few] I have [or think so] some info on lift reduction with fender louvers. Mayee in Mcbeeth's text, or another. Do have some radiator inner body cooling info - but not 'vette specific.

I have reversed the gills on the side of the fenders and am mounting 2 coolers inside that region.
for exactly these reasons too:

Keep the coolers hot air out of the radiator airflow
Shorter oil lines (less weight, less pressure drop, less area for failure)
Keep the coolers (weight) inside the wheelbase for better vehicle balance.
I'll snap a pic and send along too. Progress is slow, but "She Who Must be Obeyed" is pushing me along.

Cheers - Jim

phantomjock 10-30-2018 12:34 PM

From Simon McBeeth Competition Car Aerodynamics (second edition) 2009

Miscellaneous Devices (pg 172-3)

With closed-wheelers the situation might appear to be simpler, but can in fact be more complex than open-wheelers. Enveloping body work obviously shrouds the wheels from much of the air that they would otherwise encounter; and this is the basic reason we expect a lower drag from a closed-wheeler, but air still enters the wheel wells from in front, underneath and out to the sides, and how it is dealt with can have significant effects. It is also the case that rotating wheels can act as pumps in semi-enclosed wheel wells, an effect that increases with vehicle speed to the extent that air can be drawn into the wheel wells from the rear and pumped out at the front, according to one former wind tunnel operator writing to Racecar Engineering.
Often such efforts to improve things will involveducts, cutaways and louvers that in general help air to escape from wheel wells. The louvers on top of sports prototype wheelarches are present to help reduce the differential that can exist between the low static pressure over such a curved wheelarch compared to the the static pressure in the wheel well, a potential source of undesirable lift. The cutaways often present on the vehicle sides behind the front wheels also assist air to escape not only from the wheel wells, but also from under the front of the car. As these features often work in concert with front splitters and diffusers, providing an exit for air that has been exploited further forwards on the car. [\QUOTE]

Simon has now images other than the word picture above. I'll focus initially on Radiator Airflow, and as I find the other (louver) details will bring those in too.

Internal cooling is the single largest contributor to drag:

Below is a crude representation of C3 radiator flow:

But it can be adjusted through configuration changes/modifications. Just the orientation for the radiator and exhaust air can make a difference:

Note that "C" has lowest drag and highest volume of air of the 4 configurations.

Here is a good example of radiator ducting (on a C-5):

Some work could be made to improve its flow - but better than stock configuration.
An example from John Greenwood:

When I say "work to improve the flow" note the relationships described below:

Full open exit is not optimum, but closed in relationship to the entrance. I recall 50%, but may be off on that. You can imagine it could/would act as a venturi, but with a choked exit to keep exhaust flow speed up. The venerable P-51 Mustang used a variable exit on the radiator (located under the belly) and made it pump as a small "jet" increasing speed, by using the heated air from the oil cooler(s).

NOTE: That would constitute "Active Aerodynamics" and may not be permitted by some race organizations.

NOTE: it did offset the drag due to cooling by about 90%. SIGNIFICANT when you look back to figure 1 above.

Hope this is at least mildly interesting and useful.

Cheers - Jim

mfain 10-30-2018 05:34 PM

2 Attachment(s)
Here are a couple of my lessons-learned regarding dumping air pressure from the wheel wells and associated lift/downforce. The Viper ACR has a large louver over the front wheel with a removable grill. Using yarn tufts on the grill at speed I see almost no airflow exiting above the fender. Of interest, doing ink drops behind the rearward facing fender scoop on a 69 Dodge Dayton Charger at 250 mph also shows no exit airflow. Removing the vent grills on the ACR results in significant airflow out of the wheel well and a noticeable increase in front downforce. You also get a lot of tire rubber and track debris being thrown straight up into the air out of the holes. LOL I recently cut the ACR vent holes in a buddy's $20K SRT hood (did not come with the vents) to increase the front downforce on his track car - worked as advertised. Also, the front canards (dive planes) on the ACR have a fence at the outer edge that creates a vortex. The vortexes create a low pressure outside of the wheel well as the air passes outside the tire which helps pull air outward from the well through the brakes for cooling.


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vette427sbc 10-30-2018 07:24 PM

All excellent info!

69427- I like your idea of closing off as much of the front as possible. In fact, I did the same thing with the grilles when you first posted this mod years ago. If you have any pictures of how much you have closed off the underside vents too that would be great! I think with a more efficient airflow design/ducting after those inlets, a smaller (ie: lighter) radiator could be used also. The airflow in the nose pre-radiator has got to be a turbulent mess.

RTJ- The fender louvers you opened up were 68-69 gills? or the 73+ vent? The gills look like they may not be optimal for higher speed exiting airflow...

Jim- Ive been trying to figure out ideal duct placement, shape and size to maximize the wheel well evacuation. It seems apparent that what works on another car, may be ineffective on another, even with the same configuration (but on a different design ie: different cars with different wheel tub shapes). Funny, I was reading about the variable P51 duct in my Carol Smith book!
The new front clip on my car should make it far easier to lean my radiator forward for a hood exit while still being able to retain my stock-hinged hood.

I was thinking for the wheel well evacuation, a large opening in the rear with large radii on the top, sides and bottom feeding a duct the size of the cooler with some kind of vanes to help straighten the airflow. After looking at the available space, it looks like by necessity, the duct will have to neck down (after the cooler) anyway to fit around the forward-most body mounts which should help speed the air back up. To maximize the efficiency of both the cooler and the wheel well evac, someone that passed their fluid dynamics class :tomato: might have to give some pointers on vane design and how to make the air exiting the gills move as fast and clean as possible

vette427sbc 10-30-2018 07:31 PM

Pappy- So the silver car showed no/minimal airflow out of the fender and the same car with no gills/grilles showed a notable difference?
Interesting considering you would think the gills/grilles would create a low pressure area being that they are under the (assuming) attached airflow over the fender

phantomjock 10-30-2018 09:06 PM

Radiator Ducting - here is a fairly comprehensive set of images (eye candy) to enjoy on the topic:

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Hood Louvers

Once you "solve" radiator ducting, there might not be any requirement for hood louvers. The remaining air pressures in the engine bay may be significantly lowered -- especially if you augment with a front splitter. On the other hand, if you'd rather have a more stock radiator setup, you could measure the optimum location for louvers as the Miata gang have done:

Would take some drilling, a number of taps, plastic hose, a magnehelic gauge, and some test runs with a partner to record data at say 40 and 60 mph to get a a good data set. The lower the numbers on the grid would represent lower pressure on the hood, and a good place to exit under hood air with a hood louver. A lot of holes - so find a scrap hood to work with!

On to Fender Louvers...

This treatment on a 2003 Lister Prototype shows effective treatment of the wheel well, fender and surrounds. Not exactly a C3, but this focusses soley on a sealed wheel well - with no engine bay to deal with.

Note the location of the louvers on the fender arch. Not more 1/4 wheel diameter forward of wheel centerline. I wouldn't suggest that is based on data, but the out flow including the induced flow from wheel rotation would exit before getting too far forward in the wheel well. The louvers standing proud, rather than flush would (perhaps) also act as gurney flaps decreasing flow separation, and creating a low pressure area to withdraw the air in the wheel well.

The wheel well is basically sealed - except for suspension components.

The small "splitter/tray" behind the wheel/tire provides some downforce. Note the vortex generating side skirts. Cool huh?

So how does this fit a Corvette? A C3 with a duct/airbox radiator venting through the hood, inner fenders as on a C5+, and a splitter would isolate the wheel well from the engine bay. Fender louvers and other mods to this area could substantially reduce drag, pressure build up and downforce.

I agree with Pappy - no grill work in the louvers! I saw the same situation on teh original set of fender louvers on my 78 vette. I put in the grills as a daily driver, but are now gone as I continue to build Elvira.

On my little sports racer (Lolita), the wheel wells are isolated from the radiator airbox, and have a wide open exit for exhausting air from the well. I added the louvers.

No grill work, and they are "smallish louvers" and teh tuft tests showed flow. No data pulled to provide downforce details.

I have some pressure taps on hand, and it is on my list to capture some data, but am encumbered with other projects just now. I am considering modifying the area to include a "shelf" and vane/fence system as well. Kinda like this:

Not exactly applicable to a C3, but similar results can be made, but may look a little weird:

I just found my copy of 'Katz' so I'll do a quick review and see if there is any more to add.

Pappy - nice work with the cut-off wheel!

vette427sbc -- I'll re-read your post. I was just loading this one!

Cheers - Jim

phantomjock 10-30-2018 09:19 PM


I'll have to dig through a couple of hdd to find the pics - or uncover all the junk on the chassis to get some pics.

Got one:

I bought a set of reverse gills to replace the stock gills. They basically look like the ones on the Lister Fender above, but only 4 (or so) but stand proud. I plan on mounting the oil coolers vertically behind that area - opened up a space in the chassis and birdcage - that may need a pic. Hadn't planned on ducting it - but use fans to push or pull through using the "residual" air from under hood/enginebay.
On the original front clip - I used a set of Ecklers louvers complete with inner liners. I don't recall where I got this set - but I think Ecklers was out of stock. Doing those again, I opt for standing proud as mentioned in earlier post.

I think I understand your idea - and it makes me look at the radical Miata and think that is more or less what you mean or like the #49 ESR.

Cheers - Jim

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