Extractor Hood vs. Radiator Outlet Duct
 

With my last engine configuration in the RX7, I had a much more open area around the radiator to duct out the hood, which I did via a v-box outlet type configuration. With the new setup, there is now a much larger accessory drive, dry sump pump, air inlet tubing, etc. that occupies the same space. It's going to make creating a sealed outlet duct to the radiator a much more difficult proposition, and will also make things much more difficult to service on race day (I need access to the dry sump pump to prime it, etc.)

Has anyone done any A/B comparisons between a sealed radiator outlet duct, vs. an extractor type hood? I will have the ability to mold a fairly wide exit "scoop" into the hood that drops down several inches, and I can vent the hell out of the rest of it, but I wonder just how much I'm "losing" in terms of cooling efficiency or turbulence without the sealed duct. I have open front wheel wells due to a round tube front end, so maybe it'll help relieve some underhood pressure compared to the sealed duct?

The minimal research I did into this a couple of years ago indicated that the shape and design of the outlet duct is dramatically more important than the shape of the inlet duct. It is entirely possible to just make things worse with a poorly-designed outlet duct vs. having nothing at all. I would avoid it if you can, personally.

Thanks. I will probably end up just having to duct much further forward on the hood. I always question whether it's too far forward (it will have to be close to the leading edge), but the old RX7 pressure diagram still shows that as being a big low pressure area, so it sounds like downsizing the duct a bit and moving it further forward with a raised leading edge will still do the trick.

I read the exact opposite, fyi. I don't have a dog in the fight, just saying I read the opposite.

Me too.

Interesting. It was a while ago so maybe prevailing wisdom has changed. I recall the logic being that you could do more damage to the flow exiting the radiator than you would get back in gains from sealing it to the hood.

I think there's some miscommunication here, at least in my comment. Supe's reply reads to me like he thought Sav was advocating FOR a poorly designed duct over no duct, when he was actually saying the opposite. I thought Luckykid was attempting to correct Supe's interpretation, not Sav's post.

I read that no duct is better, let the air find it's own way.

Any pro level race car is going to have carefully shaped ducting, not just holes/louvers for venting. One of the main reasons is to reduce drag, the other is to reduce and control the speed of the flow through heat exchangers. Check out Tune to Win by Carrol Smith or Racecar Aerodynamics by Katz you'll find some info on ducting. From memory you usually have a smaller inlet that expands as it moves to the heat exchanger (radiator, IC, etc). Then the exit duct usually converges to a smaller exit, however I believe the exit is less critical and can even be about the same size as the flow area of the exchanger. Also for drag reduction having inlet edges that are smooth or radiused are better than sharp/abrupt edges. This doesn't mean that louvers and such won't work, they are better than letting all the flow go under the car and may be your only option due to space constraints. It just means ducts are likely to be a more efficient solution when packaged correctly.

I am so lost. :magna:

It should go without saying that a decently designed exit duct is much more preferable to an unducted vented hood, at least in terms of drag and heat exchanger efficiency. Radiators get all their flow from pressure drop across the core, and decreasing pressure at the exit is going to be just as effective as a similar increase in pressure at the entrance. Avoid step changes and create a design that will smoothly accelerate the airflow back up to the external airflow speed, or at least as close as possible. The typical rule of thumb is entries and exits should be 1/3 the size of the radiator. But other than basic guidelines ad without a CFD model or windtunnel time, no one is going to be able to give you a solid answer as to how much losing a duct will hurt your downforce or lap times. Aerodynamics are an extremely complicated field where everything you do effects downstream, and even upstream flow. For example, look at the pressure maps you see of Miata hoods. The leading edge of the first vent flick and the air coming out of it is going to completely throw off every pressure reading behind it. You can't assume that the pressure delta is going to be the same. An area with more obstruction below the hood, but a lower pressure reading, may well be a worse place for a duct than a clearer area with not as good of a pressure differential.

I think you'd have to make a pretty poor duct for it to be worse than just venting the hood, but that's just a rectal extraction and there's other things to consider like serviceability and if your fabrication time would be better spent on other mods. I'm a big fan of making your mods easy to work with, on, and around.

I have just recently installed my hood vents and have been spending my daily drive time monitoring its new effects. At this time the car has its original intake system with a K&N air filter, but no belly pan, no fender liners. On the very first drive it had more snap to it like I was running a fresh air intake for the motor. At 70mph+ it does generate a good bit of draft on the engine compartment, enough to start pulling on the wrapping tape on the wire loom. Now at speed there's a six inch piece of black tape sticking up waving hello.

Aerodynamics and thermodynamics can be quite similar in their functions, you can learn just enough to screw your lights out, but once you learn a little more your good enough to go try stuff.
https://cimg1.ibsrv.net/gimg/www.mia...89b4a3fa7c.jpg
Fresh install of hood vents.