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infinitenexus
infinitenexus Dork
4/16/21 1:29 p.m.

I'll be picking up a potential challenge car today and will likely take it on a few track days, so I got to thinking about aerodynamics, and something I saw steered me in this direction.  

From what I've read, boxing in your radiator and then venting the proper part of your hood can result in lower water temps as well as some frontal downforce.  I've also read that it requires a front air dam to work correctly, and a front splitter that extends back a ways also strongly helps by helping reduce high pressured air in the engine compartment escaping underneath the car.  I've also read that the placement of the hood vents is critical - it needs to be in a lower pressure zone, more towards the front of the hood.  While some do this with thin aluminum, I also saw recommended the coroplast signs (easy to get for free around an election!) as a decent material to box in the radiator.

I think I have a decent basic understanding of what to do here, but could someone learn me better so when I decide to take the plunge I'll actually know what I'm doing?  I guess I could always just copy Vorshlaq's 2011 Mustang, they did an awesome job of this.

sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
4/17/21 3:44 a.m.
infinitenexus said:

 From what I've read, boxing in your radiator and then venting the proper part of your hood can result in lower water temps as well as some frontal downforce.  I've also read that it requires a front air dam to work correctly, and a front splitter that extends back a ways also strongly helps by helping reduce high pressured air in the engine compartment escaping underneath the car.  I've also read that the placement of the hood vents is critical - it needs to be in a lower pressure zone, more towards the front of the hood.  While some do this with thin aluminum, I also saw recommended the coroplast signs (easy to get for free around an election!) as a decent material to box in the radiator.

so, first thing first:
(imho) it's helpful to think about Aerodynamics kinda like the "Pirate Code" in that they're "more what'd you call 'guidelines' than actual rules."
Fortunately, unlike the Pirate Code, these guidelines can be applied by anyone.

so.  yes, and airdam, a splitter, or a combination of both, can help a "shrouded" (i.e. ducted) radiator be more effective.  If you're ducting the inlet and the exit (especially if it's exiting above the hood/bonnet)... then how far the splitter extends back has much more to do with downforce than it does with the exit flow of the radiator.  This is based on the assumption that the ducting is well sealed to: the bumper, it's self, the radiator, and the hood;  at which point, the pressure in the engine bay is close to meaningless to the 'radiator system'.

one benefit of doing this, and placing the inlet in an airdam at the very front of the vehicle is that it will place the inlet at the highest pressure the vehicle likely generates.  So, while yes, the system will benefit from having an exit at low pressure... the reality is that it's mostly important that the exit be in a place of "not high pressure"... at which point the high pressure of the inlet well help things "operate".

the very front of the hood where it turns from "vertical-ish" to "horizontal-ish" is where there'll be the lowest pressure.  But, it's very difficult to creat an exit there, and it'll probably means that the radiator flow will exit more "up" than "up and back"... which will create more drag.  if you're going slow, that'll be ok; not so much the higher your average speed.  You can help create your own low pressure zone by placing 'kruger flaps' (i.e. wickers, front gurney flaps) at the front of your radiator exit; but those create lots of drag.  ymmv.

so, mostly have the exit not placed in the center of the base of the windshield.

if you build such a system, you want to have the inlet area sized for between 30-50% the area of the core face.  If it's for challenge only, I'd probably hedge towards 50% because the speeds are slower.  If you're taking the car to Road America, then I'd hedge towards 30%.  There's a benefit to this, in that the area change from 30-50% to the face of the core's 100% will take the high pressure at the "inlet" and further increase it at the radiator face, while beneficially reducing the flow's speed so it has more time to effect heat transfer through the radiator.

could you do this with coroplast?
Probably.  Depends on the engine bay, and the likelihood that some "very hot part of the engine, or fluids" might come in contact with the coroplast and melt it.  So, if you can minimize that chance, you'll be more likely for it to be successful.  Although, it's possible you'll have a harder time having it be successful at track speeds, than at autocross/drag speeds.

I think that's the short version.  there's lots of permutations that are possible, depending on 1) rules, 2) venue, 3) platform, 4) compromises you're ok with , 5) budget, 6) skill, etc ad nuaseam.

stafford1500
stafford1500 Dork
4/17/21 7:33 a.m.

A really big driver for underhood pressure is the front wheel openings. If there is not a significant wheel tub/fender liner the under hood pressure will be very low, almost as low as the pressure under a splitter. Most of the race cars i have worked on over the past few years have the under hood open to the wheel opening and the flow tends to blow gear box oil leaks up onto the intake manifolds.

Ducting you rad cooling out of the hood is generally complicated by the limted space behind a radiator in the engine bay. Splitting the exit flow toward the sides of the hood and having two exits can help with real space issues.

infinitenexus
infinitenexus Dork
4/17/21 11:12 a.m.

Thanks for the tips guys!

infinitenexus
infinitenexus Dork
4/17/21 12:45 p.m.

So if I had a splitter that extended back to the front axle and a good front air dam, and then ducted my front bumper/grill openings to the radiator but left the rest alone would I have any issues or just increased cooling at speed?  Or would there be too much pressure in the engine compartment?  Keep in mind I'm not doing this to be competitive, or to make a full-fledged race car. This is a cheap challenge-ish car that I bought and I just want to do it because I think it's awesome and would be a fun experience. I'll still be driving it to work. 

Keith Tanner
Keith Tanner MegaDork
4/17/21 1:24 p.m.
stafford1500 said:

A really big driver for underhood pressure is the front wheel openings. If there is not a significant wheel tub/fender liner the under hood pressure will be very low, almost as low as the pressure under a splitter. Most of the race cars i have worked on over the past few years have the under hood open to the wheel opening and the flow tends to blow gear box oil leaks up onto the intake manifolds.

Ducting you rad cooling out of the hood is generally complicated by the limted space behind a radiator in the engine bay. Splitting the exit flow toward the sides of the hood and having two exits can help with real space issues.

Interesting!

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