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JustaMiata New Reader
11/23/22 8:25 p.m.

NA Miata, boosted, all that jazz... another forum post about radiator ducting and keeping engines cool.


this season was the cars first season after the build. Initially, I had coolant temp issues where I would see coolant temps rise after some pulls to a number I was not happy to see. These temps dropped quickly after normal driving. After a 50 second autocross run, coolant temps would rise from 185ish to 235 and would quickly cool back to 185 after popping the hood in the pits. 

I wrapped the exhaust manifold and exhaust to try and keep under hood temps down. Still had a issue. So I directed my actions towards air flow through the core. That's where my issues start. 

I have a thick stack up for cooling. A 18 row oil cooler sticks out front, then a 4" thick intercooler, followed by a Mishimoto 3 row radiator.

To help with airflow and have a proof of concept that I needed ducting, I used aluminum ducting tape to seal all the coolers together, added tape from the coolers to the bottom side of the front bumper inlet, and blocked the holes on the top of the radiator support. 


of course this reduced temps to what I'd consider acceptable. Now, after a autox run my temps go from 185 to 205 max which is pretty good considering the ecoboost tends to run warm. 

after discovering (the obvious...) the lack of proper cooler airflow, I am looking for some ideas/help with planning the cooler ducting. 
with the intercooler, it poses issues with the charge pipes and ducting. Then I have the oil cooler and factory crash bar which I'm not sure what I want to do. All I know is basic principles that say inlet duct sizing should be 1/3-1/2 sizing of radiator area. 

radiator core is 12" tall, 24" wide. 
Usable bumper inlet seems to be 5" tall, 23" wide.

I have a couple ways I could route but im not sure what's best. 

here are some pictures (probably worthless) but it may help give y'all a picture of my setup and usable space.


so I can try trimming the crash bar down to just what's needed for a oil cooler mount and then go from the top of the bumper inlet to the top of radiator core. Try to make this as smooth transition as possible and airflow around the crash bar would be turbulent. 

I'm thinking it would look similar to this:

but the crash bar would be in there.

not sure how much the Miata requires any crash bar or structure there. If I remove this and figure a oil cooler top mount off the intercooler, I think it could work. PS- the intercooler is mounted between the frame rails and could act as support for the frame rails. Mounts are 1/8" steel and bolt to the cast end tanks of the intercooler.

my other thought would be to have a duct go straight back to the bottom side of the crash bar and then expand back to the top of the radiator. 

Like this: 

this doesn't seem ideal for airflow. 

im open to suggestions, info, design criteria, etc. 

JustaMiata New Reader
11/23/22 8:32 p.m.

on the bottom side, the coolers sit below the bottom side of the bumper inlet. This is what I believe would be needed to help guide air. 

Keith Tanner
Keith Tanner MegaDork
11/23/22 8:54 p.m.

The "crash bar" is structural, it closes the box of the engine bay. Lots of people misunderstand that and end up making the front of their cars into noodles. If you do cut into it, weld in some extra metal. I've got a piece of roll bar tubing replacing mine.

You just need to close the gaps. The most obvious one is around the sides of the intercooler. Air is lazy, it will go around instead of going through if it can. The factory air duct in the nose isn't half bad, you can install it and trim it.

You've got the pressure on the backside sorted out fairly well with your hood vents, but make sure you've got an undertray in place under the engine. Adding a splitter will help put more air into the inlet while also likely giving you a bit of front downforce.

Autox is tough for cooling because it's high power, low speed, but it's also short duration. After a run, get a fan in the nose to cool the water in the radiator as best you can if you're doing lots of runs back to back.

JustaMiata New Reader
11/23/22 9:03 p.m.

I noticed some improvement with adding the splitter. It extends to the front edge of the subframe.

MyMiatas Reader
11/23/22 10:57 p.m.
Keith Tanner
Keith Tanner MegaDork
11/24/22 11:28 a.m.

In reply to MyMiatas :

That's the exact opposite of what you'll need. It will increase air pressure behind the heat exchangers, which means less flow through them. Put your headlights all the way down too. 

You're not interested in engine bay temps. You're interested in engine bay pressure. In fact, as you improve the ability of the heat exchangers to dump heat and drop coolant/IC outlet temps you may actually increase your engine bay temps because that heat has to go somewhere. 

DrMikeCSI Reader
11/24/22 2:21 p.m.

Shouldn't the oil cooler be behind the Intercooler? If you have a 200 degree oil temperature this looks like it will pre heat the intake air at least on the 50% of the intercooler that is behind the oil cooler. 

JustaMiata New Reader
11/24/22 8:10 p.m.

In reply to DrMikeCSI :

charge temps are 3-5* warmer than ambient after a run. Doesn't seem to affect temps much.

JustaMiata New Reader
11/24/22 8:36 p.m.


this is what I am starting to think will be enough for the sides. 

JustaMiata New Reader
11/25/22 1:23 a.m.


and this is what we came up with. .020" galvanized sheet. Pretty simple design, let's hope it works. It's better than the aluminum ducting tape I had before. 

Keith Tanner
Keith Tanner MegaDork
11/25/22 1:50 a.m.

That'll help a lot, but I can still see the tire through the mouth of the car :) Try to close off the sides so that any air coming in the mouth has to go through the exchanger stack. 

Apexcarver UltimaDork
11/25/22 9:37 a.m.

I know this might sound silly, but you don't have anything like that plastic undertray it had stock. We had a few people try deleting that in the autocross club I used to run with and it resulted in cooling problems. I am pretty sure that it helped evac hot air from behind the radiator similarity to what Keith was talking about. 


Might be worth reinstalling or making similar?

Keith Tanner
Keith Tanner MegaDork
11/25/22 10:01 a.m.

In reply to Apexcarver :

Apparently the splitter goes back to the subframe, so the bottom is taken care of. I'd want to close off the sides if possible.

JustaMiata New Reader
11/25/22 11:55 a.m.


and yes, I don't have any factory undertray, nothing would fit anyway. 

Apexcarver UltimaDork
11/25/22 2:25 p.m.

In reply to Keith Tanner :

Ah, gotcha 

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