NACA duct for air intake?

Question for my TVR project - I'm trying to nail down a location for the air intake, and the real-estate in the front is getting full.

Quick overview - the car is a 1973 TVR 2500M, and the engine is a 2.0 EcoBoost.  Rough numbers - 250 hp / 360 ft-lb @ 21 psi, running the Ford Performance control pack.

There are two pretty nicely shaped NACA ducts on the hood, and the PS one lines up pretty well with the turbo intake.  Could I make a simple air-box that seals tight to the hood?

Looks like this:

Front of the car is to the right.  NACA duct has the screen on it.  I'm just propping up the air intake with the oil-filter bracket.  I know it looks weird.

Duct is 5" wide x 1.25" tall.  The outside of the hood looks like this:

Another view of the ducts from farther away:

I'm not a professional, but those naca ducts do not look big enough for the job. What is the area of that naca duct and what is the area of say your turbo inlet or your throttle body?

I would want the naca duct opening to be physically bigger than the turbo inlet for sure. Even just ignoring pressure and assuming everything is basically atmospheric a small opening is called a "throttle", and you only want one of those.

But also, if it's easy, just do it for now. Intake tweaking is something you can do on a running car while also having fun driving it.

Oh, and make sure water doesn't drip directly onto your filter through the naca duct AND that your airbox has a drain at the bottom to let water out.

In reply to Robbie :

Thanks for the feedback.  The filter and turbo inlet are both 2.5" diameter, so nominally we can call it 4.9 sq.in. area.  Compare that to 5" X 1.25" which gives 6.25 sq.in.  So the NACA duct is bigger, but not a ton.

I'm definitely at the point of E36 M3 or get off the pot on some of these design decisions.  I'm ready to start fabricating some stuff with an eye toward keeping forward progress.

It sounds like you will probably be ok then! I have heard mesh like that can be surprisingly resistant to letting air pass through at speed, but the mesh is probably easily tested and removed if needed.

mesh does take up available area, so it might be closer to 1:1

once you build a box...and get that all sorted, you can play around with measuring pressure (drop or rise) of the inside of the air box, but outside the filter, with like a Dwyer manometer to see if the opening needs to be manipulated in any way, larger, longer, gurney flap etc etc

I've heard the same thing about mesh.  I can remove it without much trouble.

Worst case, I can tap the wheel-well in addition to or instead of the NACA duct.

Scott,

The NACA is sized adequately for your intake. The mesh will have to be removed to get the best performance from the duct (there is significant area restriction through that mesh). The duct itself, while looking right, is not exactly the correct shape to do the job they were initially designed for. Typically NACA (more precisely - Submerged) ducts are really only going to get to ~95% pressure recovery. That may be overcome by the slightly higher than car speed due to the volumetric effect of having a car in the region and getting slightly higher effective air speed at the duct. The flanges the mesh are attached to are flow killers as well. You want those to be trimmed flush to the sidewalls of the duct. The duct upper edges should be sharp/squared off for the best inflow off the hood surface.

The NACA (National Advisory Council on Aeronautics) paper that describes the proper design/function of these duct was published in ~1938 and the original use was to get the pressure recovery from aircraft skins that were approaching sonic conditions without the possible implications of shock /expansion waves in the inlet. The sharp outer edges roll a vortex up on the sidewalls that helps pull the flow down/in to the duct. Once the flow is ingested, the counter-rotating vortices mostly cancel each other out.

Your air box does not have to be (and should not be) completely watertight. For most of the operating range of the engine the duct will be able to supply more flow than needed. Make sure to have a baffle that keeps the inflow from directly hitting the inlet filter (keeps it from getting waterlogged if you drive in the rain) and make sure there is a small drain point near the bottom for any water to escape (1/4" diameter or so, to reduce debris from blocking it).

In intakes, you really want a Scoop. 

When we were testing the NP01 we found a 12hp gain from extending the intake scoop into the free airflow.  (you can see the extended scoop below)  Naca's would be good for letting in cooler air and reducing intake temp but won't provide the "force" to get extra hp at speed. 

Johnny_at_NineLives said:

In intakes, you really want a Scoop. 

When we were testing the NP01 we found a 12hp gain from extending the intake scoop into the free airflow.  (you can see the extended scoop below)  Naca's would be good for letting in cooler air and reducing intake temp but won't provide the "force" to get extra hp at speed. 

Sure, generally, intakes are better for engine air intakes.

However, context about Scott’s TVR is important:

 1) it’s turbo charged, whereas the NP01 is NA, so the turbo can overcome some of the pressure losses that might strangle the NP01, warranting the NP01 favoring a scoop.

2) The TVR’s mission, isn’t competition... although it might occasionally see some.  So, ultimate performance isn’t the priority.

3) Looks matter, especially when considering #2... thus, grafting a scoop, or two scoops; would probably kill the hood look of the TVR.

Stafford’s right, though, about cleaning up the shape, and getting sharp angles between the hood-line and the vertical sides of the duct opening.  In doing so, you could always open up the width of the duct if need be.

Great info, everyone, thanks.  I'm going to move forward with cleaning up the NACA duct profile, and then designing an airbox for my filter.  I've already got some ideas in mind.

In reply to sleepyhead the buffalo :

You nailed the trade-offs I'm facing on the project.  Definitely some give and take.  BTW, love the new nickname, and the story behind it!

If this intake really doesn't work well then I can graft in a bigger duct, add some sort of scoop, or tap the wheel-well for high-pressure air.  Or do something else that I've not thought about yet.  But at the moment I'm in E36 M3 or get off the pot mode, and I'm ready to stop vacillating and build stuff!

sleepyhead the buffalo said:

Johnny_at_NineLives said:

In intakes, you really want a Scoop. 

When we were testing the NP01 we found a 12hp gain from extending the intake scoop into the free airflow.  (you can see the extended scoop below)  Naca's would be good for letting in cooler air and reducing intake temp but won't provide the "force" to get extra hp at speed. 

Sure, generally, intakes are better for engine air intakes.

However, context about Scott’s TVR is important:

 1) it’s turbo charged, whereas the NP01 is NA, so the turbo can overcome some of the pressure losses that might strangle the NP01, warranting the NP01 favoring a scoop.

2) The TVR’s mission, isn’t competition... although it might occasionally see some.  So, ultimate performance isn’t the priority.

3) Looks matter, especially when considering #2... thus, grafting a scoop, or two scoops; would probably kill the hood look of the TVR.

Stafford’s right, though, about cleaning up the shape, and getting sharp angles between the hood-line and the vertical sides of the duct opening.  In doing so, you could always open up the width of the duct if need be.

Roger that, just sharing our findings as well. 

Johnny, what's your thoughts on exhausting hot air from the radiator and intercooler? 

Can I duct it down and to the back of the engine compartment?  Maybe up to the louvers on the sides of the fenders?

I can doodle up some concepts if that would help.

It is far easier to shield the air filter from engine heat sources with a box or some such. Maybe even a heat shield for the box on the exhaust side.

Again duct sizing will need to be adjusted to allow enough incoming air to overcome the box "leakage".

Air blowing directly on filter media will likely accelerate debris right through the filter media. And fracture or break down filter media due to buffeting.

In reply to bentwrench :

For sure.  Right now I'm envisioning a D-shaped housing for the filter.  The flat of the D will face up and will have a rubber seal to close off against the hood.

Maybe aluminum, probably more likely carbon fiber.  The carbon would certainly be better from a reduced heat transfer point of view.

Busy doing paid-work right now, but I want to put together a working concept soon.

Made up the intake tube today and worked up a cardboard model for an airbox.  This is what I came up with:

The NACA duct will match up with the flange on the right (the seltzer box).  So no direct line for water to get to the filter.  I'll pop a hole in the bottom for a drain.

Assembly and mounting are looking reasonable at this point.

Look like it'll flow reasonably well?

Looks like it'll suck hot engine compartment air in the hole in the side of the air box by the fender. Can you make box longer on fender side with no hole in it so all intake air comes from the NACA duct into sealed box but can still enter the end of the filter?
 

In reply to NOT A TA :

I should have explained the hole. It's there just to help me line things up.  It wouldn't be there at all in the final version.

In reply to TVR Scott :

Looks awesome!

Looks good. I used the CAD technique on my air box as well. :) with a NACA duct. I wonder if the box and intake air would be cooler if I had some sort of flow through set up. An outlet for the incoming air. Thoughts?

looks very good. the 90 degree turn will help knock any debris out of the airstream and let water flow toward your drain hole. you will probably wind up with dirt/dust there too, so check and clean often.

Cool deal.  I shall proceed with fabrication.