Arca_ex

iTrader: (2)

Those suggestions for size of the opening are usually for a race car application. Where the point or goal is to make the opening smaller in order to gain an aerodynamic benefit of less drag, while still maintaining enough of an opening to still adequately cool the car.

lbatalha

I have recently measured this using a differential manometer.

My NB has good ducting, the top of the bumper has a plate covering the holes, a 50mm Ebay radiator, big intercooler (550x230x65 core), stock NB bumper tunnel, one stock fan.
Both tubes perpendicular to airflow. First tube is in front of Intercooler (about 2cm away, attached to one of the mounts for my rotrex cooler, its directly visible from the front
The second tube is behind the radiator, level just below the top end-tank attached to a rotrex oil hose between then radiator and the front plastic cam cover on the driver's side

At 80kph, 3000rpm, 32C Ambient at sea level (100kpa measured with engine off):
- Normal hood gap: 0.21kpa difference
- Sealed hood gap: 0.27kpa difference

This is a 23% difference

I found ~80km/h to be a point where the pressure difference starts increasing dramatically, at 70kph its something like 0.11kpa.
At about 100km/h it is around 0.34kpa

I was unable to see any temperature difference from sealed hood in a ~10-15 minute highway cruise at 80km/h since CLT was 86-87C, with this low of a delta between ambient I doubt it would matter much.
I also discovered that the threshold where my stock fan stops mattering is 70kph, thats the fastest speed where I can still measure a difference in CLT while cruising.
I will attempt to test this whenever I get to drive on track while this hot.

23% is nothing to scoff at, singular hood louvres claim 50% pressure improvement IIRC

DNMakinson

iTrader: (3)

How important is adding some form of radiator cover like this, as opposed to only foam between radiator and metal cross member? No hood vents.

gooflophaze

I've spent a few hours looking at NACA and FSAE documents, can't find a source for the rule of thumb - but the closest I've come to understanding is that it depends on speed to a large part. You can optimize inlet size (and duct shape) based on a nominal speed and delta P across the radiator, there's a 7 degree rule of thumb for duct walls to expand (assuming straight shot / low resistance like an air intake / cabin cooler) that goes out the wall when facing something as constrictive as a radiator - 15, 20 degrees would be okay in those situations. Larger area for slower speeds, smaller for faster. But then there's also Reynolds and Prandtl numbers involved in optimizing heat transfer.. so yeah, I'm probably just overthinking my ducting.

I haven't fabbed my undertray / ducting yet, but I'm trying to optimize my A/C and wondering if going to an NB2 condenser (which is apparently 2-pass, with the drier working as an accumulator, and post drier - about 1/3rd of the height - working as additional cooling for the liquid). Looks like most OEM's moved to that design in the mid 00's, wonder if that would improve the NA's a/c appreciably, or if it needs the cartridge-type expansion valve as well - or if the NB2 A/C is really "better" than the NA's.. but that's probably another thread.

But now I'm wondering if adding a gurney flap to the bottom of the undertray would appreciably lower the air pressure inside the engine bay..

emilio700

iTrader: (3)

All that matters is what the air does. To evaluate your mods, check if air can get past the core.

Don't get distracted by the direction the air is moving. It's slow enough that even the slightest pressure change will steer
the air mass.

Gee Emm

Quoted for truth.

On my MSM, there is (was, now) a massive exit path forward, around the inside of the nosecone, out through the guards. Repeating Emilio's advice check if air can get past the core - start with your core, and work backwards to the entry of the air through the nosecone, looking to make sure that ALL the air entering goes through the radiator. Because the radiator core resists the free passage of air, the air will seek an 'easier' route - your job is to make sure there is no easier route.

lbatalha

How important is ducting post heat exchangers? IE the gaps around the IC piping and sway bar after the radiator, near the wishbones? Do these need to be fully sealed? do the wheel wells have higher pressure than a well ducted engine compartment? I can see this working both ways, as it could help pressure escape even if to a lesser extent than hood extractors.

emilio700

iTrader: (3)

It all adds up. Again, focus on pressure differentials, not the relative inertia of a given air mass. When the air mass is moving a high velocity relative the vehicle at say, the rear wing, it begins to matter more.
But in the engine bay and in the nose, the relative velocities are very small so difference in pressure have the greatest effect on where the air mass decides to go.

We work to utilize the relatively low pressure under the car to help extract air from the engine bay. Natural exit path on the stock NA/NB:

- Around transmission and down in center of car. This is aided by a fuller coverage front under tray
- Gap between tub and subframe behind wheel. Not much to be done here unless you start cutting away or venting the fender aft of the wheel.

Because neither of these paths flow enough for a 300whp Miata when pushed hard, hood vents are needed.

The only source of incoming air mass in the engine bay should be the heat exchangers. So look carefully at the engine bay ahead of the front axle centerline to identify areas where high pressure air mass might infiltrate.. and block them off.

DNMakinson

iTrader: (3)

Which I guess means a proper cold air box so that intake air (hole behind / under NB headlight) is captured and none bleeds into the rest of the compartment.

emilio700

iTrader: (3)

Ideally, yes. That power improves by feeding the engine ambient air is a bonus.

lbatalha

Noted, will duct that as well as I did the rest. Once ducted perhaps I will measure the pressure differential like I did before for the radiators and report back.

In this case I don't have cold air intake (the NB headlight hole is completely filled with the hotside IC tubing (and some neoprene filler), so I'm sucking in only engine bay air for now which would actually help a bit with reducing pressure :P (with the trackdog header heat shield and good ducting I'm seeing mostly 8C over ambient which is not too bad, perhaps I will build a box and get air from cowl area)

sometorque

iTrader: (1)

In theory, ducting should also help with air hitting the AC condenser and thus help AC performance correct?

emilio700

iTrader: (3)

=46° F
Yuck. On a <150whp N/A engine that's 3-5whp. On an F/I engine that could be 10-30whp

concealer404

iTrader: (3)

That's not how temperature units work.

emilio700

iTrader: (3)

Regardless of the bad conversion, the point is the same: anything over ambient IAT is not as good as ambient IAT

concealer404

iTrader: (3)

100% agree.

~15F difference (depending on starting ambient point) under boost in a turbo car isn't what i'd call atrocious either, though.

Gee Emm

When looking at 'escape routes' for air entering the mouth, I found massive paths forward into the nosecone exiting into the wheel wells (amongst many others, of course, I mention this one because I don't see it discussed often).

This is a road or road/race car issue, only for those who do not go down the full custom ducting path. I built bulkheads in the nosecone to seal these off, together with side plates on the chassis.

sixshooter

iTrader: (12)

I use pieces of cut upholstery foam to seal those paths in the nose because it pulls tight when you mount the nose.

Gee Emm

The large section foams I looked at were either cheap and really porous, or very expensive and fairly non-porous. I had enough sheet aluminium so went with that.

However I never looked at upholstery foams as such, that is something worth checking out next time. Best part is you can probably get it for free by scrounging at the tip!! Recycling is good, right?!

sixshooter

iTrader: (12)

Yeah, I went to a furniture upholstery shop and they had a pile of scraps bigger than my truck and let me take what I wanted just for asking nicely. I shape it with a razor or large scissors.