Testing the stock vented underpanels/undertray
 

I recently got my hands on a spare stock aluminium underpanel, and I wanted to explore the potential cooling effects of making ventilation slats, similar to the Beatrush/Carbing aftermarket vented underpanels sold.

The premise of this test is to identify if venting the underpanel will improve coolant/oil temperatures by allowing air past the radiator to exit the engine bay instead of being recirculated/stagnated. With less pressure built up in the bay, there should be better flow/pressure at high speeds going across the condenser/radiator stack. I chose the stock panel to work on as everyone already has one and reversing it is as straightforward as finding another.

The test methodology would be to capture relevant temperatures and sensors in both stock and vented configurations, at idle and 110km/h speeds in 6th gear with the list below:

Coolant temperature
Oil temperature
Intake air temperature (not expecting this to change much given the intake is outside of the engine bay)
Ambient temperature

The laser cuts made on the panel were planned out to vent as much as possible without reducing too much rigidity (I did not account for one issue, see if you can find it). The slats are bent at a 45 degree angle to allow the widest opening on each vent.

Discussions previously on this forum have been polarizing, so I'll touch on the potential downsides as well. Venting hot air from the engine bay to the underneath will reduce its stability as more airflow now travels underneath the car, increasing lift. Not sure if that additional airflow would be marginal or significant to impact stability, however this will be difficult to measure/quantify regardless so I will be going by feel.

Relevant modifications to the car include a 82C thermostat, 30/70 Denso tropical coolant, Forester water to oil cooler and a tune, otherwise stock for the most part. Will take my time with this but will pen down my thoughts and findings as I go along.

https://i.imgur.com/VlTh23P.jpg

https://i.imgur.com/QSflVQ2.jpg

https://i.imgur.com/4T43z0C.jpg

https://i.imgur.com/ZRd6sbf.jpg

You had me at "laser cuts".

Something to consider: Is the pressure under the car less that the pressure in the engine bay?

Can't answer that for sure, but up to a certain speed, the air in the engine bay will need to exit somewhere.

Verus has pressure plots on their website in the information packet pdf on their product page. Looks like it will be okay.

https://www.verus-engineering.com/sh...egory=40#attr=

I think for the venting to work optimally, a front splitter is needed.

First set of logs here with some mountain pass driving and high speed highway driving with unmodified vents. Temps are in C, and I live in tropical climate.

https://datazap.me/u/compelica/under...y?log=0&data=4

Will get idle numbers and steady highway speeds at a later time.

But now more air under the car = less down force. It is better to vent up.

In all likelihood yes, however venting the undertray is a far more easier and reversible solution compared to cutting the hood and dealing with rainwater, etc.

The goal is to see if underpanel venting can be a viable solution when dealing with engine bay temps.

This mostly likely removes most of the downforce from the undertray in exchange for more cooling capacity, might be worth it depending on how you use the car :). The std undertray on my porsche 924 turbo looks pretty much exactly like that.

Log of consistent driving with the unvented panel at 110 kmh in 6th gear - did not cut the logs so you'll have to zoom into the two sections where the steady driving was done.

Idle is captured at the end as well, for 10 minutes with the AC off and fans forced at full speed.

https://datazap.me/u/compelica/under...ta=0-1-2-3-4-5

While the idle test was going on I observed that air was exiting from the radiator via these areas:

1. Left and right cowls
2. The gaps between the hood and fender
3. Behind the front wheels, near the anti roll bar mounts
4. Underneath the driver and passenger sills
5. Rear bumper (between the exhaust tips)

My venting solution may not suit all tastes but it does move air out of the engine compartment.

New underpanel is finally up - placed lacy pink ribbons to view the airflow.

Testing to follow.

https://i.imgur.com/E0SzR5j.jpg

https://i.imgur.com/YQBr6nO.jpg

Started the car up, grabbed some videos and took the car for a drive to a valley the morning after.

First was to see if airflow was actually exiting the vents during idle. This was done with radiator fans at full speed, with and without the hood closed.

https://www.youtube.com/watch?v=rVDa9afx2e0

https://www.youtube.com/watch?v=5TTl-GSkqIk

The ribbons do see some movement on the first few rows but not the second and third. I added ribbons in a 3x3 layout (except for the middle which has only 2), shown in the above posts. The airflow escaping via the left and right cowls have decreased slightly, but there is a lot more air escaping via the sides of the car. Good sign overall.

In city driving, coolant temps saw a drop of about 2C. I would see 89C before venting on a very hot day when going about in town; it now stays at 87C. On highway speeds it would hover around 88C when previously it was at 89-90C.

Oil temperature did not change much (or at all) which is expected.

Stability wise, there was no apparent difference, despite at one point driving at very illegal speeds.

Takeaways/observations:

1. A 2C drop in coolant temps probably means a lot more in terms of reduction in engine bay temperatures, since if the coolant temperatures are dropping due to venting then there is less stagnation of air in the engine bay.

2. The car now has a lot of leaves/debris stuck at the grille area - this is likely due to the now lower pressure in the engine bay; also with the assumption that before venting, the engine bay would be pressurized enough that the leaves/debris go around/under the car at high speeds instead of getting trapped.

3. Those with thicker radiator stacks (oil cooler, aftermarket radiators) might also see this as a potential benefit due to the increased pressure which may help air flow thru the stack.

4. The stock header's heatshield, is rather close to the underpanel, about 1-1.5cm away from the heatshield's lowest point. Those running aftermarket headers may see even less pressure buildup due to a more open layout without the shields.

Would be a good and cheap mod if you have access to a few hand tools, although bending would be a pain which is why I had it sent to a shop, don't see any downsides so far but I'll update here if I encounter any.

Have you considered creating a cover plate for the oil pan opening like the GR86 has, or even some of the aftermarket options? Could potentially help with overall flow.

Personally I’m part of the hood vent gang (with a gr86 skid plate on my 2013) but I’m really interested in what you have going on here.

https://i.ebayimg.com/images/g/aigAA...DX/s-l1200.jpg

https://cdn11.bigcommerce.com/s-p02z...1680653383.jpg

Ah wasn't aware that the GR86 underpanel is a direct fit for the older twins.

Not sure what you meant by improving overall flow, as the ZN6 oil pan opening would also serve as a outlet for air to flow out of the engine bay.

On creating a cover - probably not worth the work at this point (and you could just get a used GR86 panel anyway).

It's not exactly a direct fit, you need to get the GR86's front sway bar mounting brackets and it's OEM front chassis bracing ~ $120 for all that.

You could be right about the oil pan opening venting heat, but in my amateur opinion it seems like the opening is far too big to create a pressure differential that will allow effective air evacuation, especially compared with the "louvered" flaps you have created. I'm just thinking that if that area was covered, your louvers would siphon out heat even more than they already are. Could be completely wrong about that though. Would only require some sheet metal and 3 screws for testing though!

I see your point on the oil pan opening. Possibly that may happen on idle or low speed driving, but once highway speeds are attained that oil pan opening would possibly help evacuate air out of the bay.

A smoke machine would be great to see how the air exits...

@Compelica, I really appreciate your job, but would't it be easier to buy the vented Beatrush panel?

Given the price of the Beatrush panel including shipping, getting the stock one done at a shop will be less than 10% of the overall cost.

$200+ plus shipping vs some side cutters.
Guess your time/effort isn't worth much.

Where I live, labor is in a way, affordable but products are expensive - it cost me approx $24 to get it done at a metal works shop with laser cutting and a machine to do the bending.

Unfortunately, labour price is not the same everywhere...

My point being this something someone could do on their own for free.

It wouldn’t be pretty but I could replicate that with my 4 1/2” cutoff pretty quick.

I did this a long time ago just for fun. Also needed to bang a bend in the thing to stop rubbing on the jdl header. I also drilled holes in my side emblem thing. Not sure it does anything but I have felt heat coming out of it parked after driving. Just street, no track so I don’t see any ill effects. Thanks for your testing.