Look out, Hugh is ridiculously capable of overwhelming argument, in a way only an engineer can.

 

Well it's true. You probably didnt notice the hood dance at high speeds however I did along with the dirtier engine bay. This if fact and I've personally proven it.
I did however came up with a fairly inexpensive option, a reinforced particle board covered in Flex seal. Still to this day I laugh due to the smooth process that lead to the result.
My shield has been on since August. Sees about 250 miles of driving a week, in Florida. Went through plenty of floods and has been seeing a couple of drips of hot coolant on it and it's still there.
Unlike those out there mine is a one piece design and it's not difficult to remove at all(using a electric tools that is). I've had to take it off 10+ times and again it's still there.
Once the car is lowered I'll find out how close the sway bar gets to it however at stock height and at the biggest suspension bound it does not even come close to touching the sway.

Call me cheap, unorthodox, etc the theory worked, the result was a lightweight, waterproof tray that performs. The only thing I couldnt incorporate was exhaust vents/louvers like some of the trays however I'm wondering if the Aluminum contributes to even more engine heat, afterall the oem is plastic.


V2 template made out of compressed board

V2 template made out of compressed board

As you can see the brown completley seals the mouth area of the front bumper forcing air to go through the cores instead of out of the bumper.

Crazy that all of this is supposed to be covered.

Here's the template with 2 coats of FLEX SEAL. This stuff is A MAZING.

Satisfied with the end result

 

Okay, that is cool. I’ve driven without it as well many times, yet I never noticed any of those problems you mentioned, even on older cars with flimsy metal hoods. The air pressure won’t change from this loose, flimsy, plastic. All temperatures were the same without it as well and with the metal undertray I have. I only hear dealers try and justify it with air pressure garbage that doesn’t make any sense at all if we’re using basic laws of physics. OP could drive without the little door free of any problems to be frank, unless he lives in a snowy/rainy area.

 

Id does make the difference or high speed cars would spend thousands in their carbon ones. Every single high speed car will have one. Hell even my wife's rav4 brought one and here is the reason.
Between the radiator and the front of the engine usually there is space. The air in this space is low velocity because it slows down when it hits the condenser and radiator cores, on top of that this air is hot which means it will not by any chance want to exit out the bottom. Granted some of the air does excape however not enough. Air pressure from the incoming air will always trump this space because as it has to bend under the front part of the bumper if you look at my pictures anywhere you see brown is where air can come in and create pressure.
Older cars are worse with this any honda owner will attest to this.

 

I only agree with you that it keeps dirt, snow, rain out of there to prevent any problems with components like belts. Most cars have splash shields for that reason, as well as to slightly (just slightly) reduce the drag and mpg by maybe at most 1 mpg. In case with the engine in our cars heat almost always goes up. Do you really think it a flimsy plastic will keep it from escaping? Either way, below the engine is not as hot as the exhaust. By your argument, we should then cover the whole exhaust as well? I've driven so many cars that never had splash shields from factory and they were fine at high speeds, like Saabs, Ladas, and Datsun Sunny+Z. I think why your hood was dancing was because a snowbird sneaked into the engine bay (joking). Gs aren't really high speed cars if you mean by 180 mph and over. I have a thermal picture of a car driving along. Notice how the temperature is much higher on top of the engine and where the exhaust flows (heads).

 

Picture provides some of an explanation however the biggest difference comes with the design of the bay of that truck.

While Idling
For further examples the G with the AC off hangs between 200F-215F which in 80F weather
With AC on Coolant temps stays around 185-195F
With the AC off Intake temps can climb upt to 140-150F on stock intakes
AC on will average 100-110 on 80F weather.

With the shield the only ones affected were my intake temps while idling with the AC off, they saw an increase of about 4F however as soon as I started driving they climbed down faster than without the shield. An uncalculated effect was a higher efficiency of the AC which I suspect is because the front mouth of the car is sealed maximizing the pull of the fans.

 

There are plenty of factors that support the need of the tray that by themselves dont account for much however in the bigger picture it does. Other than pricing and possibly the not want to take it off to work on under the car should be the only reasons of why not have one.
If this one survives the year and lower ride height I might consider making fiber copies which should meet all of the perspectives and offer a bit more affordable option to this.

One of my issues I was confronted with while modifying this car is how I underated minimal things like this and their importance. Example: We know the windshield cowl of most cars is a low area what I discovered is that the wipers on this car play a huge role into this area. I taped some strings in this area and found that air does not go into the cowl as suspected and as some older cars do. Air does go into the cowl as soon as the wipers come on. Next time you're going on a drive with climate control on fresh and off you'll feel more air coming in from the outside as the wipers go up.
As soon as they rest airflow goes over them smoothly and transition towards the windshield. Because of this test I now make sure I get dont get cheap wipers or hollow frame ones.

The tech is there for a reason, the more you compare these to other sports cars you'll find more and more similarities in them confirming these theories.

 

Those plastic shields and covers are basically useless. Anyone from the north can tell you they fill up with snow. They are far from weather tight and the only real advantage of even having one is to stop something from bouncing up and hitting the engine or radiator

 

In stock form they are pretty fragile. To do oil changes most service places end up bending the metal supports to get in and out shortening the life of them. Add some hot oil into the mix and they'll turn into a mushy combo.
It still does not take away from their function when they are new/good condition.

 

lol...didn't know I was so obvious!

Its not that flimsy, is it bruv? Its got some substance to it.

Plus, see below....we're both saying the same thing.

I was voted "most agreeable guy ever made" once.


There's several benefits, as we both said, to keeping splash guards. As a sum, they render the splash shields/under trays as useful.

If Nissan needed to make the decision solely based on aerodynamics...or solely based on thermal control...maybe they'd think its not worth it.

However, summarizing the benefits would prove that the splash shield is:
-needed
-worth the expense
-not flimsy, but robust enough to last the reasonable lifespan of the car (otherwise it would fall apart and all the benefits would be lost)
-etc.

You also have to remember that it is part of a system of shields. It interfaces with the bumper (guides air to the radiator, stiffens the bumper, prevents stuff from getting behind the bumper and ripping it out if backing up over snow or mud or something). It interfaces with the wheel-well splash guards for more reasons than I can list. And so on.

The car is designed as a system. The splash shield is an integral part of that system.

The aero, heat, corrosion, protection, ergonomic, and other benefits of that system require all parts to be working as intended.

The result of these effects is also something to consider. Why is slightly controlling air flow advantageous? You stated MPG due to drag already.

But there are other benefits, right?

Turbulence abatement for wind noise is one.

A lot of products in the engine bay are designed to meet some fire standard too. Note the itchy blanket held above the engine by melt clips. The splash shield blocks air flow. They work together to produce the same effect as closing the upper and lower vent on a charcoal grill. #systems

I could go on...but you'd be bored. Plus, you sound like you've already thought of all this, judging by those thermal images you obviously screen shotted from the movie Predator.

 

I'm from the like...Mid north. No, they don't fill up with snow as long as they're properly attached.

Even if they would collect snow...isn't it better than ripping the bumper off because you backed up over a snow pile? Using MASSIVE hyperbole, that's like saying seatbelts are useless because they might slightly itch your neck when worn incorrectly.

And yes, they're incredibly useful and necessary.

Why would the manufacturer spend all them tooling dollars, engineering dollars, warehousing dollars, and many other versions of dollars (actually Yen) on some useless part? They test these cars in the snow too, you know....

Also...think about this...
In its function, how different is the splash shield than the engine hood?

And why does literally every single modern/semi modern car come with a system of splash shields?

Its going to be easy to get caught up with disagreeing with me. But think about some of the above arguments stating the part is useful and ask whether you've put that much thought into the decision that the part is useless.

 

Conventional Wisdom is almost always more about convenience than expertise. It is neither a compliment nor a supporting argument, which makes it that much more ironic when used that way.

Now, nobody actually used the phrase here... but people were thinking it, LOL.

 

Ha! no doubt.

I didn't mean to come off like I was advising people on what to do to THEIR cars.

Maybe some people hate removing the guard for oil changes so much that they dont care about any of the splash shield's benefits.

I was only disagreeing on the statement that its a useless item.

 

Not sure what the deal was, but I got my sedan with 98k on it and when I went to do the first oil change it basically just broke off. I haven't replaced it and now the splash shield in general is starting to sag, so it seems like the open patch where the hatch door was is pulling down on it since the flap isn't there to keep the wind out... I wouldn't have thought it would effect anything either, but with as much as I drive, it has sadly. Although I must say that it is staying very clean under there still. But I do make a point not to drive on gravel or freshly chip and sealed roads unless my life depends on it...
Since I can see it sagging under the car, I'm probably going to be replacing it with some sort of aftermarket version in the spring/summer.

 

Are you a car engineer? I also studied engineering in USSR before it broke apart, but I know how much it matters for us in that field to argue. The Nissan undertray is probably the flimsiest plastic I have witnessed on all the cars I have owned, so I really doubt it will do anything about air pressure, if anything at all. Even with my newer Q50, they are crappy and bendable while not lasting that long. And no, I don't run over debris for it to break. I only have metal ones from TBW installed to prevent my engine bay from getting dirty and to protect the engine from possible debris while making it easy to change oil.


Again, problem is that it is flimsy. You expect me to believe a crappy plastic will change air pressure of the engine compartment? Very unlikely. I don't live in the snowy area anymore, but when I used to and had cars without splash shields none of that happened. I believe it is nonsense because snow and debris will not rip out bumper unless it is screwed in terribly. But I agree that it connects to the fender liners, but it depends on the make because my other German car's fender liners do not connect to the splash shield like my Japanese Infiniti does.

Not to sound rude, but that sound silly. Can we take a step back and re-think this? A very thin piece of plastic made in Japan or China, with many gaps in between the screws, that weighs only a couple grams, is an integral part of the car? Wind noise was never affected for me with it off. Only thing I had to use it for was to prevent debris, water from entering, and that is it. I'd rather have it to prevent that and that's it. Air pressure is different thing and nonsense for this plastic undertray.

I need to stop you right there at the itchy blanket (hood insulation) on the bonnet. That is an old wive's tail. It is only there to make less noise, it has no fire retardant ability whatsoever and I experienced this firsthand. But image is from google. I would be more glad to take my G37 to a wind tunnel but everything is closed.