Flying Finn

I have two additional points that I don't think has beed discussed yet, and these are actually 100% true, and obviously very scientific:

-Removing the engine tray saves weight.
-Your car is more cool (pun intended) when the tray is removed.

I removed mine I think the next day I got my 993 because: the two reasons I just mentioned, without it, you'll "catch" oil leaks quicker, jacking the rear up is easier and Bruce Andersson (& various other Porsche gurus) say so.

TheOtherEric

TJ90,
Here's how I interpret your data. Please tell me if I'm wrong.

NO TRAY:
Tamb=20c, after a drive Thead=125c (thermocouple). Temp Rise=105c

WITH TRAY:
Tamb=12c, after a drive Thead=120c. Temp Rise = 108c

This tells me that your cylinder head will run about 3 deg C hotter with the tray in place. Yet you are concluding that there's no merit to the argument that the tray causes hotter engine temps?

p.s. regarding your heat gun measurements, are the odd readings due to the spread? Remember that at a couple feet, your measurement area may be a few square inches. The exact specs should be written on the gun. This has caused me problems before.

tj90

Flying:
What do you mean the car is more cool? i understand and appreciate your other good points (except for the bruce anderson point - as brave heart said, does bruce stand 10 feet tall and have lightning bolts flying oiut of his ****?) He might be right, but Id like to see his data....

tj90

I will make my final conclusions when I can retest at the same exact ambient temps. The diff is confounding the results. Assuming that everything was equal, would you bet your house that 3C really makes a difference? That is within the error of my test setup so its not statistically significant.

Good points on the gun. The Fluke IR gun is 10:1. At 10 in, the spot size is one inch. I did not write that in my original post, but I attempted to keep the measurement distance as close as possible.... you are right -however, another source of error in the IR measurements...

jnx

Lets take a closer look at that photo. Only one exhaust pipe?

It also looks like there are 2 bright spots under the car. My guess is that its originating from the heat exchanger outlets.

Going back to my original post - Porsche designed the tray to isolate the engine from the heat exchanger gases. Without the tray you are mixing outside air with this exchanger gas. Im wondering if you are increasing temps down below the engine without the tray...[/QUOTE]

That passenger mirror is burning up!

Dudley

I think that is a 964 in the photo.

TheOtherEric

I agree that testing at the same ambient is preferred, but it's certainly not necessary. I did a LOT of this exact work as a development engineer on electric motors, where operating temps determined whether or not you could ship the product.

tj90

Good point again! I would have to agree that ambient has less of a factor than other factors like loading the engine etc. Driving around and droping thermal resistance of air by changing the convection coefficient of from free to forced convection is huge in thermal transfer (typically 10X more efficient).

To further validate your point, I found that cylinder head temp was close to 120C just idling for a few minutes. Everytime you start your car, with tray on or off, the cylinder will start to get very hot if idling longer than 2-3 min. Look at my 150C temp reading after 20 min of idiling. However at 3k RPMs for 30 min, the cylinder head temp was about 25C cooler.... Heat transfer via natural convection is not that great. A 10C drop in ambient air temp DOES NOT translate into a 10C drop in head temp. There are too many variables (oil temp, forced air) etc that probabvly have a bigger role in dictating head temps...

Ray Calvo

You think too much.

Porsche ONLY installed these when some crybaby Swiss complained about engine noise. Then more greenie-weenies moaned about engine noise and Porche installed them on several more select market cars. Eventually, it got easier to include them on all cars than to install them selectively. Now, Porsche had to come up with creative reasons as to why they installed them - and noise abatement wouldn't fly so they came up with others.

I took mine off when car had less than 10K miles; it now has 93K. Am getting over 3K miles per quart of 0W40 oil and engine underside is dry as a bone. So, I feel I did right. That stupid tray stays off.

ZombiePorsche44

After replying twice and reading four pages of everyone elses replies to this thread, it hit me like a ton of bricks........... does it really make a big difference not having the tray? not really! Does my car run cooler without the tray? I think so. Does tj90's car run cooler with the tray? He thinks so.

We will never agree and could do this till 2009 and still never really know the "TRUTH", if there is any. And even if we did, what would it really matter?

As for me, I'm just going to drive my car without the tray and not worry about it..............ZP44

tj90

ZP: I was laughing reading your reply. You probably captured the eventual punchline to this thread.

I would agree with you on everything except one thing - at this point, I think that my car may run just as hot as yours but not cooler...

I became obsessive over this issue, because I have been running with a tray and wanted to see if I was an idiot for doing so. It bugged me that the common advice flies in the face of the design intent of the car...

Now I have to get back to drilling out my airbox!

ZombiePorsche44

I have to respect the fact that it appears when you get something started you have the ability to stay with it to the finish, which is admirable, but I couldn't resist the opportunity to subliminaly suggest AR Syndrome.....hope you know i was just goofin'.

FYI, I meant your car runs cooler than without, not a comparison to my car......ZP44

By the way I love your kid in the helmet, I have the same picture of my kid too. I'll look you up at the next auto-x at the Q.....Peace

Adrienne

I've always thought (glibly) it was Porsche's solution to the 911 series' inevitable oil leaks. It's essentially a fancy bed pan for the aged Porsche's embarrassing leaks.

Notwithstanding, I think it is always a good idea to test the validity of assumptions. Thanks for running some tests for us TJ90. Unfortunately it's only one data point, so we really can't draw broad conclusions from it.

Flying Finn

No, he does not stand 10 feet tall but I'm sure he has forgotten more about Porsches than you, I, braveheart and eveyone else in this thread combined (except Steve W.) knows.

Jack of Hearts

[QUOTE=JasonAndreas]The two primary purposes as designed were to improve underbody aerodynamics and for the directed cooling of the engine and transmission. [QUOT


This patent makes very interesting reading. Michael Preiss specifically claims that the transmission and drive axles are cooled by the tray.

What we really need is a rolling-road dyno test of the aerodynamics, with and without the tray.