Why I think removing engine tray is no good....
 

Before you flame me - I did do a Rennlist search and other web resources and found that EVERYONE recommends removing engine tray. After a year of ownership, I did for this past weekend track day at Willow Springs. What I found is that oil temps were not any cooler and maybe a little higher. I know there are 100s of factors that dictate oil temp, but I did not see any improvement...

Sitting in the paddock, i thought about this harder and convinced myself that no engine tray may be bad. Heres why:
1) The engine tray keeps airflow under the car from upsetting the balance and flow of the fan in front of the alternator. Think about it, If I have an 80 mph wind blowing against the fan, I am creating eddys and potential hot areas of the engine for proper heat transfer.
2) There are 2 hot air ducts that route below the engine and out the outlets of the engine tray. Keeping the tray on keeps the hot air from entering the engine compartment. Kinda like insulating your air intake on a conventional car.

I am an engineer and actually do CFD thermal transfer modelling for a living. I would love to model the engine, car and simulate air flows to be data driven instead of asking others their opinion. You could imagine the amount of time, energy this would take outside my job so it wont be happening.

For now, I have to trust Porsche that they did this modelling and empirical analysis so my tray will be going back on... I just dont want to upset the manufacturers intentions on how airflow is supposed to move anound the engine. Think about the cooling fan. Im sure Dietrich in Germany slaved on the proper fan design for adequate airflows... My '95 motor has 100k miles and I assume that the engine tray has been on from day one.

What do you all think? I would think that an AUX oil cooler would do more for engine life that removing a cover...

Many (most?) would say that the tray is there mainly for noise suppression and to a lesser extent for corrosion protection. Look at the factory based 964 and 993 racecars (the RS and GT2 variants) - did any of those use a tray? Nope. Where performace was the primary criteria it appears Dietrich took a pass.

"What do you all think? I would think that an AUX oil cooler would do more for engine life that removing a cover..."

I agree with that point. I think the big issue is the extra heat that is trapped due with the cover causes wear on the valve guides.

also, if you have ever been to the track on a very hot and STILL day, (ZERO WIND/Breeze) then you realize and see all the heat UNDER the car without the tray.

If I didn't go to DEs, I would leave the cover on myself.. but because I do participate in those 20 minute sessions, I figured it best to leave the cover off AND open the rear engine hood on hotter days for more 'air cooling' while parked.

Each to their own I guess...


I've heard repeatedly it was added for NOISE supression.. I wonder if that is the real reason or not?

Hi:

The engine undertrays were installed solely for sound surpression to meet very strict Swiss government drive-by noise ordinances. Thats it. No aerodynamic benefits. :)

All you do is cook the engine and reduce its lifespan when you retain that undertray.

Adding a left-side, front oil cooler is always a good idea on these cars to maximize engine longevity.

AFAIK, the Cup cars never came with them......

TJ90-
As you know, you are going against general consensus here. So any convincing argument would have to include a study using a couple cylinder head temperatur sensors, recording temps with and without the tray at multiple road speeds. Without that, it's just pure speculation.

Otherwise, we'd all agree that the aux oil cooler has a huge impact on oil temps. BTDT. Removing the engine tray doesn't affect the oil temps significantly.

But again, neither air flow nor oil temps are really relevant here IMHO. Just the cylinder head temps.

On a recent tech inspection my twin car same color etc.. was placed on the lift. On removal of the tray quite a bit of oil was found laying on tray, and I think some even fell to the ground. It looked like it was leaking from the valve covers and maybe the filter. Could of been leaking for awhile. The owner was surpised to see the top of tray. If the tray had not been inplace he would off seen the oil leak a lot sooner.

It just seems weird that porsche would dump hot air ducts right under the engine - hot air rises! Do the cup cars have these ducts? If I designed the engine, I would have extended the ducts another 12" and dump the hot air aft the engine compartment near the mufflers....

Honestly, I dont even know where these ducts are coming from - oil cooler? - but there is hot air that is coming from there...

I have heard the noise law explanation, but honestly, I dont notice a difference. Im sure there is someone with a sound meter... And I dont have varioram.

Opening the deck lid would increase cooling effects of engine at rest for sure.

I wonder if cup cars do not have the tray just to make it easier on the pit crews. Not having the tray allows easy access to the engine and helps diagonse leaks etc faster (as the previous poster indicated). I would think long term reliability is not as much as a cup car - remember, cup cars are rearely sitting in rush hour traffic like a daily driver would be...

Hot air rises but your car also moves. I do not believe Cup Cars have these ducts as they do not have heat exchangers.
The sound difference is from the outside, not inside of the car.

I've removed the tray and installed a Cargraphic oil cooler. In addition to helping with thermal issues, removing the tray makes jacking the car ten times easier, for I, despite my workaholic tendencies, am basically lazy by nature.

Honestly, I'm not convinced it's that useful. It never feels very hot up top when you open the decklid, plus our decklids already have huge vents to let any hot air escape. I've seen few people bother to open their decklids. Leaving the key int he ignition with the fan running would do a lot, but will also drain your battery pretty quick...

There is an engineer in the Seattle area that instumented his 993 so that there were some facts about this subject. I will ping him to see if he has any data. I don't think he spends any time on Rennlist anymore but I will see if he is collected data yet.

Cheers

Maybe your right - I was assuming that the lid was restricting airflow.

My intention is not to be controversial, but I just did not want to take the standard line about noise laws... Ive never seen anything official from Porsche on this subject, but then again Ive only been in the porsche community for 1.5 years now...

Adding thermocouples to the heads as was suggested would be very interesting - that is something that I can do at my next track event and daily driving...

I have my rear Moton nitrogen reservoirs mounted above the engine and sometimes open the engine lid to reduce heat soak on really hot days. But even on the hottest days, the canisters stay cool immediately after zipping around on the track, a testament to all of the air whirling around there.

The way I see it is that if Bruce Anderson says to remove them, I listen. While you may not see a substantial difference every little bit helps. Heat is the enemy of the aircooled engine. My SC did not have it so why should my 993? Also, unlike the transmission tray I do not recall seeing any scoops to help ram cool air into the engine. They essentially serve no purpose other than a sound barrier. If you do not believe this then maybe you can explain what they are there for?????????

The two primary purposes as designed were to improve underbody aerodynamics and for the directed cooling of the engine and transmission. That is the only official engineering data I've ever seen, the designer (Michael Preiss) still works at PAG so theoretically we should be able to confirm that the swiss noise regulations reasoning is an old wives tale that originated in Panorama (I have the issue). Obviously the engineers at PAG would have taken noise abatement into consideration but if the swiss noise regulations were the only or primary reason for the undertray than PAG would have limited the part to Switzerland like they did with certain exhaust parts, the added costs are too high. They also probably would have used a different material.

same here... :)

http://www.imaging1.com/images/1_porsche_irmg.gif

wow.. nice heat photo!

Keep in mind my car has never seen a track day or auto-x and is my daily driver here in So Cal traffic.

I drove my car with the tray from August to December then had Mark @ Black Forest (no affil) remove it in January......I now see engine temps of approx 200 deg when with the tray, I saw approx 210-215 deg. That may not seem like a lot, but I'll take whatever reduction I can get without the expense or hassle of plumbing in another cooler.

And before anyone points out it's winter, I'd remind you that I live in San Diego, Ca where 50 degrees at night is cold.

I also believe that with the tray removed your lower valves will not heat soak as bad upon shut down due to the heat being trapped between the tray and headers/heat exchangers.
.
Ever see gears melt in a tranny on the street? I never have, but I've seen and had mine do so when my tranny cooler pump failed in The POC Enduro in 98'.
You cannot compare any temps when track driving with normal street use as track temps in the engine and tranny far exceed any normal street driving.

ZP44

Somebody needs to tell the Factory Techs that the Swiss noise regs cause for the under engine tray is a "wives tale". I have personally been told that by Factory Techs while in the US doing training. I suspect that is the same place that Bruce Anderson and the Tech Committee rep who was quoted in Panorama received their information.

Take it off, lower the engine temps, and reduce the incidence of leaking valve cover gaskets.

Is an oil cooler actually necessary? with a few de's a year and some around town driving in the north east would it make a difference in engine life? If so than what would be the difference? Mileage...examples... Any one?

Ryan :)

Good question - maybe for around town driving maybe not necessary. You already have 12 quarts of oil circulating in an air cooler. The fact that I have 12 spark plugs, 2 oil filters and 8 brake lines tells me that Porsche designed these cars to be bulletproof. I believe Porsche would have put a 2nd cooler in the cars if they felt it was necessary. (Track may be a different story...)

To answer one of the posts above - you dont see air scoops on the tray because it would interfere with the cooling from the fan in front of the alternator. The fan is pushing air forwards and your car is pushing air back. This was the reason why I posted the original question. Porsche could have reversed fan direction, used scoops from below and push air out the rear. But then again, it might interfere with the downforce and airflow on the spoiler.

Wow, I just had a thought - what it all you guys without a tray are killing your spoiler downforce cause now you are picking up airflow from below and pushing it out the rear of the car. The escaping hot air can potentially cause your alternator to fail, increasing the potentials for eddying to occur above the heads, and the warm air is less dense so now your spoiler is not providing sufficient downforce! :p

I guess you can argue this thing from many different angles and OF COURSE OVERANALYZE this....

http://www.imaging1.com/images/1_porsche_irmg.gif[/QUOTE]

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...

The car in that picture is a 964.

Im not familiar with the 964 - anyone want to comment on why there are 2 hot spots...

The image was taken of a 964 which is where the swiss noise regulation idea comes from and the patent I posted. The undertray has two small openings for directing air to the cylinder air deflectors (by the oil crossover tubes), there is also another opening at the very front where it meets the transmission undertray.

I'm not sure I understand.

Are you talking about the spots on the ground? The catalytic converter runs the width of the rear bumper and it might be the heat radiating from that, or the heat exchanger vents.

If Steve Weiner (head rennlist tech guru and master Porsche mechanic) says to take off the tray due to excessive engine heat and wear, then that is good enough for me. Add to the fact that not a single 993 or 964 race car made had this panel and it is an easy decision to take the tray off. A race team would gladly trade a few hours of engine life for extra downforce if it was possible.

I'm pretty sure the muffler on a 964 is in the rear of the car. I also think, but I may be wrong....that the muffler would emit heat???

So in theory....if the pic was of a 993 you would have 3 spots? cause I am pretty sure a 993 has two mufflers at the rear of the car, and both emit heat, right?

in regards to the engine tray being off, and the aerodynamics....I took my tray off for one purpose only. I was affraid with it on......that at speed...The rear of the car would lift....from the air travleing under the car....and then the rear would flip to where the front was.....without the tray on....the air has nothing to "grab" onto. :cool:

This is what Im going to do - I will drive my car, temp humidity etc will be logged. I will insure that I drive the same exact course same RPMs as close as I can. I will take my IR gun (accurate +/- 1 C) and measure different locations of the engine. I will do both an idle test as well as driving test with tray on and off. I will post the results/pictures of the test.

The only unforeseen difficultly will be to measure the block under the car with the tray on. I need line of sight for the IR gun to work. Maybe I can drop the tray and quickly - within 15 seconds, measure block temperature.

TJ, a couple of reasons you're wrong about the heat tray. 1) My own experience is that I saw an immediate reduction in temps sitting in traffic, where temps are highest, after removing the tray. The engine runs cooler at speed, and I did not noticed any difference in temps there, but I didn't have the car long before I took the tray off to get a feel for temps at speed. 2) You're misunderstanding the airflow. You said that air coming up under the car is pushing up, working against the fan. In fact it's not. The air under the car is flowing past the engine, creating a vacuum that pulls the air down. It's like blowing across a drinking straw and thereby drawing the liquid up the straw.

I'd be surprised to see you be able to use an IR gun to collect meaningful cylinder head temperatures given the crowded conditions inside the car. You really need to thermocouple the engine in multiple places and run the car in a controlled experiment with a laptop as a data logger.

You also need to drive under various load and traffic conditions to draw useful data. All comparisons need to be done such that ambient temperatures are the same and driving conditions duplicated.

Given that performing a reliable and robust scientific test is daunting and the evidence to date overwhelmingly supports removal, mine is in the shed and has been there since I took possession of the car.

Hmm - its hard to say that you are correct. The venturi effect may not be occuring if there is enough turbulence. The flat panels for aerodynamics promote smooth airflow once they are off, all those sharp edges as well as airflow above perturb this flow.

Can someone with a wind tunnel put the car in there and tell us what is happening to the airflow please? :bigbye:

I have been driving around for a week as well as 1 track event and have not noticed any difference in oil temp - around 7:30-8 pm cruising and 9 pm if stuck in traffic. The track event with and without the tray was around 9:30 after my session. Only one time I remember that I was at 9:30 sitting in traffic on the 405. As soon as I started moving, needle would drop to 9 so I was not too worried - but I was still watching closely...

Marv:
Im wondering if anyone has data that you suggest? I would like to see data, not opinion.

Don't be surprised if you get run off the board when you post your results , ask Arjan B. :) (I realize its not very scientific but...)

At some races (like LeMans) the undertray was used and for others it wasn't.

Your last sentence is an understatement at best. No matter how we analyze this, I still know MY car runs cooler without the tray.

Summer's coming! you may want to think about opting for cooler than slicker..........................ZP44

I absolutely agree with you. I am a data driven guy.

However, lacking hard data I made a choice based on preponderance of evidence that seems to support a mild benefit.

On the plus side, at least we are not talking about oil additives! Whoops, I said too much! :icon501:

For the 993? The 996 has a ACO mandated underbody tray, but never seen that on a 993 that ran in IMSA under (supposedly the same rules)

Off like a prom dress. :bigbye:

Scott :burnout:

I may be off the wall here, but the 993 mechanic at High Performance House in Redwood City told me that the origin of the tray was was to prevent oil drips on the streets of Germany. I don't if this is b.s. but supposedly, people got fined $500 for oil drips on the streets. He said that Porsche tried to design the system so that if there were leaks, it would first drop onto the exhaust system and burn off. And, any leak overs would be caught by the tray...

Just thought I would throw that out there.

Any argument for it keeping the underside of the engine cleaner?

OK here are the 1st round results. I took Arjans initial 964 data and expanded it to 993. THIS DATA IS 993 WITHOUT ENGINE TRAY. I will post the second half of the results tomorrow night. I measured the temperatures at 3 locations - 1) Bottom of cylinder head, 2) engine block close to drain plug and 3) exhaust manifold (near the center where the 2 pipes come together). I used a type k thermocouple for the bottom of the cylinder head in addition to IR gun for all measurements. I found that the IR gun was more consistant (maybe due to averaging) than if I tried to move T/C around. Slight shifts in the end of the TC caused results to swing wildly. I did insure that the IR gun and TC readings were in agreement since part 2 of the tests with tray will only be with TC at cylinder heads.

I first idled the car, then took it on a drive and measured it the moment I came back home, I then monitored during cool down. Unfortunately, due to test setup problems I could not read temps while the car was actually driving (could not see DMM).

No tray - Outside air temp was 20C, humidity ~90%
Time;T/C Head;I/R Head;I/R Block;I/R exhaust manifold
car off (ambient):
0min; 020C; 021C; 021C; 021C

immediately start car idle and read at 12 and 24 min:
12min; 098C; --- C; 100C; 200C
24min; 126C; 156C; 112C; 230C

drive car on desolate road for 30 min, keep in 2nd and 3rd gear, maintain 3k RPMs, come home:
53min; 125C; 130C; 115C; 300C

shut car off, cool with deck lid closed, read at 63 & 73 min:
63min; 114C; 114C; 105C; 146C
73min; 106C; 108C; 095C; 095C

Take a look at the temp readings of the head at 24 min. There is 30C disagreement between the TC and IR gun. My first thought was that the gun, must have read inadvertently caught an exhaust piece. After reviewing arjans data on 964, this may be a correct reading and the TC may be incorrect. I jammed the TC up between the cooling fins so there is no way to tell if the end is actually touching the block or not. The low reading of the TC may be due to the end not actually touching the engine - a mm above... If I realized there was this much diagreement between the gun and the TC, I would have retested that moment... I tried taping the TC in place with Kapton, by the cosmoline and oil would not allow a good bond.

Interesting to see how the engine temps (with the exception of the exhaust) stay pretty consistant whether you are idling or cruising the car. the airflow definitely keeps temps in control.

I copied the post from Arjans 964 data - pretty good agreement with the 993 -
~~~~~~~~~~~~~~~~~~~~``
his morning I did a test with a so called 'heat gun'.
Here are the data, I hope we can discuss it.

Outside temp. 25 degr. Celc.
20 km road use
Thermostat opened at normal level/temp.

Temp. measured at Block, cilinder, oil tank.

Measuring directly after trip,

Oil tank: 80 degr. Celc.
Engine block, near oil drain stop: 98 Degr. Celc.
Cilinder head [the first best accesable: 125 degr. Celc.
Valve head covers: 100 degr. Celc.

After running the engine in idle for 1/2 hour.

Oil tank : 105 degr. Celc./ 95 degr. Celc. after fan stops running.
Engine block: 130 dgr. Celc.
Cilinder Head: 150 degr. Celc.
Valve head covers: 125 degr. Celc.

Jason, That was very cool to post the patent data. That is also the first IR photo I have seen on a 911. Thanks

Sonic, Here in New England they throw a lot of sand on the roads every winter. I leave my undertray on until the sand is picked up in the spring. I think it helps to keep the engine cleaner. I remove it for the summer months of July and August.

Hey Scott...you are definitely not going to heaven!!! :roflmao:


My instinct says the engine cools faster without the cover. I live in South Florida and keep my cover off...Miami's latitude is 25 degrees north of the Equador and my cover removal is due to this fact...probably if I lived 35 degrees north and up, I'd leave the cover there!

My cover is off because it makes jacking up the car onto 4 jackstands ~much~ easier.

Interim report - guys with "no tray" position for cooler heads are in trouble...
 

As I said, I will do a more thorough job tonight, but driving to work this morning (30 min commute) oil needle at ~8:30. I took a few more TC measurements WITH THE TRAY ON.

Here is what I found:
30 min drive, 12C ambient, ~89% humidity, oil guage at 8:30
measure immediately after stopping, TC head reading 120C
After 10 minute cool down, TC head reading 109C
After 20 minute cool down, TC head reading 101C

The readings with the tray in place are slightly lower than last night - makes sense, oil temp and ambient air was cooler.

Looks like the cylinder head temp diff after 20 min.of cooling is less than 5C. Additional "valve guide wear" due to thermal soak with the tray on?? sounds like a bunch of rubbish...

Guys with data please convince me otherwise. Of course, I will post the data tonight regardless of the results (even though its probably obvious to you where my position is).

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.

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? :confused:

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.

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 arse?) :roflmao: He might be right, but Id like to see his data....

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...

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!

I think that is a 964 in the photo.

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.

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...

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.

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 :rockon:

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! :thumbsup:

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

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.

http://forums.rennlist.com/upload/08..._bypass_13.jpg


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.

[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.

Any reduction in temperature is more than made up by all the hot air being generated here.

ps mine's off

Final results!
 

OK - I repeated last nights driving test with the engine tray on. For easy comparison, I relisted the results. Fortunately, ambient conditions are exactly the same as yesterday (thank God I live in San Diego!)

Time / Tray On / Tray Off
00min / 21C / 21C
Start car and idle
12min / 106C / 98C
24min / 140C / 126-156?C
Drive car for 30 minutes, stop, immediately take measurements
54min / 126C / 125C
Stop car and cool down
64min / 117C / 114C
74min / 109C / 106C

After reviewing the results, I wish that I had a better 24min idle temp measurement.

1) It appears that the idle temps were higher (~8C) with the tray installed. Hard to say what is happening at 24min of idle, I think the TC may have been faulty- no tray to keep it positioned on the block and slight movements caused wild temp swings.
2) While driving the car, no difference of temp at the head. Both cases, much cooler than extended idle.
3) Cool down with the tray in place caused head temps to be higher by 3C.

I guess Im eating some crow - removing the tray keep things slightly cooler in idle and cool down.

However, I still have my dignity because the tray does not cause hotter head temps while driving (where I spend 95% of my time). Also, the idle differences measured only differ by a maximum of 8C - about the temp difference between your monitor screen and the room.... It would be hard to convince me that this is responsible for added wear of the valve guides, but maybe Im completely wrong... Leaving the tray on maintains under car aerodynamics and allows airflow around the 993 engine and tranny as Porsche intended by the aforementioned patent cited above.

The take away for me is that there is negligible differences with head temps whether the tray is on or off, but what really generates heat is extended idle. Even without the tray, head temp climb 20-30C higher than if we cruise at 45 mph. This is illustrated by the immediate drop in oil temp just by driving like a grandma after extended idle.

BTW, the 24 min of idle cooresponded to oil needle at ~9pm. I have driven to 9:30 tracking the car and sitting in 405 traffic. I would shut down the motor, or stop it with fan still on, if I got to 10 pm. Keep in mind that you dont hit the red until 11pm. Maybe the motor can get much hotter than we give it credit.

Thanks for the 'test' result.. very nice to see :)

Um... do you have it backwards? Your Tray Off temps are higher. Should your columns be
Time / Tray On / Tray Off?

Oops - your right. the columns were swapped. I edited the original post so that the columns read Time / Tray On / Tray Off. Thanks for catching the typo.

Mines done 155k miles with the tray on without engine being opened. I'm leaving mine on ! Results very interesting. One positive for tray being on must be quicker warm up which can only be good for the engine especially in moderate climates.

Good job on the data - Great to know

Scott

Thanks for posting all this, it's very interesting. It actually makes me consider reinstalling my cover for DE events in order to get the underbody protection and aero benefits. I say this because there seems to be negligible difference when driving; the car only runs warmer at idle, which is not much of an issue at DE events. The cool-down curve is a little extended but that's largely irrelevant.

Thanks - Im glad to help. What I find interesting is that the data I collected with and without tray is very consistent with the 964 data collected by Arjans. My understanding is that the 964 engine is different than the 993, yet head temps are very consistent. Arjans conclusion was that heads dont get any hotter with the tray installed.

I also forgot to mention that my original concerns of the heat exchanger outputs right under the engine do not seem to add any more heat to the motor. I measured the output of the exchanger outputs and it was much cooler (70C) than the heads. I am thinking now that the reason porsche vented exchanger outputs to the exterior of the tray was to promote venturi effect that was mentioned above. Maybe they get better flow thru the heater system...

I encourage others to investigate this - the more data the better. It would be great if someone can figure out how to instrument and log data while the car is driving. Maybe some can resurrect old data collected or contact the patent holder Preiss at Porsche AG to comment....

Great effort, tj! I've been reading this thread with interest.

Actually, it would be pretty easy for those of us who use data loggers. There's already an intake air temp that's easy to tap into and log. Unfortunately, there's no cylinder head temp sensor (AFAIK) so that's the tough one.

EDIT: I think actually we have cylinder head temp sensors...see below.

The challenge with the TC was that there are coolling fins around the head. I could not see the end of the tip and where it was positioned on the head. Between the oil and cosmoline, kapton tape was useless (even after cleaning with alcohol). Maybe cementing it to the cylinder would make the measurement more accurate (conduction vs convection) but I doubt few of us would sign up for that experiment. My TC wire was too short so I had to drive with out the DMM which logs max and min temps. Not quite a logger, but min temps would give you an idea on how cool you can get the head when driving...

I have already been accused of being too anal about this (which I dont refute), so I will let another person on the board continue the experiment.

Ok, actually I think our cars monitor cylinder head temps already, but I'm having a hard time figuring out what Porsche calls it. I think pin 45, labeled "ECTS II" and also referred to as "engine temperature" must be it. If so, it would be easy to log intake air temp and cylinder temp to do this study. Can anyone confirm what "ECTS II" is?

I tried to locate the older ACO regulations from 91-96 but the web archives only go back to 98-99 which was after the Mercedes flips when the aerodynamic regulations were changed? The IMSA & ACO rules weren't "harmonized" till after Don Panoz got involved after 2000, I'm also looking for the BPR rules. One (not-so-surprising) thing I found, some people had commented that the english translation of the french ACO regulations were not always exactly the same so the french based teams theoretically had an advantage. :)

Reflection from the sun...

The differences are so minor to be negligible.

With a Bosch Hammer or PST2 you can drive around watching the output from the CHT sensor on the #3 cylinder. My results (as posted last year) were pretty much the same. The temperatures were between 228F - 300F and the oil temperature stabilized ~194F, regardless of ambient air temperature. At a stop light (and at idle) the CHT would initially drop to ~238F and then creep upward. Knock-sensor activity was non-existant.

How-to Add a Cylinder Head Temperature Gauge or use the slightly less accurate Bosch Hammer method.

TJ, thanks for the effort, makes me step back and think about this again. Assuming that temp is not as big a difference as once thought, the ability to detect leaks vs losing aerodynamic benefit/purpose of tray according to patent is now bothering me. Probably will go with it again and just watch leaking when doing oil changes, etc.

Also want to commend you on ability to take on topic that I sure was going to end up in a flamewar and actually turned out to be civil and informative - good job all around.

rkass - Im glad that you and others found this useful - I certainly did. I started this because I did not want to knowingly shorten engine life by driving around with the tray.

You should also thank JasonAndreas who is obviously very knowledgeable on this topic. Im a schmuck with a DMM, he provided much background information - patents, race history - to refute the Swiss noise law and Cup Car reasonings that we keep hearing about.

Im glad that the partcipants on the forum create an environment to debate these sorts or things. We can all have a "virtual beer" and still be all friends even though we disagree.

rkass - Im glad that you and others found this useful - I certainly did. I started this because I did not want to knowingly shorten engine life by driving around with the tray.

You should also thank JasonAndreas who is obviously very knowledgeable and educated on this topic. Im a schmuck with a DMM, he provided much background information - patents, race history - to refute the Swiss noise law and Cup Car reasonings that we keep hearing about.

Im glad that the partcipants on the forum create an environment to debate these sorts or things. We can all have a "virtual beer" and still be all friends even though we disagree.

Great job on this thread. :bowdown:

Your findings are not inconsistant with I have been told by three different Porsche Mechanics I have known for years. Cars in So Cal that sit in traffic a lot should really remove the cover. The cars that are in their shops with worn valves (mainly exhaust) and collapsed lifters and CEL issues are almost always cars with the cover in place. So I think the conclusions you have drawn are accurate in that Porsche made the cover Aerodynamic and used it to clean up the bottom of the car and improve perfomance at speed (A 94 Issue of Excellence Magazine makes this claim as well). As an engineer I always try to get a design to do more than one thing if I can. That is why I would not be suprised if the cover has a "secondary" purpose related to sound regs in Europe.

Since I am one of those So Cal drivers I have removed it. I also like the accessabilty I have to the engine with it removed (I'm sure that is why racing teams remove it). Anyhow good job!!!

:cheers:

Thanks for this thread ... I too found it very thought-provoking

_________________________________________________________________
Does truth ever matter?
Usually, but often the path we travel in search of "truth" is quite interesting and well worth the effort.

As a newbie, i'm reading old threads and find this to be one of the most interesting I've come across.
Lottsa Science, some emotion, large doses of passion.
Most Excellent.

Maybe the trays should be manufactured out of a nearly perfectly conductve material (perhaps lead or gold) and left ON. :rolleyes:
That would have the bonus affect of lowering the center of gravity. :) :)

Thanks

Craig

Nice theory but as Steve W and others have pointed out it's sole purpose is to meet drive by noise regulations....that's all. Take it off if you want to extend the life of your engine.

oh no...

Please... no....


NO, NO, NO, NO!

I will paraphrase what I've been told since my last post in this thread.

1. Michael Preiss designed the undertray for aerodynamic improvements.
2. Other engineers in another department took the undertray and added the padding to comply with Swiss transmission/engine noise regulations.
3. The sealed underside allowed Porsche to offer the 10 year/long life guarantee because the sealed panels were supposed to reduce corrosion problems.
4. It was originally going to be fitted to the Swiss 964 but because of the expense and engine protection they decided to add it to all type 964, speading the cost out.
5. The primary function of the side panels is for shielding the lower ignition wires and connectors from heat radiated by the catalytic converter and mufflers. Padding was added for the same reason as mentioned above.

jasonAndreas - thanks again for your summary - I did not consider the thermal radiation of the mufflers to the ignition wires. The exhaust is the hottest part of the car - seems to make sense to shield it from rubber components (ignition wire etc). Also, Interesting to note that the cat is outside the tray in the rear - probably for the same reason...

Tray on or off - dont extend your idling and avoid traffic at all costs if you are concerned with engine life. Dont kid yourself that taking a tray off and idling around in traffic is "lengthening" the life of your motor. The temperatures in the engine climb dramatically - tray on or off - with no forced airflow from driving around.

Excellent thread. Txs tj90 for the time and thourough approach. I did a mid way on mine. Its clear that long idling is a bad idea on an air cooled engine. And its possible that the tray may increase heat somewhat. I open the spoiler in these conditions (odd that nobody mentioned the manual oil cooler fan override). I also use synthetic oil.
I keep my tray on for several reasons: aerodynamics, protection (I would really hate a bolt or stone damaging my block!) and neatness. If you hoist the car you have to admire the smooth underside of it. I also cut two intakes on the front lip of the tray in line with the exhausts manifolds/exchangers so as to direct underside rushing air into them. I reckon if these run cooler while in motion, I buy some extra time at idle by starting with a lower temp.
And as far as who 'won', I think we all did.

Txs again.

Still the best thread on this topic over a decade on......keeping my trays but minimizing idling.

Some of you guys are unbelievable, thanks for the years of knowledge.

Anyone remove the tray covering the trans? I have mine on, but may take it off this spring when I get the oil changed. I liked having it on when I drove in the winter- but haven't done that for 2 years.

The upside of tray:retention:

• Protects the oil return tubes and heater flap valve housings on the bottom of the engine from debris impact damage, my personal experience not using the tray
• Keeps minor oil leaks from messing up your garage floor
• Avoids setting piles of leaves on fire if you happen back over them, Personal experience w/o the tray
• May provide additional protection to the bottom of rear bumper cover from grounding as the tray probably will hit the ground first

The downside of tray:retention:

• Pain in the ass to remove and replace it for every oil changes and other engine servicing
• Pain in the ass when putting the car up on jack stands for rear wheel removal
• Common sense says removing the tray improves cooling, although this can be debated.
• Hides oil leaks

I got rid of mine and would like one back!
If anyone has a 4S 96 compatible tray, I'll take it.
Cheers.

Wow
TJ
your thread still lives.
TJ still has his aventurine green 95, but doesn't spend much time here on RL anymore.