Marv

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.

tj90

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?

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...

tj90

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

JasonAndreas

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.

ZombiePorsche44

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

Marv

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!

Greg Fishman

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)

Scott 1996 993c2

Off like a prom dress.

Scott

mtbryder

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.

Sonic dB

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

tj90

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.

Dudley

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.

Pedro356C

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


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!

InTheAir

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

tj90

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).