Shall i leave it off.What are the pros and cons?

 

It's a GREAT question. Pour yourself a tall one and search "undertray", and settle in for some entertaining readin'.

 

And when you're read all the threads in the 964 sub forum, you can read the ones in the 993 sub forum.

 

mine is off based on my trusty mechanic.

 

Yes, "undertray" and "oil" will lead you down some rabbit holes for sure.

 

Found this somewhere in the depths of an old thread. Not wanting to get sucked into something like I did in the oil thread, but fascinating from both points of view. Don't know if it helps you LuckyLondon but interesting none the less...




The yellow marked text translation is: the under engine reduces the rear lift force. Well, in Germany we would say rear axle lift force.
The title says measurements of achieved aerodynamic improvements, or aerodynamic resistance.
The numbers are the portion of each measurement in %.
- Rounded front section (front bumper) 22%
- Cooler output in front of the front tires 11%
- Rocker panels 4%
- New rear bumper form 7%
- Moved out front windshield 10%
- Reducing the width of the rain gutter on A-Column 16%
- Front under tray 6%
- Middle under tray 7%
- Rear under tray 15%
- Engine Under tray (reduces primarily the rear lift force) 2%

the 2nd page:
- The rearspoiler reduces the rear lift force to 0 in up position. it will go down be a speed below 10 km/h
- The complete new concept of both oil cooler and condensator reduce the drag or aerodynamic resistance by 3%

 

Good info (and translation) Unkle

 

lol

Don't shoot the messenger, i just thought it was interesting.


@Laker: Translation wasn't mine but from the original article

 

I agree, I think the entire graphic is interesting - I've never seen it before. Thanks for posting this.

Of course, I think the opposing viewpoint on this subject is based on a different factor: the detriments of a higher avg. engine temp., from the shield trapping engine heat in.

Move the lighter a little closer to the gasoline...

 

This should help to bust the myth that the tray increases engine temperatures. The low pressure region under the tray no only lowers aerodynamic lift, but also helps pull more air through the cooling circuit (follow the cooling air flow arrows on the drawing).

So for around town and low-speed autocross, the tray can safely come off for weight reduction. For highway and track speeds above 80 km/hr, there are both aerodynamic and engine cooling benefits from the tray.

 

That seems to sum it up. And I don't have to spend three days reading different threads.

 

Great info.Thanks

 

Interesting article Unkle! Good find.

How does it bust the myth that the tray doesn't increase temps? Nothing about temperature is mentioned in the article...I only see airflow measurements?

by 2% (good to know. That's way my car felt squirrely at 140 at VIR)

I understand how the engine tray could be designed to create a low pressure spot designed to pull air out of the engine bay...but air is already flowing through the engine bay from the engine grill and the engine fan is also pushing TONS of air through the compartment...

Tray on or off, I don't see any problems with engine heat...while the car is moving. I would worry about the heat when sitting in traffic or when the engine is shut off and sitting in a hot garage...

Fortunately there's a good margin of error built into theses cars - tray on or off.

 

To be honest I think it helps both sides of the argument

Tray off:
Possibly indicates that the undertray has a small effect on reducing lift but may have a larger effect on temp at slow speeds and heavy traffic with heat being unable to dissipate more easily.

Tray on:
Possibly shows the tray does contribute to the overall aero package, and the rear wing on its own may not reduce aero lift to 0 at the rear fully without it. It may also be safe to assume that at speed you could see a benefit from hot air removal without turbulent air hitting the engine at force and working against the fan.

I think this is all beneficial information depending on the location and use of the 964. Hot areas with town driving may consider it beneficial to remove and the rest may want to leave it on.



On a side note. I am interested in the hot air force out to the front wheels, this is a very turbulent area due to wheel rotation requiring winglets i understand (see below). and i wonder if this was thought of at the time, it seems to create a soft version of the forward winglet, or it was completely accidental. The problem with trying to create the DIY winglets for the rear is that if they are too big they create a pressure area forward of the wheel resulting in lift (not ideal). but a couple of example are below

If anyone works in CFD I would love to here their views


I can't find them now, but i had CFD pictures from Mercedes showing the cooling with an undertray on. Didn't say whether it was at speed, TBH it must have been as it indicated that it assisted in drawing the heat out due to air rushing past the outlets, instead of pushing air up into the engine bay without it, now that may seem like its best to keep the tray on, but these all have different high and low pressure points and a million variables including the engine in the front of the Merc.

 

During a visit to the Porsche Museum I copied this interesting display video of airflow over/under a 997....I'm sure a 964 is quite different, but it gives you somewhat of an idea....(airflow under that car at 16 seconds)
http://picasaweb.google.com/udoreisi...58158963647074
(sorry I can't embed the video)