John Miles

I understood that the underpan was specifically designed by Porsche to smooth airflow under the car and to create some ground effect thus reducing lift. As Paul Frere states "The good Cd of 0.32 achieved by the 964 series must be due to the credit of the smooth underpan....".



Why is everyone in such a rush to remove the underpan and thus remove the benefit of better handling (and fuel consumption)?

Whenever I hear of over-heating problems it usually always turns out to be a problem with the oil cooling system. I have never got my 964 to overheat even on the track (excluding discs of course!)

I heard somewhere that this tip was all the fault of yet another Bruce Anderson proclamation that turned into gospel. Perhaps this is why the habit is so common in N. America?

John
All undertrays and tin intact!

Richard H

Interesting....
I removed mine during the summer(!) and the engine definately ran slightly cooler. I have however refitted it for the winter to stop all the dirt e.t.c. covering the engine. Also as the car is mainly used for short journeys I think the tray will actually improve warm up time.

Richard 1990 C2 (Guards Red)

phil@tech9

John,

The undertrays in your picture should NOT be removed. These are very important as part of the cars aerodynamics.

The undertray that everyone talks about is the engine undertray, which is not depicted and I recommend that it is removed, especially in hot climates, track days, city traffic etc.

It is very common for owners to refit this tray over the winter, when temps. are cooler and to keep road salt and general grime off the underside of the engine.

John Miles

Picture not supposed to be illustrative. Just one I found.

Why do you feel that the engine undertray is not part of Porsche's aerodynamic intent?

John

phil@tech9

John,

The rear engine undertray is part of Porsche's aerodynamic intent. But reality is that it causes more harm than good, when a car is subjected to high engine bay temperatures.

Christer

Let me join in:
but if the temp gauge is still pointing to half way even after half an hour in a traffic jam on a warm, what's the point? I mean surely it's only harmful if the engine actually overheats. What's 15degC between friends? I suppose in a country/area where the climate is very hot it would be a thing to consider, but even then only when the temp gauge rises to the point where the oil cooler is running continuously.

Living in England where we have approx 2 months of the year above 25degC, I think I'll leave mine on for now.

Adrian, does your engine still have the nappy on?

Robert Coats

Steve @ Rennsport Systems told me the engine pan is there to reduce noise levels to comply with certain European nation's regulations.

I think we all agree there is little doubt the pan also contributes to the aerodynamics as well.

I have to also imagine the engine should, in theory, run a bit cooler with the pan off.

Bill Gregory

John,

There should be no question that Porsche engineered the undertray to aid aerodynamic affects on the 964. That's pretty well documented.

As Phil mentioned, the undertray is composed of 4 sections: One under the engine, one under the transmission, the long thin one, and one under the front suspension. When people take off their undertray, they should only be taking off the portion under the engine. There's no reason to remove any other portions of the undertray. The problem you're attempting to solve by removing the engine undertray is not an oil overheating problem, rather, cooling localized temps around the valves. I have had discussions with several mechanics who feel that for 964's on the track, that valve guide wear is a concern, and localized heat exasperates that.

So, I guess in the end, I would be hard pressed to see any disadvantages from removing (only) the engine undertray. Could it have some impact on lift at 150 mph? Maybe. Could it have some impact on fuel economy? I'd be surprised, but perhaps there is some impact to the right of the decimal. Could it lessen localized heat around the valve guides, potentially extending the time when a top-end or engine rebuild is required? Quite probably so. And for me at least, who is planning on keeping my 964 for a long time, this is a fair set of trade offs.

J-McDonald

I agree with Bill G. -- the tradeoff of a very slight (if any) decrease in aerodynamics and slight increase in noise (particularly noticeable when driving next to high curbs or walls that reflect the sound) verses an increase in engine longevity due to cooler running conditions is not even close.

Not mentioned in the above posts are two additional points that I've heard raised in previous discussions of this issue -- one is that the 964 engine was purposely designed to run hotter to clean up emissions and increase fuel economy, as compared to previous 911 motors -- which is all the more reason to remove the tray. Air-cooled engines need air to cool them. There's a reason that the 993's decklid air intake is larger than the 964's decklid air intake --cooler engines last longer and perform better.

The second reason is that the aerodynamic advantages of having the tray on are minimal, at best, as is evidenced by Porsche not putting the under engine trays on its factory race cars of the time period (i.e., euro 964 RS, Cup Car, etc.). Any aerodynamic effect would presumably be greatest at the higher operating speeds that these race cars were designed to run at (especially as compared to a street car like the "regular" 964), yet Porsche decided *not* to put the trays on the race cars.

The reason that the trays were put on 964s was to reduce drive-by noise, to comply with strict Swiss noise limitations. As was mentioned in Frere's book, these Swiss standards were so onerous that Porsche was also required to change the gearing in the 964's gearbox to lower engine revs at a given speed to reduce noise (I believe that 2nd, 3rd and 4th are numerically lower). As Frere pointed out, taller gearing in a mountainous country like Switzerland makes little sense, other than to reduce noise.

GeoC2cab

High,
Anybody try this trick?, I drilled Holes in mine, about 10 at the front of the tray so more air would flow into the engine compartment and I cut off about 4-6" of the part that covers the muffler.

Planter91C2

I was just thinking the same thing! I have mine off now but was thinking of modifying it with holes or louvers. I guess experimentation is the only way to figure this out.

I took mine off reluctantly. I believed Porche put it there for good reason.

Jim Michaels

Of course Porsche put the tray there for a reason; to catch drops of oil leaking from the engine.

GeoC2cab

High,
I'd rather have the drips on the tray than the garage floor anyway, I watch oil consumption and if there are drips through the pan then u know when somethings wrong, and removal and inspection during oil changes too!, no prob. yet, I think the modification worked, my car does run a touch cooler since I drilled and cut it.

Thom Fitzpatrick

I replaced mine with a K&N tray, although I've been thinking of getting a Mass Airflow Tray instead.

J richard

Mine runs cooler without the tray. I will say that If you compare a 964 to a earlier car the whole underpan is suprisingly clean and smooth, it's obvious they did spend more time on cleaning up the greasy side. However, I think Jim Michaels hit the nail on the head, I have never seen an engine underpan on any GT or other porsche 911 race car (save the GT1 if you call that 911 based)so it can't be aerodynamics. I'm putting pretty good money on the fact that Porsche would see a lot less warrantee and maintenance complaints with that diaper under the car. On the noise, I didn't notice any difference with it removed or installed. But then again, I'm not Swiss...

regards