A large air space is created between the intake air horn opening and the grill opening when you have installed a fix tail/spoiler system on a normally aspirated 993, after some testing I discovered that this added air space increases the air temperature of the air entering the intake, which in turn reduces the horse power of the car.  Obviously the difference is minor, but every little bit help, right?

Originally I didn't give much thoughts about the added air space between the intake opening and the grill of the tail after I installed a factory twin turbo tail to my NA 993, I figured that there was plenty of fresh cold air entering the engine bay area from the grill opening of the tail.  But after some road test with the OBD2 scanner/recorder I noticed that the IAT (Intake Air Temperature) was at least 10+ degrees hotter than the outside ambient air temperature and sometime even 20+ degrees hotter when the car is in traffic.  The recording results leads me to think that the intake is receiving hot air from the engine bay area that is why there was such a drastic temperature difference. 

So the search for the solution to lower the intake temperature started.  Shortly after acquiring the microfiche parts reader I noticed that there is an extra elbow tube (intake trumpet) that can be fitted over the standard air box cover that was originally made for the twin turbo version of the 993 car.  The reason for the added elbow tube (intake trumpet) for the twin turbo is strictly for the sake of cold air.  If the intake was sucking the air after the intercooler it would drastically reduce the performance of the engine.  So I ordered that intake (elbow) trumpet to be fitted on my own 993's air box for some experiment.

The Intake trumpet's part number is 993.110.360.51 cost is around $55.  It is basically a 90 degree plastic tube that can be fitted over a 993 intake air box cover horn opening after you remove the horn from the intake air box cover.  No "Mickey mouse" modification is required over the original air box cover.

After some back to back road test I found a 6 degree temperature difference between the two configurations.  Below is the OBD2 scanner/recorder screen shot.  (I did the original airbox test first and then the intake trumpet addition).  To quickly able to change over the air box cover I pulled off the clutch ventillation snorkel tube from the air box and placed some duct tape over the hole left open on the air box to prevent hot air from entering the intake from that opening.  I also had an aid of a spare air box cover during the testing to speed up the exchange process. 
 
 

Original air box

Intake trumpet added

Here is a picture of my engine bay with the intake (elbow) trumpet installed.  The opening of the intake trumpet extend the intake opening all the way up towards the grill of the tail, so now more cold air can enter the intake opening directly without first entering the engine bay area to mix with the warmer air that is in the engine bay area. 

The procedure to remove the original horn from the air box cover is fairly easy.  But it would require the use of a total of 4 flat screw drivers at the same time.  The flat screw driver will need to be inserted where the horn's tab meets the air box cover.  And once the tabs can be cleared you can just pull the horn forward to get it off the air box cover.

P.S. 

993 NA owners with the factory retractable tail will not to be concerned about this.  If you look closely at your grill in relation to the intake opening you will understand what I mean.  Also when the tail is extended out there is a plastic wall in the back of the tail that directs cold air to enter the intake opening.  This addition is mainly for 993 NA cars with a fix tail installed on their 993.   I do recommend this addition to be added to your 993 to ensure that you get as much cold air entering the intake as possible.  as a reference (For every 10 degree change in air temperature there is 1% change in power)