On the N/A Exige (no supercharger) the roof top scoop is plugged, but on the Exige S (supercharger) the scoop is open and air is forced in (scooped in by the forward motion of the car). The air then travels through the channel and across the intercooler.
With the roof on, the scoop has a larger profile and is situated closer to the front where one would assume the aerodynamic properties are such, that more air would enter the channel. This in turn, would provide a larger amount of air across the intercooler thus allowing more heat to be transferred out of the compressed hot intake air.
With the roof off, the cross section of the now open channel is smaller and situated just aft of the open cockpit area. Because of the smaller opening and turbulent air flow resulting from the open cabin, less air is likely to enter the channel and flow across the intercooler. Even with a soft top fitted, the air flow may be better (less turbulent), but you still have a smaller "scoop" that is situated further back.
To understand more about why you would need to cool the intake air in the first place, visit:
I also asked Lotus and got the following official reply - from the UK. As you can see they hate to admit you actually can drive it open...
Your email was forwarded onto me by my colleagues in England.
First of all congratulations on taking possession (or nearly taking
possession) of an Exige S, I had the opportunity to drive one for the first
time last week and they are absolutely fantastic fun to drive.
As far as your enquiry is concerned, it is indeed technically possible to
take the roof off of an Exige and drive without it, but what you can do and
what you should do are two very different issues.
The Exige requires the roof for optimum performance, and in effect the whole
body shape and contours contribute to aid the exceptional down force of the
vehicle, which is one of the main attributes of the Exige and Exige S.
Also, after you have removed and refitted the roof, you may find that there
could be some quite time consuming 'tweaking' involved to ensure that you do
not suffer from any water ingress, rattles and squeaks or wind noise,
obviously these are all items that are attended to at the time of the
vehicles production or at time of PDI.
Any Elise supplied with a hardtop also has a softtop which can be stored in
the luggage compartment of the trunk in the event that protection is
required either due to weather conditions or added security when the vehicle
is parked. Obviously the Exige does not come with a soft top and you cannot
store the hardtop in the rear whilst you are driving. With much fussing and
fettling it may be possible to 'retrofit' the hardware to accept a soft top,
but you still have to purchase a softtop and then start making holes in the
rear cover shroud of your new vehicle to make it fit. Obviously I cannot
condone this even though it may be technically possible.
I hope you do not find my comments too negative, because you are in for a
real treat when you start driving your Exige, but I have to make you aware
of the facts, also I have to state that removing the hard top does not void
the limited vehicle warranty, but Lotus Cars USA always reserves the right
not to accept claims for adjustments or repairs that can be proved to be
directly caused by unauthorized modifications. Although it is highly
unlikely to have warranty issues arise of any major consequence due to
removing the roof it is obviously my duty to make you aware of it so that
you can make any decisions about modifications based on all the facts and
the consequences that they may have.
Sorry to give you such a long answer to what was a very short, simple and
Lotus Cars USA, Inc.
2236 Northmont Parkway, Duluth, GA 30096
Phone (770) 476- 6564, Fax (770) 476-6541
Fun Fact. The part numbers are exactly the same for the current models as they are for the S2 Elise and Exige.It can come off the S3 as well...