IMHO, on most ordinary production cars, spoilers are simply unnecessary as far as roadholding and speed are concerned. They are attractive fashion accessory ornaments and if race cars have them, then my road cars must have them ... sort of thing...
Over the years several of my cars where I have both spoiler and no-spoiler versions, the non-spoiler version has proved to be both more stable AND faster. In addition, all my MG Montegos had/have OE lip spoilers and they tend to allow much road filth to accumulate on the boot lid below the spoiler. The pattern of the accumulated road filth suggests the spoiler version is less efficient from drag aspects. The plain bootlid on the 1.6LX Montego used by myself and my wife for thirteen years did not have a lip spoiler and the bootlid always remained relatively clean and clear. This 1.6 car would cruise in excess of three figures when circumstances allowed with four adults on board and their luggage.
One final observation. My bog standard TomCat Turbo Coupe on one occasion had the needle right round the the dial and was still accelerating! The car was surprisingly very stable at that speed! On that one occasion, I had removed the OE Standard rear spoiler from the boot lid ... coincidence... I do not think so.
Is there any substantial evidence to show that bog standard spoilers on production cars are actually beneficial from a stability and performance viewpoint. I wonder how extensively manufacturers research and explore these things with wind tunnel use? Yes. I wonder if they even do that or simply bolt them on purely from a styling and appearance sales tool viewpoint.
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