Keep in mind that is with a windshield. They had a similar discussion on the
SCCA production car forum. This is what was said
"1.) An open car with no windscreen and a fairing and/or tonneau to smooth the
air across the cockpit is the fastest of the bunch.
2.) a closed car with a well-designed rear window area (like a fastback or a
little lip wing) is second fastest.
3.) an open car with a full-height windscreen and an open cockpit is at a
disadvantage as is a closed car with a poorly designed rear window (think of a
510)
The reasons are that the back side of the windscreen in an open car becomes a
low pressure area. Air rushing over the car is sucked down into the cockpit
creating turbulence and drag. The same is true of the poorly designed closed
car - it's just that the drag is above the trunk. A Fastback helps close the
airstream over the car and creates less turbulence. The open car with no
windshield (just a fairing or tonneau) has the least frontal area and no place
for turbulence or drag to slow the car except behind the rear bumper. I hope
that helps. If I were driving a Spec Miata, I'd want the top on. but if I were
driving an EP/FP Miata with no upright windshield, It would be a roadster.
-Basil"
I hope that helps.
-Bob Adams
emanteno@comcast.net wrote:
-------------- Original message from Larry Young --------------
> While we're on this subject, does anyone know whether a hardtop would reduce
>drag on a TR3 or TR4?
While this isn't racing data, I did check fuel economy with the top up and down
on a road trip to a VTR Convention in my TR6 . With it up, the mileage went up
4 mpg. I attributed that to better aerodynamics with the top up, but that's
just an uneducated guess on my part.
I was also able to observe that a TR4 will lose 500 rpm on the straight at Road
America if the trunk lid comes up and stays up, but that's another story.
:-)
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