Hi,
I also think the idea of fender vents is a very good one. (I'm installing them on my TR4.)
They are not just for looks, since they will help the engine compartment stay cooler and supply the carbs with plenty of nice, fresh air.
There is quite a bit of air pressure trying to escape from an engine compartment, more when in motion. But even if stopped and idling, with the cooling fan operating, some means of escape for air should be useful. Anywhere along the flat area of the fender behind the wheel well is a low pressure area that should help pull air out, when the car is at speed.
Style is a matter of personal preference and taste, of course. On the Michelotti cars, I prefer something a bit cruder, more in keeping with what was used on various race cars of that era. On the Karman-body TR6, I think a little more modern and finished vent looks fine (Hey, it looks good in the tree, too!). IMHO, you should choose what you like. You can find a lot of photos out on the Internet, on Triumph and many other cars.
The usual "Shell Rally" triangular vents right behind the wheel well on TR4 aren't my personal favorite. I've looked at 6 or 7 other possible designs that I think can be physically be fitted to the car and have narrowed it down to something along the line of the TRS LeMans cars some sort of louvering. Louvers can be protruding or indented, vertical or angled "slashes".
I have definitely decided to use the TRS-style location, no matter what the final vent looks like. I like that it is farther rearward in the engine compartment, and a little higher up. I suspect that will be at least a little more efficient than the slightly lower, more forward "Shell" vents, where the battery/wiper motor/footwell shelves and firewall can still tend to trap some air. The difference is likely minimal, since there's got to be a lot of air swirling around under the hood when the car is in motion.
One thing to think about with the more rearward location is water drainage. The area between the inner and outer fender is already a bit of a problem, because of the drain leading in there from the scuttle vents. If using this location, some thought needs to be given to sealing the vents or providing good drainage. Drainage is less of a concern with the "Shell" style vents, although mud an muck thrown up by the tire might tend to get caught and trapped around the vent ducting.
At any rate, the removeable TR fenders make for a relatively easy installation of vents in either location.
Another approach, either instead of or along with fender vents, is louvering the hood. That can get a bit tricky. Too far back and the effect of the windshield will be to trap air, at speed. Also, hood vents might give a nasty surprise if there is ever a serious coolant or oil leak under the hood. There's a some chance that hood vents would direct any fluids right onto the windshield!
Further forward on the hood louvers might help air flow through the radiator, but would do little for air trapped further back. Also, I'd be a bit concerned about positioning louvers over the ignition system, that rain might get in there too easily and foul the electrics.
Have fun!
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