Interesting thing I discovered because it has been fifteen years since I had an RX-7 that I could drive over 70, and the speed limits were a lot lower then.
The vehicle is an early FB RX-7. The thing I noticed is that the car takes noticeably less power to drive at 80 than at 75. Like, I cross a threshold and it feels like the car is getting sucked along down the road and I only need a feather touch on the throttle to maintain speed. Exhaust gets noticeably quieter, etc. I just did a trip to Columbus and back and fuel stop to fuel stop I did nearly 30mpg driving this way.
Driving at 65mph or so requires dipping almost into the secondaries. Fuel economy is closer to 25mpg.
It's not an engine thing, because I tried keeping it in fourth gear and running around that critical engine speed and didn't feel that sensation of the engine getting a lot less loaded.
Could there be some weirdness where there are vortices or something at certain speeds that disappear at higher speeds?
The only other trivia is that I have louvers over the hatch, and if I have the sunroof cracked open the louvers rattle agaist the glass at 70, closing the sunroof makes it stop.
More likely it was an outside variable like a slight downhill grade or a tail wind.
I initially said that cD is constant but it does vary with speed.
It sounds like it's stupid but it's really not. I don't know, but I'm curious now.
tuna55
MegaDork
9/13/22 12:46 p.m.
I saw something with data with an old Civic. Tunawife had a 98 sedan with a five speed, and we took long drives often. I tried it once, and it got better fuel economy on the same 1000 mile stretch at 80 mph than it did at 70 mph. Since the thing made no power anywhere, but definitely slightly more in the upper revs, I attributed it to the ease with which it climbed the mountains on the eastern seaboard, and through Pennsylvania. It could have been aero, but it never occured to me as I was taught that cD was constant.
jmabarone said:
More likely it was an outside variable like a slight downhill grade or a tail wind.
I initially said that cD is constant but it does vary with speed.
Tail wind or grade doesn't affect it. I noted that it took less throttle to go uphill at 80 than flat ground at 70. You could feel the effect even when going uphill as you accelerated past the speed. (There is no appreciable flat ground between Cleveland and Columbus, it is all gentle grades up and down)
It is very curious.
I realize that while I checked for any kind of engine speed related anomaly, I did not try driving at 75 to 80 in 4th to see if I could feel the same effect. Hmm.
what's the cooling setup on a FB RX-7... or this FB RX-7 in particular?
crank-pulley fan? electric fans?
It's because you you're planing at 80 mph :)
Seriously, though, I can see how various deflected airflows could interact with the body at different points as your speed changes - or, if not at different points, then certainly with different levels of energy that might energize something else. I'm trying to figure out if the high pressure zones would move and I'm not sure.
I wonder if some flow somewhere detaches more cleanly at some particular speed instead of getting sucked down into turbulent but mostly adhered flow?
I don't know jack, just applying a recent review item from a Superfast Matt video...
Keith Tanner said:
It's because you you're planing at 80 mph :)
Seriously, though, I can see how various deflected airflows could interact with the body at different points as your speed changes - or, if not at different points, then certainly with different levels of energy that might energize something else. I'm trying to figure out if the high pressure zones would move and I'm not sure.
That's what I was thinking however I'm inclined to think it's happening on the underbelly of the car.
Jesse Ransom said:
I wonder if some flow somewhere detaches more cleanly at some particular speed instead of getting sucked down into turbulent but mostly adhered flow?
My guess, is that it's maybe a combination of a couple things...
1) it could be that the cooling system is getting enough flow at 80mph that the fan shuts off... or the fan helps evacuate the flow out on the car in some advantageous way.
2) the rear louvers below 70mph are likely creating extra drag by the flow attaching and re-attaching on each louver (or some other turbulent flow mechanism... potentially multiple different turbulent flow states as the speed changes between 30mph and 60mph); but that at ~80mph the flow is detaching 'clean' where it can't re-attach to the louvers below it, resulting in lower drag.
it'd be easy to test the 2) theory by removing the louvers, and seeing if the reduced drag effect is still there at 80mph
Technically yes, like others are saying I'd wager theres something going on where a change in the flow characteristics at that speed suddenly and sharply reduces flow separation or causes it to do so in a way that reduces drag. In fluid dynamics there is a thing call Reynolds number which is used to characterize fluid flows and its proportional to the fluid density and velocity (among other things). You can kind of think of it as air moving straight 1000 times faster than water moving the same way will show the same flow characteristics. This means air moving at different speeds are technically different fluids which will change Cd but at the speeds cars operate at the change in theory isn't going to be very drastic, its more of a thing as you start to approach trans/supersonic speeds (IE planes). But it does change, plenty of papers/data out there on this and Cd is calculated/measure by measuring drag forces so if you are actually feeling a sharp reduction in drag, you would see that on a plot of Cd vs Re for your car.
I'd be interested to see a video of some tufts taped to the roof/hatch of your car and either have some one follow you while you hit that transitioning speed or see if you can attach a camera that somehow wont disturb the flow too much.
That goes beyond idle curiosity, though 
Testing in 4th gear proved pointless because at that engine speed the carb runs stumblingly lean on the primaries. It's a Nikki 4bbl thing.
The idea that it could be related to the louvers is plausible, this is my first car with them.
An aside: On my first one, there was a little spring loaded air deflector for the sunroof hole. Over 85mph the wind resistance would push it down and it made a noticeable difference when that happened. (I found the car's top speed or close to it most days. I grew up since then) The other interesting thing is that car generated a lot more underhood pressure than the later chassis that did not have the same nose/air dam shape. Pop the hood on it and it would hover 4" away from the latch. The later cars don't do that...
Our Odyssey gets better mileage at 75 MPH than 70 MPH. I chalked it up to the engine being in an RPM range where it made more power (and was likely more efficient). Plus it rarely downshifts on hills at 75 MPH, compared to downshifting all the time at 70 MPH. In your case it sounds like something else is going on from an aero perspective.
