Occam’s Racer

Those are good examples of convertibles vs fastbacks, and it surprises me that the fastback makes about no difference in top speed. I wonder if there are cars that go faster with the fastback, like maybe a Corvette or something? Anyway, it's great info to support the status quo and beat down the new guy, which is always a fun game.

For me, there were a a few good reasons to build a fastback. For one, I could sell my old hardtop and build a fastback for half the price. It was also less than half the weight. And I think it looks cool, which was maybe most of it.

Since we race with windows down, I also wanted to optimize airflow around the open windows. The stock hardtop is wider at the rear than the windows, and I'd imagine this acts as an air scoop, feeding more air into the cabin. My hardtop is very different in this regard, and I have some vents to extract cabin air as well. Does it work? I don' t know, the only evidence I have is my last race at WGI where the engine was running terribly. However, an e30 was clearly out accelerating me and walking away on corner exits, but every lap I'd catch him on the back straight and pass him going into the inner loop. I don't know if it was that his aero was terrible, but mine was clearly better. How much of that was the fastback is debatable, maybe a lot, maybe none.

I live 25 miles from WGI, and I have a factory hardtop on my street car, so I could do an A/B test and see what the difference is. There's a foot of snow on the ground, so not tomorrow, but maybe this summer I'll get around to it. It would be interesting to find out. I won't be happy to be proven wrong and start over, but the point of innovating is failing and improving.

To change the topic slightly, what are people doing to optimize aerodynamics when running with the windows open?

concealer404

iTrader: (3)

Sounds lighter than a CCP oem shape + oem trunk lid according to my scale.

The more people say this, the more i think i got a bad one. My CCP is the hottest of trash.

Yep. I got a bad one. Fitment is 2/10 would not bang if OEM is perfect 5/7.

#justbenthings

Guess i should consider the fastback if my lack of quality issue is an anomaly.

rrjwilson

Top speed is a function of gearing and engine RPM (as well as having as little drag to be not limited by that).
There will be (like ours with top down and top on/up) but they will be drag limited not power limited.
This may be sarcasm but it may not. I would not believe anyone is beating down the new guy stipulation of information and facts is hardly bad. This forum tends to be totally up front with accurate tested material which people often dislike but it is very rarely incorrect. That is why it is the place to come for advice for a miata.
Very impressed with your construction. Did you ever ponder the connection between your fastback and the softop edges being smoothed?
There are still quite a lot of discussions about this although they seem to cycle and not listen to posts mostly. The hardtop certainly scoops but its full effect may be less than thought. Vents will help but placement is paramount as the boot has high pressure areas resulting in reverse flow on the standard hardtop.
This would be a great piece of comparable information for the discussion and very welcome. I would suspect that fastback > hardtop > softtop > topdown. The difference between hardtop and softtop may be difficult to gauge but I'd believe the luffing from the canvas or mohair would increase drag. However the difference in top speed will be none from the softtop upwards as we are only slowed by drag (and lack of power) when the roof is down. What you should see is the time to peak will be shorter.

endura

-re. open windows racing, I've eliminated rear window completely. haven't done a apples to apples comparison yet. It was much faster in cold weather but I don't have a baseline from temps. that low.
-disagree on 1.6 miata under 100mph top down/128mph top up. (the top up part only)
-I agree fastback should have less drag, I just think it's not as much as everyone thinks/assumes.
-OP DIY fastback is fantastic, wish I had his skills/resources.
-all the speed figures are either from car and driver.or manufacturer. before, when the magazine was much thicker, they would do actual top speed runs on their own. I'm assuming their budget cuts killed that in the past 7-8 years.
-everything is drag limited. no governor/redline limits.
found the corvette.
-1998 C5 coupe Cd .29 171mph, conv. Cd .32, 167mph. (160mph top down, back when C&D budget permitted top speed testing.)
-2009 coupe 190mph. convertible 185mph.

rrjwilson

Thats like saying everything is drag limited when you drop it from 30,000ft. Correct but not helpful.
This is only true because you have removed the things manufacturers and tuners use to stop bad things happening. Specifications are there for a reason.

With a particular RPM limit and gearing you can only do a certain speed regardless of the output of the engine. You just get there quicker.
Once you get obnoxious power levels top speed will be limited by RPM and gearing only. This is likely why the R8 is exactly the same but the Mustang is not.
Emilio's Vegas in standard trim rather than aero would rip to the limiter without difficulty but a more subtle tune would (with the same gearing) just get there slower. Neither will be drag limited.

That is not to say reducing drag on a car that isn't drag limited is not a worthy thing to do. It is. Very worthwhile really as the air can be used more efficiently with other devices.

Whilst the speedometer is likely inaccurate on our car I have hit the limiter in 5th and the reading was just shy of 130 with top up.
The top down speed is a hilarious moment I my driving history having only done this back to back over the course of an abandon airfield "run what you brung" day.

If specifications for the engine and powertrain are the same this demonstrates that the convertible has more drag.
It doesn't demonstrate the coupe is drag limited though.

I'd love to see more information on the 2005 Mustang.
Assuming it has the same specifications the fact the softtop has higher top speed means it has less drag.
What amuses me is that means for some reason the coupe with all the smooth panel design is worse that the badly shaped canvas top for drag which is impressively bad given the funding supplied to manufacturers.

Occam’s Racer

I'm not sure what you mean by soft top edges. I don't have a soft top in either car, so I can't test that vs other tops.

rrjwilson

The edges of where the soft top used to be not covered by your main fastback area. I've highlight in green the locations i mean.

Soft top edges

jspeed.713

Something to keep in mind, drag differences is only one aspect to consider. Buffeting and inconsistent flow structures have great impacts on performance. Buffeting generates oscillating downforce which in turn increases tire contact patch load variation, which is typically a negative thing for performance.

endura

Confirmed best i could to compare cars with no governors and theyre not hitting redline in top gear/top speed. That, to me, leaves only aero drag. If (otherwise identucal) corvette coupe with ideal fastback and convertible with miataesque rear window slant/long rear deck have a 5mph difference at 190mph, what am i gaining on my 250 whp miata at a fast track where I'm hitting 130mph? 2-3 mph tops.? I agree that it will gain, just disagree that it's significant. The gains are probably much more significant with a proper wing /fastback combo. If the wing on a fastback can get clean air/lower, drag should go down significantly compared to hardtop with wing at roofline for clean air. Regardless, it's all somewhat educated guesswork without a wind tunnel or actual testing. Also, I'm going to build a fastback this winter as a-even if it's 1-2 mph gain, I'll take it, b-I love how a fastback miata looks.

Occam’s Racer

Ah, thanks for the illustration. Those ares are somewhat smoothed out, I used leftover plastic from my airdam. I think that area you highlighted might be a very important part of the drag reduction. Just look how much wider the soft top (and hard top) is at that green location! And the stock configuration has virtually no taper from that point rearward, it's very abrupt. The fastback is so narrow at this spot, and has the ideal 12 degree taper going back. If there isn't a benefit, I'll be very surprised.

Occam’s Racer

Looking forward to seeing what you come up with!

rrjwilson

Agreed but I think the reduction is due to reducing the separation of flow via smooth transition.
Maintenance of the corner between window and panel as you have done should allow air to feed along better than standard but I would be it will cause more separation than desired.
Smoothing of the corner into the fastback shape would likely cause less separation and reduce drag for those areas.
Downstream effects may be worse off but as far as the hardtop goes that would likely be better.

Occam’s Racer

jspeed.713

Lots of people spend a ton of resources and collaboration on big engine builds but ignore aero. Using the relationship between drag and hp required to over come it, Hp required=(Frontal Area^Coeff of Drag*.00256*MPH^3)/375, using this we can see that power needed is proportional to mph^3, so to increase top speed by 10% i.e. to 1.1 times its original top speed we would need 1.1^3 amount of hp to overcome the drag, I.e 1.33 original hp. To summarize it takes 33% more power to achieve 10% more top speed. This is a slippery slope and puts quite a bit of strain on powertrain reliability and increased vehicle consumables. The coeff of drag is not cubed, it is proportional to Hp required, reducing drag is effectively dynamic horsepower, power that doesn't show up on the dyno when classing your car in race series that use adjusted pwr/weight ratios. If you do the calcs for a car with frontal area of 20f^2, at 85mph (assumed avg track speed), and reduce coeff of drag by .05 it is worth ~4hp which is important in some race series, Assuming the engine on avg uses .6 lbs fuel/hp/hour we can relate this reduction in drag to an increased fuel economy of 2mpg, In a longer race this is critical, the fuel consumption relationship to vehicle weight distribution can have noticeable impacts on vehicle balance at grip thresholds, (over/understeer at the limit as weight distribution shifts foward assuming fuel cell is near rear axle) All these benefits with very little impact on powertrain reliability and rate of vehicle consumables.

Make Sense? Did I miss something?

shindul

Occam’s Racer

Let me share a couple online tools I use. I don't know how accurate this stuff is, but it's fun to mess around with.

First, this HP Wizard calculator allows you to play with different body shapes and estimate the total drag. I chose some values to represent a Miata and got Cd of .376, which is close enough to published figures of .38. I messed around with the shape of the top and got these values:

.376 Standard top, closed windows
.395 Fastback, open windows
.424 Standard top, open windows
.462 Convertible top down

So then I go to this other top speed calculator (scroll to the bottom and use 18 sq ft for the frontal area and 2400 lbs). Using the open-window drag figures, here's some interesting results.

To go 120 mph, the convertible needs 123 hp, the standard top needs 114 hp, and the fastback need 107 hp. You can reverse the whole process and find top speed. So let's say the same Miata with 123 hp, the calculated top speed of the convertible is 120 mph, the standard top is 123 mph, and the fastback is 126 mph. The standard top with closed windows is 128 mph.

These are big numbers for a Miata, so let's take a look at what happens at 90 mph. The convertible needs 55 hp to get there, while the standard top uses 51 hp, so it has another 4 hp available. And the fastback uses only 48 hp to go 90 mph, and so it has another 3 hp available over the standard top, and 7 over the convertible.

If you have a 300 hp Miata, then you may not care about single-digit hp. But if you're endurance racing a stock-ish Miata, you take all the free HP you can get. In addition, a 10% reduction in drag is about a 4% increase in fuel economy (at 70 mph), which can be a big deal if you're racing on a stock gas tank.

nigelt

iTrader: (10)

What do you all think about the difference between the OEM-style hardtop and the treasure coast chop top? With windows open, I'd imagine it wouldn't be all that different - lose some smoothness from the over the top air, but reduce the drag of the open window-cabin air interface.

Occam’s Racer

I bought two of those half tops because it was the same price to ship one or two. If nobody else has done this test, I can try open top vs hardtop vs half top next spring.

endura

https://s3.amazonaws.com/amv-prod-ca...pless-toys.pdf

From Aug. 2001 Car&Driver. Corvette and Boxster S are ungoverned. Top speed, top up/down.
Corvette 162/158
Boxster S 155/144

No thoughts, just more info for the pile.

moocow

I'd be careful generalizing a figure without understanding the assumptions that went into it. These are usually done on simple 2-D profiles for simplicity sake, whereas our cars are highly three dimensional.
This random paper shows that the notchback (45d) was only 10% lower Cd than the fastback (21d), .28 vs .31, and the squareback (no window, flat to the rear of the car) was 35% more draggy @ .38. In reality, the curved edges of the OEM glass would further reduce the difference. One of the big fastback advantages might be lower drag in corners.
As rrjwilson pointed out, the other benefits of the fastback are there, but i doubt the shape alone is as big a reduction as most of us think.