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Air flow question
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Cusser
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PostPosted: Sun Oct 26, 2014 11:02 am    Post subject: Air flow question Reply with quote

I've run 1835cc DP engine with external cooler and hood standoffs since way back in 1976 in Arizona, have never had any cooling issues, going across desert in mid-day, mid-summer, etc.

About 6 years ago I installed the Yesterday, drove 30 miles to Volkstock with top down in 90F temperatures, noticed oil temperature (drain plug sensor) read about 30F higher driving at 75-82mph than at steady 70mph. After returning home (noticed same, 94F temperature), realized that I had never driven that engine that far in warm temperatures with the top down.

So I was wondering if having the top down (covered by boot) disrupts the air flow enough that I noticed higher than expected temperatures. It seems to me that this would create a diversion of air so doesn't get rammed in with the hood standoffs. Anyone KNOW?
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Brian
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PostPosted: Sun Oct 26, 2014 11:22 am    Post subject: Reply with quote

I don't know. But it seems like there is no direct flow as opposed to a hard top.
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gt1953
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PostPosted: Sun Oct 26, 2014 11:37 am    Post subject: Reply with quote

Looking at your car yesterday, I would have to say the top down looks like a wing on the back that would deflect the air up and away from the deck lid intake. With go out today and try it with top up, I bet the engine oil will be cooler.
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Cusser
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PostPosted: Sun Oct 26, 2014 12:23 pm    Post subject: Reply with quote

gt1953 wrote:
Looking at your car yesterday, I would have to say the top down looks like a wing on the back that would deflect the air up and away from the deck lid intake.


That's what I was thinkin'....
This is from the gallery, not mine (wish it was !!!), but shows the issue I'm describing
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gt1953 wrote:
go out today and try it with top up, I bet the engine oil will be cooler.


I'm going to the Steelers bar today - not far away enough to make a true test - need to do extended drive at 75-80mph with top up and 90F day.
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PostPosted: Sun Oct 26, 2014 8:03 pm    Post subject: Reply with quote

The top does a lot for the aerodynamics. Although I didn't have a temp gauge to monitor the temps, I used to drive from SF to LA quite often in my '79 convertible. Sometimes with and sometimes without the top up. What I noticed was how much the gas mileage suffered. Top up and I could cruise at around 28mpg, with the top down I'd be lucky to break 20mpg.

-jeffrey
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Aussiebug
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PostPosted: Mon Oct 27, 2014 2:16 am    Post subject: Reply with quote

Interesting observation Cusser.

The airflow over the top of a hardtop sedan curves fairly smoothly down the back of the car, and so there is good airflow past the cooling vents under the window, and in the engine lid or through the slot created by standoff hinges. In fact if you look at the car side on, the shape is similar to a fat wing (curved on top, flat or almost so underneath). for it's day, the bug had quite a low coefficient of friction - about 0.33 (drag only 0.33 that of a flat plate of the same frontal area). Many cars of 30s/40s design were closer to 0.5.

With the top folded back as in the pics above, there would be a general disruption of airflow over the top of the open car (as there is with any convertible), and around the folded roof - it would act something like a deflector sending air up and away from the air inlets for the engine. The airflow into the engine venting would only come from the sides - not from over the top of the car.

I bet if you have a sensitive pressure gauge mounted near the air vents for the engine you'd see a reduced pressure with the top folded like that, so the fan would have a harder time pulling in cooling air. And it needs a LOT or air - the doghouse fan uses 25cf/sec or 42,500 litres per minute.

If you look at the whale-tail on the back of many Porsches, you'll see a similar shape to the roof-line of the folded bug roof, but those are mounted under/behind the engine air vents - not above them, so the effect there is to increase the air pressure at the engine venting for better airflow into the engine area.

The speed you travel will have a lot to do with it too. It takes about 25hp for a bug to cruise at 60mph, but it takes another 40hp to get it up to just over 80mph. Since rolling resistance is linear, the difference is in drag, which increases as a square of the speed - double the speed and you quadruple the air resistance.

So I would expect that if you did a test at say 50mph, you'd note only a tiny difference in engine temps with the top up/down, but at 80 the difference would be much larger.

I used to fly aircraft, and aerodynamics is still an interest of mine.
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Cusser
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PostPosted: Mon Oct 27, 2014 6:44 am    Post subject: Reply with quote

Yeah, when I dropped down to 70 mph, would cool the oil from "reading" of 280F at the drain plug to 220F at the drain plug (before it goes through the oil cooler. So I definitely know it's speed-related, but first time I've gone extended time (40 minutes) at high speed in it with top down, if I recall correctly.

I say "read" because it is a VDO sender and a Motometer gauge from the 1970s, so not sure that the ohms resistance is the same, just use it to note changes.
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VolkswagenVMan
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PostPosted: Mon Oct 27, 2014 10:20 am    Post subject: Reply with quote

Yes it does, My friend has the same issue when he has the sunroof open. The sunroof disrupts the airflow over the car. I have also heard this with bay window buses. VW moved the air intake up on the bay windows and if you have the pop out windows open it disrupts the airflow that way. So yes it does!
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PostPosted: Mon Oct 27, 2014 12:29 pm    Post subject: Reply with quote

Great thread, great observation. I certainly believe putting the top down disrupts the smooth laminar airflow over the car "body". The down top seems to act as an engine stator.- redirecting airflow. Also, as mentioned above, it also increases fuel consumption- additional parasitic drag with less cooling and engine working harder.

I will often lift the lid- when really hot and top down-in parking lot.

Keith
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Mr.Duncan
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PostPosted: Mon Oct 27, 2014 1:11 pm    Post subject: Reply with quote

You can see in this STD hard top beetle a vacuum/drag forming right where the decklid slots WOULD be.

I assume having the convertible top DOWN (folded in the boot) makes this vacuum/drag even more.

Causing the engine fan to work harder to suck air in.

Assuming.. as im no expert.


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For fun, here is a toy car...


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Cusser
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PostPosted: Mon Oct 27, 2014 3:21 pm    Post subject: Reply with quote

I think my question has been answered, and more, so thanks all Yes, the top down will disrupt the flow of cooling air, and such disruption will also negatively affect mileage, and maybe top speed !!!

I just never had seen this question addressed before, and not common for me to run at such fast speeds on highway in relatively warm temperatures.

Here in Arizona, "hot" starts when it reaches like 108F.
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gt1953
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PostPosted: Mon Oct 27, 2014 5:09 pm    Post subject: Reply with quote

Here in Arizona, "hot" starts when it reaches like 108F. That is for the flatlanders. In the mountains it is 86 degrees F.
I did like the wind tunnel link.
Simple answer is slow down.
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Her74buggy
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PostPosted: Mon Oct 27, 2014 5:44 pm    Post subject: Reply with quote

Top flow is understandable and you really starve the vents if it's creating a vacuum. I wonder what the airflow is from the sides about door handle level and if it's wrapping around to the vents possibly breaking up the dead space. I guess the vw engineers figured it sufficient but maybe 95 degrees and 80 mph was outside their theory of normal conditions😋
Cool post though, thanks for bringing it up and all the input. Very interesting stuff.
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PostPosted: Tue Oct 28, 2014 10:13 am    Post subject: Reply with quote

There could be airflow issues, but I think it can't be that detrimental. I have a 1915 engine and drive 70miles doing 80mph with the top down. I haven't even come close to having any cooling issues.
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Cusser
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PostPosted: Tue Oct 28, 2014 11:37 am    Post subject: Reply with quote

Yehan73 wrote:
There could be airflow issues, but I think it can't be that detrimental. I have a 1915 engine and drive 70miles doing 80mph with the top down. I haven't even come close to having any cooling issues.


I didn't say that I was having cooling issues, just that the oil temperature reads higher with the top down, and reads significantly higher at 75-80 than at 70 mph.

And one cannot compare Colorado temperatures to Arizona desert regions.....
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PostPosted: Wed Oct 29, 2014 4:55 am    Post subject: Reply with quote

Cusser wrote:
I didn't say that I was having cooling issues, just that the oil temperature reads higher with the top down, and reads significantly higher at 75-80 than at 70 mph.


You do know that at 80mph your engine is doing a lot more work than at 70mph? I don't know what the coefficient of drag and crosseccional area are for a converitble Bug is but the math is the same, power requirements increase by the cube of the velocity.
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