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Date:         Sat, 4 Sep 2004 11:48:56 -0700
Reply-To:     Alistair Bell <albell@UVIC.CA>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Alistair Bell <albell@UVIC.CA>
Subject:      vanagon top speed - more info
Content-Type: text/plain; charset=US-ASCII; format=flowed

some list members have thougth that my extrapolation of Martin Jagersand's drag measurements is in error. I did state that it was a rough estimate, and was more of an illustration of increase power required to move our big boxes at speeds near or over 100 mph (160 kph).

So I re-examined the approach...

first the "original" data:

Martin's data:

Converted into Imperial units:

Speed (mph) Total drag (hp)

33.67 -10.72 44.89 -26.41 56.11 -46.59 67.33 -71.25

78.55 -102.59 89.78 -138.00 101.00 -180.49 112.22 -225.29

And now the new approach:

The SA magazine "Car" tested the SA 5 cyl passenger Vanagon back in '91. That van had an engine output of 100 kW (134 hp) and 200 Nm (147.5 lb ft).

Their tested top speed was 159 km/h (99 mph).

The "theoretical" top speed, in 5th at 6250 rpm was calculated to be 188 km/h (117 mph).

From an old Road and Track test of the 1980 Vanagon I found a Cd figure of 0.44, and from the above article I found the frontal area of the SA van measured as 3.59 m2 (38.6 ft2). Using a weight approximation of 1720 kg (3792 lbs) I used the calculator on this webpage:

http://www.davewin.com/tech/horsepower_calc.shtml

to arrive at these results:

speed (mph) power required (hp)

50 19.3 60 30.6 70 46.1 80 66.4 90 92.2 100 124.3 110 163.2 120 209.7 130 264.6 140 328.5

these results show a much lower power requirement than the extrapolation of M. Jagersand's data and also his original data. It does again show the massive increase of power required to move the van above 100 mph. It also is in close agreement to the road test results from SA.

I do wonder about the drag calculation, but for purpose of this discussion I'll leave it alone.

The original claim of 112 mph then would be estimated to require 171.9 hp. To push the van to 115 mph would need 13.4 more hp. Another's member's claim of 140 kph (87.23 mph) would require 84.5 hp. To push it to the round imperial figure of 90 mph would require and additional 7.7 hp.

I would opine that a stock wasserboxer Vanagon cannot exceed 100 mph on level ground, and in still air. Safe assumption seeing as the SA 5cyl couldn't break the 100 mph barrier.

So send those photos in showing speedometers pegged :)

But an interesting side note, if one could reduce the Cd from 0.44 to 0.40, then the power required at 100 mph drops from 124.3 to 114.1.

And if we could really streamline the box to give a Cd of 0.30 (without increasing frontal area) then you would only need 88.6 hp to do 100 mph.

Fun eh?

Alistair

'82 westy, diesel converted to gas in '94

http://www.members.shaw.ca/albell/


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