CROSSPOST; from Off-Topic Tech

[freebeard]

THIS IS MY FAVORITE CAR, AT LEAST FOR TODAY.
Lawrence's 1959 Porsche 356 Speedster Replica!!

Lawrence's 1959 Porsche 356 Speedster Replica!!

A replica 356 is the only thing attainable these days.

This is fiberglass with an aluminium tonneau and wind split. The shift knob is Balsa wood! The foot pedals are dimple died. The weight is ~1300 LB

4:11 .... the Porsche
4:14 philosophy is if you make it a third
4:15 lighter and a third smaller it should go
4:17 a third faster and pretty much holds
4:19 true yeah I think you're absolutely right

It's mostly interview, they never show the engine or even talk about the sway bars, so one is free to speculate.

I'd want a 3/4 tonneau and since it's fiberglass, a slot in the cowl so the windshield would roll down. With skirts instead of flares, the frontal area would be approximately the rectangle around the front tires -- but shaped like a half-bar of hand soap.

This would take a 25-50 MPG Beetle to 100 MPG*. And that's before the electric front axle between Rabbit axles (Superbeetle struts and 75 or later rack and pinion?) in front to make a through-the-road hybrid.

*(cube vs square)

[aerohead]

'Numbers' would be helpful.

[freebeard]

All we have to go on is the video. 2/3rds the weight, maybe 2/3rds the frontal area, 1641cc engine with indeterminate output.

...and the quote from Porsche.

[aerohead]

My home library paid off on some 'numbers.'
1) Speedsters have a 170-pound weight advantage, compared to coupes.
2) The mutilation of the Speedster roofline, compared to the Coupe results in a 10.4% Cd penalty.
3) The added mutilation of lowering the roof and side curtains of the Speedster results in a 18.6% Cd penalty, compared to the Coupe.
4) Top speed of the Speedster, compared to the Coupe suffers a 2.65% loss.
5) Fuel economy is 10.2% lower than the Coupe.
6) The Speedster does have an acceleration advantage up to 28-seconds elapsed time, then is overtaken by the Coupe all the way up to terminal velocity.
7) In the 1/4-mile, the Speedster is 0.6-seconds 'quicker', and 2-mph 'faster' than the Coupe.
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For the 356-to-Beetle comparison, things get interesting:
8) The Beetle has a CdA 0.891 meters-squared vs CdA 0.4635 meters-squared for the Speedster.
9) The Beetle's test weight was 2,130-pounds vs 2,110-pounds for the Speedster.
10) The Beetle' Road Load @ 60-mph = 161.6-pounds vs 102.0-pounds for the Speedster.
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In 1952, The 356-A could hit 103-mph on 65-bhp.
MGs and Austin-Healeys of the same 'size' required 100-bhp for the same speed.
* The #44, Porsche 550 Renn Sport, two-man 'COUPE', by race builder Walter Glocker was clocking 116.88-mph at the Nurburgring. This car was reported @ Cd 0.232, by Erwin Komenda, in scale wind tunnel testing, and Cd 0.36 @ full-scale, vs Cd 0.45 with the 'Spyder' ( roadster )[ Hucho ].
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To have a chance at 100-mpg, you'd want to ditch the 'full' wind screen ( Windschutz-scheibe ), the Porsche 'aero-screen', and go with Walter Korff's single-occupant 'bubble' screen, full side side windows ( seitenfenster ), full head fairing, slow diffuser, and boat tail, as Knot Farrington did with his Ford Thunderbird Bonneville streamliner 'Hel's Angel'.

[freebeard]

2) The mutilation of the Speedster roofline, compared to the Coupe results in a 10.4% Cd penalty.

I find this assertion surprising. Does your home library give a CdA for the coupe? What do you mean by mutilation? How much mitigation with a 3/4 tonneau? I proposed a roll-down windscreen.

1) Speedsters have a 170-pound weight advantage, compared to coupes.

What weight baseline? I see reports from 1698 to 1874lb. This car's owner estimates 1300lb. That's 400-570lb difference. Did you notice all the speed holes in the seats, pedal and brake handle?

100 MPG was an aspirational goal. Has anyone achieved that other than Aerocivic?

[aerohead]

Originally Posted by freebeard

I find this assertion surprising. Does your home library give a CdA for the coupe? What do you mean by mutilation? How much mitigation with a 3/4 tonneau? I proposed a roll-down windscreen.



What weight baseline? I see reports from 1698 to 1874lb. This car's owner estimates 1300lb. That's 400-570lb difference. Did you notice all the speed holes in the seats, pedal and brake handle?

100 MPG was an aspirational goal. Has anyone achieved that other than Aerocivic?

1) According to Hucho, who ran VW's climatic wind tunnel for a decade, the Coupe has Cd 0.34, Af 1.61-meters-squared, for a CdA 0.5474 m-sq. The Speedster, with it's ragtop up,with it's side curtains has CdA 0.6046 m-sq )
2) The Speedster gives a 'virtual' Cd 0.4032, when the ragtop is lowered and side curtains removed compared to the Coupe, @ CdA 6.9873-sq-ft ( 0.6491-m-sq ).
3) The curb weight for Coupe = 1920-lb, Speedster = 1750.
4) The 550 RS was 1,300-lb 'dry', 1,510-lb with fuel, plus 370-lbs for two occupants
I'm out of time, be back Thursday.

[aerohead]

The expression was coined by Fredrick Lanchester, in his 1907 book on aerodynamics, which I shared here at the Forum a number of years ago.
1) The 'roofline' of a road vehicle, in 'ground reflection, is the most important element as far as 'aerodynamics' goes.
2) The contour of the 356 Coupe is already extremely compromised ( mutilated ).
3) The contour of the Speedster takes the injury to an extreme level, both roofline and rear deck area ( it's completely different from the Coupe ), a double-whammy.
4) When the roof of the Speedster is lowered, and side curtains absent, you're looking at something in the neighborhood of Cd 0.5184, the trifecta of aero abuse.
5) The 'buttocks' and 'tail' of the car are essentially 'gone.'
6) Dr. Komenda's wind tunnel schematic images depict complete flow separation at the top of the wind screen, with no reattachment, anywhere on the upper body; violating Hucho's ground rules for fluid mechanics.

[aerohead]

1) For all I have, I have no drag data for empirical measurements of that specific body configuration.
2) Komenda did test the 550/1500RS Spyder, with no windscreen, no driver or passenger, 100% tonneau, and no head fairing, which came out at Cd 0.247.
3) This Cd 0.247, when adjusted to Hucho's reporting for 'actual' full-scale measurements might reflect a 'true' Cd 0.3003, which is higher than the 550 RS Coupe, at an adjusted, Cd 0.2821.
4) Your 356 Speedster, replicar, with 'no windscreen,75% tonneau, driver's head protrusion excrescense into the airstream, and head fairing, would fall somewhere a bit higher than Cd 0.3003.Can't be more specific.
5) We have no specific, isolated data, for any aerodynamic contribution which can be attributed solely to the driver's head fairing.

[aerohead]

At 60-mph, with drivers, the weight advantage of the Speedster, compared to the Coupe would knock off 0.8494- hp from the car's road load. About 0.9036-bhp.
If you already know the BSFC for the VW's 1582-cc engine, you can figure the mpg savings at that road speed. It won't be much.
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The Speedster returned 31.6-mpg actual HWY.
The Coupe saw 35.2-mpg, with greater weight, and larger frontal area.

[aerohead]

Some consideration:
1) The 1961 Karmann-Ghia was Cd 0.39, and Af 18.793 sq-ft, CdA 7.32927-sq-ft ( 0.68091 m-sq ).
2) The test weight was 2,089-lbs.
30-mph = 56.3 mpg hwy
40-mph = 51.3 mpg
50-mph = 45.4 mpg
60-mph = 39.2 mpg
70-mph = 31.5 mpg
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The 1992 GM ULTRALITE, Cd 0.192, at 50-mph = 100 mpg.
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* The rolling force coefficient for the early Porsches was Cf 0.020832195
* Bridgestone Ecopias are Cf 0.0055
* The Beetle-based project could compensate for some of the mediocre 'Porsche 356' aerodynamics simply by incorporating the modern Low R-R tires.
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The BSFC of the air-cooled engine, from there on out, will determine the CdA you'd have to come up with in order to hit the 100-mpg target.
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Figure for a mechanical efficiency of 94% for VW's powertrain to get to your useful horsepower available at the traction interface of the rear tires.
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From this 'net' road power, subtract the horsepower absorbed by the tires, leaving what's left for aero.
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Knowing this horsepower, and your target road speed, run your drag power formula 'backwards' to solve for the target Road Load Force.
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Once you have this, allowing for the dynamic pressure/ air density/ velocity-squared, your target CdA will fall out of the math.
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Dividing the CdA by frontal area, yields your target Cd.
Then it's a matter of using 'known-knowns' from the aerodynamic bag-of-tricks reported in the last 102-years to get you to that 100-mpg.