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Top speeds and Cds?

Question came up today that got me wondering.

Anyone care to share their actual top speeds?

As a basis for performance an old Paul Frere article listed a 1974 3.0 RS with 220 bhp in a 2410# car as 149mph in a short (.0724) 5th gear @ 6300 rpm. 6300 is a full 1000 rpm under its working shift point. (basic RSR/turbo body kit on a mid '70s car)

SCRS (another turbo body kit with short gears and 255 bhp @ the crank) was listed by the factory as a 159 mph top end.

The highest listed narrow body production car was a Club Sport @ 231 bhp and 156 top end.

By today's standard none of these cars are fast, let alone quick. Nothing breaking even 5 sec 0-60.

Top end from a 425 hp Ruf RCT EVO in a narrow body 964 C2 form and a Turbo tail is 190 mph. Cd no worse than .350 with a turbo tail.

Top end of the narrow body CRT Yellowbird, Carrera tail with 469hp is 212.


356 had a .360 Cd.

early 911 was .381 Cd.

Duck tail with a deeper front spoiler ran a Cd of .363 when tested by Porsche.

Duck with a standard front spoiler ran .414 Cd
The Mary Stuart duck RSR ran .413 Cd
'74 RSR ran a .421 Cd
934s ran a .434 Cd
935s at LeMans ran a .390 Cd.
Carrear front spoiler and Carrera tail .414 Cd.
964 went down to .320 Cd with the automated spoiler
993 went up to .330 Cd automated spoiler and higher than a 964.
996 and GT 3 are down to a .300 Cd.
996 Turbo at .310
996 GT2 up again to .340

Gotta wonder what pulling the mirrors, rain gutters and window wipers would do to these Cd numbers? That plastic sheet Jack O is using under the car is looking pretty good right now for a high speed run.

Any one care to comment?

Last edited by rdane; 06-20-2005 at 10:58 AM..
Old 06-19-2005, 10:37 PM
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It's not so much the weight, but the Cd. vs. HP. Having a light car such as yours, gets to top speed quicker, question is will your 280 rwhp get you to 175. Judging from the list you've provided, no.
Compare the top speeds of the GT3 and 2, look at their Cd and HP (400 & 456). The GT2 is rated on the conservative side.
j.p.
Old 06-19-2005, 11:04 PM
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Really? If it is all about Cd and HP I would have to see some better figures before making a judgement. Dropping weight effectively gains real world HP.

1974 3.0 RS, 230 bhp , 2338#, Cd .421 and 149mph

SCRS 255 bhp, 2110#, 159 mph on short gears, Cd of .421 (all Porsche numbers BTW)

Club Sport 231 bhp, 2442#, 156 mph and a narrow body .414 Cd.

964 with 250 hp, 2970#, .320 Cd and a top end of 162mph
11.83 lb/hp

993 with 280 hp, 3014#, .330 Cd and a top end of 168 mph
10.77 lb/hp

997 turbo, 408hp, 3300#, .310 Cd and a top end of 180 mph
8.08 lb/hp

I would guess my aero package is under .363 Cd. Any of the engineers here care to toss out what the Cd numbers might be? I already know 160mph is easy enough which the SCRS numbers seem to back up.

Duk is a conservative 280hp @ crank, 2200# and under .363 Cd.
7.85 hp/lb

Last edited by rdane; 06-20-2005 at 12:28 AM..
Old 06-20-2005, 12:04 AM
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You cannot just stare at Cd and power. Just having a Cd coeficient and max power isn't enough to allow you to calculate top speed.

A brick will have much higher Cd than 911 but will achieve higher speed when propelled by same amount of Joule/second (=watts).

What you need to know to begin with is frontal area of the car. Total drag will be Cd x A[m^2].

Also, you need to know exact gearing at highest gear and torque curve of your particular engine. In most extreme case, your car won't be able to achieve it's calculated top speed due to:
1. Extremly peaky torque curve where mid-range torque isn't enough to push it trough untill torque starts to rise.
2. Bad gearing where tallest gear is mismatched to Cd x A either by being too short (car revs itself trough it's most effective rev-band where it produces most power) or too tall (car "hits the wall" before it hits it's peak power).

Optimal gearing for achieving max speed will make the car rev exactly at it's power peak when aerodynamic drag equals torque at wheels -> propelling force.

Weight of the car is of little importance...heavier car will induce somewhat higher rolling resistance but it's impact is minimal. Only thing that happends is that car will take longer time to accellerate to it's top speed.

Here is a good page on topic:

http://aerodyn.org/Drag/speed-drag.html
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Last edited by beepbeep; 06-20-2005 at 09:12 AM..
Old 06-20-2005, 03:20 AM
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The 993 and 964 #'s are a good comparison for the 280 hp range; both have a better Cd and don't achieve 175.
I'm not trying to lessen the car's ability, I really like the body engine combo you've put together, I'm just trying to find the actual top speed it can achieve.
j.p.
Ps. Notice that in the examples you've given; only the 400+ hp cars break the 170 mark.

Last edited by jpahemi; 06-20-2005 at 03:56 AM..
Old 06-20-2005, 03:53 AM
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Quote:
Originally posted by beepbeep


Optimal gearing for achieving max speed will make the car rev exactly at it's power peak when aerodynamic drag equals torque at wheels -> propelling force.

So in theory if we could calculate the aerodynamic drag we'd be able to find what the theoretical top speed for Dane's car would be (engine and drag are given... we could change gearing).

Correct?
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Old 06-20-2005, 07:03 AM
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My old car, now Craig's, I saw 176 indicated on the very long front stretch at Spokane. From riding on the freeway, the indicated speed is pessimistic if anything. It was still climbing slowly as I ran out of track. If I had to speculate, judging from the rate of acceleration at 176, I would guess the top speed would be somewhere around 185.

This is an old 930 body, with a Ruf valance, 275/315 tire package, 993 aero mirrors, full rain gutters, large oil cooler openings and brake cooling ducts, and a 3.8 RS tail set at max angle and a 1-1/4 inch gurney flap set at 70 degrees.

The torque curve is very flat and consistent, starting at the wheels at 325 ft/lbs at 3100 revs, and climbing to 371 at 5000, but staying over 325 until 6800 revs. Peak power is 418 hp at the wheels, with a fairly shallow rise, making over 200 RWHP at 3200 revs.

The gearing was stock G50/50, with a theoretical max of 196 at 6800 revs.

FWIW, the car does make a big hole in the air with the wing, other cars would regularly pick up 10 mph several car lengths back if they drafted at all.

Just for reference, Spokane is a good place to test the actual top speed of most cars.
Old 06-20-2005, 07:34 AM
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Quote:
Originally posted by beepbeep
You cannot just stare at Cd and power. Just having a Cd coeficient and max power isn't enough to allow you to calculate top speed.

A brick will have much higher Cd than 911 but will achieve higher speed when propelled by same amount of Joule/second (=watts).

What you need to know to begin with is frontal area of the car. Total drag will be Cd x A[m^2]. .. ..
Yep; and, excellent post.

However, 911's are pretty damn close in frontal area. (except turbos).

So, for a first order aprox (is anyone really expecting more than that?) comparing power to Cd here isn't so out of line.

Oh, fwiw (re, Cd,brick) my fluids book has the example, the Cd's of a both an old VW bug, and a VW bus. Guess which one has a lower Cd?


hint: it's not hte smaller one.
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Old 06-20-2005, 07:34 AM
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Quote:
So in theory if we could calculate the aerodynamic drag we'd be able to find what the theoretical top speed for Dane's car would be (engine and drag are given... we could change gearing).
Yes. probably be able to get pretty close I would imagine.

Check out this thread for formula and examples

Drag/CD scenario.......a question, a challenge..

My old '88 topped out at redline in 5th. With an extra gear or different gearing under ideal conditions I think a 3.2L car would hit 160. The problem is that most of the 3.2L cars are gearing limited and it's difficult to find a place to test this. I used I-10 in AL or MS.
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Old 06-20-2005, 08:00 AM
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OK thanks, that thread made my head hurt
But all good stuff guys, thanks!

How about in theory ?

A 300 hp engine in a car that has a .350 Cd ?
Gearing physically is maxed out at 181.

What else do ya need?

Last edited by rdane; 06-20-2005 at 09:14 AM..
Old 06-20-2005, 09:06 AM
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Steve

How do you get to the 160mph for a 3.2 with ideal gearing?
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Old 06-20-2005, 09:19 AM
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What island is alluding to ... and what most here are ignoring [or unaware of] is that Cd is not used in motion calculations for the real world, but rather Cx! Cx is the product of Cd and frontal area ... and, YES, a VW 1st gen. bus had lower Cd [drag coefficient] than a beetle, but Cx was a different matter!
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Old 06-20-2005, 09:28 AM
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Porsche lists for the '88 3.2, 231hp, a narrow body .414 Cd with both front and rear spoilers and a 156mph top speed.

Quote:
My old '88 topped out at redline in 5th. With an extra gear or different gearing under ideal conditions I think a 3.2L car would hit 160. The problem is that most of the 3.2L cars are gearing limited
Close enough to the always conservative Porsche factory numbers for me.

So now can anyone give me the math and answer for this one?

"A 300 hp engine in a car that has a .350 Cd ?
Gearing physically is maxed out at 181."
Old 06-20-2005, 09:32 AM
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Ok... say we have the Cx.

How do we relate HP/torque to Cx in an equation to come out to a theoretical Vmax (where optimal gearing takes the engine to the sweet spot)?
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Old 06-20-2005, 09:32 AM
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Quote:
Originally posted by Early_S_Man
What island is alluding to ... and what most here are ignoring [or unaware of] is that Cd is not used in motion calculations for the real world, but rather Cx! Cx is the product of Cd and frontal area ... and, YES, a VW 1st gen. bus had lower Cd [drag coefficient] than a beetle, but Cx was a different matter!
The problem is the only numbers I can easily gather from Porsche is Cd.

Can you do basic calculations from that and HP alone?
Old 06-20-2005, 09:34 AM
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Quote:
Originally posted by rdane
Porsche lists for the '88 3.2, 231hp, a narrow body .414 Cd with both front and rear spoilers and a 156mph top speed.



Close enough to the always conservative Porsche factory numbers for me.

So now can anyone give me the math and answer for this one?

"A 300 hp engine in a car that has a .350 Cd ?
Gearing physically is maxed out at 181."
Assuming you had the same engine as the 88 3.2 then using Steve's formulas you get:

169 mph.

Now since you have a better engine I just don't know... But should be higher than that. Are us ure the 88 3.2 had a cd of .414?
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Old 06-20-2005, 09:36 AM
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.414 * 69.738^2 = .35*V^2

V=169mph

You have an extra 100 RWHP over the 3.2. But I have no idea of where that puts u.
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Old 06-20-2005, 09:39 AM
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Quote:
Originally posted by 911teo
Are u sure the 88 3.2 had a cd of .414?
Porsche's number for the H Series '74/'75 Carrera with larger factory tail and front spoiler is .414 Cd. According to Paul Frere (where I pulled the info) the car's stayed pretty much the same until the 964 came out at .320 Cd in '89.

Closest thing to my car is a standard duck in racing form @ .363Cd. But it had a less aerodynamic front and rear bumpers, one mirror, rain gutters and 2 wipers.

Also saw a Porsche or Ruf comment in one of the posts last night or this morning that said the flag mirrors alone were worth 5mph on a top end past 150. No idea if that is actually true.

Last edited by rdane; 06-20-2005 at 10:19 AM..
Old 06-20-2005, 09:55 AM
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The funny thing is that using the 993 as a base case (which would be perfect as the engines are similar) with a .33 cd and 168 mph top. would put you at:

163 mph

.33 * 75.1^2 = .35 * V^2

Which makes me think that the 993 was greatly short geared to favor acceleration over top speed.
Any thoughts?
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Old 06-20-2005, 10:01 AM
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Thanks Matteo.

Would you mind running these numbers?

"300 hp in a car that has a .350 Cd"

My seat of the pants guess from driving the car was it would hit just over 175 and come close to maxing the gearing. The difference in air pressure and drag over 130mph is something I have no experience with so it was only a guess on my part. To be honest I am in no hurry to run the car to 150+ again. Just too many risks.

But I am curious.
Old 06-20-2005, 10:07 AM
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