Rennteam 2.0 - EN - Forum - Re: Speed vs. RPM - Page1

Mar 5, 2007 7:04:31 PM

Dubai: Newbie; Loc: Dubai; Posts: 14, Gallery; Registered on: Jan 8, 2007

Speed vs. RPM

guys,

cars performance depend on several factors like luck,engine break-in, etc...

does anyone know what speed is his 997 be at different RPMs and gears?

for example: 997S 200 kph 6th gear 4000 rpm

or : 997 30 mph 2nd gear 3000 rpm (these figures are not real)

i think that this will be a great reference to all rennteames, also please mention if u have x51 or pccb!

Thanks

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Mar 5, 2007 8:51:36 PM

69bossnine: Rennteam Pro; Loc: FL, USA , United States; Posts: 2028, Gallery; Registered on: Jan 27, 2005; Reply to: Dubai

Re: Speed vs. RPM

Dubai, the relationship between car speed and RPM is direct, mechanical, fixed...

In other words, it wouldn't matter if one 997 was making 330 h.p., and the other was making 400 h.p., they would still do the same speed at the same rpm. The difference would be found in the rate at which they could achieve those speeds under full-throttle.

The only thing we would possibly prove if we all submitted data to you, is whether there's any overt speedometer accuracy varience from car to car.

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Mar 5, 2007 9:05:39 PM

Dan L: Expert; Loc: Santa Barbara, CA , United States; Posts: 1957, Gallery; Registered on: Jan 2, 2006; Reply to: 69bossnine

Re: Speed vs. RPM

Unfortunately, another variable presents itself,--tires.

If you look, however, in the back of your owners manual they have a speed and HP curve for both engines posted. The gear factor is displayed as well.

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Mar 5, 2007 9:40:46 PM

69bossnine: Rennteam Pro; Loc: FL, USA , United States; Posts: 2028, Gallery; Registered on: Jan 27, 2005; Reply to: Dan L

Re: Speed vs. RPM

Yes, tire wear changes speedometer accuracy, or lack thereof. My post was of the assumption that two compared cars had identical factory-spec/size tires with identical wear. A Rennteamer with new tires would show slower speeds at a given RPM than a Rennteamer with bald tires.

All I was really trying to explain, was that power had nothing to do with the ratio of speed/rpm.

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Mar 5, 2007 10:25:58 PM

Grant: Rennteam VIP; Loc: Denver, CO , United States; Posts: 12586, Gallery; Registered on: Aug 21, 2002; Reply to: 69bossnine

Re: Speed vs. RPM

Quote:
69bossnine said:All I was really trying to explain, was that power had nothing to do with the ratio of speed/rpm.

Exactly - this experiment is nothing more than a test of gearing (and all factors that affect gearing, like tire diameter)...

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Mar 6, 2007 1:01:58 AM

ewi: Pilot; Loc: Germany; Posts: 430, Gallery; Registered on: Apr 21, 2005; Reply to: 69bossnine

Re: Speed vs. RPM

Quote:
69bossnine said:
...
A Rennteamer with new tires would show slower speeds at a given RPM than a Rennteamer with bald tires.
...

Shouldn't it be the other way round?
speed = wheel RPM * wheel circumference
and wheel RPM is mechanically fixed to the engine RPM in each gear.
So, if RPM is the same for both cars but wheel circumference is greater for new tires the car with the new tires should achieve higher actual speeds. Don't forget to break in the new tires if you try that yourself

The displayed speed is another thing and depends on the circumference that is used in the software. Assuming that they use a fixed circumference of a new tire and not some characteristic curve dependent on tire wear (would be interesting to know how they would try to calculate that) and speed itself (wheel circumference increases with speed) the displayed speed does not depend on tire wear. In that case the displayed speed of the two cars would be the same.

997S: 300km/h @ 7200RPM, obviously in 6th gear

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Mar 6, 2007 5:29:00 AM

Dubai: Newbie; Loc: Dubai; Posts: 14, Gallery; Registered on: Jan 8, 2007; Reply to: ewi

Re: Speed vs. RPM

Thanks everyone for the info!

im still breakin-in my 997s, but it doesnt feel like it can reach the 300km/h, i have pushed it to 200 km/h at 6th gear, around 5000 rpm.

overall, the car feels weak below 4000-4500 rpm

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Mar 6, 2007 6:37:30 AM

dasams: Junior; Loc:; Posts: 55, Gallery; Registered on: May 1, 2004; Reply to: ewi

Re: Speed vs. RPM

Quote:
ewi said:
Quote:
69bossnine said:
...
A Rennteamer with new tires would show slower speeds at a given RPM than a Rennteamer with bald tires.
...

Shouldn't it be the other way round?
speed = wheel RPM * wheel circumference
and wheel RPM is mechanically fixed to the engine RPM in each gear.
So, if RPM is the same for both cars but wheel circumference is greater for new tires the car with the new tires should achieve higher actual speeds. Don't forget to break in the new tires if you try that yourself

The displayed speed is another thing and depends on the circumference that is used in the software. Assuming that they use a fixed circumference of a new tire and not some characteristic curve dependent on tire wear (would be interesting to know how they would try to calculate that) and speed itself (wheel circumference increases with speed) the displayed speed does not depend on tire wear. In that case the displayed speed of the two cars would be the same.

997S: 300km/h @ 7200RPM, obviously in 6th gear

You are right, of course, but so is 69. He's just hard to understand sometimes.

Shade tree mechanic out...

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Mar 6, 2007 3:56:03 PM

69bossnine: Rennteam Pro; Loc: FL, USA , United States; Posts: 2028, Gallery; Registered on: Jan 27, 2005; Reply to: dasams

Re: Speed vs. RPM

Yes, you are right, and I was right. We're both saying the same thing, from opposite directions.

What I meant, is that as you decrease wheel diameter (i.e. tire wear), your "actual" speed would decrease congruent to the wear when compared to a fixed or "target" displayed speed, assuming the speedometer is working on a static algorithm or mechanical/electronic trigger that is directly calibrated to gearing, as most speedometers are.

Now if the speedometers were using GPS or radar technology for determining speed, gearing and tire wear would be irrelevant.

If you buy the car new, with new tires, and when your speedometer reads 80mph, you are actually travelling 80mph, then I would ball-park estimate that once you've worn the tires to almost-bald, you're actually only travelling 78 mph with the needle on 80 on the dash.

BTW, if you want to hear something funny, the speedometer on our 1911 White Touring works like this..

A cable, with a gear on the end of it, engages a sprocket on the LH front wheel. At the other end of the cable, at the dash, the cable spins a little FAN that blows air into a tiny metal horn, that is attached to the needle's axle, which is spring-loaded. The harder the fan blows air into the horn, the further the needle is pushed around the gauge. The calibration is all in the pitch of the fan, and the shape of the horn!

Amazing how things that seem so "standard" to us today, were being done in any number of crazy ways back when everything was a new invention, somebody's unique "idea" of how to accomplish something.

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Mar 6, 2007 4:02:07 PM

Dan L: Expert; Loc: Santa Barbara, CA , United States; Posts: 1957, Gallery; Registered on: Jan 2, 2006; Reply to: Dubai

Re: Speed vs. RPM

Quote:
Dubai said:
Thanks everyone for the info!

im still breakin-in my 997s, but it doesnt feel like it can reach the 300km/h, i have pushed it to 200 km/h at 6th gear, around 5000 rpm.

overall, the car feels weak below 4000-4500 rpm

it will take on a new feeling once broken in. Be patient.

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Mar 6, 2007 9:44:29 PM

function_analysi: Pilot; Loc: Jeddah, Athens , Greece; Posts: 352, Gallery; Registered on: Aug 19, 2006; Reply to: ewi

Re: Speed vs. RPM

Quote:
ewi said:
Quote:
69bossnine said:
...
A Rennteamer with new tires would show slower speeds at a given RPM than a Rennteamer with bald tires.
...

Shouldn't it be the other way round?
speed = wheel RPM * wheel circumference
and wheel RPM is mechanically fixed to the engine RPM in each gear.
So, if RPM is the same for both cars but wheel circumference is greater for new tires the car with the new tires should achieve higher actual speeds. Don't forget to break in the new tires if you try that yourself

The displayed speed is another thing and depends on the circumference that is used in the software. Assuming that they use a fixed circumference of a new tire and not some characteristic curve dependent on tire wear (would be interesting to know how they would try to calculate that) and speed itself (wheel circumference increases with speed) the displayed speed does not depend on tire wear. In that case the displayed speed of the two cars would be the same.

997S: 300km/h @ 7200RPM, obviously in 6th gear

Theoretically speaking you're absolutely right, although the equation v=w*R (w stands for the rotational speed) is just a simplified algebraic expression of the circular motion and NOT the algorithmic model used by nowdays manufacturers.

Besides, the principle of the car speedometer as described doesn't actually incubate the modern forms of digital devices that have NOTHING to do with the wheel diameter just becuase they aren't directly linked with it.

In fact current speedos employ a rotation sensor, to be found on one of the driveshafts -not on any of the wheels. By definition, that sensor generates a series of electronic pulses whose frequency corresponds to the rotational speed of the driveshaft. The signal is then transmitted to the ECM that converts the repetitive pulses of any frequency to a meaningful speed value before being displayed either on an electronically-controlled needle reading or a digital output display.

Hope this brings our understanding closer to modern cars reality.

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Mar 6, 2007 9:58:53 PM

dasams: Junior; Loc:; Posts: 55, Gallery; Registered on: May 1, 2004; Reply to: 69bossnine

Re: Speed vs. RPM

Quote:
69bossnine said:
the cable spins a little FAN that blows air into a tiny metal horn, that is attached to the needle's axle, which is spring-loaded. The harder the fan blows air into the horn, the further the needle is pushed around the gauge. The calibration is all in the pitch of the fan, and the shape of the horn!

Interesting. Sort of reminds me of a pitot tube.

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Mar 7, 2007 2:59:27 AM

ewi: Pilot; Loc: Germany; Posts: 430, Gallery; Registered on: Apr 21, 2005; Reply to: Dubai

Re: Speed vs. RPM

Quote:
Dubai said:
Thanks everyone for the info!

im still breakin-in my 997s, but it doesnt feel like it can reach the 300km/h, i have pushed it to 200 km/h at 6th gear, around 5000 rpm.

overall, the car feels weak below 4000-4500 rpm

Don't worry, that's normal. At 3500km my 997S reached 288km/h only before I was stopped by traffic, with not much left to come (PSE retrofitted at around 1700km). It took some 1000km's more until it reached 300km/h and above.

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Mar 7, 2007 3:41:42 PM

69bossnine: Rennteam Pro; Loc: FL, USA , United States; Posts: 2028, Gallery; Registered on: Jan 27, 2005; Reply to: ewi

Re: Speed vs. RPM

Regardless of where the speedometer is counting its rotations from, a change in wheel and/or tire size requires a comparable change in the program code settings that run the dash gauge.

In the old days, you changed gear diameters or gear teeth to adjust up or down to correct for ring & pinion swaps or wheel-tire swaps.

Today, you recalibrate with a laptop.

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Mar 7, 2007 8:54:18 PM

ewi: Pilot; Loc: Germany; Posts: 430, Gallery; Registered on: Apr 21, 2005; Reply to: 69bossnine

Re: Speed vs. RPM

That's right. Therefore, I would like to know, how the ECM can calculate the speed just from a rotation sensor at the driveshaft.

All I know is that many cars with ABS still use wheel speed and wheel circumference to calculate the speed. The ABS/brake control unit calculates an average wheel speed from the ABS sensors at the driven wheels. The instrument cluster/speedo takes that and in most cases a constant wheel circumference to calculate the speed for its analog and/or digital speed display. This is how it's done e.g. in a Lambo Gallardo or Porsche Cayenne. So, what function_analysi(s) is trying to say is that these are no modern cars.

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Mar 8, 2007 1:21:44 PM

69bossnine: Rennteam Pro; Loc: FL, USA , United States; Posts: 2028, Gallery; Registered on: Jan 27, 2005; Reply to: ewi

Re: Speed vs. RPM

Quote:
I would like to know, how the ECM can calculate the speed just from a rotation sensor at the driveshaft.

The driveshaft is connected to the wheel, the wheel is connected to the tire, the tire rolls down the road...

I'm not sure what your puzzlement is. All the ECM needs to know is all the fixed measurements/ratios between the rotation sensor and the road. The rate of rotation gets plugged into a formula that's probably so short you could write it on the palm of your hand to cheat on your math exam. (I never did that...yes I did...)

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Mar 8, 2007 10:05:59 PM

ewi: Pilot; Loc: Germany; Posts: 430, Gallery; Registered on: Apr 21, 2005; Reply to: 69bossnine

Re: Speed vs. RPM

I was thinking about function_analysi's statement that modern speedometers have nothing to do with wheel circumference. At the moment I don't see a way to calculate the speed without it.
I assume that the rotation sensor generates a known constant number of pulses per km.
In that case the pulses received in a defined time frame can be converted to the speed. At first sight there is no circumference needed, but it is hidden in the pulses per km constant. The constant is just a pulses per rotation constant that is converted to pulses per km via the wheel circumference.

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Mar 8, 2007 10:31:08 PM

Grant: Rennteam VIP; Loc: Denver, CO , United States; Posts: 12586, Gallery; Registered on: Aug 21, 2002; Reply to: ewi

Re: Speed vs. RPM

Ewi - you are correct - the speed sensor only measures the rpm of the axle and without a value for the circumference of the tire, this is useless. Changes in tire size will make the speedo innaccurate. Until units rely on GPS, this will continue to be the case...

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