Speed Limiter Curiosity - S8
05-07-2007, 05:41 AM
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Speed Limiter Curiosity - S8
With the break-in period over and fresh oil in the engine, I've had a chance to open it up a bit.
The book says the top speed is limited to 250Km/h, but it turns out that the speed limiter engages at an indicated speed of exactly 274Km/h (!!!).
I tried it repeatedly (yes, of course it was a private road with no traffic, etc.) and every single time, the speed limiter engaged at exactly 274Km/h (digital indication).
I am wondering if the speedometer is off by 10%, or if the speed limiter is set to over 250Km/h and this is not documented - either one is quite telling of Audi's approach to customers and also quite disturbing.
To satisfy my curiosity I placed a portable GPS unit on the dashboard to see my true over-the-ground speed, but that didn't work - apparently there's some coating on the windshield which prohibited the GPS from getting any satellite signals (my toll ez-pass unit also doesn't work through the windshield).
Before I go out and buy a GPS with external antenna (NO, I'm not going to try holding the GPS out the window or duct-tape it to the roof) or read the ECU speed from VAG-COM, does anyone have any idea what's going on?
Thanks,
L.
The book says the top speed is limited to 250Km/h, but it turns out that the speed limiter engages at an indicated speed of exactly 274Km/h (!!!).
I tried it repeatedly (yes, of course it was a private road with no traffic, etc.) and every single time, the speed limiter engaged at exactly 274Km/h (digital indication).
I am wondering if the speedometer is off by 10%, or if the speed limiter is set to over 250Km/h and this is not documented - either one is quite telling of Audi's approach to customers and also quite disturbing.
To satisfy my curiosity I placed a portable GPS unit on the dashboard to see my true over-the-ground speed, but that didn't work - apparently there's some coating on the windshield which prohibited the GPS from getting any satellite signals (my toll ez-pass unit also doesn't work through the windshield).
Before I go out and buy a GPS with external antenna (NO, I'm not going to try holding the GPS out the window or duct-tape it to the roof) or read the ECU speed from VAG-COM, does anyone have any idea what's going on?
Thanks,
L.
05-07-2007, 07:35 AM
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There's a chart somewhere on Tirerack...
that shows how different wheel/tire combinations effect accuracy of the speedometer. Since the chassis/electronics were developed for the A8 with a different size wheel/tire combo in mind, and you're off by less than 2%, my guess is that its just because of that.
05-07-2007, 09:25 AM
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Re: GPS signal
The coating is the UV/heat reflective coating. There should be a spot with a small uncoated square, possibly on the rear screen. This is for a re-radiating antenna.
for a more permenant GPS install, you might be able to tap the GPS antenna and run a second cable to your device.
With regard to speed, does the DIS speed display match the speedo display accurately? When I changed up to 275/35 R20 tyres, I noticed an increase in speedo accuracy with the slightly larger rolling circumference offsetting the slight under-reading of the speedo.
for a more permenant GPS install, you might be able to tap the GPS antenna and run a second cable to your device.
With regard to speed, does the DIS speed display match the speedo display accurately? When I changed up to 275/35 R20 tyres, I noticed an increase in speedo accuracy with the slightly larger rolling circumference offsetting the slight under-reading of the speedo.
05-07-2007, 11:24 AM
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Re: GPS signal
I have the stock 265/35 R20 tires.
The digital display on the DIS matches the analog speedo very accurately, but my feeling is they're both off by a considerable margin.
What's a re-radiating antenna?
The digital display on the DIS matches the analog speedo very accurately, but my feeling is they're both off by a considerable margin.
What's a re-radiating antenna?
05-07-2007, 11:42 AM
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Re: Not exactly...
Well, I'm off by 10% (274Km/h vs 250Km/h), but my point was that if the speed limiter is working as it says on the book (@250Km/h), the ECU has a different/better idea of the actual speed and the speedometer is intentionally set wrong by Audi.
Moreover, you can't calculate speed accurately based on tire size.
At an indicated 274Km/h, the tacho showed about 5700-5750 rpms (hard to tell exactly, given the speed).
265/35 R20 tires typically have a circumference between 2.10 and 2.20m depending on brand, model and probably pressure.
The 6th gear ratio is 0.691:1 and the final drive ratio is 4.055:1.
Thus, assuming the RPMs indicated were accurate, true speed could be anywhere between 256Km/h (5700rpms, 2.10m tire circumference) and 271Km/h (5750rpms, 2.20m tire circumference).
L.
Moreover, you can't calculate speed accurately based on tire size.
At an indicated 274Km/h, the tacho showed about 5700-5750 rpms (hard to tell exactly, given the speed).
265/35 R20 tires typically have a circumference between 2.10 and 2.20m depending on brand, model and probably pressure.
The 6th gear ratio is 0.691:1 and the final drive ratio is 4.055:1.
Thus, assuming the RPMs indicated were accurate, true speed could be anywhere between 256Km/h (5700rpms, 2.10m tire circumference) and 271Km/h (5750rpms, 2.20m tire circumference).
L.
05-07-2007, 07:21 PM
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The speedo is about 5% optimistic
If you want to find your real speed w/o using a gps unit, reset the average speed display on the dash. It will show you true speed (unless otherwise effected by tire size, etc). Look at my pic poster, there's a chart/graph I made of acutal vs indicated speed.
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05-08-2007, 01:33 AM
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Re: re-radiating antenna
Think of it as an extension cable for the GPS antenna. If the car has a small untreated patch of front or rear screen that the GPS signal can get through, it's not practical to mount your GPS device right underneath that untreated square (for example, on Renault cars with UV-reflective windscreens, it's under the rear-view mirror, and I believe it's in one corner of the front or rear screen in the A8).
By placing the receiver end of a re-radiating antenna in the untreated patch, and placing the transmitter side of it right next to the built-in antenna on your GPS device, you can geta clear signal inside a car that normally blocks GPS signals.
By placing the receiver end of a re-radiating antenna in the untreated patch, and placing the transmitter side of it right next to the built-in antenna on your GPS device, you can geta clear signal inside a car that normally blocks GPS signals.
