More exact vehicle speed logging

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mituc
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More exact vehicle speed logging

Post by mituc »

I'd like to know if it's possible to add one or even two decimals to the speed PID, if it is reported properly through the OBD bus.

Especially when logging in mph (which is 1.6x kmph) we see no increase in the vehicle speed when we start accelerating while the RPM (which given it nature is more accurately reported from a time relation standpoint) is increasing. Because of this the speed/RPM correlation is broken which leads to weird curves in torque/power analizing software such as VirtualDyno which may trick us into thinking we have clutch or wheel slip.
2008 Cosmic Blue Mazda 3MPS
Build engine + WMI + GTX3071, ~476WHP @31-32PSI
2008 Icy Blue Mazda CX7
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Ugnius @ VersaTune
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Re: More exact vehicle speed logging

Post by Ugnius @ VersaTune »

The ECU returns vehicle speed as a single byte with 256 different values from 0 to 255, which corresponds to the speed in km/h. As a result, we cannot increase the accuracy of that.
The speed is calculated by the ABS control module and passed on to the ECU. The speed signal is filtered, that's why there is always a delay between the actual speed and reported speed. As far as I know, Virtual Dyno software doesn't use speed measurements at all. It calculates the speed from RPM and the gear ratio.

JohnnyT
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Joined: July 6th, 2017, 4:18 pm

Re: More exact vehicle speed logging

Post by JohnnyT »

Image

He is asking because he has been helping me with the tune and I keep seeing this in all my logs.
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mituc
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Re: More exact vehicle speed logging

Post by mituc »

Indeed, and we can't figure what's that abrupt line right before 3k and then after 4k rpm... and we don't want to make assumptions. It may look like a bit of clutch slip right between those RPM points or just something VD would react to form the logs, as the RPM/speed is kind of smooth, and so is the RPM and speed curve.
However sometimes RPM increases by 25-ish RPM between each line, sometimes by 50-ish RPM and that may cause it.
2008 Cosmic Blue Mazda 3MPS
Build engine + WMI + GTX3071, ~476WHP @31-32PSI
2008 Icy Blue Mazda CX7
Stock engine and exhaust (YES!!), JBR3" + GTX2867 gen2 + Autotech HPFP, self-tuned to 320-ish BHP

equinox92
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Joined: December 3rd, 2019, 2:48 am
Location: Detroit, MI

Re: More exact vehicle speed logging

Post by equinox92 »

Ugnius @ VersaTune wrote:The ECU returns vehicle speed as a single byte with 256 different values from 0 to 255, which corresponds to the speed in km/h. As a result, we cannot increase the accuracy of that.
The speed is calculated by the ABS control module and passed on to the ECU. The speed signal is filtered, that's why there is always a delay between the actual speed and reported speed. As far as I know, Virtual Dyno software doesn't use speed measurements at all. It calculates the speed from RPM and the gear ratio.
Why depend on a value return from the ECU? Looking at the bus the vehicle speed is updated around every 20ms with an unsigned two byte value. This two byte value is divided by 100 and gives you speed in kph.

You could easily log a super fast vehicle speed with a decimal place of accuracy.

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Ugnius @ VersaTune
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Re: More exact vehicle speed logging

Post by Ugnius @ VersaTune »

equinox92 wrote:Why depend on a value return from the ECU? Looking at the bus the vehicle speed is updated around every 20ms with an unsigned two byte value. This two byte value is divided by 100 and gives you speed in kph.

You could easily log a super fast vehicle speed with a decimal place of accuracy.
The vehicle interfaces that we support (J2534 and OBDLink) do not allow simultaneously querying control modules using a high level protocol like ISO 15765 and monitoring the CAN bus using a low level protocol like ISO 11898.

equinox92
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Joined: December 3rd, 2019, 2:48 am
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Re: More exact vehicle speed logging

Post by equinox92 »

Ah, that makes sense to retain compatibility with all J2534 devices.

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