speedo accuracy or lack thereof

by **jp928** » Mon Mar 17, 2008 9:50 am

Put the ecudatascan on the gtb at the weekend. Couple of oddities came up - it shows the speedo is overreading at least 5%, even at 60kmh , same rate at 100 indicated. No wonder I couldnt keep up with the flow at legal limits.
Also, in scanning through the items available on the datascan, Knock is not listed - ign (advance ) is shown and seems correct, but no sign of Knc parameter. Anybody know how this ecu does its ign corrections ?

jp 08 gtb 6mt wagon

by **tangcla** » Mon Mar 17, 2008 9:53 am

So the speedo is reading less than what the ECU reads? odd.

Knock would be controlled with the electronic drive-by-wire throttle. It will (partially?) close the throttle for you if knock is detected.

by **Resonate** » Mon Mar 17, 2008 10:16 am

Yeah this is very common amongst the liberties, mines also off.

Here's some links to other similar threads for your reading pleasure:

Link

Link

by **sooobi** » Mon Mar 17, 2008 10:25 am

Lose the speedo over-reading part and this could be an interesting thread..

sadly, the TANGLA has already made an appearance..

by **tangcla** » Mon Mar 17, 2008 10:52 am

sooobi wrote:sadly, the TANGLA has already made an appearance..

boredom at work = checking the forum every half hour for useful/interesting threads to lend a helping e-hand.

You also mis-spelt my username.

by **Ric** » Mon Mar 17, 2008 11:09 am

jp928 wrote:Put the ecudatascan on the gtb at the weekend. Couple of oddities came up - it shows the speedo is overreading at least 5%, even at 60kmh , same rate at 100 indicated. No wonder I couldnt keep up with the flow at legal limits.
Also, in scanning through the items available on the datascan, Knock is not listed - ign (advance ) is shown and seems correct, but no sign of Knc parameter. Anybody know how this ecu does its ign corrections ?

jp 08 gtb 6mt wagon

All manufacturers deliberately make their speedos read a little high.
It's interesting that the ECU data appears lower. Iwonder how accurate it really is, would be interesting to compare against a GPS navigator

by **jp928** » Mon Mar 17, 2008 12:31 pm

Note that I said the speedo reads HIGH, not low, which I know they all do to protect us from speeding tickets, but it irritates me a fair bit.
I have a Garmin GPS, so will check its speed reading against both next opportunity. I prefer to have a pax on board to do these sort of tests so I only have to look at one thing at a time.

jp 08 gtb 6mt wagon.

by **Ric** » Mon Mar 17, 2008 12:33 pm

tangcla wrote:...
Knock would be controlled with the electronic drive-by-wire throttle. It will (partially?) close the throttle for you if knock is detected.

Are you sure about that?
Most traditional anti-knock systems retard the ignition timing when they detect knocking.

by **tangcla** » Mon Mar 17, 2008 12:53 pm

Ric wrote:Are you sure about that?
Most traditional anti-knock systems retard the ignition timing when they detect knocking.

Am I sure? no. That's what I heard from my tuner though, because I wanted to ask him to tune the throttle response to be more responsive; this is the reason he gave me as to why it can't be done overly aggressively.

Perhaps it's a combination of both?

by **jp928** » Mon Mar 17, 2008 2:29 pm

My understanding of trying to get the best out of a motor was aggressive advance (ie suited to high octane fuel) modified by knock triggered retard, particularly in a turbo where the pressures can get very high. Reducing throttle opening would help some, but its not the right solution. Even the Outback showed knock figures on the Ecudatascan, so it was doing it.
more research needed.
jp

by **CJ1177** » Mon Mar 17, 2008 9:18 pm

While researching getting new wheels, & knowing my new wheels would be a larger diameter than stock. My stock wheels are 17" & the tyres size i was looking at was slightly larger than the 18" rolling diameter i figured to try & keep my speedo close to accurate to change the speed sensor, change setting in the ECU or speedo, or some thing like that.
Any way i got hold of a good Subaru techo who did some research for me & come up with, "all the speedos are inaccurate & have the same settings no matter the wheel size".
So if you have 17" wheels there more inaccurate than those with 18" rims, for those that don't know both rims come with 215-45 tyres

by **jp928** » Tue Mar 18, 2008 8:53 am

Whatever way the speedo is set up, why is its reading different from the ECU's output? Where is the ECU getting its data from? Many cars now with ABS use that sensor on one wheel to supply speed data, as its already reading notches, but it wouldnt care about accuracy at all. I will have to see if the ECU figure matches the GPS - if it does, then the speedo is either purposely high, or its getting its data from a different source from the ECU.
Next time I get under I will see if there is a recognisable speed transducer on the gearbox anywhere. Anybody have any manuals that can shed light on this?
jp 08 gtb 6mt wagon

by **tangcla** » Tue Mar 18, 2008 9:07 am

Unless, it's just the speedo needle that's not calibrated?

Though that sounds kinda dodgy, if that's teh case...

by **mianos** » Tue Mar 18, 2008 10:38 am

It would be a real surprise of the speedo needle in my car (MY08 3RB) was out, as it seems to be NC. That's how it does that 'ricey' rise and fall when you start the car systems. I assume it's all digital and they simply fudge it down a bit. More likely also considering everyone seems to have a similar degree of over reporting.

by **jp928** » Tue Mar 18, 2008 12:31 pm

If the needle was offset on the shaft, it would be out a fixed amount no matter wat the indicated speed. Mine acts like a 5% error, not a fixed amount, ie 3kph@ 60, 5@100
jp

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