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      04-13-2010, 02:10 PM   #1
OpenFlash
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Exclamation Reading DME Data

Originally posted by Mike@n54tuning in this thread.
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Originally Posted by shiv@vishnu
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More misleading info. None of those parameters are read by the jb3. DME actual boost is not read either. What is read by the jb3 is the desired MAP voltage output it is sending to the dme. The actual voltage being fed into the dme and what boost actual reading the DME translates this voltage to is NOT read by the jb3. These values are all internal to the DME. Reading with an external device (jb3) is only possible by reading data off of the CAN network.
Shiv you're getting pretty ridiculous now. The JB3 is generating the voltage signal, so of course it knows what voltage it is creating. It's loggable in the JB3 interface as ECU PSI. The DME itself converts that voltage to kpa. Since the V4 logs PSI like the JB3 instead of kpa it must not be reading the value either right? Pretty silly argument.
I don't know why that thread was closed. I thought the purpose of this forum was to discuss tech.

So here goes:

No, Mike. I'll try to explain it in another way. When a piggyback feeds a MAP sensor voltage to the DME, the DME interprets the voltage as a certain MAP/boost reading. I think we can all agree on that.

However, the voltage that is being fed to the DME is not measured/interpreted equally by the DME and Piggyback. For these next scenarios, let's say the piggyback wants to output 2.5v to the DME.

Due to calibration/hardware differences between DME units, some DMEs may measure this as low as say 2.45v while others as high as 2.55v (just estimates).

Similarly, due to calibration variance between different units of that same piggybacks, the actual voltage output will naturally have an output variance range as well of say, 2.45 to 2.55v (again, just estimates).

This measurement variance is normal for such electronic devices. This is also the same reason you can measure the same signal on 3 different voltmeter and see 3 slightly different values.

So when you factor in that no two piggbacks units (of the same type) will, despite the same mapped output, give exactly the same voltage AND that no two DMEs will measure the same input signal to be the exact same voltage reading, it's a fact that you are going to see cumulative differences of a fraction of a volt. And each tenth of a volt is 0.75 psi. This is not an insignificant increment.

The reason we know this is because we saw this first hand. When we were in the early development stages of v4, we were only testing on my car. And, in doing so, the tune would perfectly satisfy the DME's boost target requirement. This means that DME boost target and DME boost actual would be right on top of each other as one would want. This was great.

However, we we tested the same exact map/firmware another car, we noticed that there was measurable negative boost error. So much, in fact, that we would through an underboost code during sustained load.

And then we tested in another car. This time it would show a positive boost error and trigger throttle closure and an overboost error during sustained load.

Then we looked at the actual DME readings for boost actual and we noticed that they weren't the same for the reasons mentioned above.

To solve this, we had to develop an auto-calibration routine that start every time you start the car. It compares the boost value that the Procede *thinks* it is outputting to the CAN DME boost actual that the DME is seeing. And applies a running average correction to keep the two within a hair of each other. You can view both the instantaneous voltage error and the average average voltage error in the datalogging system.

This is why I take objection to suggesting that the JB3 can read DME boost actual. Because it doesn't/can't. The JB3 only knows the voltage that it is mapped to output. The actual voltage AND how that voltage is interpreted by the DME are unknowns. The latter will always be unknown until it is read off the CANbus.

I hope that explains things...

Shiv

PS. Mike, you are more than welcome to post in this thread.

Last edited by OpenFlash; 04-13-2010 at 05:10 PM..
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