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Practical Dyno Tuning

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Hi everyone, I have recently installed on my mx-5 (Na) a megasquirt Ms3 ecu, this is my first experience with a programmable ecu, the car works, and I am using the auto map function to create the Ve table, after every set-up I look at the logs to check that everything is normal, but I noticed that with a cold engine I have a minimum depression of 28Kpa, which become 38Kpa with a warm engine. I don't think it's normal, what could be the problem? thank you all

Why don't you think it's normal? Under what conditions do you see this "minimum depression"? Are you talking about the engine at idle?

What kind of dyno are you using?

Yes, I'm talking about the engine at idle, starting from a clt at 35 °C up to 85 °C, I think that an old (100k miles) and cold engine has a low depression which improves with thermal expansion. these are the first road tests, I haven't been on the dyno yet

Is there Idle Air Control done by the ECU? That could result in a different MAP reading as it attempts to control the idle using a minimum spark advance, but more air bypass.

Looking at your logs -- is the Ignition Timing the same at 28kPa and 38kPa?

Is the RPM exactly the same -- a higher RPM will have a lower manifold pressure for a given throttle position. Often an ECU with idle control will request a faster engine idle speed when cold and reduce this as it warms up.

To answer your original question -- I don't think you need to be concerned.

Is there Idle Air Control done by the ECU? Yes

Looking at your logs -- is the Ignition Timing the same at 28kPa and 38kPa? Yes

Is the RPM exactly the same -- they are the same

looking at the attached photo is everything normal?

Attached Files

The Lambda at idle seem pretty rich (0.83, particularly if that EGO Correction of .85 indicates a -15% trim). The target AFR looks to be Lambda 1.0 (14.7 AFR), so your VE table value is too large for this condition.

WTH? I had already commented but it's not here - been a few of those of late...

To iterate, when cold the engine oil is more viscous (thicker) and there are two reasons this will affect manifold depression - possible both. As the engine warms up, so does the oil and so the drag is reduced and manifold vacuum/depression is increased as less power is required to overcome the drag.

More internal drag means the rpm will be pulled down untill the engine torque can equalise the drag and, for the same throttle opening, this will mean less vacuum.

Second is if the engine has an active idle control of some kind - I think your does? - in this instance, to counter the increased drag from the oil the ECU will open the throttle slightly to provide a little more power to maintain the idle speed. This will also reduce the vacuum in the manifold.

Perfectly normal and not something to be concerned about.