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jibberjive
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« on: December 05, 2011, 11:31:41 PM »
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One question that I've gone on multiple search excursions for and can't seem to nail down regarding tuning for MBT, is MBT boost dependent? For example, if I were to tune for MBT on E85 at 13psi of boost, then eventually I do another tune at 32psi on the same fuel/setup, would the optimal timing advance be the same? Or does one absolutely need to retune the timing tables for specific boost profiles?
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« Last Edit: December 06, 2011, 12:11:20 AM by jibberjive »
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judeisnotobscure
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« Reply #1 on: December 06, 2011, 03:26:52 AM »
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Surely it must be boost dependent. Simply think about mbt
cylinder pressure will affect burn time, more pressure= faster flame front which means less timing advance needed to meet mbt
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« Last Edit: December 06, 2011, 03:37:44 AM by judeisnotobscure »
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I have a b5 s4
but i just want to dance.
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robin
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« Reply #2 on: December 06, 2011, 06:26:55 AM »
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If your MAF scaling is accurate and you have real airflow measurements... then ME7 will drop the timing request accordingly with more boost. 'Optimal' timing advance is just that.
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jibberjive
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« Reply #3 on: December 06, 2011, 12:53:25 PM »
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So, let's take ME7 out of the equation for a simple hypothetical.
Let's say, a bare bones car (non-Audi) on bare-bones standalone. This is on alcohol, so there are no pre-detonation (or any other) concerns limiting boost, timing or fueling. You tune the car on a dyno and magically precisely find MBT for 5000RPM and 15psi of boost. You up the boost by 15psi, so lets say you're now holding 30psi completely flat to redline. So, at 5000RPM and now 30psi of boost, is the value of MBT the same?
Anyone by chance have links to any technical papers or the like?
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« Last Edit: December 06, 2011, 12:58:18 PM by jibberjive »
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elRey
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« Reply #4 on: December 06, 2011, 02:40:41 PM »
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No.
More boost = denser AF mixture (more or less) = faster burner which requires less timing to reach same peak cylinder pressure.
So, no. MBT @ one boost/RPM != MBT @ a different boost/RPM
BUT!!!
And good timing table will have have load (boost) vs RPM.
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jibberjive
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« Reply #5 on: December 06, 2011, 06:03:13 PM »
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Yeah, thinking about the fundamentals more, it is kind of a dumb question. So many places though you hear of people tuning for E85 by tuning for MBT, then adjusting boost afterwards, which doesn't make much sense if MBT is affected much by the boost pressure. Maybe it's not so big of a deal.
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Rick
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« Reply #6 on: December 10, 2011, 07:20:39 PM »
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It is effected, of course it is, E85 is nowhere near knock proof anyway, just better than unleaded.
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jibberjive
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« Reply #7 on: December 11, 2011, 03:47:27 AM »
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Most (basically all) of what I've seen with E85 is that people are able to reach MBT before reaching the knock limit. Have you had a different experience?
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fredrik_a
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« Reply #8 on: December 11, 2011, 02:24:31 PM »
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Most (basically all) of what I've seen with E85 is that people are able to reach MBT before reaching the knock limit. Have you had a different experience?
I've never had any issues with knock on E85 before reaching MBT in the dyno compared to regular petrol. Also, heavy fuel enrichment for knock/ping prevention using E85 is extremely rare in my experience and I've calibrated quite a few cars on both petrol and E85. Fuel enrichment for regular pump gas is usually necessary at 600 mg/str but E85 you can usually run up to 900-1.000 mg/str without any direct need for enrichment. In my experience this has been a good rule of thumb. |
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