Load to boost/airflow scaling

Moreload

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Mar 23, 2024
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First post so hello everyone

Ive been studying mevd and mg1 bosch on bmw trying to understand how this all works but I am lost on one area.

I understand the ecu's are torque based and using diop table torque is converted to load and at some point the table is used in reverse to calculate back to torque.

I understand load is not boost but is cylinder fill/ volumetric efficiency and boost/positive pressure is used to achieve higher load numbers. In the case an engine had higher compression, more ignition timing due to high octane fuel then i assume less boost would be required to reach target load.

What table/ tables are used to calculate the amount of airflow/ positive pressure for a given load. This is theoretical as i am still stock turbo (b58) but in the case of a high compression engine or larger turbo and someone wanting to keep correct logic some airflow to load rescaling would be required?
 

wheela

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Welcome to spoolstreet!

As long as you don't change the housing your MAF sensor sits in you should be fine. The DME calculates the load% based on the measured airflow into your engine and rpm. So as long as your MAF readings remain correct, your load% should also be correct.

Here's another thread with some information about load%.


I don't know the technical details, but the DME also calculates MAF independent of the MAF sensor (some kind of speed density, or volumetric efficiency based calculation) as a sanity check for the MAF reading from the MAF sensor. If the two don't match within a certain threshold, you'll get implausible airflow codes. Also, at least on n55, if you disconnect the MAF sensor, I believe the DME will default to using the sanity check airflow calculation for calculating load actual%. (and throw a code)

If you change something that significantly impacts volumetric efficiency (i.e. ported head or cams), that would throw off the sanity check calculation and cause it to not agree with the MAF reading. I'm not sure what tables would need to be adjusted in that case (assuming you want to keep your MAF reading accurate).

Similarily, if you change the housing the MAF is in, now your MAF reading could be incorrect, so load actual% could also be incorrect, and that reading may not match the sanity check airflow calculation. In this case, for an n55, the HFM (Hot Film Massflow) tables would need to be adjusted to re-calibrate the MAF to read correctly in the different housing.

Long story short, unless you're going MAFless, or porting your head, or installing bigger cams, I don't think you'll have to worry about making updates to the sanity check calculations. If you change your MAF housing, then you'll want to update the b58 equivalent of HFM tables to recalibrate your MAF.
 
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Moreload

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Mar 23, 2024
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Thank you so much for this detailed answer. I had searched high and low and had come across that post. Was useful but not as useful as your answer. My reasoning is more theoretical. I assume some these newer engines like the s63t4 and s58 use this calculation at all times as they are maf-less but most their tables remain maf based calculations.

Im miles away from needing this but I see some cars making 40-50psi on built engines and wonder if they rescale load- airflow as im not sure the max load would have the headroom to go so high without being adjusted.
 

wheela

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Not sure about people running 40-50 psi. I'm still on an OTS tune, and only plan on running up to around 20psi on 93 when I get my larger turbo on.

For the MAF based cars like n55, I'm under the impression the breakpoints of the tables that reference load% and MAF can be updated to larger values to expand the operating range. But I'm sure the DME has upper limits of what values can be entered, and the MAF sensor itself must also have an upper range it can't measure beyond. I don't know what those limits are.

For MAFless, my understanding for the n54 guys is when the run the n20 tmap for higher boost, they split its range up into two halves, and re-scale the top half to a different slope. This lets them run higher boost, but when they do this, if that's all they change, I don't think the DME knows the actual boost, and being MAFless I think the DME would mis-calculate MAF and load actual% in that re-scaled top half of the tmap range. That means they need to update/resale the corresponding top halves of all the other tables that reference load and MAF. But n54's don't run Bosch DME's - not sure how different this is for the other MAFless BMW engines you mentioned?

I'd love for some guys to comment on this/correct anything I've gotten wrong, as I'm still on an OTS tune and haven't actually self-tuned my car yet, so I could certainly be getting some things wrong.
 
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Moreload

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Mar 23, 2024
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I know 40-50 is abit of a reach just using as example for numbers above normal load calculation. Load to Maf tables makes complete sense and even on mafless cars I would not be surprised if the tables exist as they still use maf for their compressor tables etc.

Dont suppose you know the name of any tables that could possibly be for load/ maf lookup