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PostPosted: Tue Dec 10, 2013 11:48 am 
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From the MR2OC.

jon.sole wrote:
The ST205 ECU has a lower fuel cut at higher revs. The Rev 3 MR2 ECU has the same fuel cut across the rev range.


jon.sole wrote:
Both ECUs have a table of 3 values that controls the fuel cut limit at 3 rev ranges.
On the MR2 ECU the 3 values are the same:

Code:
ROM:C1EF table_boost_limit:     .dw 0200h
ROM:C1EF                               ; Boost cut lookup table
ROM:C1F1                               .db 02h
ROM:C1F2                               .db 218, 218, 218          ; 255 on TechTom ECUs
ROM:C1F5

On the ST205 the last entry is reduced, which has the effect of reducing fuel cut progressively from 4800rpm to 6400rpm:

Code:
ROM:C1B8                 .dw 0200h
ROM:C1BA                 .db 02h
ROM:C1BB                 .db 218, 218, 205

I don't know the exact conversion from the raw values in the table to PSI, but it is around a 5% drop at 6400rpm. I'm guessing the lower fuel cut at higher RPM is to protect the transmission.

Modified ECUs with a TechTom board fitted change the values in the table to 255 to effectively remove fuel cut.


shinny wrote:
Assuming 0 means 0V from the map sensor and 255 means 5V and based upon Richard Doig's formulas (http://gtfour.supras.org.nz/mapsensor.htm) I calculated the MAP sensor's voltages...

1.31bar -> 19psi -> 4.27V -> 218
1.14bar -> 16.5psi -> 4.02V -> 205

That puts the ST205's fuel cut at 1.14 bar, which is lower than I expected! I didn't have trouble running 1.15 bar on my original external wastegate setup and the ST205 ECU, so perhaps Richard's formula is fractionally incorrect?

Edit: This page suggests the ST205 has fuel cut set at 17psi / 1.17bar: http://k1ano.co.uk/boostcontrol/rvgt4specifics.htm

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PostPosted: Tue Dec 10, 2013 11:47 pm 
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Interesting post.....not sure exactly what it all means in reality. Needs a universal translator or a cold cloth on the forehead :mrgreen:

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PostPosted: Tue Dec 10, 2013 11:52 pm 
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Yep very interesting. I'm not sure that any of the measurement kit most of us have fitted will have greater real time accuracy than about 0.1 bar so 1.14 1.15 I don't think there is any real way of telling.

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PostPosted: Wed Dec 11, 2013 10:27 am 
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Interesting info
There could be a host of reasons. Gearboxes are normally rated for maximum torque capability I thought with only cooling requirements being greater at higher rpm?

I would initially suspect it is more to do with the turbo efficiency and preventing lots of hot air entering the engine.

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PostPosted: Wed Dec 11, 2013 11:25 am 
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I've already commented on the MR2 thread that I don't think it's transmission limited.

I think your turbo theory makes sense, but then why would the MR2 not be limited.

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PostPosted: Wed Dec 11, 2013 12:30 pm 
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As the mr2 runs less power it might well be consuming the same amount of air at higher boost than compared to a 4


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