limit of the std ECU

dynamix · 12-19-2007, 05:40 PM

I am just about to commence a built 2.5 litre probably mated with an FP red and wanted to know at what point will the MAF based system become a limiting factor in it.

I am contemplating other ecu's but would rather stay with std ecu because of the excellent knock control strategy in place.

The maf scaling at present should allow resolution up to 460 gram/sec although the top end of this scale has not been tested for obvious reasons.

Power / Torque targets are over 500/500 at the flywheel running 99 octane and 10% in tank methanol mix

other bits:

APS CAI 70mm
APS DR725 FMIC
840cc injectors
3inch full decat

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Master2192 · 12-19-2007, 05:59 PM

With that BigMAF intake you got, you will max out the fuel pump (assuming a walbro 255) before you max out the MAF sensor.

The various sensors are pretty much the only restriction of the stock ECU. MAF sensor being the main one, MAP sensor if you are using the stock boost control at boost above 26 psi. IIRC people have tuned up to 1200cc injectors on the stock ecu without issue.

dynamix · 12-20-2007, 04:09 AM

Yes, Walbro 255 is in situ.

This is my maths to see what it could do but dont know the limits of the ECU at this end of the scale (yet)

255L/h = 4.25 litres per min
4.25 litres per min = 3.2045 kg/min (at 0.754 specific gravity)

Assuming my injectors are not going to change:

858cc x 4 = 3.432 Litres/min
Mass of which is 2.588 kg/min

Assuming I still target 11:1 afr

The injectors at 85% IDC out will require: 24.2 kg/min of Air (403 g/sec)
The pump maxxed out will require: 35.250 kg/min of Air (474.50 g/sec)

With UK pump fuel you end up with somewhere between 20 and 22 BHP for every kg/min of air that you move through the engine. This could yield somewhere between 484 - 532 bhp

Anyone got any views on the maths stuff to start with and any specific issues that I will encounter on the ecu tuning of it?

Master2192 · 12-20-2007, 09:23 AM

Quote:

Originally Posted by dynamix

255L/h = 4.25 litres per min

This value...isn't going to help you much. The flow goes down with pressure, since the car uses a 1:1 FPR set at 43.5 psi base, at 30 psi of boost you should have a fuel pressure of 73.5. Look how fast that blue line drops after 60 psi.

dynamix · 12-21-2007, 12:42 PM

thanks - useful info

I may need to think about a heavier duty fuel pump then.

WeldingHank · 12-21-2007, 01:37 PM

or possibly supplying it with more voltage. or a combination of the 2.

DaFastOne · 12-28-2007, 04:00 AM

dynamix what ecu are you planning to run w/? anyone know anything about the jdm sti ecus?

dynamix · 12-30-2007, 11:19 AM

Quote:

Originally Posted by DaFastOne

dynamix what ecu are you planning to run w/? anyone know anything about the jdm sti ecus?

I will be running the std 32 bit ECU

The JDM STi ecu is the same but with slightly different maps in it from what I know.

Freon · 12-30-2007, 01:08 PM

The 32bit ECU itself has no technical limit on the airflow, load, injectors, etc. it can precisely calculate. Almost everything is done in IEEE-754 floating point precision. You could run millions of grams per second with 20,000cc injectors on it and you would not run into a mathematical limit on the ECU. Certainly it would take an inordinate amount of tweaking, but it would calculate a correct load, airflow, injector pulse width, etc.

The limits of the stock ECU have more to do with getting good sensitivity out of the physical MAF element itself with huge jumps from idle to max airflow. Also, no current stock ECU reflash lets you add extra injector outputs or dual boost control solenoid outputs, etc. Also, there seems to be no controls for ignition dwell, or at least I don't think any reflash has found it if it is possible.

12-19-2007, 05:40 PM	#1
dynamix Scooby Newbie Member#: 122603 Join Date: Aug 2006	limit of the std ECU I am just about to commence a built 2.5 litre probably mated with an FP red and wanted to know at what point will the MAF based system become a limiting factor in it. I am contemplating other ecu's but would rather stay with std ecu because of the excellent knock control strategy in place. The maf scaling at present should allow resolution up to 460 gram/sec although the top end of this scale has not been tested for obvious reasons. Power / Torque targets are over 500/500 at the flywheel running 99 octane and 10% in tank methanol mix other bits: APS CAI 70mm APS DR725 FMIC 840cc injectors 3inch full decat * Registered users of the site do not see these ads.

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12-19-2007, 05:59 PM	#2
Master2192 RIP 10/12/83 to 02/10/08 Scooby Specialist Member#: 83254 Join Date: Mar 2005 Chapter/Region: NWIC Location: Roy, Washington Vehicle: 98 2.5 RS 04 WRX	With that BigMAF intake you got, you will max out the fuel pump (assuming a walbro 255) before you max out the MAF sensor. The various sensors are pretty much the only restriction of the stock ECU. MAF sensor being the main one, MAP sensor if you are using the stock boost control at boost above 26 psi. IIRC people have tuned up to 1200cc injectors on the stock ecu without issue.

12-20-2007, 04:09 AM	#3
dynamix Scooby Newbie Member#: 122603 Join Date: Aug 2006	Yes, Walbro 255 is in situ. This is my maths to see what it could do but dont know the limits of the ECU at this end of the scale (yet) 255L/h = 4.25 litres per min 4.25 litres per min = 3.2045 kg/min (at 0.754 specific gravity) Assuming my injectors are not going to change: 858cc x 4 = 3.432 Litres/min Mass of which is 2.588 kg/min Assuming I still target 11:1 afr The injectors at 85% IDC out will require: 24.2 kg/min of Air (403 g/sec) The pump maxxed out will require: 35.250 kg/min of Air (474.50 g/sec) With UK pump fuel you end up with somewhere between 20 and 22 BHP for every kg/min of air that you move through the engine. This could yield somewhere between 484 - 532 bhp Anyone got any views on the maths stuff to start with and any specific issues that I will encounter on the ecu tuning of it?

12-21-2007, 12:42 PM	#5
dynamix Scooby Newbie Member#: 122603 Join Date: Aug 2006	thanks - useful info I may need to think about a heavier duty fuel pump then.

12-21-2007, 01:37 PM	#6
WeldingHank Scooby Specialist Member#: 98645 Join Date: Oct 2005 Chapter/Region: NESIC Location: Lawrence,ma Vehicle: 2002 WRX Wagon PSM	or possibly supplying it with more voltage. or a combination of the 2.

12-28-2007, 04:00 AM	#7
DaFastOne Scooby Newbie Member#: 19396 Join Date: May 2002 Location: BAY AREA, CA Vehicle: 02/6 FrankensteinWRX black	dynamix what ecu are you planning to run w/? anyone know anything about the jdm sti ecus?

12-30-2007, 01:08 PM	#9
Freon Scooby Specialist Member#: 88322 Join Date: Jun 2005 Chapter/Region: MWSOC Location: Indianapolis, IN Vehicle: 2009 BMW 135i	The 32bit ECU itself has no technical limit on the airflow, load, injectors, etc. it can precisely calculate. Almost everything is done in IEEE-754 floating point precision. You could run millions of grams per second with 20,000cc injectors on it and you would not run into a mathematical limit on the ECU. Certainly it would take an inordinate amount of tweaking, but it would calculate a correct load, airflow, injector pulse width, etc. The limits of the stock ECU have more to do with getting good sensitivity out of the physical MAF element itself with huge jumps from idle to max airflow. Also, no current stock ECU reflash lets you add extra injector outputs or dual boost control solenoid outputs, etc. Also, there seems to be no controls for ignition dwell, or at least I don't think any reflash has found it if it is possible.