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MHI Small 16g vs. MHI Evo III 16g Air/Fuel Ratio

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Smallfry311

10+ Year Contributor
52
0
Jan 23, 2009
Franklin, Tennessee
I just recently swapped an Evo III 16g turbo for my small 16g in my 2g GST. I was wondering if I had my A/F ratio tuned for 19psi with the small 16g, could I still run the Evo III 16g at the same pressure and achieve the same A/F ratio results? Or does the increased mass flow rate affect the A/F ratio therefore making it run too lean at 19psi?

Thanks for the comments.
 
This is an interesting question. In my own opinion I would think it doesn't because the air intake that's calculated my the maf and the ecu is trying to make a certin af mixture with it using the injectors pulse.. if your running link I think it would be the same but deff post results I'm interested in knowing
 
My wide-band O2 sensor just recently went out, so I'm just trying to be safe. I am tuning with an SAFC right now, but I think I'm going to dyno tune it asap. By the definition of AF ratio (AF = mdot_air/mdot_fuel) it should increase no matter what pressure I run I think.
 
Target AFR is controlled by load condition and RPM so I think the difference would be because of the difference in spool time and not airflow right? Because the ECU will determine the injector duty cycle to achieve the AFR set in the tables?

This is my understanding, please correct me if I'm wrong. This is a good question
 
Target AFR is controlled by load condition and RPM so I think the difference would be because of the difference in spool time and not airflow right? Because the ECU will determine the injector duty cycle to achieve the AFR set in the tables?










You bring up a good point with the targeted af ration under a load condition. But the spool time would make no differance being it's judging the air intake through the maf, brings back to the idea of the amount of air comming througg the maf will be slightly less and the ecu will compensate for less air with less fuel but he's using a safc so even the way I'm thinking may be off because I use link and have no experience with a safc but it works by making the ecu thinks theirs less or more air then their actually is so if the settings are the same and injectors are the same I can't see it being much differant I'm sure it would be able to drive and find out would deff get the wideban uo and running for it though
 
This is a great question that I am interested in finding an answer to as well. Not sure if this helps the conversation, but I believe I've read before that you should re-tune a car depending on if its winter or summer. The theory is that in the winter the colder, denser air packs more air molecules, therefore requiring more fuel to be burnt. While in the summer, the hotter, thinner air makes the engine more prone to knock, therefore requiring you to retard timing.

However, wouldn't the MAF and barometer make adjustments to these conditions? Just thinking out loud. Someone please correct me if I'm wrong.
 
As long as you are running a maf, and have a large enough fuel system you shouldn't have any issues. This is the beauty of maf based tuning. The airflow is actually measured(instead of calculated in sd), so changes in airflow are taken into account by the ecu and fueling is changed to meet the demands.
 
As long as you are running a maf, and have a large enough fuel system you shouldn't have any issues. This is the beauty of maf based tuning. The airflow is actually measured(instead of calculated in sd), so changes in airflow are taken into account by the ecu and fueling is changed to meet the demands.

That is true for closed loop operation.
I think the biggest issue would be a maf calibration issue at target hrtz due to a bigger flowing turbo even if it's slightly bigger. So where if it was calibrated prior and his target a/f is met with the first turbo, now the maf calibration might be off (probably not by a lot) and the target vs wideband a/f will differ.
 
That is true for closed loop operation.
I think the biggest issue would be a maf calibration issue at target hrtz due to a bigger flowing turbo even if it's slightly bigger. So where if it was calibrated prior and his target a/f is met with the first turbo, now the maf calibration might be off (probably not by a lot) and the target vs wideband a/f will differ.
Changing the turbo does nothing to maf calibration, it actually measures the mass of the air. In a speed density setup, changes in volumetric efficiency can change fueling, such as a different size turbo, cams, exhaust, etc.

In a maf system, as long as the fuel system can flow enough, and you aren't out of the range of the maf sensor(around 50lbs/min for a 2g) fueling will be fine.

Closed loop operation uses the o2 sensor feedback to trim fueling.

Changing the turbo will change how much air is flowing through the engine. But the maf measures it, and changes injector pulse to compensate and still hit the target afr.

If you weigh yourself on a scale you get a number. Take that number and use it in a calculation. Now drink a gallon of water, and do the same calculation, you'll get a different number.
 
Changing the turbo does nothing to maf calibration, it actually measures the mass of the air. In a speed density setup, changes in volumetric efficiency can change fueling, such as a different size turbo, cams, exhaust, etc.

In a maf system, as long as the fuel system can flow enough, and you aren't out of the range of the maf sensor(around 50lbs/min for a 2g) fueling will be fine.

Closed loop operation uses the o2 sensor feedback to trim fueling.

Changing the turbo will change how much air is flowing through the engine. But the maf measures it, and changes injector pulse to compensate and still hit the target afr.

If you weigh yourself on a scale you get a number. Take that number and use it in a calculation. Now drink a gallon of water, and do the same calculation, you'll get a different number.

That makes more sense.
could it be true that maybe changing the target a/f is needed if lets say it was targeted for 11.5:1 with a 40lb/min turbo then a 60lb/min turbo could be causing more knock due to more air and not enough fuel at the same target a/f of 11:5:1?

Is it also safe to say once a maf has been calibrated then with a bigger turbo all is needed to play with target a/f if needed and timing assuming you have enough fuel for the turbo upgrade?

Thanks
 
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That makes more sense.
could it be true that maybe changing the target a/f is needed if lets say it was targeted for 11.5:1 with a 40lb/min turbo then a 60lb/min turbo could be causing more knock due to more air and not enough fuel at the same target a/f of 11:5:1?

Is it also safe to say once a maf has been calibrated then with a bigger turbo all is needed to play with target a/f if needed and timing assuming you have enough fuel for the turbo upgrade?

Thanks
A maf doesn't care what turbo is behind it, the airflow is measured the same, as long as the airflow is within the design parameters of the meter.

The only way you would have a situation where more air led to not enough fuel is if the fuel system lacks the flow capabilities to met the demands of the ecu.
 
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