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EQ Tuning · 2007-11-12, 06:32 PM

As for some useful info, I do prefer blow-through MAF housings with just about any FMIC setup. Here are some of the main advantages:

-- Air metering dellay
When using a FMIC with a standard pull-through type setup, you're adding a lot of volume between the MAF sensor and the throttle body that cannot be accounted for in the ECU. This added volume introduces an inherent delay in the air metering and fueling system. In simpler terms, the air gets metered by the MAF sensor, then travels through the turbo, through the long FMIC piping, and the big fmic core, before finally getting to the throttle body. Since the ECU isn't set up for this much volume between the MAF and throttle body, it ends up reacting to the metered air before that air actually gets into the engine. This causes inconsistent fueling and the common rich spike during quick boost transitions generally associated with FMIC's.

Using a blow-through setup moves the MAF sensor much closer to the throttle body and eliminates this delay. The end result of this is more accurate fueling, no more rich spike on quick boost transitions, and much crisper throttle response. Drivability in general is vastly improved if tuned correctly.

-- Intake temp measurement
In most Subaru's the intake temp sensor is integrated into the MAF sensor. When the MAF sensor is used in a pull-through arrangement, the measured temperatures are heavily influenced by underhood temps rather than intercooler efficiency. This means that the ECU has no idea how hot or cold the intercooler is. The FMIC may be completely cool and the air that's actually going into the engine may be near ambient while at the same time the underhood temps can be very hot. With a pull-through setup the ECU can only see the underhood temps which makes the built in IAT trim tables somewhat useless.

With a blow-through setup, however, the intake temps as measured at the MAF sensor will actually correspond to the temperature of the air going into the throttle body. So if the FMIC is nice and cool , the ECU knows about it and doesn't pull timing. If the FMIC gets heatsoaked, the ECU knows about this as well and the ignition maps can be adjusted accordingly to pull timing before knock occurs.

-- Intake/BOV setup
Finally, using a blow-through MAF allows you to run whatever you want on the intake side... you could even just run a screen on the turbo if that's your style

. Since the MAF sensor is no longer there, there is absolutely no limit to what you can do with the intake. This is great for a lot of the rotated setups because you can now run a nice free-flowing 4" intake without any sort of restrictions.

This also means that you can run a fully atmospheric BOV without any of the associated side effects. Since the BOV is positioned before the MAF in a blow-through setup, the air that the BOV is venting has not yet been metered by the MAF sensor and won't have any impact on the resulting a/f ratio and drivability.

All of that being said, remember that the final results will really depend on the tuning. Blow-through MAF Setups take a slightly different approach to tune properly. I've seen some very poorly running cars with blow-through simply because the tune wasn't right. If done correctly, however, a blow-through setup will provide significantly better overall drivability, response, and performance than any FMIC setup with a pull-through setup.

BTW, Perrin developed their blow-through MAF based on some discussions and data I gave them after putting together a custom setup for a customer with a GT30R. We used the standard Perrin Big MAF intake housing as a starting point for this setup:

Thanks
-- Ed

2007-11-12, 06:32 PM	#17
EQ Tuning EJ22 Join Date: Mar 2006 Posts: 132	As for some useful info, I do prefer blow-through MAF housings with just about any FMIC setup. Here are some of the main advantages: -- Air metering dellay When using a FMIC with a standard pull-through type setup, you're adding a lot of volume between the MAF sensor and the throttle body that cannot be accounted for in the ECU. This added volume introduces an inherent delay in the air metering and fueling system. In simpler terms, the air gets metered by the MAF sensor, then travels through the turbo, through the long FMIC piping, and the big fmic core, before finally getting to the throttle body. Since the ECU isn't set up for this much volume between the MAF and throttle body, it ends up reacting to the metered air before that air actually gets into the engine. This causes inconsistent fueling and the common rich spike during quick boost transitions generally associated with FMIC's. Using a blow-through setup moves the MAF sensor much closer to the throttle body and eliminates this delay. The end result of this is more accurate fueling, no more rich spike on quick boost transitions, and much crisper throttle response. Drivability in general is vastly improved if tuned correctly. -- Intake temp measurement In most Subaru's the intake temp sensor is integrated into the MAF sensor. When the MAF sensor is used in a pull-through arrangement, the measured temperatures are heavily influenced by underhood temps rather than intercooler efficiency. This means that the ECU has no idea how hot or cold the intercooler is. The FMIC may be completely cool and the air that's actually going into the engine may be near ambient while at the same time the underhood temps can be very hot. With a pull-through setup the ECU can only see the underhood temps which makes the built in IAT trim tables somewhat useless. With a blow-through setup, however, the intake temps as measured at the MAF sensor will actually correspond to the temperature of the air going into the throttle body. So if the FMIC is nice and cool , the ECU knows about it and doesn't pull timing. If the FMIC gets heatsoaked, the ECU knows about this as well and the ignition maps can be adjusted accordingly to pull timing before knock occurs. -- Intake/BOV setup Finally, using a blow-through MAF allows you to run whatever you want on the intake side... you could even just run a screen on the turbo if that's your style . Since the MAF sensor is no longer there, there is absolutely no limit to what you can do with the intake. This is great for a lot of the rotated setups because you can now run a nice free-flowing 4" intake without any sort of restrictions. This also means that you can run a fully atmospheric BOV without any of the associated side effects. Since the BOV is positioned before the MAF in a blow-through setup, the air that the BOV is venting has not yet been metered by the MAF sensor and won't have any impact on the resulting a/f ratio and drivability. All of that being said, remember that the final results will really depend on the tuning. Blow-through MAF Setups take a slightly different approach to tune properly. I've seen some very poorly running cars with blow-through simply because the tune wasn't right. If done correctly, however, a blow-through setup will provide significantly better overall drivability, response, and performance than any FMIC setup with a pull-through setup. BTW, Perrin developed their blow-through MAF based on some discussions and data I gave them after putting together a custom setup for a customer with a GT30R. We used the standard Perrin Big MAF intake housing as a starting point for this setup: Thanks -- Ed Last edited by EQ Tuning; 2007-11-13 at 03:41 PM.