Understanding draw-through MAF systems with centri

40th GT

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So I installed a 10% OD crank pulley for my Vortech V1 setup, and it's currently not tuned for that. For obvious reasons I have not attempted to see what the new boost level is, but I noticed while datalogging I have LTFTs climbing to 1.25 on both banks and they quickly settle closer to 1.0 as soon as revs are raised. I've checked all vacuum lines, intake manifold bolts and throttle body bolts/gasket for leaks. There were two evap lines coming out of the charcoal canister that looked suspect so I replaced those. Car no longer smells like fumes but the LTFT behavior hasn't changed. Short term trims average right at 1.0. It's not giving me any codes. The only symptom is this high LTFT.

My question is, since the supercharger discharge is after the MAF, did I induce unmetered air by changing pulley sizes? This is my first time working with draw through systems so sorry for the noobish question but I want to understand fundamentals.
 

robvas

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You didn't change much by changing pulley sizes. If it ran before, it should run now. Are you overrunning the MAF?

What do you mean your supercharger discharge is after the MAF? Any (compressed) air that comes out of the blower, comes through the inlet of the blower so it has gone through your MAF already.
 

40th GT

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I was also thinking the MAF needs to be rescaled because MAF counts were at 850 at 5500rpm. I didn't take it any further.

Sorry for the weird wording. I was thinking since the blower is blowing air after the MAF, that amount of air is unmetered. But you're saying it's the same amount of air compressed so it's metered.

I have to figure out where this vacuum leak is. o_O
 

DSG2003Mach1

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the blower is still spinning faster at all rpms, therefore drawing more across the maf at any given rpm.

I don't know if it could have anything to do with what it's learned in the KAM or not. Maybe unhook the battery for a few, then fire the car up and let it idle for 5-10 minutes with ac off then another 5-10 with ac on and see if it changes anything?
 

01yellercobra

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I was also thinking the MAF needs to be rescaled because MAF counts were at 850 at 5500rpm. I didn't take it any further.

Sorry for the weird wording. I was thinking since the blower is blowing air after the MAF, that amount of air is unmetered. But you're saying it's the same amount of air compressed so it's metered.

I have to figure out where this vacuum leak is. o_O
You don't rescale the MAF. You're probably reaching a different part of the curve now so you might have to tweak the upper end. Really the low end shouldn't change. I'd say you have a leak somewhere. Or the tune was done around the leaking hoses you had and now needs to be readjusted because everything is sealed as it should be.

As long as there is no leak after the MAF all the air post MAF is metered. That's why you have to run a BPV. If you switched to a draw through set up you could run a BOV vented to atmosphere. That's my preference with centri set ups.
 

JGDogg10

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If the scaling was correctly done in the original tune, adding a few more pounds of boost shouldn’t cause the tune to need to be changed. The bpv should be directing air back in to the pipe between the MAF and the blower inlet, if it’s too close to the MAF and with the increased airflow you could be sending air back towards the MAF and causing misreadings. It does kinda sound like a leak of some sort, but you could log your o2 voltage as well just to make sure they are functioning properly and there isn’t an issue there
 

4sdvenom

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You don't rescale the MAF. You're probably reaching a different part of the curve now so you might have to tweak the upper end. Really the low end shouldn't change. I'd say you have a leak somewhere. Or the tune was done around the leaking hoses you had and now needs to be readjusted because everything is sealed as it should be.

As long as there is no leak after the MAF all the air post MAF is metered. That's why you have to run a BPV. If you switched to a draw through set up you could run a BOV vented to atmosphere. That's my preference with centri set ups.
Your wording is off in the last paragraph. I think you meant to say if you changed to a “blow through” set up you could run a BOV vented to atmosphere as long as the BOV is located after the supercharger and before the MAF.

OP, how is your BOV routed as of now in your draw through set up?
 

01yellercobra

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Your wording is off in the last paragraph. I think you meant to say if you changed to a “blow through” set up you could run a BOV vented to atmosphere as long as the BOV is located after the supercharger and before the MAF.

OP, how is your BOV routed as of now in your draw through set up?

You would be correct on that. I'm a blow through fan.
 

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