SLP 85MM MAF - bag it?
08-04-2003, 10:06 AM
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SLP 85MM MAF - bag it? ATAP says I'm rich - mods are H/C, full bolt-ons...no tuning
I keep reading about people who recommend bagging the SLP 85MM MAF. ...I installed this before the heads and cam and LT headers; it never gave me any problems....
...the rich condition seemed to arise after LT headers but before the heads and cam....
Question: Should I go back to the stock MAF unit to see if I can get it to run leaner?
I keep reading about people who recommend bagging the SLP 85MM MAF. ...I installed this before the heads and cam and LT headers; it never gave me any problems....
...the rich condition seemed to arise after LT headers but before the heads and cam....
Question: Should I go back to the stock MAF unit to see if I can get it to run leaner?
08-04-2003, 01:44 PM
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Re: SLP 85MM MAF - bag it? I don't think you should trust ATAP. Just a bit
of lowered temp on the sensor will make more O2
volts change than a bit more fuel, when you're up
on the flat end of the transfer curve. Look at
these:
http://community.webshots.com/album/74289894mbMeMp
You can see that the AFR that gave you (say) 900mV
with a 750C EGT would give you 980mV with a 500C
exhaust gas stream. You'd have to be way, way
below 10:1 at that original 750C to get the same
voltage.
I think this is where ATAP is coming from, it
knows nothing about the gas temp and makes some
assumption. Then, everybody gets a reported-rich
value.
But only the wideband is trustworthy.
'98-'00 model SLP MAF, if anything, should have
leaned you out. '01-'02 in my experience is dead
neutral (relative to stocker, going by effect on
LTFTs - that being, nothing).
of lowered temp on the sensor will make more O2
volts change than a bit more fuel, when you're up
on the flat end of the transfer curve. Look at
these:
http://community.webshots.com/album/74289894mbMeMp
You can see that the AFR that gave you (say) 900mV
with a 750C EGT would give you 980mV with a 500C
exhaust gas stream. You'd have to be way, way
below 10:1 at that original 750C to get the same
voltage.
I think this is where ATAP is coming from, it
knows nothing about the gas temp and makes some
assumption. Then, everybody gets a reported-rich
value.
But only the wideband is trustworthy.
'98-'00 model SLP MAF, if anything, should have
leaned you out. '01-'02 in my experience is dead
neutral (relative to stocker, going by effect on
LTFTs - that being, nothing).
08-04-2003, 01:52 PM
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Re: SLP 85MM MAF - bag it? Oh - Thanks!
At idle, I can smell unburned fuel from the exhaust...so this comfirms a rich condition, right?
My software is OBD2 not ATAP; does this make any difference?
Car is 99 SS.
At idle, I can smell unburned fuel from the exhaust...so this comfirms a rich condition, right?
My software is OBD2 not ATAP; does this make any difference?
Car is 99 SS.
08-04-2003, 03:39 PM
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Re: SLP 85MM MAF - bag it? I think stockers & my SLP MAF all run richer down
low. I have something like -10 idle LTFTs. But
this fades to a much more reasonable number in
mid-throttle and I'd sure rather be off (rich)
at idle than off (lean) where I'm trying to make
power.
The smell of gas means unburned hydrocarbons to
me. HC goes up when gross rich, yes, but it's more
often a symptom of lean misfire, partial burns.
Your fuel mixture at idle is really a yo-yo, look
at how that O2 voltage jerks and you can see that
the vehicle is bumping rich-lean-rich. The cats
should chew up the low flow of misburnt stuff but
won't get it all.
OBD2 or ATAP, if it's giving you a mixture from
O2 mV it's making a calculation that still has to
presuppose a particular EGT, and it's very likely
to be a little wrong for a "normal" car, and more
so as you modify the exhaust thermals.
low. I have something like -10 idle LTFTs. But
this fades to a much more reasonable number in
mid-throttle and I'd sure rather be off (rich)
at idle than off (lean) where I'm trying to make
power.
The smell of gas means unburned hydrocarbons to
me. HC goes up when gross rich, yes, but it's more
often a symptom of lean misfire, partial burns.
Your fuel mixture at idle is really a yo-yo, look
at how that O2 voltage jerks and you can see that
the vehicle is bumping rich-lean-rich. The cats
should chew up the low flow of misburnt stuff but
won't get it all.
OBD2 or ATAP, if it's giving you a mixture from
O2 mV it's making a calculation that still has to
presuppose a particular EGT, and it's very likely
to be a little wrong for a "normal" car, and more
so as you modify the exhaust thermals.
