installed headers, no tune - running leaner now - normal?
Thread Starter
Joined: Nov 2001
Posts: 1,794
Likes: 1
From: Slate Hill, NY
installed headers, no tune - running leaner now - normal? Headers and aluminum flywheel gave 25 hp and 14 ft/lbs of torque but made the car very lean (13.7 a/f ratio now but was 13.0 before headers, as measured at tailpipes). Just a guess that richening it up to 12.8'ish would give add'l 5-8 hp. Waiting on LS6 intake and bigger throttle body before getting a tune.
But i thought headers would've richened up the mixture.
Is it normal that I'm leaner now?
But i thought headers would've richened up the mixture.
Is it normal that I'm leaner now?
Pretty normal as I've never heard of headers richening up the mixture but instead leaning it a bit since they allow more air through the engine for the same amount of fuel. More air, same fuel = leaner.
Staging Lane
Joined: Dec 2014
Posts: 97
Likes: 0
From: Mobile, AL
I concur with this statement. The ECU will read a lean condition since the cats are not existent. To compensate the ECU will add more fuel causing it to be a little richer. Granted I am new to the LS platform but pretty sure that a lean condition is not the norm. Someone else chime in with more information but that is my 0.02 cents.
Doesn't that "more air through the engine" go through the MAF sensor?
I concur with this statement. The ECU will read a lean condition since the cats are not existent. To compensate the ECU will add more fuel causing it to be a little richer. Granted I am new to the LS platform but pretty sure that a lean condition is not the norm. Someone else chime in with more information but that is my 0.02 cents.
A more open exhaust will raise the actual VE and so will
resonant effects (the primary point of long tube headers).
To the extent that air mass figuring involves the speed
density (dynamic) airflow, and you didn't tell it, the net
result will be bent lean. You expect the MAF to be the
primary measurement above 4000RPM, below this the
SD model still has some influence. You don't say where
the lean reading is taken, or how consistent it is vs RPM.
Even if you are dead-nuts on air mass, having more air
mass requires more fuel and fuel fade in these dead-head
setups can be significant. What do you know about fuel
pressure vs RPM, case in hand? The 25HP pickup is about
8-9%-ish (air), your lean drift is 5%-ish (as reported), so
it could well be as simple as a 17-year-old fuel filter and
crusty fuel pump electrical feed. A decent fuel pressure
reading at the big end would be interesting / useful.
resonant effects (the primary point of long tube headers).
To the extent that air mass figuring involves the speed
density (dynamic) airflow, and you didn't tell it, the net
result will be bent lean. You expect the MAF to be the
primary measurement above 4000RPM, below this the
SD model still has some influence. You don't say where
the lean reading is taken, or how consistent it is vs RPM.
Even if you are dead-nuts on air mass, having more air
mass requires more fuel and fuel fade in these dead-head
setups can be significant. What do you know about fuel
pressure vs RPM, case in hand? The 25HP pickup is about
8-9%-ish (air), your lean drift is 5%-ish (as reported), so
it could well be as simple as a 17-year-old fuel filter and
crusty fuel pump electrical feed. A decent fuel pressure
reading at the big end would be interesting / useful.
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Staging Lane
Joined: Dec 2014
Posts: 97
Likes: 0
From: Mobile, AL
Basically the O2 sensors will relay a signal to the ECU. When the headers are added the catalytic converters are removed from the exhaust system. If the O2 sensors gets confused it can cause the ECU to add fuel if the AFR is to lean.
I might not be explaining everything well but the point is the O2 sensors are getting a signal they are not set up for so the computer has to compensate. If the OP were to get a tune then the problem would be averted. Of course what I am stating does not seem to the be the case for his car.
I hope that clears some things up.
EDIT: I also failed to mention that if there was an exhaust leak then the O2 sensor would read lean causing more fuel to be added. In this case is seems like there is not enough fuel being added. Pretty sure if anything I had mentioned were the case a CEL would be on.
I might not be explaining everything well but the point is the O2 sensors are getting a signal they are not set up for so the computer has to compensate. If the OP were to get a tune then the problem would be averted. Of course what I am stating does not seem to the be the case for his car.
I hope that clears some things up.
EDIT: I also failed to mention that if there was an exhaust leak then the O2 sensor would read lean causing more fuel to be added. In this case is seems like there is not enough fuel being added. Pretty sure if anything I had mentioned were the case a CEL would be on.
Last edited by Jake89; Dec 23, 2014 at 02:12 PM.
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In closed-loop it will be rich from a false lean caused by the o2 sensors no longer being accurate. At WOT it should be slightly leaner because the o2 sensors don't matter and it pulls from the VE table mostly.
Basically the O2 sensors will relay a signal to the ECU. When the headers are added the catalytic converters are removed from the exhaust system. If the O2 sensors gets confused it can cause the ECU to add fuel if the AFR is to lean.
I might not be explaining everything well but the point is the O2 sensors are getting a signal they are not set up for so the computer has to compensate. If the OP were to get a tune then the problem would be averted. Of course what I am stating does not seem to the be the case for his car.
I hope that clears some things up.
EDIT: I also failed to mention that if there was an exhaust leak then the O2 sensor would read lean causing more fuel to be added. In this case is seems like there is not enough fuel being added. Pretty sure if anything I had mentioned were the case a CEL would be on.
I might not be explaining everything well but the point is the O2 sensors are getting a signal they are not set up for so the computer has to compensate. If the OP were to get a tune then the problem would be averted. Of course what I am stating does not seem to the be the case for his car.
I hope that clears some things up.
EDIT: I also failed to mention that if there was an exhaust leak then the O2 sensor would read lean causing more fuel to be added. In this case is seems like there is not enough fuel being added. Pretty sure if anything I had mentioned were the case a CEL would be on.
TECH Senior Member
Joined: Apr 2002
Posts: 6,080
Likes: 17
From: So.Cal.
TECH Senior Member
Joined: Apr 2002
Posts: 6,080
Likes: 17
From: So.Cal.
I concur with this statement. The ECU will read a lean condition since the cats are not existent. To compensate the ECU will add more fuel causing it to be a little richer. Granted I am new to the LS platform but pretty sure that a lean condition is not the norm. Someone else chime in with more information but that is my 0.02 cents.
Basically the O2 sensors will relay a signal to the ECU. When the headers are added the catalytic converters are removed from the exhaust system. If the O2 sensors gets confused it can cause the ECU to add fuel if the AFR is to lean.
I might not be explaining everything well but the point is the O2 sensors are getting a signal they are not set up for so the computer has to compensate. If the OP were to get a tune then the problem would be averted. Of course what I am stating does not seem to the be the case for his car.
I hope that clears some things up.
EDIT: I also failed to mention that if there was an exhaust leak then the O2 sensor would read lean causing more fuel to be added. In this case is seems like there is not enough fuel being added. Pretty sure if anything I had mentioned were the case a CEL would be on.
I might not be explaining everything well but the point is the O2 sensors are getting a signal they are not set up for so the computer has to compensate. If the OP were to get a tune then the problem would be averted. Of course what I am stating does not seem to the be the case for his car.
I hope that clears some things up.
EDIT: I also failed to mention that if there was an exhaust leak then the O2 sensor would read lean causing more fuel to be added. In this case is seems like there is not enough fuel being added. Pretty sure if anything I had mentioned were the case a CEL would be on.
If the LTFT's do not peg, then the CEL/MIL will not necessarily be on.
Staging Lane
Joined: Dec 2014
Posts: 97
Likes: 0
From: Mobile, AL
The front O2S are ahead of the cats. The front O2S still function without cats... with long tube headers, the front O2S are further away from the combustion chambers, so the delay will be a bit longer, but they can still function ok regardless of cats (as long as they can stay hot).
The front O2S sensors report a voltage indicating which side of stoich (richer or leaner) the exhaust gas oxygen content indicates that fueling is... the O2S do not detect a signal, but they produce a signal for the PCM to use... the PCM uses this signal to trim to stoich (by fueling in the opposite direction), it does not "compensate".
If the LTFT's do not peg, then the CEL/MIL will not necessarily be on.
The front O2S sensors report a voltage indicating which side of stoich (richer or leaner) the exhaust gas oxygen content indicates that fueling is... the O2S do not detect a signal, but they produce a signal for the PCM to use... the PCM uses this signal to trim to stoich (by fueling in the opposite direction), it does not "compensate".
If the LTFT's do not peg, then the CEL/MIL will not necessarily be on.
Thread Starter
Joined: Nov 2001
Posts: 1,794
Likes: 1
From: Slate Hill, NY
i don't have a check engine light; we turned off the rear cats using HP Tuners.
Still about 3 weeks away from installing new intake/throttle body and tune on dynojet.
Still about 3 weeks away from installing new intake/throttle body and tune on dynojet.
TECH Senior Member
Joined: Apr 2002
Posts: 6,080
Likes: 17
From: So.Cal.
$0.02
Thread Starter
Joined: Nov 2001
Posts: 1,794
Likes: 1
From: Slate Hill, NY
updates from the header dyno...
Instead of just tuning for the headers, I had cam and fast intake installed and then tuned. Mods:
-speed engineering 1 7/8" headers, TSP 3" off-road Y, stock 98 ws6 muffler/tailpipe
-92mm fast intake, 92mm NW Tbody, 85mm MAF from c5z06
-aluminum flywheel and c6z06 clutch
-SSRA
-new bosch O2's, stock 98 injectors (99% duty cycle on last pull)
-fuel pressure at rail was 55-60 during the pull
-idle is rough and stalls often (6M)
-I think the low RPM torque is off and needs more attention
403 hp and 370 torque. I have new 36 lbs injectors waiting to be installed and will try again.
Instead of just tuning for the headers, I had cam and fast intake installed and then tuned. Mods:
-speed engineering 1 7/8" headers, TSP 3" off-road Y, stock 98 ws6 muffler/tailpipe
-92mm fast intake, 92mm NW Tbody, 85mm MAF from c5z06
-aluminum flywheel and c6z06 clutch
-SSRA
-new bosch O2's, stock 98 injectors (99% duty cycle on last pull)
-fuel pressure at rail was 55-60 during the pull
-idle is rough and stalls often (6M)
-I think the low RPM torque is off and needs more attention
403 hp and 370 torque. I have new 36 lbs injectors waiting to be installed and will try again.