soundengineer

iTrader: (16)

I'm putting on Hooker Longtubes over the Labor Day Weekend.....And I'm Still tryin to figure out what they will do to my Air/Fuel Mixture....lean it out??? richen it up???? and Why..... I have this thing: Exhaust Backpressure Document. and I know that our PCM is always trying to adjust..... But what I used to know from carb days is that headers would lean you out a bit...so you would have to adjust your carb so you wouldnt burn valves and cause detonation from being lean..... So somebody Please Explain this better for me... obviously the carb thing is different than the PCM thing.

wrencher

iTrader: (2)

This is a deep question lot of factors in play.
First thing O2's work from exhaust heat,& they produce voltage from the amount of o2 content in the exhaust(outside of porcelain) In relation to the content of o2 outside the sensor(inside the porcelain)
In a nut shell, the pcm is programmed with the stock location of O2's.(closer)
Manifolds hold heat much more than headers, where headers dissipate it quickly.
These + a few other factors affect the closed loop fuel control & cause the L/T cars to run on the rich side.

Mustang Hat'r 2

iTrader: (24)

Not exactly. They lean you out. During cruising the PCM can adjust this. Since the PCM thinks its lean it will add that same % of fuel to WOT making you rich. Thats why headers will give you little to no gains until you fix the problem.
Make sense?

jimmyblue

iTrader: (11)

Headers were generally considered to make a car
run leaner, back in the carbureted days. This
really relates to the changes in volumetric
efficiency - more exhaust gas extraction makes
more cylinder draw, and the mix you pull in
is more completely "fresh meat" and less inert
gasses.

A MAF-based system -should- have the right intake
air info to compute proper fuel delivery. However,
the MAF computation is "corrupted" by bad loop
closure from colder O2 sensors, which deliver
depressed voltage when cold, which is interpreted
as a lean mixture by the PCM. This then jacks the
LTFTs around, especially at low exhaust flow
rates, making an actual rich condition. See many
posts on headers, O2 heater codes and rich running
with positive LTFTs.

The MAP based "backstop" calculation would want a
proper tweaking of the VE maps to properly model
the enhanced exhaust-side flow's contribution to
engine pumping efficiency. Many header / cam cars
need this at the low end just to idle well, though
this pertains more to overlap than the headers'
extraction. But this seems to be an art that has
not been widely developed, and which the MAF (when
properly read and not skewed by other sensor
issues) takes care of by itself.

If you were going to calibrate VE, I suppose you
would want to log MAF values, RPM, MAP and then
error-beat the MAF vs the speed-density algorithm
MAP*RPM*VE*displacement/2 (or some such), and fit
VE cellwise until the SD calculation matches.

But this is a lot of work for a bolt-on.

I'm wondering when some bright boy is going to
start selling an aftermarket harness that applies
heater power full-time to the O2s, and fools the
PCM into thinking it has authorita, so the O2
readings are valid in remote, cooler long-tube
bung locations.

wrencher

iTrader: (2)

So the end result is it makes you run richer.
Exactly