I'm going to attempt something similar to tilting at windmills or
explaining that the either the millenium ends at one second after 23:59:59
on December 31, 2000 or you started counting at Year Zero. Yep, I'm going
to try to shatter some misconceptions that have taken hold and are accepted
as Fact.
We're going to talk about OBD-II, what it does, and what it doesn't do.
Josh Sirota and I had this conversation off-list yesterday, and it was
during that exchange that I realized that a lot of people confuse adaptive
PCM (Powertrain Control Module) strategy with OBD-II. This seems
particularly prevalent among BMW owners for some reason, at least based on
my admittedly non-scientific observations.
OBD-II, for those of you wondering just what the heck we're all talking
about, is an acronym for On-Board Diagnostics, Phase II. It's EPA
regulation regarding emissions controls and their performance. This
started to appear in some cars during the 1994 model year, and was required
on all cars and light trucks sold in the US starting with the 1996 model
year. It's basically an extension and expansion of the OBD-I system that
just about anyone with a computerized car has encountered at some point --
ever had the tech "read the codes" in your car? That was OBD, either -I or
-II.
OBD-II added a bunch more parameters to monitor more areas of emissions
performance than OBD-I, enough that I'm not able to list them all from
memory, but there are several prominent features. For example, OBD-II's
fuel system integrity test has lead to a huge number of warranty claims for
MILs traced to loose gas caps. If you don't tighten the gas cap all the
way, the fuel system doesn't pass the vacuum integrity test that's
performed after each engine start and the Malfunction Indicator Lamp (MIL)
is lit. Why is this a big deal? Most hydrocarbon emissions are now from
evaporative emissions, not from tailpipe emissions, and EPA has a standard.
There are other monitors that may prove useful -- for example, if the
thermostat sticks open, the car takes longer to warm up to optimum
operating temperature, heater performance suffers, and if it sticks closed,
it will get too hot very quickly. Either way, OBD-II will set a code and
light the MIL lamp, potentially providing the owner with a way to prevent
an expensive overheat.
One of the trickier bits of OBD-II is the detection of individual cylinder
misfires. This is done by measuring crankshaft angular acceleration and
position (to pinpoint the cylinder that is misfiring). Get enough misfires
in a certain time window, and OBD-II lights the MIL.
The one monitoring aspect of OBD-II that seems to have everyone in the
aftermarket up in arms is the catalyst monitoring system. This consists of
another HEGO (Heated Exhaust Gas Oxygen, you Eurpoean car owners may know
them as "Lambda sensors") sensor or sensors downstream of the catalytic
converter(s). The upstream HEGO continues in its role of air/fuel ratio
monitoring, allowint the PCM to adjust the mixture on the fly, the
downstream HEGO simply checks post-cat oxygen levels to make sure the
catalyst is still performing as the manufacturer intended. If the cat
performance drops off, OBD-II will detect it and light the MIL. This can
actually be a benefit to the performance-oriented driver: a failed cat
typically has *more* backpressure than an operating cat, and early
detection may prevent or limit the performance dropoff.
The oft-rumored "backpressure monitor" that counters any attempt at freeing
up the exhaust system is a myth. Some cars with EGR (exhaust gas
recirculation) systems use a backpressure feedback in the EGR feed tube to
control the amount of EGR valve operation, but there isn't a main exhaust
system backpressure monitor required by OBD-II to the best of my knowledge,
and I have yet to physically see one on any of the 50-plus different
vehicles I've examined since OBD-II came about.
Which brings us to the adaptive strategy. In many cars, some form of
adaptive strategy has been in place since the advent of digital electronics
for engine management in the early '80s. OBD-II, however, required enough
extra processor muscle and operational memory that many manufacturers took
advantage of the needed redesign to incorporate much more sophisticated
adaptive strategy capability to their PCMs. The timing for OBD-II and much
more capable adaptive strategy was a coincidence. Whether that coincidence
is happy or unhappy depends on whether you view it as evil or not.
Adaptive strategies simply introduce a capability to "bias" the programmed
fuel and spark (and sometimes shift strategy, in the case of automatic
transmissions controlled by the PCM) maps that the PCM uses to control the
engine. Some cars now have "drive by wire" throttles that may also use
adaptive strategy to adjust to the driver's usage.
What does this mean? Drive the car hard, and the PCM will bias the fuel,
spark, shift, throttle, etc. maps toward a slightly more 'performance'
orientation. Balloon-foot it, and the PCM will decide you are a Yuppoid
who bought the car for its "cachet", not its performance, and it'll bias
the maps toward a more "boulevard cruiser" orientation.
What's important to keep in mind, though, is that the operational "window"
that the PCM can operate in and still meet emissions standards is pretty
narrow, so this bias is pretty small potatoes. Engine performance for the
most part is still governed by the amount of fuel you can burn in a unit
time and how efficient the engine is in getting energy from the fuel to the
wheels. Which means that an adaptive-strategy-equipped car will still
respond to the traditional Stock-legal breathing mods of a cat-back exhaust
and lower-restriction air filter element.
How MUCH it responds depends on a lot of variables, particularly how close
to optimum the OEM exhaust system is anyway. I recall tales from the M3
crowd that they just didn't respond to cat-back exhaust system changes.
This is likely because BMW got it right the first time, but it was rapidly
attributed to the evil "OBD-II" by a lot of folks. Most cars, though, do
have enough "cushion" built in to their PCM strategy that they can readily
accept such things as a cat-back exhaust system and benefit from the
improved breathing.
Sometimes, particularly in the case of exhaust systems on basically stock
cars, bigger is not always better. Using my Probe GT as an example, the
stock system seems to run out of steam about 6500 rpm, 500 rpm short of
redline. I went to a custom 2.5" cat-back system with a SuperTrapp. Pull
the discs off the 'Trapp, and it pulls up to about 7500 rpm, but below
about 3200, there's nothing really exciting going on. Switch from that
system to the very popular Probe/MX-6 Borla system, and you give up just a
little at the top end (it starts to fall off about 7200), but the gains in
midrange and bottom end, particularly from 2500 rpm - 6000, where we spent
most of our time on an autocross course, more than make up for it.
If I'd opened up the intake system, put headers and a free-flow Y-pipe on
it, and maybe tweaked the fuel and spark maps a little bit to optimize them
for the better breathing capabilities and allow the engine to rev past 8500
rpm, the bigger system would have likely paid off better, and I've have an
incredibly uncompetitive ESP car instead of a still-competitive G Stock
ride. :)
But I digress. The basic point is that OBD-II is an emissions-control
monitoring system, nothing more. Adaptive strategy, which has been around
for quite a while, got a major boost in its capabilities due to the extra
processor muscle and memory requirements that OBD-II needed in the PCM that
was simply not there before. I don't think either system need be
"dreaded", as the added capabilites allow the PCM considerably more
flexibility in responding to physical changes (such as that exhaust system)
than before.
Jim Crider
Will engineer engine parts for salary & benefits
autojim@delphi.com
(No, that's not Delphi Automotive. That's Delphi Internet Services. We
had the name first.)
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