I've been tuning for a number of years. I've seen some very inconsistent information here and in other forums. This post is to give the guys who want to start tuning a basic understanding of what conditions require what changes and why. If it seems to miss something or seems to jump around I'm sorry. There's so much that can be covered but I'm trying to just get the basics.
Now that there is support for a large number of J car OBD II PCMs there will be many new concepts discussed on the .org. While much of this is new to this forum and many of the members, GM ECM/PCM tuning has been around for other people for many years. OEM PCM tuning became popular in the mid 90's when several tools became available in a relatively short time for OBDI computers. OBDII tuning was slower to come out due to the increased complexity of the code and the major design change of the PCM’s it was used in. Despite these differences there are some common rules for both OBDI and OBDII which are worth noting.
The GM PCM has one primary job and two secondary jobs. First and foremost its job is to cause the engine to comply with federal regulations regarding emissions and diagnostics. The secondary jobs are only slightly less important, providing a level of safety that keeps the bean counters from paying for warranty work, and providing power and performance that keeps the driver happy. It is important to keep this order in mind when you are first starting to tune. It helps explain why things are the way they are.
Since the number one goal is emissions compliance, it makes sense that the engine will be guided toward conditions which provide the lowest emissions whenever possible. The current configuration for most cars requires that a catalytic converter be installed and be at a minimum operating temperature, the combustion chamber be within a certain temperature range, and that the air and fuel must be delivered in a balanced ratio. These conditions allow tailpipe emissions to be at their lowest. How the PCM reacts when these conditions are not met is important to you as a tuner. You will experience the most problems with your car when one or more of these conditions is not met!
What happens when everything is right:
When everything is right, the catalytic converter is at operating temperature, the combustion chamber temps are balanced, and engine rpm is fixed. The air:fuel needs to be at the magic 14.7:1 AFR so that fuel is delivered based on calculations. In a speed density system, the calculations are (1) how much air is entering the engine, and (2) how much fuel the injectors can deliver. The PCM has to know these two values to get it right. The air calculation is based on engine temperature and volumetric efficiency. Volumetric efficiency is obtained from a lookup table that is indexed by engine RPM and MAP. Fuel delivery will come from either an injector constant or a base pulse width constant. If you lie to the PCM about the VE or the injector size, the PCM will lie to you when it tells you what the AFR is It's dumb enough to trust you to be honest with these numbers.
Why we have closed loop and the O2 sensor:
In short, nothing's perfect. Engines wear over time. Timing belts and chains stretch. Fuel filters get plugged. Intake manifolds develop carbon buildup. Even the PCM’s own calculations for the amount of air and fuel getting to the cylinders are not perfect. Enter the O2 sensor and Closed Loop operation. The O2 sensor provides feedback to the PCM about the calculated amount of fuel it's delivering. The PCM makes it's best guess and then asks the O2 sensor "how much was I off by?" The O2 reading is used as a correction, it's added to or subtracted from the calculated fuel value to provide the best balance of air and fuel it can. This looping game of guessing and correcting with O2 feedback is "closed loop." One limit is that the O2 sensor will only give correct readings when it's warm enough, about 800 deg F. But the narrow band O2 sensor used in most cars has no way to report temperature to the PCM. So GM uses other values to guess when the O2 sensor is ready to send a signal. For example, the PCM won't use O2 feedback if the engine coolant is below a certain temperature. If the engine temp is say 40 degrees F, it's not realistic to expect the O2 sensor to be at 800 deg F. In some calibrations there's a certain amount of time that has to pass after startup before the O2 sensor is used. This makes sure that if the car has been shut off for a while and then is restarted, the O2 sensor has plenty of time to heat up. Another condition of the narrow band O2 sensor is that it's only correct at 14.7:1 AFR. At AFR's even a little richer or a little leaner, the sensor can't be trusted. That's why the PCM doesn't actually use the O2 reading to deliver an exact AFR, it uses the O2 readings to make educated guesses.
How do changing conditions affect closed loop?
If the engine was running at a fixed RPM, if the intake and exhaust temperature never changed, if the amount of humidity never changed, if the engine never experienced wear, the corrections from the O2 could be stored once and they'd never have to be changed. But we're running an engine in a car, in conditions that change constantly. Closed loop feedback is stored as short term and long term corrections. Depending on the calibration there may be more or less of these corrections. They cover a range of conditions, several hundred rpm and a variation of 10-20 kPa MAP is common. The O2 feedback is first put into short term correction. The PCM waits to see if it's adjusting for a temporary condition, maybe a sudden blast of cold air. If it continues to need short term corrections the PCM begins to move those corrections into the long term correction cells. Those are corrections that occur because of engine condition, exhaust or intake changes, or days with more humidity. Conditions that are not going to change from minute to minute. When the guessed AFR is off because of something that lasts for a long time, the PCM will eventually try and shift all the correction values to the long term correction area and will try to reset the short term corrections to zero. But this takes time. It takes time for the PCM to ask the O2 sensor how things are doing, to make a change the fuel delivery, to ask the O2 again, to move the values to short term correction, and then to long term correction. This doesn't happen fast. And the more often rpm changes the less chance the PCM has to get any of the specific long term correction cells "perfect." In this case the PCM is always playing catch up. And to make it worse there are some changes that are just too dang fast for the closed loop guessing game to work. In these cases the PCM uses special tricks to keep things right.
Acceleration Enrichment and Deceleration Enleanment:
These are two "special cases" in the PCM world. They handle fast changes in throttle opening or manifold pressure. When you snap the throttle open the air density in the intake manifold suddenly increases. This throws the magic AFR balance off so fast that the PCM can't calculate and deliver the new amount of fuel in time for it to get to the engine with the extra air. Since the amount of air increases, we need to increase or enrich the fuel, hence the name Acceleration Enrichment, or AE. The plan here is simple: if the throttle suddenly snaps open a certain amount, pulse the injectors a certain amount. AE can also be activated by MAP sensor changes. If MAP suddenly increases by a certain amount, pulse the injectors a certain amount. This fuel is in addition to the amount of fuel calculated based on the VE. But the only reason it's there is to cover the extra fuel need during the time when the PCM is working out the new calculated amount of fuel. So AE should be kept to a minimum. You only want to add enough fuel to cover for the PCM. You don't need to drown the engine. Deceleration Enleanment is just the opposite. When the throttle is snapped closed the air density in the manifold suddenly decreases. There's too much fuel, so the idea is to shut the injectors off for a certain amount of time. This is only to cover until the PCM calculates the correct fuel delivery, so it's not a very long time. One thing that really affects AE and DE is the amount of fuel the injector flows. AE and DE are simply timed pulses. A bigger injector will deliver more fuel than a smaller injector will, if both are open for the same amount of time. If you put large injectors on your engine without adjusting the AE pulses downward you might notice puffs of black smoke and even sluggish performance when you snap the throttle. This is because closed loop can't adjust for these changes and no one's made the adjustment in the PCM.
Power Enrichment is not a special case. Power enrichment is the same game as “When Everything Works Right.” The only difference is that a richer non emissions friendly AFR is desired to help the vehicle accelerate. The PCM still calculates the amount of air getting into the cylinder based on the VE table. And the PCM still calculates how long it needs to open the injector, based on the size it thinks the injector is, to deliver the desired amount of fuel. The difference between PE and Non PE mode in terms of fueling is that the O2 sensor can't be used to provide feedback. It's not accurate at AFR's richer than 14.7:1. So the PCM calculates the amount of air getting into the cylinder, looks at the table where the desired AFR is kept, then calculates the amount of fuel to provide that AFR. If you lie to the PCM about injector size and/or engine VE, it will lie to you about the AFR. It will tell you it's delivering what you asked for, but it will have no way to know whether or not it is delivering the right amount. Now there are some calibrations that play a little trick during PE. Some calibrations look at the long term corrections stored by checking the O2 sensor in closed loop. They'll use the LT correction to adjust the delivered amount of fuel in PE mode. This doesn't happen often because it's not a great idea. What if the long term correction is wrong? What if silicone or anti-freeze has contaminated the O2 sensor and it's saying the engine's too rich when it's not? Well, you'd go into PE mode and the AFR could stay the same or even get leaner than 14.7:1. That's not a chance you want to take if you are trying to prevent warranty work.
Startup Enrichment, Cranking Fuel, Choke Effect, open loop AFR
An engine just starting requires more fuel than an engine running. If the temperature is really cold AFR's of 6:1 or lower may be needed to get the engine to fire. The engine must be given enough fuel to fire but not so much that the plugs get fouled. After the engine starts it must run leaner than it was during cranking but richer than it would if it were warm. The amount of cranking fuel and the choke effect will be reduced as the engine warms up. The AFR must be calculated at all times, and open loop AFR tables are where the desired results come from. These tables usually specify an AFR richer than 14.7:1 for colder engines but they get to 14.7 as quickly as possible to keep emissions as low as possible. Again, AFR tables are affected by injector size. If the PCM doesn't know the correct injector size it can't calculate the correct amount of time to open the injector to give the desired AFR. And it has no way to tell how it's doing in open loop.
Let's cheat to save some gas!
One final fuel mode is Decel Fuel Cut Out. The idea is if you've let off the gas completely at, say, 65 mph and you're coasting along the highway, you're trying to slow down. So if you're trying to slow down, why deliver any fuel to the engine. So DFCO is triggered either by a certain throttle position, a certain MAP value, or both. And it's usually only allowed after a certain amount of wait time. Most people do not have to adjust DFCO although this can become important if you change tranny types or final drive ratios.
Although the tuning of OBDII J cars is brand new, the tools used for tuning are not. The narrow band O2 sensor hasn't changed since the 90's. It can't be and hasn't ever been used during open loop or PE mode fueling. Fuel injectors have had some similar characteristics since the 70's. The MAP sensor used on the J car is similar in function to ones GM introduced in the '80s. Yes OBDII has added complexity and a bunch more rules, but the basic job of the pcm is still the same.
It might be a good idea to talk about the PCM software here or in a "tuning" thread rather than in the HPTuners FAQ since HPTuners won't be the only player in this market forever. And there's enough confusion about subjects like replacing a 1 BAR sensor with a 2 or 3 BAR version that the subject deserves it's own discussion. For instance there are very good reasons why a 1 BAR table shouldn't be used with a 3 BAR MAP sensor even if it is rescaled. There's also characteristics related to how injectors function which can play a large part in the around town driveability and emissions of your J car. And there's an entire world of spark which I didn't talk about. Engine tuning can be a hobby for life. I've been doing it for over 15 years and there's still plenty to learn.
Edited 1 time(s). Last edited Tuesday, December 07, 2010 6:29 AM
Always trying to out-do yourself . Thanks for pointing out to many of the people driving around in 'tuner' cars that they actually need to be tuned, now lets hope people aren't too ignorant to read through this. I third the sticky vote, and am interested to know more about why rescaling the MAP lookups would be a bad idea.
fortune cookie say:
better a delay than a disaster.
C Smyth, you've got it easier than the OBDII guys. The emulator is way cool but the simple chip programmer and a couple of flash chips works very well. I used an eprom programmer and a pile of UV erase chips for many years,
The OBDII guys won't know what they're missing in real time tuning.
Nice write-up. There's a lot to learn, even for someone like myself who has been around "tuner cars" for ten years or for many of us, longer than that, and the new OBD II software is going to be an all new learning experience in and of itself.
The OBDII guys won't know what they're missing in real time tuning.
thats why the HPtuners software is out
No. The HPTuners software is cool, but it does not tune in real time. Real time tuning is driving down the road, reaching over to a laptop or controller, making an adjustment to a value, and having the results show up right now. With current stock and aftermarket tuning programs, the engine must not be running when a calibration is uploaded to a GM OBDII pcm. Not running engine = not real time tuning.
Here's a guy selling LS1 OBDII pcm's modified for real time tuning. http://www.moates.net/roadrunner/
You can even use bluetooth technology to tune this thing live, in real time, from a remote location outside the vehicle! Techies and tuners alike should understand that this thing has a cool factor of 50 on a scale of 1 to 10.
This is a much better write-up than the good ol' "General Tuning" crap that Dom of Genertec wrote, and was on teamforwardmotion's website for a long long time... Let's not forget, that Dom's "performance" engines were ultimately stock....
Real time tuning is driving down the road, reaching over to a laptop or controller, making an adjustment to a value, and having the results show up right now.
so you are looking at a laptop that is in your passenger seat instead of the road?
you get someone who is knowlegable in tunig to ride with you
one drives while the other makes on the spot corrections to feul and spark maps. this is very effective for tuning a cars part throttle and crusing behavior. very cool if you have ever done it
I don't actually read your posts
I stare them down until they give me the information I require
if I had read this when I first started messing with feul injection I would probobly be a little farther along today
I don't actually read your posts
I stare them down until they give me the information I require
great write up man!
now i have a question... I just got a wideband and was wondering how do i know what A/F mixture should i run at?
how can i change this mixture at partial throttle or wot?
will the safc do the job? how about the adjustable fpr?
i have a 95 ecu with mantapart HO chip in it. now at wot i'm at about 12.8 a/f ratio.
Okay now I'm completely baffled. Here it is...I'm a journalist...I know words.... All the rest of this is freaking greek to me.
So...in LAYMAN'S terms...can someone please tell me what I need to have and or do to "tune " my car...and if it even needs it at this point.
I have a '96 Sunfire GT 2.4L9...AEM short-ram intake, "secret" cams...the ones the DON'T require the head swap...Magna-flo High-flo cat converter and custom 2.5" cat back into cannisters.
NGK tridium or something other spark plugs...and it's an auto.
Everything else is stock, although I'm collecting parts for the HO swap...if I can ever figure out what I really need.
I occasionally (this weekend) run it on the track (15.85, 15.94, 16.00)
so before all that, am I killing my engine with what I've done? Do I need to "tune" it and if so, what is the easiest...ie read: made for mechanical idiots...device/program/whatever that I need to make sure I don't blow my engine up?
Thanks. G shattered-concepts.com shattered-concepts.com/forum
"Best Crew" Import Face Off, Nov 12, 2006 "Best Domestic" LS-1 'vette "Best Import (Other)" '05 Soobie STi "Best Street Beat" spl 130.8 "Best Overall" spl 142.6 single 12" in a '03 Cavvy
If i wanted a civic i'd have had japanese parents! now figure out how to make this fit!!
well like the guy said, ur ecm can can MINOR air fuel changes, if its with in reason itll be ok but if your motor is doing more air than fuel it might be problem u might have to look into ur fuel. theres alot of different products out there. an apexi afc is very popular easy to tune. i think the new one go -50%/+50% on the maps. and the new ones have 22 points to change ur fuel map vs the old ones had 16 12 and 8 for the analog dial ins
Forum Post / Reply
You must log in before you can post or reply to messages.