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PCM's & Tuning Discussions regarding PCM's and tuning

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Old 11-16-2004, 11:23 AM
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Basic PCM and sensor functions and tuning 101!

This is a "tuning 101" guide for supercharged 3800 front-wheel-drive applications. Most of this can be applied to the naturally aspirated motors, and you Aussie guys too!

Enjoy!


Mass Air Flow (MAF): Detects the amount of air flowing into the motor. This should be viewed in your scan tool in "frequency", not "lbs/min". You should never see over 11,500Hz, if you are getting into the 11,000Hz area then you should think about an upgraded MAF and table or custom PCM tuning.

Injector Pulse Widths (IPW): Shown in milliseconds, 22 is the max you should to see and is considered approaching static (injectors cannot flow any more fuel). Anything over 22 milliseconds and you should consider bigger injectors.

Manifold Absolute Pressure (MAP): This should be viewed in your scan tool in "PSI". Really, the MAP is almost useless on the supercharged motors. We have a 2-bar MAP sensor and most 3.20" and smaller pulley cars max out the MAP sensor almost immediately. It’s good to watch if you think you might be having boost dumping issues (the PCM is overriding the boost bypass valve on the supercharger, causing it to open and bypass the boost before it reaches the motor). You will see a drop in this value if you are losing boost. When the sensor is maxed out it will show a value of 29psi, which obviously is wrong, but when you know what to look for with this sensor it can be a useful problem solving tool.

Oxygen sensor, bank one - sensor one (O2B1S1): You have two oxygen sensors in your front-wheel-drive 3800. Bank one - sensor one is the O2 sensor that is in the rear manifold. This is the sensor that your PCM uses for almost all of its calculations when it relates to air and fuel. You will view this sensor in your scantool in “millivolts.” This sensor is of the “narrow band” design, and consequentially, is rather inaccurate when it comes to very fine tuning. But, most light to moderately modified cars can use this sensor to gauge the basic air and fuel health of their motor. It is my opinion that a non-intercooled 3800 should be in the .940-.950 range at wide open throttle, and an intercooled 3800 should be in the .920-.930 range. This goes against what most people will tell you, because to a point the leaner a car is the more power it makes. However, I have made well over 400 front wheel horsepower with a stock 115,000 mile shortblock – and I didn’t do it by running my motor lean. I would rather be safe and lose 5 horsepower due to running my motor a little on the rich side, than to run it on the edge and take out a piston or two due to a sensor hiccup. This is the other reason for running your motor to the rich side, if you have a sensor hiccup or a small tuning error the motor will not be on the edge and will probably make it through the weak moment without damage. I don’t believe in squeezing out every possible horsepower in every possible manner, motors aren’t cheap and I don’t like constantly replacing them.

On your front-wheel-drive 3800 you have a second O2 sensor, bank one – sensor two (O2B1S2), which is directly after your catalytic converter is used only for emissions detections. That is why you can disable the bank one – sensor two oxygen sensor and have no ill performance or reliability effects. There is no need to monitor this sensor.

Knock Retard: Contrary to popular belief, knock retard is a natural occurrence in almost every non-intercooled 3800. What, you say your car never has knock retard, and you don’t have an intercooler? Sounds like you don’t scan your car enough, or you need to drop a pulley size! Knock retard is a function of pre-detonation. Pre detonation (sometimes called pre-ignition or knock – hence “knock retard”) is when the air and fuel mixture in a cylinder catches on fire before the spark plug is ready to ignite it. This can be traced to many different factors: the wrong octane fuel is used, bad fuel, too small of a pulley for the motor, too much timing advance, etc. The important thing to remember is that on a non-intercooled 3800 a few degrees of knock retard is healthy. One of the biggest questions I am asked is, when someone has 2 or 3 degrees of knock retard what in the heck should they do (like the sky is falling)? I tell them that it’s perfectly normal. Our stock blower setup is not optimized for higher boost, and you will see a little knock retard now and again. What I always is explain is that up to 4 degrees of knock retard is considered healthy, 5 to 6 is a little excessive but other factors should be considered at this level. Is it really hot outside? Have you been beating on the motor pretty heavy in the last few minutes, bringing the temperature up and heat soaking it? If you consistently see more than 6 degrees of knock retard than it’s time to consider your setup. What size pulley are you running? Keep in mind, that in temperate climates (like Ohio) you may be able to run a 3.00” pulley in the winter when it’s 30 degrees outside, but when it gets into the 80’s and 90’s in the summer you may have to back that pulley off. It’s very common to change to a larger pulley in the summertime due to the hotter climate. Do you usually see 3 to 4, but spikes of up to 8 or 9 on hotter days? Then it’s time to consider a few smaller knock fighting modifications. Colder thermostats, colder plugs, exhaust modifications (including higher ratio rockers), even up to adding an intercooler, will all help to lessen or eliminate your knock retard problem.

"Alright smartass, how do exhaust modifications help to remove knock retard?"

Good question! When you are running boost into your motor, the more air that your motor can process then a lower amount of boost has to be stored in your lower intake – this boost is also becoming superheated with every turn of the blower because a natural byproduct of compression is friction, which creates heat. Hotter air makes a hotter air and fuel mixture, and, you guessed it, makes that mixture easier to pre-ignite. This is why an intercooler works so well, it cools the heat off of the compressed air which makes the mixture much cooler. Anyway, when you add better flowing exhaust components like higher ratio rockers, headers, cams, heads, etc. then the more air your motor can process and the chances of pre-ignition lessen.

Fuel Trims:Fuel trims are the PCM’s way of adapting and changing to different fuel flow demands from one second to another. They are the reason that your car runs well in 20 degree weather, and still runs as well in 100 degree weather (or with bad gas, a small vacuum leak, etc). It’s the PCM’s way of estimating what fuel the motor will need to stay running properly. 0% is near perfect fuel delivery, the PCM is not compensating in either the rich or lean direction. If you see a negative value, it means the PCM is removing fuel (the car is rich), a positive value means the PCM is adding fuel (the car is lean). The PCM can accurately compensate up to 16% in either direction to keep the motor from running too lean or rich – however, if you are beyond 8% in either direction you really should be doing a little tuning to get your trim numbers back toward 0%.

"Okay, what is the difference in short term (STFT) and long term (LTFT) fuel trims?"

Generally, you will not concern yourself with the short term fuel trims. The short term fuel trims will bounce around and are directly affected by the real-time response of the O2B1S1. The long term fuel trims are estimations the PCM makes based on the values read from the short term fuel trims. Basically, the long term trims are the corrections the PCM estimates the motor will need in the future. Suddenly going wide open throttle will cause the PCM to engage the long term fuel trim values.

I hope this helps some of you in your tuning quests.
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Old 11-16-2004, 05:25 PM
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That is by far the best and most understandable explanation for tuning basics I have ever read. I'm sure I can speak for more than a few people when I say that we can all pretty much do most of the mods to our cars ourselves, but when it comes to tuning we are lost. And with all the time and money we spend on these vehicles, it's nice to know that they're running right.

Thanks
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Old 11-16-2004, 06:39 PM
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Quote:
Originally Posted by k0bun
That is by far the best and most understandable explanation for tuning basics I have ever read. I'm sure I can speak for more than a few people when I say that we can all pretty much do most of the mods to our cars ourselves, but when it comes to tuning we are lost. And with all the time and money we spend on these vehicles, it's nice to know that they're running right.

Thanks
No problem, I'm glad that it helps you to understand what the heck is going on!

I'll write up a few more as time progresses.
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Old 11-18-2004, 02:44 AM
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Question

Thanks for the lesson! I was confused about fuel trims, I understand much better now. BUT... Could you please give a better explaination as to why 2-4 degrees of knock retard is good, or normal for the 3800. I have never heard of anyone saying this... I was always told that any amount of knock is not normal, and should not be allowed to occur, be it a forced induction street machine, or a high compression racing engine. Also, how does "pinging" relate to knock? Also, do you know how many degrees of KR = audible knocking, becuase I have heard the heavy knocking sound before... but never it in my regal, even though it has had 8* of KR in the past. Thanks.

Last edited by VT Regal; 11-18-2004 at 02:48 AM.
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Old 11-18-2004, 09:13 AM
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Brian, I think what he's trying to say is that because we use a roots blower, it is much more highly susceptible to knock. Pretty much everyone here should have at one point in time seen KR if they were scanning on a regular basis. If you have never seen a blip of KR, then the pulley you're running is too small. What that means is that you can push your engine further. You should run the smallest pulley you can without knock. However, there will always be days where your engine does not perform as well as other days, usually due to weather conditions, and your engine is experiencing knock.

So bottom line is that 2-4* of KR is not good. The more power you push with these engines, the more susceptible your pistons are to breaking with even a small amount of knock over an extended period of time.
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Old 11-18-2004, 09:28 AM
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Quote:
Originally Posted by Excell
The important thing to remember is that on a non-intercooled 3800 a few degrees of knock retard is healthy. One of the biggest questions I am asked is, when someone has 2 or 3 degrees of knock retard what in the heck should they do (like the sky is falling)? I tell them that it’s perfectly normal. Our stock blower setup is not optimized for higher boost, and you will see a little knock retard now and again. What I always is explain is that up to 4 degrees of knock retard is considered healthy, 5 to 6 is a little excessive but other factors should be considered at this level.

If you consistently see more than 6 degrees of knock retard than it’s time to consider your setup.
Normal? Yes. Healthy? No. I disagree that ANY amount of knock is 'healthy'. I DO agree that it is normal and almost impossible to run with 0 KR, 100% of the time. If you are seeing ANY KR (assuming it's real) then your engine is knocking causeing a lot of extra stress on it's parts. Extra stress equals shorter engine life. If your engine is consistantly knocking, even if it's only knocking enough to retard timing 2-4°, that's still not a good thing. That's the way I see it. I would like to hear your reasoning behind smaller amounts of knock being healthy though.
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Old 11-18-2004, 08:45 PM
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Well written Chris!

I think this may be a good thread to continually expand upon...
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Old 11-18-2004, 09:38 PM
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Let me expand on Knock Retard...

I'm excited to see some questions! I was hoping the knock retard portion would inspire some good discussion.

Our knock detection system is one of the most advanced and quickest responding systems ever designed. For good reason! The reason I say that a couple degrees of knock is normal and healthy is because every car is going to get a little knock. I have seen 100% stock cars with up to 6 degrees (or spikes of more!) consistently. General Motors would not have over designed a system without a cause, and that cause is because they knew that these cars would see some detonation.

However, using the term "healthy" may have been a little misleading. What I was trying to express is that if you see a few degrees of knock retard that you should not worry about your motor exploding. I get several paranoid e-mails a week asking about a few degrees of knock retard. While it's not "healthy" in the technical sense, it is healthy in the literal sense. Any knock is undesirable; however a small amount of knock is not going to hurt anything. We have a very advanced system in place that does a damn good job of saving your motor from small amounts of pre-ignition.

"Pinging": Before the days of advanced knock retard systems (think pre-1996) your engine and/or computer had no way to battle against pre-ignition. Pre-ignition commonly causes a "ping" or a knocking sound when it is occurring. Our knock retard system listens for the specific sound frequency of this knock, and categorizes it in degrees by the amplitude (how strong) of the sound. For every "degree" of knock it detects, one degree of ignition timing is retarded ("knock retard" ). Retarding the ignition timing causes the spark plug to fire sooner, and earlier in the piston's compression stroke. This catches the pre-ignition before it occurs, but it also produces less horsepower since the mixture is not being compressed as much when the ignition sequence takes place. This is the reason that you do not hear pinging or knocking, but you have knock retard. The PCM hears the knock before you do, and is already compensating for it.

Now, let's take my late 1988 Mustang 5.0L (R.I.P. ). Those cars do not have a computer controlled ignition system like our newer vehicles have. You had to increase timing manually by turning the distributor. You found where your timing advance limit is by advancing two degrees, driving the car, and listening for pinging. You just kept adding timing until you hear pinging, and then back the timing off two degrees. Instant horsepower!

I hope that answered your question as to why you don't hear the "pinging."
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Old 11-19-2004, 12:37 AM
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Thanks for the explaination. Its good to know that the few degrees of knock I have now is not as serious as I thought. There seams to be reports of blown pistons every few weeks on ClubGP, it really scares me away from further modding.

So, pinging is just a lesser form of knock, right? I remember hearing pinging in my old 93' Regal (series I 3800) quite often... I knew it was not good but the owners manual said that pinging was normal, but on the same page it also warned about knocking and how bad it was for the engine. I guess my 93' did not have a good knock supression system.

Another thing that GM had to compensate for with good knock supression equipment is people putting regular gas in instead of premium. We all know how cheap old people are (I have a GS ). The owners manual says nothing less than 91, but some people will surely put the cheapest they can find in. I think is is really due to poor education. GM should devote a page in the manual to explaining what can go wrong when you put crappy gas in, and exactly why most cars can run 87, and why we need nothing less than 91.

Last edited by VT Regal; 11-19-2004 at 12:39 AM.
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