How far can the stock PCM go?
03-02-2008, 08:32 AM
#1
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How far can the stock PCM go? im doing a 370/76MM turbo build and plan to keep the stock PCM. The setup will be ran @ 15PSI making around 770WHP, but with more boost could support much more.
Im just wondering what the limiting factor in the stock PCM is? and how far you can take it? I plan to use low-impedance injectors with the AEM driver box.
Im just wondering what the limiting factor in the stock PCM is? and how far you can take it? I plan to use low-impedance injectors with the AEM driver box.
03-02-2008, 09:51 AM
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no not at all, im asking when the use off a standalone PCM becomes mandatory. I know the 98 PCM can only do 1 bar, so after 14.7PSI of boost I would either need a standalone or need to repin to a 99+ PCM, so on my PCM the limiting factor would be that I can only boost to 14.7PSI. What about the 99+ PCM, if i where to repin to the 99+ PCM how far could that take me, and what would eventually lead me to need a standalone over that setup?
03-02-2008, 01:20 PM
#7
The only kind of tune you can do with a 98 pcm is a mass airflow tune as there is no 2 bar (14.7psi of boost) or 3 bar (29.4 lbs of boost) OS for them (edit: I take that back, I think HPT might have a 2 bar OS of the 98 pcm). With this kind of tune, you depend on the MAF to know how much air the motor is flowing in volume and add fuel as such. You can run boost this way, but once you max out the 512 g/sec that the MAF can read, then you have to use a little trickery in the pcm to add fuel. It can be done though.
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03-02-2008, 03:32 PM
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The 98 doesn't have a 14.7 boost limit. 1 bar is NA. We live in 1 bar of atmosphere. 1 bar is 0 vac to 14.7lbs of atmosphere that is considered NA.
The only kind of tune you can do with a 98 pcm is a mass airflow tune as there is no 2 bar (14.7psi of boost) or 3 bar (29.4 lbs of boost) OS for them (edit: I take that back, I think HPT might have a 2 bar OS of the 98 pcm). With this kind of tune, you depend on the MAF to know how much air the motor is flowing in volume and add fuel as such. You can run boost this way, but once you max out the 512 g/sec that the MAF can read, then you have to use a little trickery in the pcm to add fuel. It can be done though.
The only kind of tune you can do with a 98 pcm is a mass airflow tune as there is no 2 bar (14.7psi of boost) or 3 bar (29.4 lbs of boost) OS for them (edit: I take that back, I think HPT might have a 2 bar OS of the 98 pcm). With this kind of tune, you depend on the MAF to know how much air the motor is flowing in volume and add fuel as such. You can run boost this way, but once you max out the 512 g/sec that the MAF can read, then you have to use a little trickery in the pcm to add fuel. It can be done though.
03-02-2008, 03:35 PM
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The absolute best answer to your problem is to swap to the newer PCM, use a 2 or 3 bar (2 bar for under 14.7 PSI, 3 bar for over 14.7) MAP sensor to control your fuel in a speed density setup, do not use a MAF, it can only read 512 grams of AIR which is easily surpassed with a turbo.
03-02-2008, 03:45 PM
#10
The MAF limit is a hard limit in the PCM, that can't be over come. Like said, your probable better off doing the 99+ swap, but it is possible to do what you want with a 98 PCM, just have to do a little trickery in the pcm, and might not be as smooth and perfect of a tune. But 14.7 PSI in a 370 is a LOT of power.
03-02-2008, 03:51 PM
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The absolute best answer to your problem is to swap to the newer PCM, use a 2 or 3 bar (2 bar for under 14.7 PSI, 3 bar for over 14.7) MAP sensor to control your fuel in a speed density setup, do not use a MAF, it can only read 512 grams of AIR which is easily surpassed with a turbo.
So basically, someone planning to boost their engine could do so with a 98' PCM using the HPT 2bar OS and a speed density tune if they go over 512G/sec on the MAF, they would just have less resolution in the tuning tables
On the 99+ PCM the max on the MAF is what? or i would still need to do the SD tune, only because the have the higher resolution tuning tables i would end up with a better overall tune?
03-02-2008, 03:54 PM
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The MAF limit is a hard limit in the PCM, that can't be over come. Like said, your probable better off doing the 99+ swap, but it is possible to do what you want with a 98 PCM, just have to do a little trickery in the pcm, and might not be as smooth and perfect of a tune. But 14.7 PSI in a 370 is a LOT of power.
What are these little tricks for the MAF?
03-02-2008, 04:13 PM
#13
The 512 MAF limit is in all the GM pcms, not just a 98 only thing 
. So you pretty much have to go SD for any real seirous boost levels. I can get about 10 psi out of my 4.8 and 6200 rpm before I max the MAF. Thats about 475rwhp in my truck.
Some of the tricks are bumping up the last row of the VE, MAF and using lower values in the PE tables and injector flow rate table to "trick" the PCM into squirting more fuel to get your desired AFR.
. So you pretty much have to go SD for any real seirous boost levels. I can get about 10 psi out of my 4.8 and 6200 rpm before I max the MAF. Thats about 475rwhp in my truck.Some of the tricks are bumping up the last row of the VE, MAF and using lower values in the PE tables and injector flow rate table to "trick" the PCM into squirting more fuel to get your desired AFR.
03-02-2008, 04:42 PM
#14
HPTuners has a 2-bar OS for the 98 PCM. I have tuned a car to 17psi on a 98 with a 2-bar OS. Basically, by the time you reach 15psi, at 15psi you will be just a touch rich since you have enough fuel in that area for the 17 you are going to. It only varies by a tenth or two of a point AFR-wise and it works really well. If using a nice progressive meth setup like Alkycontrol, you can probably get the AFR error to a single tenth from 15 to 17psi.
03-02-2008, 05:00 PM
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HPTuners has a 2-bar OS for the 98 PCM. I have tuned a car to 17psi on a 98 with a 2-bar OS. Basically, by the time you reach 15psi, at 15psi you will be just a touch rich since you have enough fuel in that area for the 17 you are going to. It only varies by a tenth or two of a point AFR-wise and it works really well. If using a nice progressive meth setup like Alkycontrol, you can probably get the AFR error to a single tenth from 15 to 17psi.
Last edited by LS1IMPULSE; 03-02-2008 at 05:08 PM.
03-08-2008, 04:02 PM
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Read through a couple recent tuning books and they both mention the LS1 based PCM and software is really the best from the factory, and one of the top including aftermarket. Pretty much the only thing it can't do besides the aftermarket is that a few of the aftermarkets have options for Nitrous controller built into it, also if you have two sets of injectors per runner some aftermarkets have setups for that. I am far from expert but I'd say you can safely use your factory PCM with some huge numbers with a proper setup and tune.
