Getting wet feet....junkyard TBI

[WEG1192 at aol.com]

In a message dated 5/29/2001 11:04:29 AM Eastern Daylight Time, 
John.Janssen at SomervillePackaging.com writes:

<<  Am I overanalyzing things? What you guys are working at doing is
 tuning your ECMS for MAX performance correct? but in my case just hooking
 everything up (closed loop - will do timing as well) should work fine
 correct? Will I have to do any programming?
 
 One part I need yet - the 5 prong chip that I have to swap out the GM HEI
 distributor to run closed loop timing - anyone know an application that I
 can order this from? >>

John,

For starters, get the ignition module mentioned recently and wire it to 
trigger from the pickup coil in your distributor. Then disable the mechanical 
advance in your Jeep distributor by removing the springs and weights, but 
reinstall the springs to hold the timing steady. Others on this list advocate 
tack welding the rotor mount to the distributor shaft so that there is no 
slop, but I have just put the springs back in and have had no problems. The 
only time I can think you would have any slop is when decelerating hard in 
gear, and in this case the rotor mount would have to outrun the distributor 
shaft with the spring tension fighting this motion. Again, I think its not a 
problem. Disabling the vacuum advance is done by simply leaving the rubber 
vacuum line off and plugging the line.

For spark tuning on a non-GM vehicle, I start by getting a copy of the 
factory manual showing the vacuum and mechanical advance curves. I then 
program Excel spreadsheets with the vacuum portion alone, the RPM portion 
alone, the static timing alone, and the fourth sheet being the sum of the 
first three. This sum sheet will be a table with MAP (same as inverse of 
vacuum) as the x axis, RPM as the Y axis, and the spark value as the entry in 
the table. This is a 3 dimensional table since you have two independent 
variables. This table is then programmed into the chip as a starting point 
for say a Jeep. If you want to tweak it after that, its up to you, but the 
spark table fine tuning is where the hidden power lies, that and getting the 
WOT fueling right. By the way, there is no closed loop spark tuning. This 
part of tuning is feed forward only. What you program stays that way even if 
the engine runs crappy. The exception to this is the knock sensor system, but 
this only retards timing. It doesnt add timing when a timing increase could 
probably help. The only feedback for spark tuning is in the seat of your 
pants (or an accelerometer for performance tuning).

As for fuel adjustments, start by getting a scantool and seeing where your 
BLMs are at hiway cruising speeds on a flat road. Then adjust (ie reprogram) 
the Base Pulse Width constant to get your BLMs around 124-128. Then check the 
BPW constant to make sure its not too far off from what it should be. What I 
mean by that is the BPW constant is calculated from the injector flowrate, 
the engine cubic inches, and a units conversion constant (see Tunercats help 
file for the 1227747 ECM at www.Tunercat.com). Since the cubic inches ain't 
gonna change (258 ci right) and the conversion constant is a constant, the 
only variable in the BPW constant is the injector flowrate. The 4.3L 
injectors were rated for 45 #/hr based on what I've learned and found with 
actual injectors. However, some flow more and some less. So if the BPW that 
gets you a good BLM is back calculated and the injector constant is more than 
10% away from the 45 #/hr number, then you either don't have 45#/hr injectors 
or your engine has a problem (in my case, my engine was worn out causing a 
very high injector size to be calculated; what I think was wrong was all the 
blowby I had required more gas than a fresh 360 engine would require; 
therefore I needed a very high BPW to make my Jeep run; 139 should have done 
it whereas my Jeep with AMC 360 needed a 170 BPW constant to run well with 61 
#/hr injectors).

Now assuming that the BPW that gets you a decent BLM range also yields a 
reasonable flowrate for the injectors you are supposed to have (the color 
bands on the top of the injectors tell you the flowrate), you now have two 
choices. You can tune the hell out of it like most on this list insist has to 
be done, or you can drive it awhile and let the GM closed loop logic tune the 
system for you. I have found that on a stock engine for a daily driver, that 
I don't have to do much of anything at this point except watch my BLMs with a 
scantool for a few weeks just to make sure the values aren't too far off. 
What the closed loop logic does is maintain a 14.7 AFR at all times by nature 
of the O2 sensor design, and it adjusts the fuel tables via the BLM (block 
learn mode). So no matter what the fuel table says, the BLM will steer the 
system towards an AFR of 14.7. The exception to this rule is at WOT when the 
closed loop logic is turned off. In this case, the logic looks at the highest 
BLM cell, which is assumed to yield a 14.7 AFR at WOT, and recalculates the 
AFR based on a WOT RPM vs. AFR table. For performance gurus, the WOT RPM vs. 
AFR table and the spark table are where all their efforts go. It takes a real 
good tuner and probably a dyno ( or good spark plug reader) to fine tune 
these tables for performance. But again, I have found that for a daily driver 
where MPG is more important than MPH, the closed loop logic will take care of 
everything. I don't plan on racing my Jeep Grand Wagoneer. I just let the GM 
stock WOT settings govern here.

In summary, this is the basic start for reprogramming a GM EPROM just to get 
the system running well. To get it running perfect, you have to go much 
further. Also, keep in mind that this is a simplified explanation and not all 
exceptions to the rules are covered here. JW
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