I have just finished the tuning of my base VE tables for my 383 tpi running a 1991 speed-density system with the 7730 ecm. I used the "locked BLM" method and found it to work very well. Now I am moving on to my "power enrichment" tuning. In my Tuner-cat ecm definition file I find two main PE tables: PE vs coolant temp and PE vs. rpm. As a starting point for my calibration, I am using the stock values for a 1990 Camaro 5.7L auto program. The first table is very straight forward and fully warmed up, the PE percent is +22. When I look at the PE vs. RPM, I see some funny things: at low rpm it starts off reasonable at +3 going to +6 at 1600 rpm, suddenly at 2000 rpm, the value drops to -5.5 and immediately back to 3 then 6 then 8 at 4000 rpm.
Why would there be such a big drop at 2000 rpm??? Does this make sense and I am missing something?

In addition the overall percentage increase seem a bit too high. To go from 14.7:1 to 12.5:1(max power) requires a change of 15%, yet the combined numbers from the two tables can be as high as 30%????? This seems very rich to me and would actually cost some power. Any thoughts or observations would be greatly appreciated.


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Dave Zelinka

 

Good to see you worked on the VE tables before going after the PE stuff. The PE vs. RPM changes due to the intake tuning and other engine variables (cam, heads, other stuff related to airflow). Since the computer interpolates the values between the ones listed, it's actually peaking at 1600 rpm, then dropping off to a low at 2000 rpm, then gradually increasing again up to 4000 rpm, sort of like a sine wave.

GM programmed these cars to run really rich at WOT, so you're probably seeing an A/F ratio of 10.3 with the additional 30% PE. Yep, it sure does cost power, but it's safer for GM to run rich than run lean due to poor maintenance (clogged fuel filter, dying fuel pump, etc). Grumpy can expand more on this, but hopefully this gets the point across. On my stock 305, the PE vs. coolant temp was also 22% at normal operating temps, and I had it down to 9% where I found the best power, plus anywhere between 3% and 7% for the PE vs. RPM, but this will be different for every engine.

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Greg Westphal
'87 IROC 355TPI/A4

 

Thank you Greg. I have read many of your posts in the past and appreciate your response. I figured GM runs rich for safety reasons and the cooling effect you would get if you were under continuous heavy load. I will lean it out maybe 10% to start based on where you ended up, I can probably take out even more fuel.

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Dave Zelinka

 

Dave,

did you do any Timing work after you were done with the VE table?

Greg,

Does it make sense to see if there is any gain/loss by adjusting timing @ this point B4 going into the PE adjustment? if he is fat on timing & does not need to be to get a certian hp/torque value, then editing in PE mode (taking out fuel) might start the motor detonating, when he could take some timing out now & not hurt the efficiency of the engine any. Kinda like Grumpy was talking about a week or two ago when he posted that info on playing with timing on the freeway?

Not that far yet............. in San Diego.
BW

 

BW: I worked out my spark table before I started my VE tuning. All of my VE runs showed 0 degrees knock retard so I know I was not getting any detonation. After my VE tables are retuned, I will go back and fine tune the spark tables again as fuel mixture does affect burn rate.

 

I see, Cool. let us know how it goes.

BW

 

Bob, it looks like Dave beat me to the punch, but yes, I would start with the timing first. For my recently installed 355, I started with the timing table I had for my 305, but zero'd out the PE spark advance since I wanted to add it all into the main table. The timing was pretty conservative and the PE tables rich, but I wanted to be safe until the engine was broken in and I had some good data to use. Now I'm up to 32 degrees total advance at WOT from 3200 rpm on up, but still a little on the rich side since it'll lean out some with the cooler less humid air this fall.

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Greg Westphal
'87 IROC 355TPI/A4