What exactly is boost spark fuel decay?

[BJM]

I am trying to figure out what this is, it seems to be an additive term to the timing that only occurs during boost which seems kind of odd since normally one reduces timing under boost. Is this during spool up only?

 

[dennisL]

The ecm factory program was written to do two things during boost: subtract fuel and add spark advance. Neither is used in the stock '86/'87 chip.

Those interested in how its done, read on.

For boost spark, an internal register ($66) is used as a multiplier or scaler. The data item at $3270, represents the max amount of advance to add during boost. The program could determine (but doesn't) what percentage of the max spark to add and this would go to $66. The portion of the ROM program that calculates spark advance multiplies $66 & $3270. The result is stored in $65 to be added with other variables to produce the total spark advance.

For boost fuel, the same register, $66 is used again. The contents of $3272 is multiplied with $66 to produce a number that is subtracted from the fuel trim ($91). The value in $3272 represents how much of $66's value to use for the fuel reduction.

In the factory program, $66 is always 00, so no spark is added nor is any fuel subtracted.

 

[bruce]

Interestin.
Thanks

 

[BJM]

dennisL

I read through the code and I can now see that in fact it doesn't do anything in stock form. I wonder why it was put in place in the first place.

Thanks for the answer I always enjoy reading your responses.

 

[dennisL]

I have no idea what the original intent was. The name "boost spark fuel decay" should be read as boost spark and fuel decay. The word "decay" implies a gradual reduction, but for what purpose? I just looked at the code and saw there is also $3271 which was intended to be a Boost Spark Filter Coefficient. This means the boost spark was "lag filtered". A similar use of this is with the wastegate duty-cycle where its called ramp-up ($3776)

It is interesting though, the way $66 is used. It is a multiplier when used to increase advance, but for the fuel reduction it is the value that gets reduced by a multiplier.

It is the only data item I can think of that is used this way.