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Btdc and atdc

EFI Tuning Fundamentals

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Hello everyone ,,

I am tuning 98 Mercedes E55 with natural aspirated engine 5.4

When I attempt to adjust spark advance tables I found ATDC table with minus numbers now I am.confused ! If I want to advance spark should I add positive or minus numbers to the table ??

Please I need further explination for BTDC Against ATDC .

Thanks

G'day Mohammed.

That's a little odd, but will work the same as a normal spark advance table, if you ignore the negative sign. It's just a matter of which direction you look at things from. The more common way this is setup is to have positive numbers in the table, that represent the number of crank degrees the spark occurs before the piston reaches the top of the compression stroke. The key word there is 'before', and does the same thing as the negative signs in your table. The table in your Mercedes ecu is the number of crank degrees the spark event occurs at after the piston has reached the top of the compression stroke and is on the way back down the bore. So having a negative number in the table means the spark event will occur before the piston reaches the top of its stroke.

Quick example; for my engine (with a Link ECU) to idle nicely, it likes around '+15' degrees of advance. That is, it likes the spark to occur 15 crank degrees before the piston reaches top dead center. In your table, this would be entered as a value of '-15'. If we wanted to change the number of crank degrees before top dead center that the spark occurs at to 12, we would enter a value of '+12' in the table for me engine with a Link ECU, but '-12' in your OEM Mercedes table.

Its less common, but the way Mercedes has their table setup there actually makes more sense than the usual way, which is grandfathered in from the days of distributors and carburetors.

Hope that helps! Post up if its still a little confusing and we'll try to help further :-).

Hi zac , good day to you ....

Thanks alot sir , you clear all my confusion .

Just to be more specific: going from -15 degree ATDC in your table to -18 degree ATDC advances by 3 degrees. Going to -12 ATDC retards spark 3 degrees.