Logical explanation of timing

[ss9940]

Hi all!<br /><br />I just finished my last post and asked myself a logical question: When setting timing in base timing mode, i have to set it to 2 deg ATDC (3.0 volvo). How the hell is that possible? That means that the spark ignites the fuel when the piston already goes down? Wouldn't that burn fuel when the valve is open and the fire would expand into the manifold? If i recall correctly, automotive engines NEVER use ATDC for timing, always BTDC. Am i correct?<br /><br />And how does the octane value of gas influence timing? Is there a scale how much you should add to base timing per 1 octane?<br /><br />Thanx,<br /><br />Alex

 

[Don S]

Re: Logical explanation of timing

Base timing is only used as a way to have a steady timing that the distributor can be set at, and has nothing to do with engine timing.<br />Without being in base timing mode, the timing is constantly changing.<br />If you set the timing at 2° ATDC, once you take it out of base timing the timing is actually around 6° BTDC but constantly changing + or - 2or3° on either side of 6.<br />You change the timing for the different burn rates of different octanes of fuel.