[Mgs] Engine maths...and spare time

Barney Gaylord barneymg at mgaguru.com
Wed Apr 1 21:17:18 MDT 2020


Okay, let's see if anyone knows the answer to 
this one.  Why is it that "You do eventually get 
to a point where no more advance will help".  Why 
do we stop spark advance at ~32d BTDC at road 
speed, and keep the advance the same for all higher engine speeds?


At 07:56 PM 4/1/2020, Richard Ewald via Mgs wrote:
>It takes about 3ms to burn the fuel in a 
>cylinder from when the spark ignites it.  The 
>engine designers want the peak pressure to occur at between 5-10 ATDC.
>At idle speed 3ms is about 15 degrees. Â
>So if you want your peak pressure at 5 ATDC, 
>your base timing is 10 BTDC, if  you 
>want  your peak pressure at 10 ATDC your timing is 5 BTDC.
>If you look at tune up specs for non smog 
>controlled engines, most have base timing in the 5-10 BTDC range.
>Now if you double the engine speed, it still 
>take 3ms to burn, but to keep the max pressure 
>at 5-10 ATDC, you have to light the mixture sooner.
>You do eventually get to a point where no more 
>advance will help.  That point varies with the 
>engine design,  cam profile, combustion chamber design, headers etc.  Â
>Rick
>....



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