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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