[Mgs] Engine maths...and spare time

[Max Heim]

Well, it seems to me that if you advance the spark to a point on the compression stroke where the piston is still coming up, the expanding ignited mixture would be fighting the piston, and it would actually reduce power. I’m not sure if that is what you are talking about, though.

--
Max Heim
'66 MGB

> On Apr 1, 2020, at 8:17 PM, Barney Gaylord via Mgs <mgs at autox.team.net> wrote:
> 
> 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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