[Mgs] Engine maths...and spare time

[Richard Lindsay]

Max that might be true if the ignition pressure from the combustion didn't
build so long after the spark...within reasonable limits.

On Thu, Apr 2, 2020, 12:11 PM Max Heim <mvheim at sonic.net> wrote:

> 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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