[Mgs] Engine maths...and spare time

[Richard Ewald]

Remember the intake valves stay open for a bit past BDC as the piston
starts up, also to get the mixture to go from flammable to flat out
explosive the charge has to be compressed.  so those factors tend to limit
how much effective timing you can have.
Going a little deeper when you are using a mechanical distributor with
weights there is only so much refinement you can get in your advance
curve.  Once you get into fully electronic ignition systems, there are
systems that have upward to 70 degrees of range from fully retard to full
advance (they may not use it in all applications, but the capability is
there in the electronics)
Rick

On Thu, Apr 2, 2020 at 11:33 AM Max Heim <mvheim at sonic.net> wrote:

> Well, I mean, it would be true at some extreme — for example, igniting the
> spark at the very beginning of the compression stroke. But I suspect Barney
> was getting at something else.
>
> --
> Max Heim
> '66 MGB
>
> On Apr 2, 2020, at 11:24 AM, Richard Lindsay <richardolindsay at gmail.com>
> wrote:
>
> 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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