[Diy_efi] Timing

[Bret Levandowski]

So, if I'm running a SB400 with 9.9:1 comp, a .447/.462 cam, mild port and pocket port and Afterburner headers, I'm guessing I shouldn't have too much vacuum advance in my timing table. Say maybe 5 or 6 degrees, base at 10 and mechanical at 18 or so? I'm running GM TBI.

dh at busb.com wrote:Correct. Further, reduction in advance is retard. The vacuum advance allows
the low volume (low temp) charge burn more slowly and still be at optimum
pressure in the vicinity of TDC. The throttle stab causes more air and
fuel. The immediate effect is that the mixture is more lean (due to fuel
precipitation) and therefore has a quicker flame front requiring less
advance. Some advance can be returned as the mixture balance is restored,
but the timing is probably more like 1 or 2 seconds.

Vacuum advance is for economy and removed from performance engines. The
base timing (before vacuum and 'mechanical' advance) is highest power, but
only when the throttle is wide open. Mechanical advance increases the
advance for timing due to RPM changes.

dh
----- Original Message ----- 
From: "Rick McLeod" 
To: 
Sent: Wednesday, September 28, 2005 4:55 PM
Subject: Re: [Diy_efi] Timing


> > If I make a quick stab at the
> > pedal, the only real change before downshift is
> > vacuum increase causing the vacuum component to
> > increase but I'm still reducing just the base?
>
> shouldn't this cause a reduction in vacuum? Therefore
> causing a reduction in advance toward base?
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