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Re: Vacuum rule carb sizing
Posted: Tue Oct 19, 2021 11:06 am
by skinny z
MadBill wrote: ↑Mon Oct 18, 2021 4:48 pm
All else equal, carb size has no effect on idle vacuum. What does and is frequently overlooked is idle spark advance. Running say a 15°vacuum advance system could add up to 5" Hg., even with a locked-out advance curve, thereby widening the window of possible camshaft parameters.
That's exactly what I did to help smooth the idle with a 71 degree overlap cam. Ran 14-16 initial and then another 14-16 via ported manifold vacuum. Even on my wheezy old 350 it propped up the idle vacuum by several points.
Re: Vacuum rule carb sizing
Posted: Tue Oct 19, 2021 1:39 pm
by My427stang
panic wrote: ↑Tue Oct 19, 2021 8:42 am
spread is there to minimize overlap and relatively early ICL
Those have
opposing effects
Not sure I agree, but willing to listen. Which do you think lowers vacuum of those two?
My guess is you mean too early and I need more cam to RPM, but I have always seen each of those raise idle vacuum (to a point)
Re: Vacuum rule carb sizing
Posted: Tue Oct 19, 2021 4:20 pm
by Nut124
My427stang wrote: ↑Tue Oct 19, 2021 1:39 pm
panic wrote: ↑Tue Oct 19, 2021 8:42 am
spread is there to minimize overlap and relatively early ICL
Those have
opposing effects
Not sure I agree, but willing to listen. Which do you think lowers vacuum of those two?
My guess is you mean too early and I need more cam to RPM, but I have always seen each of those raise idle vacuum (to a point)
Larger LSA delays IVC and reduces idle vacuum.
Re: Vacuum rule carb sizing
Posted: Tue Oct 19, 2021 5:27 pm
by My427stang
Nut124 wrote: ↑Tue Oct 19, 2021 4:20 pm
My427stang wrote: ↑Tue Oct 19, 2021 1:39 pm
panic wrote: ↑Tue Oct 19, 2021 8:42 am
spread is there to minimize overlap and relatively early ICL
Those have
opposing effects
Not sure I agree, but willing to listen. Which do you think lowers vacuum of those two?
My guess is you mean too early and I need more cam to RPM, but I have always seen each of those raise idle vacuum (to a point)
Larger LSA delays IVC and reduces idle vacuum.
Ahhh, I put the ICL where I want it, both starting in the custom grind itself and then when I degree, IVC is wherever I want it. Exhaust events do change significantly though of course
Re: Vacuum rule carb sizing
Posted: Sun Nov 27, 2022 10:22 am
by My427stang
This is an old post, but ended up at 241 degrees @ .050, 8 degrees split, 113 LSA, 106 ICL, conical springs and short travel Morels
434 cid FE, untouched TFS FE castings, 11:1, unported Victor, 950 QFT, gapless top, made 13 inches of vacuum at 950, and 633 hp at 6200.
Pretty happy with a relatively simple build