Vacuum rule carb sizing

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skinny z
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Re: Vacuum rule carb sizing

Post 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.
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Re: Vacuum rule carb sizing

Post 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)
Bullock's Power Service, LLC
Plattsmouth, NE
70 Mustang, 489 FE, TKO-600, Massflo SEFI, 4.11s
71 F100 SB 4x4, 461 FE, 4 speed, port injected EFI, 3.50s
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Re: Vacuum rule carb sizing

Post 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.
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Re: Vacuum rule carb sizing

Post 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
Bullock's Power Service, LLC
Plattsmouth, NE
70 Mustang, 489 FE, TKO-600, Massflo SEFI, 4.11s
71 F100 SB 4x4, 461 FE, 4 speed, port injected EFI, 3.50s
My427stang
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Re: Vacuum rule carb sizing

Post 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
Bullock's Power Service, LLC
Plattsmouth, NE
70 Mustang, 489 FE, TKO-600, Massflo SEFI, 4.11s
71 F100 SB 4x4, 461 FE, 4 speed, port injected EFI, 3.50s
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