CARB SIZE???

PKRWUD · 08-07-2001, 05:03 PM

That's where I'm confused. Manifold vacuum will always be lower at full throttle than it will at idle, no matter what the induction is like. You could put a one barrel on a 460, and have 18 inches at idle, but as soon as you crack the throttle, vacuum will drop, eventually to zero if the throttle is opened all the way. I understand what you're saying, I think, but I don't see how that's possible. At idle, the throttle is completely closed, minus a few thousanths, which will create the highest vacuum, other than when you close the throttle at high rpms, in which case it will exceed your idle vacuum by 4 to 6 inches, provided your rings are good. I really hope I'm missing something here, because trial and error, and now experience, have always been my guides for carb selection, and if you can figure it out with a vacuum gauge, I want to know how! Tell me what I'm missing. At which point during your run are you supposed to check the vacuum? And are you comparing manifold vacuum (WOT) to manifold vacuum (idle), or is ported vacuum involved here?

Very confused, but eager to learn!

Take care
~Chris

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08-07-2001, 05:03 PM	#15
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	That's where I'm confused. Manifold vacuum will always be lower at full throttle than it will at idle, no matter what the induction is like. You could put a one barrel on a 460, and have 18 inches at idle, but as soon as you crack the throttle, vacuum will drop, eventually to zero if the throttle is opened all the way. I understand what you're saying, I think, but I don't see how that's possible. At idle, the throttle is completely closed, minus a few thousanths, which will create the highest vacuum, other than when you close the throttle at high rpms, in which case it will exceed your idle vacuum by 4 to 6 inches, provided your rings are good. I really hope I'm missing something here, because trial and error, and now experience, have always been my guides for carb selection, and if you can figure it out with a vacuum gauge, I want to know how! Tell me what I'm missing. At which point during your run are you supposed to check the vacuum? And are you comparing manifold vacuum (WOT) to manifold vacuum (idle), or is ported vacuum involved here? Very confused, but eager to learn! Take care ~Chris ------------------ Retired Moderator MustangNet My site: JimPorterRacing RACECAR spelled backwards is RACECAR HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!