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01-21-2015, 07:49 AM
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#1 |
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Registered User
Join Date: Oct 2014
Location: uk
Posts: 621
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Vacuum
On an SBC 350 what creates the vacuum at the carburettor? Thanks
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01-21-2015, 09:04 AM
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#2 |
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Senior Member
 
Join Date: Jul 2000
Location: North Texas
Posts: 3,597
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Re: Vacuum
Most carbs have 3 sources of vacuum -- a large port for the power brake booster, and two smaller ports.
The large port and the small port (closest to the carb base) are manifold vacuum. The small port that's a little higher up on the carb is known as "ported" vacuum. There is little to no vacuum at this port until the throttle blades begin to open. The small ports are used for distributor vacuum advance, transmission modulator, and emission controls, depending on the vehicle. There is some debate on which small port to use for vacuum advance. Personally I've had success using both, depending on engine idle vacuum (mostly a function of camshaft duration and overlap).
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Mike 1969 C10 LWB -- owned for 35 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, recent AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then! |
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01-21-2015, 09:07 AM
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#3 |
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Registered User
Join Date: Oct 2014
Location: uk
Posts: 621
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Re: Vacuum
But where or how is the vacuum created?
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01-21-2015, 10:06 AM
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#4 |
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Registered User
Join Date: Jan 2014
Location: Indpls. , IN
Posts: 795
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Re: Vacuum
Pistons moving down in their bore with the intake valve open. This lets air rush down thru the carb from above and thru the intake and into the cyl. head past the valve and into the cylinder.
14.7 psi of atmospheric pressure above the carb and zero psi in the cylinder because the piston is moving down and creates space for the air to rush in. Air flows thru to fill that space ! Last edited by 68post; 01-21-2015 at 10:12 AM. Reason: uh-huh |
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01-21-2015, 10:44 AM
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#5 |
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Registered User
Join Date: Oct 2014
Location: uk
Posts: 621
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Re: Vacuum
Ah right thanks, low vacuum would be because if a leak or worn rings then
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01-21-2015, 10:49 AM
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#6 |
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Registered User
Join Date: Mar 2012
Location: edgartown mass
Posts: 852
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Re: Vacuum
just remember if the blads on the carb are closed you have high vacuum open no vaccum.
just like was said the piston needs to make a low pressure area and then the carb blads need to make a restriction, that gives you vacuum. this is also why diesel's dont have vacuum as they have no restriction on the intake system
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1986 c20 "the hillbilly" 350 sm465 3.42gears 21mpg at 70mph 1985 k30 "the ice cream truck from hell" no rust az truck 197-8??? c30 "ramp truck" 454 1978 monte carlo 350 350th with 2.73 gears. no rust. sc car |
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