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VACUUM?
#1377013 Mon Sep 14 2020 05:11 PM
Joined: Nov 2002
Posts: 371
O
ol' 55 Offline OP
Shop Shark
THIS IS ONE FOR YOURS TRULY, AND PROBABLY HAS BEEN DISCUSSED A TIME OR 2...… WELL OUT PLAYING POKER WITH THE FELLAHS OVER THE LABOR DAY HOLIDAYS, SO HAPPENS TO BE THE SUBJECT ON HOW VACUUM IN A NATURALLY ASPIRATED ENGINE ON GASOLINE WAS DISCUSSED...OK BOLTERS TECHNICALLY WHAT CREATES VACUUM AND WHAT IS THE OPTIMUM MEASURED FOR PERFORMANCE?

Re: VACUUM?
ol' 55 #1377017 Mon Sep 14 2020 05:36 PM
Joined: May 2015
Posts: 2,790
Housekeeping (Moderator) Making a Stovebolt Bed & Paint and Body Shop Forums
No need to shout. wink

Vacuum is produced in a gas engine when the pistons move down in the intake stroke with the intake valve open. It's caused by the restriction in the intake tract, mainly the carburetor and throttle plate. The pistons suck air in, but it's made harder by the closed (or partly closed throttle).


Kevin
First car '29 Ford Special Coupe
#2 - '29 Ford pickup restored from the ground up.
Newest Project - 51 Chevy 3100 work truck. Photos [flickr.com]
Busting rust since the mid-60's
Re: VACUUM?
ol' 55 #1377020 Mon Sep 14 2020 06:02 PM
Joined: Dec 2008
Posts: 1,484
P
Shop Shark
This is the official (and correct) explanation.
Vacuum varies with a maximum of atmospheric pressure (valve closed) or 29.92 Hg", a minimum in low performance engines (equipped with a 1 or 2 bbl. carburetor) of about 3 Hg" down to about 1.5 Hg" in high performance engines (4 bbl.). EFI can be even lower since vacuum is not needed to draw fuel into the engine.
According to David Vizard, in a race engine with intake and exhaust lengths tuned for a harmonic resonance to assist cylinder fill, the returning exhaust pulse is stronger than the piston's draw.

Re: VACUUM?
ol' 55 #1377027 Mon Sep 14 2020 07:17 PM
Joined: Feb 2004
Posts: 21,021
H
Boltergeist
Vacuum sucks!
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: VACUUM?
Hotrod Lincoln #1377056 Mon Sep 14 2020 10:30 PM
Joined: May 2015
Posts: 2,790
Housekeeping (Moderator) Making a Stovebolt Bed & Paint and Body Shop Forums
Originally Posted by panic
According to David Vizard, in a race engine with intake and exhaust lengths tuned for a harmonic resonance to assist cylinder fill, the returning exhaust pulse is stronger than the piston's draw.
HUH??
Meaning what?

Originally Posted by Hotrod Lincoln
Vacuum sucks!
Jerry
Yes, Yes it does. grin


Kevin
First car '29 Ford Special Coupe
#2 - '29 Ford pickup restored from the ground up.
Newest Project - 51 Chevy 3100 work truck. Photos [flickr.com]
Busting rust since the mid-60's
Re: VACUUM?
panic #1377066 Mon Sep 14 2020 11:24 PM
Joined: Aug 2020
Posts: 59
A
Wrench Fetcher
Originally Posted by panic
According to David Vizard, in a race engine with intake and exhaust lengths tuned for a harmonic resonance to assist cylinder fill, the returning exhaust pulse is stronger than the piston's draw.

That’s common in 2 strokes to keep the intake charge in the cylinder.


Pat
1940’s tech was great in the 40’s
Re: VACUUM?
ol' 55 #1377068 Mon Sep 14 2020 11:43 PM
Joined: Feb 2004
Posts: 21,021
H
Boltergeist
On one truck pull engine we built (a MOPAR 440) the tuned length of the header pipes we custom built were 8 feet long, and the ones for the 2nd, 3rd, and 4th. cylinders on each bank extended back into the 6" collector to keep the tuned length the same on all the cylinders. We were building for maximum low end torque, but the engine still maxed out at 714 HP at 6500 RPM on one Quadrajet carburetor and gasoline. The class we were running didn't allow racing fuel or more than one carburetor.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: VACUUM?
ol' 55 #1377125 Tue Sep 15 2020 01:15 PM
Joined: Dec 2008
Posts: 1,484
P
Shop Shark
Intake vacuum @ WOT 1.5 Hg", exhaust may pull as high as 7 Hg".

Re: VACUUM?
ol' 55 #1377132 Tue Sep 15 2020 01:58 PM
Joined: Feb 2000
Posts: 4,177
J
Shop Shark
As stated above, the vacuum is created by the pistons on the downward stroke when the carburetor throttle valve is partially closed. The more the throttle is closed, the higher the vacuum. There is some vacuum at wide open throttle, but very little and hard to measure if the carburetor is sized correctly.

The exhaust vacuum is created when the high pressure exhaust pulse is released into the port when the exhaust valve is opened. It leaves so fast it pulls a vacuum with it. With an exhaust vacuum, the engine can fill the cylinders with air and fuel above what a less tuned exhaust will. When both valves are open ( valve overlap ) the exhaust vacuum pulls fresh air / fuel from the intake ports into the cylinders, some of which goes on out the exhaust, the fresh mixture purges the cylinder of most if not all of the spent fuel from previous cycles.

Re: VACUUM?
ol' 55 #1377134 Tue Sep 15 2020 02:15 PM
Joined: Feb 2004
Posts: 21,021
H
Boltergeist
Exhaust scavenging (or lack of it) is why engines with radical cam profiles don't run well at low speed. At idle, and usually midrange speeds, when the intake valve opens while the piston is still on the exhaust upstroke, a lot of the exhaust gas is forced back into the intake manifold. That reduces vacuum in the manifold, creates a rough idle, and the situation doesn't improve until enough RPM and exhaust gas velocity is achieved to begin the exhaust scavenging process. Lumpy cams radically reduce low end torque, and the result is an engine that only runs well at or near wide open throttle. Combine a radical cam, lots of carburetion and a low restriction exhaust with a cylinder head that has lousy breathing characteristics, and you end up with the sort of problems a lot of stovebolters won't admit they have. "It runs GREAT with two Rochester B's"! Maybe at 4000 RPM!
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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