The Stovebolt Site Forums The Shop Area Engine Shop VACUUM?

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?

No need to shout.

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).

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.

Vacuum sucks!
Jerry

HUH??
Meaning what?

Yes, Yes it does.

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

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

Intake vacuum @ WOT 1.5 Hg", exhaust may pull as high as 7 Hg".

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.

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

"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?"

klhansen,

Here's what that means:

"In the case of the currently-flowing header primary, the EVO-initiated positive pressure (compression) wave is reflected back as a negative pressure (expansion) wave. If the arrival of the reflected negative pressure wave back at the exhaust valve can be arranged to occur during the latter part of the exhaust cycle, the resulting lower pressure in the port will enhance the removal of exhaust gas from the cylinder, and will reduce the pressure in the cylinder so that when the intake valve opens, the low pressure in the cylinder begins moving fresh charge into the cylinder while the piston is slowing to a stop at TDC."

The quote is from this article that fully explains how exhaust systems work and how they benefit performance when properly executed:

http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm

2-stroke expansion chamber design is based on the same principals applied differently. It's all basic physics and really interesting stuff.

Ray W

Ray
Thanks for the full story. The first statement wasn't very complete. I do understand fluid dynamics, just thought everyone would benefit by a better explanation.

The 2-stroke exhaust scavenging effect is very similar, with the idea to get all the exhaust gasses out of the cylinder as well as replacing them with as much combustible intake charge as possible.

'Ray
Thanks for the full story.'

You're very welcome. I've been interested in exhaust systems since the Summer of 1967 when I was a college kid and had a welding job at Doug's Headers. Science and engineering are always moving forward and it's been interesting watching exhaust design evolve along with other engine components like camshafts, cylinder heads, and ignitions.

Having basic welding skills makes it possible to fabricate exhaust systems to fit just about anything, accommodating the inconvenient locations of components like starter motors, steering boxes, oil filters and shift linkages, Non-welders would be pleasantly surprised how easy most welding processes are to learn, excluding tig that I would liken to learning to play a fiddle.

Ray W

Here is my take on what vacuum means to me.
When there is vacuum, your wipers work.
When you drive in the rain on the highway, there is no vacuum.
Stovebolt vacuum sucks, but only when you are parked in the rain with the wipers on.