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Re: VACUUM?
ol' 55 #1377146 Tue Sep 15 2020 03:26 PM
Joined: Mar 2010
Posts: 926
Y
yar Offline
Shop Shark
"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
Re: VACUUM?
ol' 55 #1377157 Tue Sep 15 2020 04:57 PM
Joined: May 2015
Posts: 2,814
Housekeeping (Moderator) Making a Stovebolt Bed & Paint and Body Shop Forums
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.


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 #1377174 Tue Sep 15 2020 07:33 PM
Joined: Mar 2010
Posts: 926
Y
yar Offline
Shop Shark
'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


Ray
Re: VACUUM?
ol' 55 #1377212 Wed Sep 16 2020 01:12 AM
Joined: Mar 2010
Posts: 9,446
5
Master Gabster
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.

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