Exhaust Backpressure

[Anonymous]

I found a great article on backpressure. While i'm sure half of you will dismiss it because its at a Honda forum, the rest of you might find it interesting....

http://www.hondalife.com/articles/Backp ... 20Area.htm


-=Whittey=-

 

[StrangeRanger]

The real question is WHAT were YOU doing browsing a Honda forum? Is there something you're ashamed to tell us?

 

[Anonymous]

Nah, nothing so sinnister.... Just a simple google for Larry Widmer, then Jim McFarland, then David Vizard, etc..... I believe that was a search for Widmer.


-=Whittey=-

 

[StrangeRanger]

That's what they all say.
Have you checked for the time and location of your local Ricers Anonymous Meeting? Sounds like ypu're going to be needing one.

 

[Anonymous]

Nah, I wouldn't be able to hang. For even a much-mocked ricer is faster than my beloved pickup.


-=Whittey=-

 

[xctasy]

A long time Vizard disciple, I've found that a moderately worked over head valve engine needs an internal exhast area equal to the cubic inches of the engine, divided by 60. This is based on thousands of emperical dyno runs on vehicles ranging from 50 cube Minis to 460 cube Lincolns. This guy maintained that zero backpressure is the policy for performance, but that header (extractor) selection is best done on a dyno.

Example exhast outlet internal diameters. (This assumes all bends over the differential or about the gas tank and tranny are either mild, or are of a diameter 0.25 inches greater than the size listed below):

144 cid = 2.400 sq inches ( 1.75 inch id)
170 cid = 2.833 sq inches ( 1.90 inch id)
200 cid = 3.333 sq inches ( 2.0625 inch id)
240 cid = 4.000 sq inches ( 2.25 inch id )
250 cid = 4.167 sq inches ( 2.3 inch id single, or two bigger than 1.625 id pipes)
300 cid = 5.000 sq inches ( 2.5 inch id single, or two bigger than 1.78 id pipes)

For full house engines,where low speed torque isn't an issue, you can divide the cubes by up to 35. An old Falcon campaigner, Aussie Dick Johnson, is on record for saying that the pipe diameter in a street engine must be big enough to drop backpressure, and small enough to keep the exhast drawing the spent gasses out of the exhast at the optimum speed. Lots of guys with 351 V8's found that low speed torque plumetts when a big bore extractor system goes on. Don't streach the friendship!

 

[Anonymous]

Unless your on the moon, backpressure is unavoidable. The "trick" is to minimize the "bugger" w/o reducing the exhaust flow velocity through the exhaust system.

For example, at WOT, the exhaust gas speed is near SONIC (maybe a little higher; ~500 - 600 m/sec) as the gasses exit/pass into the "open" volume of the exhaust manifold. A very substantial "diffusion" process then occurs which reduces the exhaust velocity to say 100 m/sec or so (look at the flames exhausting from a top fueler). Due to the outside atmosphere, even short open headers yield a "diffusion" that quickly translates into a "backpressure" at the exhaust valve. Then, the compression/expansion waves associated with the dynamic/acoustic behavior of the "system of pulses" and cam-timing intake actions occur. This makes the whole gas flow condition inside the exhaust system pulse or resonate in an on-going series of intake/exhaust events. Thus, we users of such systems need resonators and mufflers to "limit" the non-linear and unpleasant "noise" emmited from the tailpipe.

Ahhhh. ... the wonders of backpressure on an automotive IC engine. Get a gas turbine, at least the backpressure issues are more controllable!!! ..