...... Next time I go to the track I'm gonna take a bag of potatoes and cram 'em into the tailpipes 'til that 'ol backpressure sends me flyin' down the strip .................................

 

This is the most correct general answer, all about the actual gas velocity.

O2s are designed with a certain amount of heat in mind in the pipe. The gases need to get to a certain level, like 600F, for the voltage reading to stabilize from the sensor. If that heat isnt there, say from too big a pipe that scavenges the gas out quickly or lack of obstruction (same difference), the O2 readings force the ECM into open loop, and that kills the mileage for part throttle lugging around.

If the chip is retuned with these things in mind, the mileage will go back up and in my case even outperform its original levels.

 

This is correct and why I suggested using PipeMax. Seriously, because it is pretty difficult for a DIY guy to try to figure this stuff out let alone have the ability, time and money to build multiple systems.

In regards to mileage, my 383 with 400+rwhp/430+rwtq still got 28mpg (there) and 31mpg (back) with 3.73s and a zf6 last weekend on a 200 mile journey each way. Home tuned.

 

Sorry, but physics says otherwise. As was said earlier, an engine is an air pump. Restricting either the input or output kills torque. Torque is an indication of cylinder pressure. Restricting flow reduces VE, which reduces cylinder pressure.

It's my guess that this whole "backpressure helps low end torque" myth started way back in the early carburetor days when if someone reduced the exhaust restriction, frequently he lost some power. There wasn't some magical power thing happening because a bunch of exhaust gas got trapped in the cylinders (due to the restrictive exhaust). Carburetors meter out fuel to the cylinders in two ways. Primarily when the air is rushing downward through the carb towards the intake valve, and secondarily by when the intake manifold air gets momentarily "burped back" up the carb venturi when the intake valve opens (releasing the residual cylinder pressure, consisting of burnt gas, back into the intake manifold). When the burp happens, air that just milliseconds earlier pulled fuel out of the venturi, reverses direction and passes through the venturi again, gathering fuel again. Once the piston starts to go down, creating suction, the twice-fueled air passes the venturi again getting additional fuel. This happens all the time and is not a big deal as long as you have correctly jetted your carb. Now if you reduce the exhaust restriction, you reduce the burp magnitude. This reduces the volume/mass amount of air that gets fueled multiple times, leaning out the mixture. When you lean things out (say from 12.5A/F to 15 A/F) you're going to lose power. A lot of guys misread what was going on in the engine, and passed this misinformation on to succeeding generations. Had WBO2 sensors been commonly available back then, as they are today, the "tuners" would have been able to measure what was actually happening. But technology most often isn't around as soon as it could be used or appreciated. Back to the early tuner: If the carburetor jetting was increased after the exhaust upgrade (getting the A/F ratio back to the 12.5 target) the driver would've noticed increased power&torque at all RPMs due to the increased VE caused by better cylinder cleaning/scavenging.

That's it in a simple nutshell, from a former Alaska resident (where I bought my '69).

 

I think you need the evidence of a dyno run to prove that (need of back pressure for low end tq in stock motors)... when you dont....for good reason... its ok to shift your paradigm ... and learn / understand the gas velocity science.

 

This is some pretty excellent tech. info from all of you , totally excellent , and I am learning , which was my goal, you folks, ROCK. respectfully, bob

 

Guys, I'm not convinced that exhaust velocity is the magic thing here (regarding post-header exhaust). To get a fluid to go quickly through a pipe you need pressure. This means there is going to be a pressure drop across the length of the pipe. The head-pressure initiating this flow is called, in this topic, backpressure. We all agree that is not desirable.

I think we can also all agree that the same amount of fuel/air mass that goes into an engine has to go out of it. As long as out equals in, why do I care about the exit speed? We can all picture in our mind the idea of exhaust flow where the gas going through a "small" pipe has to go faster than the same gas going through a "large" pipe, for the same mass per second rate. Look at any engineering textbook regarding fluid flow, and the formulas show that the pressure drop is proportional to the pipe/tube length (makes sense) , to the square of the flow rate, and inversely proportional to the diameter. I think we can all envision that a larger pipe would reduce the proportion of the exhaust gas that is slowed by "wall friction", agreeing with the formula.

We can all make our own choices for our exhaust system. Obviously I have my own preferences.

 

Is it possible that the velocity component is most helpful to reduce reversion? As was previously stated, the exhaust is a pulsing flow, if the velocity is high, the exhaust mass has greater inertia with which to reduce reversion. If the velocity is too high, (small pipe) the mass flow is reduced, lessening the inertia, and allowing greater reversion. Just a guess from someone who doesn't know.

 

FWIW, here's a real world dyno comparison.........

I have a substandard exhaust system, I've always known that. I wanted to get an idea how much I could gain so I had my car strapped to a dyno a few years ago. I ran back to back, first thru my exhaust (Hookers into a 3" RT Cat, then out thru 2 1/2" Flowmasters); then dropped the exhaust and ran again straight out of my collectors. I gained 20 RWHP and increased the torque by 5lbs by dropping the exhaust. It sure proved to me that my exhaust system sucked (it still does) but what surprised me was the increase in torque. I now believe backpressure is an urban myth as frequently mentioned above. At least as far as WOT is concerned.

One of these days I'll actually get a decent exhaust on my car........ especially if I ever hook up the N20 again

 

Very well put!... Now move to the next chapter and discuss the following:

Chapter Two-- applying your knowlege of fluid dynamics -- in redneck terms.
(Keep it simple St--id. with lots of recognizeable pitures)


1. The venturi effect: does it apply in any way to a exaust system when a "tuned header" is developed for a four stroke engine?

Consider: in your two stroke dirt bike the expansion chamber of a properly tuned exaust will make a very noticable "power band".

2. What is the ratio of expansion on volume of a 14.7 fuel to air ratio when heated from 90 deg c to 900 deg c?
and how does that effect volume of gas in vs volume of gas out?
Consider: what is the effect of the rapid expaision of gasses in a gas turbine or rocket motor .... or..... your Likker stil = (Liquor Stil)
extra credit: how many times does water expand going from 90 deg c to 900 dec C..
(what happens when you build too big of a fire under your pot of beans... why did the beans boil out of the pot????)

3. For the water utility guys. Thinking of line loss, pipe size and head pressure....
Explain the "water hammer effect" and how would it relate to the intake filling of a cylinder on your next high performance engine build?
and...Can you explain volumetric efficiency?

Consider: (When your local volunteer fire department abruptly cuts off a flowing fire hydrant... why do all the houses in the local area spring leaks and/or break water lines?)

4. Could you apply the physics of siphoning gas out of you roommates z28 gas tank in college to a six figure job of increasing the output of Dale jr's motor to help in his quest to win a cup title??. Could you apply that to his exaust system?

Do you think there is a little more than just moving fluid through a pipe going on here.... thats why we call it exaust velocity.... the dynamics in the real application seldom fall under one simple formula.

Square that? or just go talk to your local speed shop and quit talking to your neighbor who still thinks his grand am is a mustang eater because he put on a K&N filter, installed platium plugs, and put 104 additive in his tank.... and thinks SBC need a little back pressure to run right...
Remember bench racers and mouthy beer are a leathal combination.
http://www.youtube.com/watch?v=O-74Z...eature=related

 

amen

 

Its all about having a "TUNED" combination( from the air cleaner or air inlet to the Exhaust header outlet or tailpipe outlet for your intended application.

There are excellent articles from Smokey Yunick & David Vizzrrd on this topic from 20 years ago.

Published papers.

I believe in a free flowing exhaust too.

Works best with a camshaft overlap of at least 60 degrees I think.

Ed Iskendarien's 5th cycle theory.

Without pointing fingers here tonight,

"The Kadance theory" has been proven not to be 100 % correct.

Finate Amplitude waves at work.
They are invisible to the human eye.
But present inside of a running 2 or 4 cycle internal combustion engine.

Have to read on your own to get a good understanding.

 

Thank you !
And there you have it fokes.... The dyno proof that myth busters would be proud of.

Next myth?

 

Help me out here. I honestly can't tell if you want a serious engineering discussion, or are attempting to be a smartass.

 

Dave D. understands best out of anyone here on C4.

He signed out.

If you eliminate mufflers from a blower car or Turbo car,

More available boost can be had at WOT.

Why guys are always installing large 5 " inch single exhaust systems on Modern Chevy Duramax & Ford 6.0 diesel trucks & Dodge Cummins.

I know everyone around the USA has witnessed that in the last 10 years.

They run pretty fast too for a 6,000lb to 10,000 Vehicles.

Just citing a daily example of free flowing exhaust at work everywhere today on the roads in the USA.