Hank to the rescue again,
I have had a belief for years about pipe size and torque and can't remember how I got that belief but this article supports it. Whew! Now I know I'm not totally crazy.
article quote:
" In practice though it is better, especially for a street-driven machine, to have pipes a little too small rather than a little too big. If the pipes are too large a fair chunk of torque can be lost without actually gaining much in the way of top-end power."
[sm=yeahsmile.gif]

 

When I asked Marty about where the torque gains were he said low to mid-range. If torque is what makes wheels spin easier and throws you back in your seat, then I gained torque from the change in exhaust. They may not have the same effect on a stock motor. I don't know.

From an article called "The Truth About Exhaust Backpressure and Torque"








Some say that "an engine needs backpressure to work correctly." Is this true?
No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.
The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.
The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.
The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.
Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.
Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.
Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.

 

More:

This was taken from ProficientPerformance.com

[blockquote]quote:

Most of the time bigger is better, but that's not always the case with exhaust. Today's late model cars are designed with several restrictions on the manufacturers. So in order to perform but still stay within the limits of emissions and other regulations, the exhaust is usually the first thing to suffer from the factory. The easy power that is found in changing the original equipment exhaust system, is just one of the many reasons there are so many aftermarket manufacturers capitalizing on this insufficiency. But just opening the exhaust doesn't always make more power, it takes the correct combination of flow capability and velocity. That's why performance exhaust manufacturers spend so much time in search of just the right combination, so that they can ensure peak power to their customers.

In this article we will discuss the effects that exhaust backpressure has on your engine. Backpressure is the restriction of the exhaust gases that are produced by your engine while they are being released through your exhaust system. Exhaust gasses are the burned and unburned gasses that are released out of the exhaust system after engine combustion happens.

The exhaust system should have minimal restriction to the exhaust flow that is being released in order to create the most amount of power from the engine. With restriction present, backpressure is developed, making the engine work much harder to release the exhaust out of the engine cylinders.

The standard for exhaust diameter is to not run too large of exhaust, for the engine needs a certain amount of backpressure. This is correct in the sense of one should not run too large of diameter exhaust tubing, but the statement of the engine needing backpressure is not. You need to have the least amount of backpressure possible to produce maximum power. Too large of diameter exhaust will cause a power loss and loss of low end torque because a larger pipe has less exhaust stream velocity than a smaller pipe. If the exhaust pipe is too large, then the exhaust flow will be slowed with less velocity. There are exceptions, forced induction or high-volume race engines require a larger diameter exhaust due to the flow characteristics of the engine. Supercharged and turbocharged vehicles can have an exhaust gas volume of 1 to 2 times more than an equivalent displacement naturally aspirated engine. In this case, one is able to use a larger diameter of exhaust for greater performance.[/blockquote]


This little tid-bit is from howstuffworks.com


[blockquote]quote:

Headers are one of the easiest bolt-on accessories you can use to improve an engine's performance. The goal of headers is to make it easier for the engine to push exhaust gases out of the cylinders.

When you look at the four-stroke cycle in How Car Engines Work, you can see that the engine produces all of its power during the power stroke. The gasoline in the cylinder burns and expands during this stroke, generating power. The other three strokes are necessary evils required to make the power stroke possible. If these three strokes consume power, they are a drain on the engine.

During the exhaust stroke, a good way for an engine to lose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the piston has to push against to force the exhaust gases out, power is wasted. Using two exhaust valves rather than one improves the flow by making the hole that the exhaust gases travel through larger.

In a normal engine, once the exhaust gases exit the cylinder they end up in the exhaust manifold. In a four-cylinder or eight-cylinder engine, there are four cylinders using the same manifold. From the manifold, the exhaust gases flow into one pipe toward the catalytic converter and the muffler. It turns out that the manifold can be an important source of back pressure because exhaust gases from one cylinder build up pressure in the manifold that affects the next cylinder that uses the manifold.

The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. These pipes come together in a larger pipe called the collector. The individual pipes are cut and bent so that each one is the same length as the others. By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector.[/blockquote]

 

The internet is full of conflicting information on this. I have read articles that swear up and down that tighter exhaust lobe seperation will cause a reduction in torque due to the reduction in backpressure when I have plainly seen dynos comparing a Comp 604 and a KRC210x. The 210x, with it tighter lobe seperation of 110*,out pulls the 604 in torque hands down. People go by experience (correctly), and while some trucks may pull better with a completely stock exhaust system, others might respond better to performance exhaust. Apparently (per Krc's dyno and my butt) , atleast two Dodge Ramsrespond well to the latter.

 

When was there any confusion about this? Years ago i read David Vizard's book How to Build Horsepower vol. 2; Carburetors & Intake Manifolds from 1996. He explained it all back then...

This "myth" probably came from someone who had no clue as to what he was talking about and it speading from there.
Most people who deal with cars, truck etc. do not know much if anything at all, although this forum is different from most...
that is why most things on the internet should be takencum grano salis.

In other forums they would say you are the one who iswrong for thinking decreasingback pressure increases torque. If you had pointed this fact out in say...VW Vortex forum you would be laughed atand harassed.

 

im sure it came from hondaforum from some guy with his "bubble hatch" installing a 4" exhaust. and since theres no torque to lose with that car anyways, the loud sound made him think it was torquier

 

Always remember - just because someone took the time to write it down, doesn't make it true.

 

Here's the Austrailan article from Julian Edgar's
21st Century Performance Book:



http://tinyurl.com/2y6ll7

quote from page 139
begin quote

"Few tests have been done that clearly show the effect of changing back
pressure. Most muffler and exhaust comparison tests change more than one
parameter simultaneously, making the identification of exhaust back pressure
as a culprit difficult. However, Wollongong (Austraila) mechanic Kevin Davis
has done extensive testing of varying back-pressure on a number of performance
engines. These range from turbocharged Subaru Liberty (Legacy) RS flat fours
to full-house traditional pushrod V8s. In not one case has he found any
improvement in any engine performance parameter with increased exhaust back
pressure.

The tests came about because Kevin has developed a patented variable-flow
exhaust that uses a butterfly within the exhaust pipe. He initially expected
to use the system to cause some back pressure at low loads 'to help torque.'
However, he soon changed his mind when any increase in back pressure proved to
decrease torque on a properly tuned engine. What increasing the back pressure
does do is dramatically quieten the exhaust.

One of the engine dyno tests carried out by Kevin was on a modified 351 4V
Cleveland V8. Following the extractors he fitted a huge exhaust that gave a
measured zero back pressure. Torque peaked at 573Nm (423 ft-lbs) at 4700 rpm,
with power a rousing 329 kW (441 hp) at 6300 rpm. He then dialed-in 1.5 psi
(10.4 kpa) back pressure. As you'll see later, very few exhausts are capable
of delivering such a low back pressure on a road car. Even with this small
amount of back pressure, peak torque dropped by 4 per cent and peak power by 5
per cent. He then cahnged the exhaust to give 2.5 psi back pressure. Torque
and power decreased again, both dropping by 7 per cent over having zero back
pressure.

Figure 6.1 {which is a graph with three lines showing HP against rpm} shows
the power curves gained in the tests. These results were achieved on a large
engine with a large overlap cam - one of the type some people suggest is
'supposed' to like back pressure.

If, in fact, power does increase with increased exhaust back pressure, it is
most likely the air/fuel ratio and/or ignition timing that are no longer
optimal for the altered state of engine tune."

end quote

 

Unfortunately
the quote from Vizard about pipe sizes is
'taken out of context'
because he is not talking about exhaust pipe sizes
he is talking about a different animal...primary header pipe diameter.

Here is the whole quote:

Headers — Primary Pipe Diameters
Big pipes flow more, so is bigger better? Answer: absolutely not. Primary pipes that are too big defeat our quest for the all-important velocity-enhanced scavenging effect. Without knowledge to the contrary, the biggest fear is that the selected tube diameters could be too small, thereby constricting flow and dropping power. Sure, if they are way under what is needed, lack of flow will cause power to suffer. In practice though it is better, especially for a street-driven machine, to have pipes a little too small rather than a little too big. If the pipes are too large a fair chunk of torque can be lost without actually gaining much in the way of top-end power.
end quote

David Vizard would agree that low backpressure is good
and does not reduce torque.

Most of the 'myth' has always involved people mistaking header primary pipe recommendations with exhaust pipe recommendations.....or with race engines running ultra-long overlap camshafts

 

glad to see you like the long tubes when i mounted mine on my motor i hadsimilar results.....deff. what the motor was needing.....

now i want to get the dual plane intake from hughes mounted up then im going to the dyno.....