hi all,i have a 94 f150 stan cab short bed 2wd with a 300 I6. right nowe its got a flowmaster 40 series muffler. i wanna upgrade to stacks. my buddy said high flow cats with straight pipes would sound awesome. i wanna upgrade but i dont wanna lose the gobs of low end torque the 6 has. id love to gain torque. would this be the best setup. i want it to sound like a trailer truck im a volunteer firefighter, so when i start up a 2am i dont wanna wake half the neighborhood.

 

my cousin had a 94 f150 with a 300 I6 4x4 that he put stacks on. He ran the exhaust regular to the bed. Then put in a 90* that brought it up in the center front of the bed. Then he put in like and air storage tank, and ran his stacks out each end of the tank. Rather quiet. Sounded no good at all.

 

The only problem with the 300 is that they usually sound like tractors, not big rigs. Whether you believe in back pressure or not, if you run any kind of cat you will be covered. After that it doesn't matter what size pipe or even if you have a muffler, the cat(hi-flo or regular) will provide all the BP and sound muffling you need. Considering the size of your truck I think you get away with a single stack and still look good(just my opinion) but dual stacks are always nice.

 

ok. i kinda believe in back pressure. im prob gunna lose the muffler, but maybe keep the stock cat. im not gunna get rid of the cat because i need to go through emissions (and pass). i might do just 1 stack, but it mght look odd. considering my trucks an XL itll prob be black stacks. i just gotta figure out a way to guard them when i load the bed (basically everyday). im thinkin a thick piece of steel with a top piece that slides down the stack so i dont lose nuts, bolts, leaves, etc behind the guard.

 

You might look into some perferated covers for sidepipes ... Might have to be cut for length tho ...

 

It has nothing to do with backpressure. Backpressure does not produce torque, and is not needed to run properly. What is needed is velocity, which is the result of smaller pipes and "restrictive" points. The only time specific backpressure is needed is either if it is an important part of EGR systems, or in certain rare circumstances, to prevent overscavenging. The reason torque is lost if the exhaust is too large is because the exhaust that comes out of the exhaust ports is very hot and is moving very fast (often 300 mph or more). If it moves into an area that is significantly larger than the ports it came from, it expands to fill that space. But as it expands, it loses speed and heat. This effect is most pronounced at lower RPM, since the exhaust pulses are spread farther apart, the temperatures are lower to begin with, and the cylinders are not fully filled, which means less exhaust volume and lower initial speeds.

The faster the exhaust is moving, the harder it is to stop, which produces a smoother flow and more consistent power, especially at low RPM and part throttle conditions. But if it is moving too slowly, and expands into too large an area, it becomes turbulent, and it slows down. This slowing creates an area of pressure which restricts further flow. It actually creates backpressure. One key difference as well is the location of this pressure. In a properly sized system, the pressure at the ports is relatively low, and at times is even a relative vacuum. The effect of a gas or a liquid flowing at high speeds through a small passageway is known as Bernoulli's principle. One of the effects this produces is vacuum, which we also cause a scavenging effect. This pulls as much of the spent exhaust from the cylinders as possible. This increases the amount of fuel and air that can enter the cylinder to produce power.

If the area the gases are flowing into is too large, the gases will expand and cool, and will also slow down. Since the gases prior to this area are still moving faster than the gases that have cooled, the result is areas of pressure. This pressure will resist motion through that area.

Backpressure can never be fully eliminated, but one can change where is occurs. As a general rule, smaller exhaust systems produce lower backpressure in the region of the exhaust ports and manifolds, and much of the front portion of the system, but can become increasingly restrictive as RPM increases. In contrast a larger system produces higher levels or backpressure in the manifold regions and more initial resistance to flow, but becomes much less restrictive as RPM increases.

Finding a balance between the velocity and flow is difficult. As a general rule, the factory size is pretty close to the ideal size and moderate increases in size will not trade off much bottom end performance. Also keeping the front portion of the exhaust smaller will keep the velocities high and maximize torque. Towards the end of the exhaust system increases in size are less significant on torque, but can support the flow of higher RPMS. Larger pipe in the back of the exhaust system also causes the sound waves to pile on each other as they slow down, creating a lower, deeper tone and also becoming louder.

One examples of smaller sized pipe being used to maximize torque is Dynatech's Merge collectors. These collectors merge the exhaust from multiple ports into a much smaller area than a conventional collector before increasing in size to the diameter of the rest of the exhaust system. The design seems to defy traditional tuning logic, but dyno tests shows significant increases and a broader smoother powerband.

 

Everything ^he^ said.

A 300 does not make a lot of noise. To make it loud you need to straight pipe it, or almost-straight pipe it (i.e., high flow cat running into straight pipe). When you do this, you only get one sound; a tractor sound. I don't think it sounds bad. I like it. I think running dual stacks that emit a tractor like sound would be cool. But that's just me.

Here's a vid of my truck with a high flow cat and straight pipe. I liked it, personally.

YouTube - New set up; High Flow Cat / tail pipe

I think running dual 2.5'' straight pipes would sound mean.