exhaust installed, little disappointed, need advice

96GT281 said:

so your saying that lt's dont improve flow? i never knew you gained hp by restricting exhaust flow

Click to expand...

Green&GoldGT said:

Longtubes make the exhaust quieter at lower rpms and louder at high rpms. Imagine that you have the same exhaust flow, but longtubes have more area inside the exhaust than a stock manifold.(like the difference between blowing air through a straw or paper towel roll). Loudness will decrease and it will sound deeper. At a high rpm, pushing out more air, longtubes will have an advantage.

Click to expand...

Thundermouse said:

I'm not exactly sure I understand where your question was directed at, but my best guess if that you are referring to my point that the engine is not going to blow any harder with a less restrictive set of headers (longtubes). I'll do my best to explain my reasoning:

Everyone will agree that longtubes offer less restrictive flow characteristics and higher top end power to the wheels. For this discussion we will accept both of those points as givens. The question to ask is where is that extra power to the wheels coming from. Power at the wheels for the purposes of this discussion can be expressed as:

Gross Energy from Combustion - Energy Lost to Heat/Friction/Light/Inefficiency = Net Energy Successfully Transmitted to Wheels

When we change a stock Mustang by adding longtubes, we are reducing the 'Lost' part of this equation. Keep in mind this is a 4 stroke combustion engine. During the 4th stroke, the engine must use energy to push exhaust out of the chamber. The exhaust stroke does not happen without force supplied to the piston, force that comes from the crankshaft and ultimately otherwise would be transmitted down the rest of the drivetrain. In other words, the exhaust stroke acts as a load on the engine much like a water pump or an alternator acts when they are driven by the serpentine belt. With a less restrictive exhaust header, less force is require to push the exhaust gas out of the chamber. Think about how hard/long you have to squeeze the air out of a ziplock bag when there is only a pin-prick sized hole versus when a quarter of the zipper is open. So with longtubes we are reducing the energy lost due to an inefficiency of the engine.

Based on the expression above, we have increased power available to the wheels since we have shrunk the reducing half of the left side of the expression. Now, the argument could be brought that longtubes will also increase the Gross Energy From Combustion part of the expression since they will provide more optimal backpressure characteristics for the higher rpm range that we are discussing, but in the case here of our otherwise stock mustang, we are only talking even at the most optimal tuning, single digit hp improvements. Important to note: that gross gain is only achieved if the optimal backpressure is achieved, ie. the right longtubes are used. Different longtubes reduce resistance to flow, thus affecting backpressure, versus shorties by different magnitudes. The magnitude of the reduction is a function of tubing size, pathway, and even interior coating. If you reduce resistance too far, you lose backpressure and potentially cost yourself some Gross Energy From Combustion. Don't go far enough, and you are where you started with the shorties. With the case of rare exceptions, the gross power gains (note: not Net Power Gains) achieved from using longtubes on otherwise stock mustangs will not be significant. The bottom line to this paragraph is, longtubes really do not have the capability to help the engine create more gross power.

Theorem 1: Longtubes increase the efficiency of the engine and raise the Net Power transmitted to the wheels, but they do not help the engine create more gross power.

Now we should be able to agree that the mustang on the dyno with longtubes puts up better peak numbers on the dyno because of the second part of the above expression. Thus, I stand by my point that: The engine is not going to "blow" exhaust into the headers with greater force just because the longtubes have less resistance to flow. I can stand by this point because "how hard the engine blows" is the force at which the piston pushes the exhaust out. The rotating assembly of the engine will only apply the minimum force required to complete the exhaust cycle, as the rest is transmitted down the drive train. So how do we know that the force to exhaust gas is the same on a stock mustang with longtubes vs a stock mustang w/ shorties? Force for this case can be expressed as:

Mass of the Exhaust Gas x Acceleration of the Gas = Force Exerted on Exhaust

Let's compare each part of this equation. Looking back to the conclusion reached in Theorem 1, the use of longtubes does not enable to engine to combust a greater amount fuel, thus the mass of gas to be removed from the chamber will remain unchanged. Acceleration of the Gas represents how quickly the exhaust gas in the chamber must be accelerated to leave the chamber in a set amount of time defined by a given rpm. The gas is accelerarated by the piston, and the piston will move at the same rate in both scenarioes (longtube equipped engine vs shorties).

The given difference noted earlier between longtubes and shorties is that shorties have greater resistance to flow. A pipe with greater resistance to flow requires that gas be accelerated through it at a quicker rate if all other conditions are to be held equal. Another way of looking at the situation could be: Think if you have a room with 30 people in it who are lined up in front of a 4 foot wide doorway, and an identical group of 30 people in an identical room lined up inside of a 10ft wide doorway. If the groups are to exit their respective rooms in the same elapsed time, the first group of people is going to have to move their feet much quicker!

So looking back to the Force expression, we have an equal part one of the expression between shorties and longtubes and a greater required acceleration (part two of the expression) for shorties. The result of the equation is greater force to be applied on the exhaust in the condition that shorties are used on the engine. (We are keeping things simple and linear for the purposes of this discussion, but the same conclusion is reached if you examined the process by calculating the integral of the change in force applied, and thus work, and thus power, over the duration of the exhaust cycle.)

Theorem 2: With longtubes on the engine, less force is applied by the engine to exhaust gases.

Are we clear now on why the engine is not going to "blow" the exhaust "harder" just because longtubes are used?

If what I discussed is not what you were referring to, then... I'm sorry I wasted your time in reading this.





I agree with your ideas. I am not taking issue with stangzilla100's conclusions on loudness, I only took issue with the thought process to get to that conclusion.

Click to expand...