Less back pressure = less Torque? Why?

[DiggerStang]

I've read more times than enough that less back pressure in the exhaust system equates to less Torque. However, I've never come across an explanation for why this is so. Can someone please fill me in? It doesn't seem to make sense.

HP=Torque*RPM/5252. Let's say a 5.0 makes 300 ft.lbs of Torque at 3200 RPM. That means about 183 HP at 3200 RPM. Now, if I get a 3" exhaust system, I'm going to lose Torque (apparently). So let's say we lose 10 ft.lbs of Torque. Now we have 177 HP. Great! We lost 6 HP! Of course, the exhaust system will allow the engine to breathe better, so we'll gain some HP from that. But we may not be much better off than where we started, right? And if we lose 30 ft.lbs of Torque, then we lose 18.5 HP! It seems what HiFlow has posted in the past is correct (of course!), there is a point where exhaust work becomes overkill. If it is true that less back pressure = less Torque.

Clearly, I'm missing something here. Maybe the less-restrictive exhaust pushes the peak HP to a higher RPM. I don't know. But if it's true that less back pressure = less Torque, why is it so? And what is the point, if you lose HP? My understanding is that the Torque peak occurs when the engine is turning at the rotational speed where the most air and fuel enters (and burns) in the cylinder. What does this have to do with the exhaust?

[Mendari]

If you've ever tried to siphon out a jacuzzi or an aquarium what I'm about to say will make more sense. I'm not sure about back pressure, but I think it has more to do with exhaust gas velocity, pipe diameter, and scavenging. A thiner pipe causes the causes the first group of gases to actually suck out the following set of gasses. This is because the expulsion of the first set of gasses leaves an empty void behind itself. The next set of gasses are pulled into the void. The third set of gasses get pulled into the void created by the second set of gasses. So on, and so on.
If the diameter of the exhaust pipe is too large, then there will be no void left by the gasses and therefore the scavanging chain will not occur.
But then again, I just may be full of ****.

[HiFlow5 0]

Originally Posted by Mendari

If you've ever tried to siphon out a jacuzzi or an aquarium what I'm about to say will make more sense. I'm not sure about back pressure, but I think it has more to do with exhaust gas velocity, pipe diameter, and scavenging. A thiner pipe causes the causes the first group of gases to actually suck out the following set of gasses. This is because the expulsion of the first set of gasses leaves an empty void behind itself. The next set of gasses are pulled into the void. The third set of gasses get pulled into the void created by the second set of gasses. So on, and so on.
If the diameter of the exhaust pipe is too large, then there will be no void left by the gasses and therefore the scavanging chain will not occur.
But then again, I just may be full of ****.

That sounds pretty good to me Mendari.

Remember, you will not lose torque all together, you will lose torque in the lower rpm band, while gaining more power higher in the rpm band. You should also notice the engine rev more freely. You are in sense un-choking the engine by going with less restrictive exhaust. The motor will be able to exhale the spent exhaust fumes much easier as the motor works harder when the rpms climb, therefor creating more power by running more performance efficient.

[DiggerStang]

I knew I was missing something! So there are basically waves of compression and rarefication in the exhaust system. Makes perfectly good sense, now that you mention it! And if you only lose low end Torque, it makes sense that you will gain HP at higher RPM. I'm not sure I entirely understand why you lose low end torque. I'll just have to run the entire intake/compression/combustion/exhaust thing through my mind and reason it out.
Thanks!

[HiFlow5 0]

OK lets try something here, I hope you follow me now. Lets pretend your mouth is an engine. You have a straw and a garden hose, both will represent 2 different size exhaust's. A glass of water which will represent exhaust fumes.

Now with the water in your mouth, blow lightly through the straw. You get a nice steady stream. Now blow hard. You get a fast sharp stream, but as you blow harder, the effort you exert becomes very strenuous and the effort doesn't really pay off for the amount of water that is coming out.

Now do the same thing, but with the garden hose. Blowing lightly will let the water more or less just dribble out. Now blowing hard will really get the water to flow fast out of your mouth, much quicker then the straw would have.

You see what I'm getting at here?

[DiggerStang]

Sure, I see what you're saying. I can see why it increases HP. The piston experiences less resistance when expelling gas through a larger exhaust pipe. Less work expelling gases means more power available to turn the wheels. That makes perfectly good sense, and I knew that much before I posted the thread. It's the Torque thing I was questioning.

[Rodek]

okay, what i get from HiFlow50's explanation is this:
the reason that there is a low end torque lost but high end HP gain is becuase at the lower rpm, we are puting out less exhaust overall in a given interval, so when you have a bigger pipe, there might not be as much of a strong vacumm sucking out the exhaust gasses from the cylinder as there would be if you had a smaller exhaust pipe. But as the rpms climb, you get more exhaust gases getting expelled at a faster rate, which is when the bigger pipe comes in handy since the air is getting sucked out appropriatly now that there is more pressure.
So what you get is this:
Before: small pipe = lower rpms needed to create good suction of exhaust gases out of cylinder but at higher rpm size becomes a problem as you reach a limit as to how much exahust gas you can expell out of the pipes
now:
big pipe = higher rpms needed to get same suction but the limit of how much exhaust gas you can expell increases so at the higher rpms you have more suction and more effeciancy.
This might not make any sesnse. I was wondering the same questions before and i think their asnwers really helped, this is kinna just what i got from it.

[HiFlow5 0]

It's the scavenging of the exhaust, where torque is lost. Referring to my previous post, the water will flow steadily though the straw, but more of a dribble through the hose with light blowing. So the exhaust fumes flow better at low rpms with a smaller exhaust cause there is more scavenging going on. With a larger exhaust, there is less scavenging at low rpms, but flows easier at higher rpms.

[HiFlow5 0]

Yeah Rodek, that's pretty much what I was getting at. I think I have explained all this correctly now. It's hard, knowing the facts then trying to explain them for others to understand.

[DiggerStang]

I'm pretty sure I've got it now. I was pretty much unaware of the concept of scavenging until now. I'm a good electronic technician (at least I like to think so...), but I still have alot to learn about mechanics. Thanks for the help, guys!

[Vroom Vroom]

One more missing piece of the puzzle with exhaust is heat. Hot gasses flow faster. To large of an exhaust allows the gasses to cool off to quickly before they get out the pipe therefore slowing them down.