That's a bunch of crap then. There's such a hodge podge of pulses from the collector back, that you could never organize them such that the decelerating gasses from one pulse causes low pressure to help draw out another. Think about the firing order: 1-8-4-3-6-5-7-2. That gives you L-R-R-L-R-L-L-R, so the pulses in one collector are not at all evenly spaced. It would be impossible to size an exhaust system to organize these such that they would tune properly to increase scavenging.

Now. . . the header primary is a whole different story. Since the pulses in the tube are very well organized (originating from one cylinder only), it is easy to tune the length and diameter to increase scavenging effect.

Besides all that, he says in the quote "small tubes", which lead me to believe he's referring to primary header tubes, not exhaust sytem piping.

Mike

 

Aren't headers supposed to "suck"?

Probably not what you meant.

 

Believe what you want but that is the way an exhaust works! If you have a good explaination(believable atleast) please offer it !

 

Bringing some more info so we all can learn!
QUOTE-----"Erase your brain and start over. RX7Speed is correct. Read his posts and learn.

There are a few fundamental ideas you need to learn.

1. How does air flow? Air flows when there is a pressure differential. It flows from a high pressure region to a low pressure region. Temperature changes play a role here but I don't want to get into that.

2. What is a restriction? A restriction is anything that impedes the flow of air. This includes anything that changes the direction of air flow such as pipe bends, muffler baffles, o2 sensors in the flow path, pipe size changes, unions, joints, etc.

3. What is “backpressure?” It is a concept and a concept ONLY! It is conceptually any form of pressure that impedes the flow of air from the engine cylinder to the tail pipe. Personally, I hate the term backpressure. In all my years of studies in engineering, including 7 courses in fluid dynamics, I never once heard the term backpressure.

Imagine for a moment you’re in the engine cylinder. The piston moves up and pushes the air out of the exhaust valve. This creates a high pressure region at the exhaust valve. Air will flow to a low pressure region. This happens to be the tail pipe. Any restrictions in the exhaust system will impede the flow of air from the exhaust valve to the tail pipe, effectively creating “backpressure” in the exhaust system. This backpressure works against the air flow. ANYTHING YOU DO TO INCREASE BACKPRESSURE WILL DECREASE FLOW RATE and decreased flow rate will reduce your power output. Additionally, the interaction of the air streams from each header tube as they merge into the exhaust pipes has an effect on the air flow.

The purpose of an exhaust system is to purge air from the cylinders. The idea is to maximize air flow from the engine to the tail pipe. Different engines move different quantities of air at different engine speeds. The quantity of air you need to move determines the capacity of the exhaust system (pipe diameter). The time you have available to purge air from the cylinder determines the design of the header tubes. Keep in mind that the time available to purge air from the cylinders is dependent upon the camshaft profile and the engine speed. The smaller the cam the less time the valve is open. The higher the engine speed the less time the valve is open.

A street engine doesn’t move much air. In order to keep the air flow rate high enough to purge the cylinder in time, a small diameter header tube is employed (1-5/8). The rest of the exhaust system is sized appropriately for the volume rate of air that the engine flows. Again, anything you do to increase backpressure will decrease flow rate and decreasing flow rate will reduce your power output. Putting 3” duals on an ordinary engine is complete overkill because the engine doesn’t need that must flow capacity. In fact, due to the dynamics of air flow it’s actually hurting performance. But that’s too complex to describe here. A race engine moves a lot of air and it does it at high rpm. The shear volume rate of air needed to be moved increases the size requirement for the exhaust system. The header tubes will have to be increased in size but it’s still important to keep them small enough to efficiently purge the engine cylinder AT THE RPM THAT THE ENGINE WILL BE USED. Everything has to be sized appropriately. Bigger is not better. Small is not better. Getting it right is better" END OF QUOTE

 

I think the point you're missing here is the importance of scavenging. The objective is not simply to "purge air from the cylinders" (or do you mean exhaust gasses?), its to remove the exhaust gasses and help draw in the fresh air charge. The smaller tube primary increases pressure losses, but its a tradeoff because of the affects of pulse flow and the ability to tune the reflective pulse for the RPM range you want to maximize.

This is why you need to break the exhaust "system" down into at least four separate sub-systems. The design criteria for each subsystem.... primary, collector, separation and tail pipe... are different. Pulse tuning in the primary and collector is a proven benefit. High velocity has a purpose. But once you reach the optimum (for your particular) length of the collector, you've done all you can do with pulse tuning. That's why you want a physical separation at the end of the collector. To establish the effective length of the collector. After the separation, the "tail pipe" part is simply a matter of moving the exhaust gasses from one point to another... less pressure loss is better. Getting rid of that part completely is even better.

Again, to repeat what I posted a couple pages ago, people are speaking too genericaly of a system as a whole, rather than looking at the specific function of each part of the system.

 

I agree scavenging plays a critical part in an "exhaust system".

 

I thought I just did. . .

If you want another explanation, just read Injuneer's post. The primary tube and collector work to tune the pulses, where "velocity [momentum] has a purpose", causing "scavenging."

But, "the tail pipe part is simply a matter of moving the exhaust gasses from one point to another... less pressure loss is better. Getting rid of that part completely is even better."

 

I just stated that scavenging is critical(important) so what's left to argue(discuss) ?

 

That the scavenging is caused by / set up by the header primary and, to a lesser extent, the collector, and not the collector-back exhaust system.

 

Oh, ok I get it now. I gave the impression that the "collector back exhaust system" had a critical part in scavening. Went back and read my replies and now I understand where I led you to believe that I believed that . and yes injuneer gave a good explaination.

 

One question so I know that we are all kind of on the same page. We all agree that you don't want to introduce backpressure in the collector back exhaust system right? Needing backpressure is a MYTH.

 

I agree with that, except in certain rare cases.

For instance, a local engine builder built a Hemi that had a cam with alot of overlap. Due to a very efficient header and head design, it would over-scavenge, that is, draw air and fuel through the chamber and out the exhaust. Backpressure would probably help this engine. However, getting a camshaft with less overlap is the "right" solution.

Also, certain supercharged cars with too much overlap can use a little backpressure to prevent blowing air and fuel out the exhaust during the overlap period.

Mike

 

you dont want RESTRICTIONS.

 

this thread is great