I was snooping around the other sections of the forum and ran across an interesting take on exhaust systems and low-end power. If you havn't read this already, you may find it interesting.
https://dodgeforum.com/forum/2nd-gen...t-low-end.html

 

It's confusing to say the least.

 

I agree with almost everything 95_318 said except for his statement, "backpressure does not help power or torque at any rpm." While technically correct, it misses the fact that backpressure is a component related to the how of why an exhaust works well at a specific RPM range and not another. The one word missing from his dissertation is "scavenging," which is the process whereby the outgoing exhaust gasses help pull the incoming fuel mixture into the cylinder. This process, when optimized, can improve performance at a given RPM range. The problem with so many of our engines is that we need them to perform across a much wider RPM range than what the exhaust system can be tuned for resulting in our having to accept a fair amount of compromise in order to have reasonable performance across a wider RPM range. The magic comes from finding the sweet spot in that compromise where the falloff in performance is minimal or where the best performance comes at an RPM range that suits the larger percentage of our driving needs. Often, the compromise leads us toward one extreme or another. We sacrifice some top end power in order to have more torque at lower RPM or vice versa. The design of the internal combustion engine and the wide RPM range it is capable of means we will never be able to squeeze every bit of power out of it across the entire RPM range. The best we can hope for is maximum performance in the powerband of choice for our needs.
Rule of thumb is that low RPM torque engines are better with smaller diameter exhaust tubes. These help the exhaust retain a certain amount of movement (called pulse) that creates a vacuum at the exhaust port that assists in pulling in the incoming fuel charge (almost all engines have an amount of valve overlap where the exhaust valve is still open while the intake valve is opening). Too large an exuast pipe and that vacuum is lost. Too small and the excess backpressure inhibits the process. In a high RPM engine the consideration is to get the exhaust out quickly and with as little restriction as possible. Look at a top fuel dragster. They often have very large diameter tubing cut extremely short. Almost no backpressure at all.
Nowhere else is compromise more evident in an engine than in the exhaust. It is almost impossible to design an exhaust that works as efficiently at 2000 RPM as it does at 5500 RPM. Performance at one or the other has to suffer, hopefully only a little, but the considerations of engineers in overall performance and cost of manufacture often creates a wider gap at the top of the RPM range than at the bottom. This is also why a small modification can yield pretty sizeable gains. Something as simple and cheap as replacing a poorly built/designed stock muffler can really wake up an engine. When considering exhaust modifications one really has to do their homework to prevent disappointment. Putting a 3" tube system on a truck you mostly rock crawl with is a bad idea. Likewise, so is putting a 2" system on a mud bogger. Compromise will always be a part of the equation. You just have to figure out where the compromise will be and whether you can live with it based on how you use your truck.

 

I agree with that Miami. IDK if you read the entire thread or just the first page, but me and hydrashocker "discussed" this topic. My final statement in the thread was that too little backpressure disrupts the intake flow (what you called scavenging), so where you may have freed power up in the exhaust, you have lost more in the intake.

I guess its the same idea with a supercharger... you have to lose power to create more!

 

Yeah, I only read your post and a few others. I will go back and read the rest. Good stuff. What most don't understand about backpressure is how it helps/hinders scavenging and how it contributes to the exhaust pulse. You probably know this, but for the benefit of others, I'll elaborate. As previously mentioned, cylinder scavenging is the process whereby the outgoing exhaust helps pull the intake charge into the cylinder. When there's too much back pressure, i.e. restriction in the exhaust, scavenging is hindered and less of the intake charge reaches the cylinder. This means less power. The same result happens when the back pressure is too little and the exhaust moves too quickly out of the cylinder to help with scavenging (the velocity you mentioned in that other thread) or it moves so fast that some of the intake charge goes right out with the exhaust. When the back pressure is just right there is a slight vacuum created at the exhaust port. This vacuum helps pull more of the intake charge into the cylinder resulting in more power. The design of a good exhaust system is aimed at getting a consistent exhaust pulse from exhaust port to tailpipe tip. That's where the term "tuned exhaust" comes from. It refers to a system that is designed so that the exhaust pulse moves consistently all the way down the pipe and through the muffler. A cast iron manifold can not do this because all the exhaust pulses from each cylinder dump into the same place and consistency is impossible to maintain. That's why a tuned length header is superior to a stock manifold. Each exhaust port has its own tube to dump into and each pulse deals only with its own environment. By the time they reach the collector (where all the exhaust from each cylinder meets) they are less affected by backpressure and have little influence on each other or the scavenging process. They now work together to continue evacuating the exhaust from the engine as it moves through the cat, muffler, and tailpipe.
Unfortunately, cost and manufacturing considerations often put a crimp in the capabilities of the stock system. Poorly designed manifolds, cheap mufflers and exhaust tubing of the wrong size with poorly executed bends leaves a lot to be desired in the performance arena. Taking the time to figure out your performance needs relative to how you use your truck will result in a better outcome for any exhaust modification.

 

I will admit, at the beginning of that thread I got too focused on just the exhaust until hydrashocker's comments got me thinking about the effects of the exhaust on the intake. I do have a different view now than when I made my first post. At the time I was thinking about high performance racing applications, where the cam and intake are designed and chosen in such a way that if you create a zero backpressure exhaust system, it will not not hinder the performance of the intake, so you don't have to lose that power on the exhaust side to create power on the intake side. In theory, that is the best solution, but in practice as far as anyone on this forum is concerned, thats impractical for a daily driver. At the time I wrote my first post, I my thoughts were that you want no backpressure, but now my thoughts are that you want as little as you can get without hindering the intake. I just wanted to clarify that. Now with that said, I doubt anybody reading this would ever spend the shop time neccessary to build an exhaust system to have no back pressure, so if you just go for minimal backpressure, chances are whats left will be enough!

Now that that is out of the way, you make some good points Miami, but there's one part that the wording confuses me:

I think you meant to say too little backpressure causes the exhaust to move too quickly out of the cylinder? The more backpressure you have, the more it slows the gasses down, the less you have, the better they can keep up their velocity through the system. Otherwise, I agree with everything you said there!

 

My bad, I fixed it. Sometimes my brain goes faster than my fingers can type.

For the record, in my other life as a magazine writer and product tester, I spent hundreds of hours on the dyno testing and tuning motorcycles (Harleys and dirt bikes). I've seen precisely how a properly designed exhaust system can be used to move the torque and horsepower curves to the desired RPM range and how performance can also be hurt by a poorly designed system. It's frustrating to see what some owners do to their vehicles on bad advice or because they see something on someone elses vehicle and think it will suit their own purposes. I know a company that sold a buttload of Harley air cleaners simply on looks whose sales fell after we did some dyno testing comparisons with them. Actually, I know a few companies that weren't too happy with me after a story I wrote.

 

ok, im going to throw a monkey wrench at y'all

so last week i deleted my cat, which lowers backpressure and actually gained low end on the butt dyno??

my only thought is this: the honeycomb had disengrated and was falling around loose in the case, so maybe it was restricting flow enough to cause too much backpressure

all i know is, my truck pulls harder now when i take off, i like it...oh and with only a glasspack for a muffler, i can scare people going under bridges when the sound bounces

 

Was it the stock cat? The stock cat is a gigantic restriction to the system. Almost as much as that water heater in the back.

 

Thats exactly my point!! The misconception and myth is that more backpressure means more low end torque. I've heard it many times and it is simply not true. If you get rid of the restrictions put in the exhaust by the factory to meet emissions and noise standards, you will gain power and torque. The point me and miami and hydrashocker were all pointing out is that if you go too low (as you approach 0 backpressure) on an otherwise factory setup, it will disrupt the intake flow and hurt power that way.... at least thats the dumbed down, simplified version of it... read miami's posts to get the full explanation.