Here we go again....

Backpressure, is used to tune the pulses. If "tubes/cats/mufflers=backpressure" is so bad, why don't Top Fuelers just run no exhaust? Straight out the heads?

Because the backpressure, TUNES the exhaust, increasing exit velocity!

 

How can a runner introduce any more backpressure than the head if its the same size as the exhaust port in the head? And yes they do use the runner to tune the engine. Go ask an old top fuel crew chief ( I have ). Since top fuel engines pretty much stay at one RPM you get hot-cold-hot sections in the runner due to the standing waves. You want to make sure the runner ends in the middle of the hot spot. If you watch a top fuel car running down the track you will also notice the waves extend past the runner into the air. They almost look like stationary smoke rings. Not to mention having no runners at all is a sure fired way to bend valves.

I'll refer to the original sticky. Its a sticky for a reason. Backpressure is bad.

 

Here we go again indeed:

This is like trying to explain to someone that you need resistance in an electrical circuit, when they have been told that resistance is bad. You are saying that tuning requires using wave theory, which is dependent on tubing and reflective features within the tubing, both of which create back pressure, and the wave tuning gives positive results not possible without the tubing and reflective features, yet you say that back pressure is bad. Don't you see, you have just contradicted yourself. You're trying to kill all the cows because you only want beef.

Top fuel cars do not need scavenging, their superchargers and incredible cylinder pressures from the Nitro blow so much pressure into and out of the cylinders, evacuation happens because of massive pressure that does not exist in any other kind of engine. In a Fuel car, nobody cares how much unburnt fuel gets pushed out the exhaust. If you think I'm wrong, stand near the pipes of one idling some time, and you'll understand what "unburnt hydrocarbons" are, just raw fuel dumped out the exhaust. Fuel cars are grossly inefficient in miles per gallon or HP/Gallon, but NOBODY CARES.

Also, since they are not required to provide any kind of efficiency or low end torque even with a blower, let alone gas mileage or ability to pull from any RPM, how or why a fuel car uses "zoomies" is also completely irrelevant in the real world and irrelevant to this discussion.

And if you don't think that those pipes represent a restriction, try running a foot race with a piece of tubing over your mouth and nose that represent the same area as your nose and mouth would be when you would run normally. Please don't pass out on me...

TA

 

My car got faster when I installed the adjustable Borla muffler.
I have headers, no cat, and the Borla.
I put down 195rwhp and 273ft/lbs torque on a stock long block 165K mile penut cammed LB9.

 

That's not a surprise to anyone. Reducing back pressure from stock while installing a tuned system is not the same as "all backpressure is bad always".

If you removed the entire system and just ran bare heads with no exhaust whatsoever, that is as close to "no back pressure" as you can get No matter how you tune it, it will not make power, any normally aspirated and most blower cars will run very badly that way, no matter what kind of technical term shell game you play.

TA

 

Then for all the people who have reduced their backpressure by gutting thier cat, may I suggest a potato in you tail pipe. Would a moderator please delete this thread.

RTFM

 

Brilliant idea to keep from learning anything, if someone doesn't agree with your opinion, you should demand that the thread be deleted!

Lets find some common ground instead of just arguing back and forth. We all agree that back pressure or restriction on a stock system is excessive to prevent noise and also due to outdated catalytic converter technology. REDUCING the back pressure, if done correctly, will result in increased power across the power band. Running a modern High-flow cat verses no cat at all will bring about the same power level, usually within one or two horsepower on a 400HP engine, less difference on less powerful engines.

We also agree that "wave tuning" of the exhaust system is highly beneficial to producing increased power, but to get wave tuning, you need tubing and something to reflect the waves and merge the different cylinders, as well as the right size/length tubes timed to get that energy back to the exhaust port when you want it to get the highly desirable scavenging effect. Whatever you use to reflect and guide the wave energy back to the exhaust valve will be a restriction or produce back pressure. If you can figure out how to return energy without taking any out of the system, I'd really like to see it.

The design of a header is specifically to maximize scavenging with a minimum of back pressure, but you notice what order those are listed. No matter what you do, you cannot have one without the other, and a little back pressure is a small price to pay for effective scavenging.

So, in the sense that back pressure or restriction will reduce power if it is not part of the tuned scavenging system, that is correct. However, my argument has always been that removing all back pressure/restriction will reduce power as well, because you destroy scavenging. That is my argument with "all back pressure is bad". That statement is not correct, no matter how many of you throw potatoes at me.

TA

 

I give up...I'm done trying to explain backpressure vs scavaging...

 

I'm beginning to agree. Apparently because something has been made a sticky means that it is absolutely true, correct and beyond question. You cannot teach anyone who does not want to learn.

There are a number of errors and oversimplifications in the sticky that negate all the pretty colored graphs and math. The statement that a simple mathematical formula proves that a pipe presents no restriction does not hold up in the real world. My experience with a flow bench proves this is wrong. I live in the real world, and a math formula is only valuable if it is proven RIGHT by real world experiments, otherwise it's 'back to the drawing board".

If a tube provided zero resistance or back pressure, than the size of the tube would be irrelevant, zero is zero. Take a tube of small diameter, stick it in your mouth and blow. What happens? Back Pressure. A larger tube provides less, and a smaller tube provides more. I thought this was common sense.

So if the sticky is wrong about this a fundamental property of physics and his basis for his simple formula, then his over-simplified formula is wrong and his conclusion is also wrong.

Go ahead, make this a sticky, I dare ya.

TA

 

Alright then. I won't give up on you yet TA. Lets try to find some common grounds we can start on.

BTW, I ask for forgiveness from the original poster of this thread. While we have ventured far from his original concern of loss of power since he gutted his cat. In response to that, I feel there is not enough evidence to suggest actual power loss. "Seat of pants" dyno results should not be enough to blame the mod. But there may be something to be said about your newly introduced expansion area (the gutted cat). Try replacing it with straight pipe and see if the power returns.

We need to make some basic assumptions about physical laws. Machines will never denounce physics, even though thier method may be complex.

I will start this discussion by saying that the only way move air from one point to the next is with a difference in pressures. This also asumes that all things equalize as quickly as possible. IE, air will flow out of the end of an open balloon simply because the elastic casing pressurizes the air to a pressure greater than the outside air. Would you agree?

 

Sorry, i do not agree. Matter, such as a gas (air, or whatever) or solid matter can be moved with other forms of energy. You can use sonic waves to move things. When your speaker cone moves in your speaker, which in turn moves your eardrum, the energy moves in a sound wave. This is the principal behind some of the scavenging, it is reflected wave energy being used to 'stuff" fresh mixture back into the cylinder just before the exhaust valve closes.

Magnetism is also capable of moving matter. A gas is matter, and particles can be moved or aimed by magnetism. This is how a CRT (your television) works.

Heat will move matter, if by the expansion of the molecules, the pressure difference is what is happening there. But heat also rises because that expansion causes there to be less matter, and thus weight, in a given area, so the gas becomes lighter, such as in a hot air balloon. In that case, the heat itself will cause the matter to move, the bottom of the balloon is wide open, so there is no pressure difference.

The entire problem with making blanket statements such as "all back pressure is bad always" or "the only way to move gas is with pressure difference" is that they tend to sound true at first glance, and that they require some genuine thinking to get your head around. Human nature is that they will take the easy way out, not think, and just accept something that "sounds right". I'm a pain in the butt to those people.

So no, I don't agree with your statement that pressure difference is the only way to move matter, well, because it's wrong.

I think you're coming to understand now why the others have given up on trying to convince me that the moon is made of green cheese.


NEXT?!

TA

 

I think your problem is you are over complicating things. If you cannot agree with me that a balloon pushes air out of its nozzle due to pressure then I cannot discuss anything with you at all. Go grab a high school physics book and start reading up.

 

YOU said "only way", and that statement is completely, totally wrong. I did in fact included a sentence on pressure differential, so I never disagreed that the balloon analogy is viable, but I'm not in the habit of agreeing with a completely false statement because some small element of it is right.

When you get into a discussion about things that you don't understand, they always seem complicated. This is not "High School Physics", so don't feel bad for not knowing this stuff. I went a lot further than High School, trust me.

And this discussion does relate exactly to why the original poster lost power when he gutted his cat and removed his muffler, because he gutted his scavenging in the process, something that the "louder is better" committee seems to hold as Heresy.

So, let the thread die. I've had my say, and if the reader doesn't get it at this point, well, you can lead a horse to water, but you can't make 'em drink.


TA

 

The only fallacy I see in one of your statements was he CRT tube. There actually is no gas in a CRT tube, it's evacuated. It's the electrons that are guided by magnets. But yes, even gas can be moved by magnets, even liquids. Hold a magnet next to a flow of liquid oxygen and it'll bend slightly. Really cool stuff. ...And no, I don't expect anyone in here to hold a magnet next to a flow of liquid oxygen...


I think we should all come to this agreement, or at least most of us. We all seem to argue with eachother, but just say the same thing in different words. Whoever agrees with:

The less back-pressure the better. The more scavenging the better. Inadvertently some back-pressure is a byproduct of sufficient scavenging.

Say "I" haha

Not everyone can be expected to have a very substantial physics background, just like not everyone can have a large computer background which is why some people choose to go carb'ed.

 

I!

I agree with you 100%.

 

Sounds accurate and to the point, a good resolution to what everyones been trying to say.

 

I really didn't read through the whole thread. But if you had a stock cat and you gutted it, it is NOT going to make you slower. It may feel like it did, but it did not. I do not agree with gutting a cat, I am just saying. Now if you gutted the cat and didn't straight pipe it, then yes, I agree it can actually do more harm than good.

It's like when some people put headers on and it feels slower. It is not, you just moved your power band up a bit.

 

Ok, Douchermann scores "brain plugged in" points!! Yes, my example of a CRT was a stretch, but I needed an example that the forum readers would be familiar with to illustrate the ability of a magnet to move or "bend" flow. There is certainly no gas, especially exhaust gas, in your Television tube. Sorry for the lame example.


"I" sorta. There is no arguing that this statement is correct, but the focus in on the WRONG THING. The reason the original starter of this thread had a problem, and the reason that many others, myself included, have done really STUPID things with their exhaust systems, is that the order and the way this statement is made implies that reducing back pressure is a higher priority than maximizing scavenging. Again, this is IMPLIED, not stated, so I'm not arguing that the statement is correct.

However, if you create ideal scavenging for YOUR combo, back pressure will be whatever it needs to be, and the car will be a rocket. If you prioritize on simply reducing back pressure, more than likely you will totally screw up your scavenging, and as the original poster did, make his car run like Buffalo Feces, and not understand why.

Let back pressure land where it will, less back pressure will not give more power every time, only when it is impeding scavenging. It is the notion that focusing on a drastic reduction of back pressure will increase performance, and that is NOT CORRECT. Back pressure in the right places at the right amounts is what causes good scavenging.

"never mind reducing back pressure, concentrate on improving SCAVENGING in your particular combination and you will increase power every time."

Everyone who just wants TA to shut up now say "I"...

TA

 

I.

The improvement in power would depend on the current setup. It sounds like you would tell me that someone (with all things remaining the same) replaced only their 2" crinkle bend catback with 2.5" mandrel would not notice a increase in power. Which they would. And this mod will have minimal effect on tuning with a significant decrease in backpressure.

Again,
Thats not to say that reducing backpressure will have no affect cause it could have a huge effect. Increasing effective tuning while reducing backpressure is important in increasing power.

 

Wow, I see why you can't understand my point. You somehow got the idea that because I disagree with "all back pressure is bad always" that you concluded that I must believe that "more back pressure is always good". Well, if that was my opinion, I'd advocate just welding the exhaust ports closed. Back pressure is just a small part of the scavenging, and unfortunately, scavenging is very complicated. Most people are not fond of complicated.

Scavenging is the evacuation of burnt and partially burnt "exhaust" gasses from the combustion chamber. Back pressure is a component of scavenging, as is everything else on the engine, including the intake tract, all of which must be balanced in the right proportions and timed properly to be most effective. The more your combo changes, the more the "tuning" of your scavenging also needs to change.

From a stock configuration, which is overly restricted for best scavenging because of noise regulations, 3rd gen cats are antique technology, and because the stock engines don't produce strong exhaust pulses, so need more aggressive "reflectors" to scavenge properly, there is usually easy power to be had in going to a slightly larger and more free-flowing cat back, I certainly do not disagree with that. But it is very easy to overdo it, because you think if less back pressure gave you power, than eliminating as much as you possibly will result in huge power gains, a misconception which a lot of people on this board seem to adamantly defend. That is why the person who originally started this thread got into trouble, and why the "backpressure is evil" contingent simply can't explain why this keeps happening.

If, instead of going up to a 2.5" cat back, you gut your cat converter and toss your muffler and run straight off the y-pipe, especially on your V6, you'll just make a lot of noise, and although you will make a bit more power than the stock system, it will make LESS power than the 2.5 cat back would have. This is the "mystery" that I've been trying to explain, but apparently I keep tripping over someone's sacred cow.

This is as easy and simple as I can make it:

Less back pressure= more power: Not always
More back pressure=less power: Not always
Better scavenging for your combo=more power: ALWAYS

TA