kinda like running with clogged cats

 

I have heard this before, but am curious if anyone knows how this is supposed to help? It seems that anything that impairs the flow of exhaust out of the engine (ie backpressure) would increase the power/torque loss from the engine as it now has to "push harder" to clear out the burned combusiton gasses.

Growlor

 

I found a couple of interesting theories (not sure if either is true.) This one sounds really whacky (that carburated engines were designed to have the intake air get slowed (and thus get more fuel "rich") from air being pushed back in the carb due to incomplete exhaust venting (http://www.uucmotorwerks.com/html_pr...torquemyth.htm)
Whereas this one actually makes some sense: that in a confined exhaust system, if the exhaust exapnds too much it will slow/cool and thus decrease the overall exhaust flow. Although I would think after the initial slowdwn the exhaust would be hot/fast again - so this might be untrue (see the comments by "doodfood" in this thread - http://forums.maxima.org/3rd-generat...nd-torque.html)

Growlor

 

if u want something that sounds good on these 4.7 dakota,get a mac performance muffler and put on it, when i got my 2 mufflers for my gt i stuck them on the tips of the truck just to see what it sounded like and i was really amazed by the low rumble it gave it,i will be puttin one on it very soon

 

Yeah. Back pressure is the culprit sometimes. I have to get my exhaust or muffler changed out as well.

 

Growlor, the second theory is correct. The rapid expansion of the hot gasses cools it quickly thereby causing it to slow. The slow exhaust gasses act to impede the path. What you're concerned with is exhaust flow velocity, as opposed to back pressure.

What you need is balance, the exhaust needs to have a diameter small enough to keep the exhaust from expanding quickly after leaving the engine and big enough to eliminate any impedace. Smoothing out the exhaust path is the best way (high flow cats, mufflers, etc.).

Hope this helps!

 

You have no idea how glad I am to hear someone actually understand exhaust fluid dynamics.
VELOCITY, NOT back pressure

Exhaust is considered a fluid, like water when put through an exhaust system. If you're moving 4.7 liters of water through your exhaust (not actually doing that, but bear with the illustration) you want the first cylinder's amount of water to just be fully exited when the next cylinder's amount starts coming through. Thus, the first one essentially pulls the second one and so on, helping the motor out. This is done by the momentum from the piston exhaling the "water".

When you reach the perfect balance, (water isn't spread out, or all jammed through together, creating a bottleneck) you have the best mileage from your exhaust.

BACKPRESSURE: This just creates a bottleneck in the pipe for your exhaust gases and decreases both your HP AND torque. It does, however, move your overall power band lower, because your motor essentially can't move the "water" through any faster than it can at say, 3000 rpm. So, while the optimum exhaust tube may be 2.5", if it puts your workable torque at 5,000 rpm, then yes, putting 2" exhaust will lower your workable torque rpms, to lets just say 4,000 rpm. However, you will lose about that much percent in torque as well!!!

SO WHAT DO I DO???: Create more exhaust gases to fill your pipe at a lower rpm. Instead of going down in size on your exhaust (at least from stock) you need to create more exhaust at lower rpms than you are doing right now. There are several ways of doing this. The easiest is probably doing a Cold Air Intake. While the air going into your motor is a few degrees cooler, using denser air, once combusted, whatever is left will expand, and there will be more air left over, and more air/fuel combusted overall, createing more exhaust. Your engine's computer will read the cooler air and give it more fuel, so yes, the power will be better down low, as well as even better higher up. Other tricks and ideas are supercharging, adjusting your timing, and richening up your fuel mixture. Most of which are not available for our trucks right now, or are probably not what you're looking to do. (although, we'd all love to see you put a supercharger on!)

Ok, i'm spent, and I plan on writing up a FULL explanation of fluid dynamics of exhaust soon, and post it like on every forum i've ever been on... and tha'ts LOTS ( to include exhaust diameters and hp ratings, exhaust lengths, forms of induction inc. nos, intake dynamics, etc.)

TY for listening....
Blu3

 

That makes more sense. So now, I think our enemy is anything that impairs the flow. This means public enemy number one is that platinumed lined wonder, the catalytic converter, correct?
I am actually old enough to remember the days before catalytic converters were common (the air in Los Angles valley was sometimes PURPLE when my parents drove us there for summer vaction trips!), so I don't think completely eliminating them is good thing. However, maybe there ought to be some serious work done to redesign those things (maybe line the whole exhaust pipe with the catalyst and induce a swirl in the flow to ensure most/all of the flow can react with the catalyst?)
I've also wondered if using modern engine controls if the actual emissions would not be improved by just eliminating the darn things entirely (ie would the increased mileage - and thus decreased amount of fuel burned - offset the "quality" loss gained by adding a cat converter?)

Growlor

 

They make high-flow cat's. Check out Summit Racing.com
I put one on my mustang with a turbo and it still works great.