Single to dual exhaust cats/sensor??

[Dirtman]

Never heard of a polish dill?

 

[19Walt93]

Nope but I bet that would work. I was there when disco arrived and detested it. After years of rap and hip hop I guess disco wasn't that bad. Actually, yes it was.

 

[Dirtman]

I'd take disco over rap any day... Don't get me wrong, they are both among mankind's most awful crimes. But disco only makes me want to rip my own ears off, Rap makes me want to run towards the nearest living thing and kill it.

 

[Rick W]

If the exhaust pipes are too big it slows down the flow of exhaust and hurts scavenging at lower RPMs. If you've heard that some engines need backpressure, it's not true, they need flow velocity.

Hmmmm, As a ChemE myself, this is not my direct area of expertise, but I believe scavenging is a myth. It is the concept that exhaust leaving a cylinder creates a pulse down the tail pipe pulling a vacuum behind it (as if it were a piston in a cylinder ). Makes no sense. While there may be pressure pulses down the tail pipe, ALL the gases down the tail pipe are EXPANDING constantly. While there may be high pressure pulses coinciding with exhaust valves opening, there is never any time when the gases are drawing any kind of vacuum behind them. The “pulses” consist of a wave of very-high pressure peaks with lower high-pressure valleys between them, never a vacuum. When I put on my gear-head turn-wrenching hat, this is supported by the fact that dragsters and other ultra high power engines have little or no tail pipe present. One of the most important purposes of tail pipes when you consider everything, is that limiting the exhaust flow allows the exhaust fumes to act as a “coolant“ so you don’t burn your exhaust valves prematurely: longevity is not a consideration in quarter-mile cars. “Back pressure” is necessary for valve cooling, and it actually hurts horsepower.

But again, not my area of expertise, I may be confusing it with something I learned about cockroach farts.

Your points are all valid BUT that doesn't change the fact that dual exhaust is just plain cooler...

I believe I have stated in almost every reference to Rick’s Ranger Rig that it has nothing to do with a need, common sense, smart spending or any other rational concept. It will be safe and functional, but it is all for therapy…

Dual tailpipes on a single exhaust is the equivalent of taping a sausage to your leg under your polyester pants before going to the disco.

I thought you were Polish, you could use a Kielbasa.

In summary, the entire project is akin to taping a sausage to your leg and going to the disco. The dual exhausts are simply a sweetener that makes my sausage hard.

and in closing:

i uważajmy na polskie żarty.

(let’s be careful with those polish cracks!)

 

[MikeG]

Wouldn't exhaust gasses be cooling as they expand? Thus less pressure down the pipe? They should be hottest right out of the cylinder, and coolest at the tailpipe..... although not sure if the cat heats them up again enough to matter?

PV = nrt?

Even cooling / shrinking, gas would still have inertial moving down the pipe.....

Just some random thoughts, pretty much hated college chem..... terrible prof, no offense to anyone

 

[Rick W]

YES, ALL CORRECT. A+. That is the “Ideal gas law.” It is the foundation, what happens to gas without any other influences, as in a temperature contained balloon. The exhaust out of the engine, although it follows that law, at any moment close to the exhaust valve is a much more dynamic situation.

The hottest place is in the combustion chamber, and then the gas exits, expands and cools all along the pipe with the exception of the cat, where trace unburied fuel is converted (burned) and the NOX is converted. I’m not sure, but I think the cat heat addition is minimal. Not that the cat doesn’t get very hot, but compared to the volume of gas, I don’t think it’s a factor compared to the combustion in the engine.

And yes, the pressure goes down as the gas moves down the pipe (expansion and cooling from the pipe). Since the gas is introduced in pulses, there are pressure waves as it flows down the pipe. If you (carefully) put your hand at the end of the pipe, you can feel the pulses. If the pipe was infinitely long, at some point the gases and pressure would equalize so the exhaust would be similar to the exhaust from a compressed air tank.

In the area where there are pulses, whether you’re in a high spot of the pulse or the valley in between, it is still all high pressure. The combustion chamber exhaust gas would be even higher pressure. The gas is all pushed down the pipe, no gas is ever sucked down the pipe from the gas in front of it from inertia.

Since so many people believe in scavenging, I can only assume that it is a balancing of the mechanical action and output of the motor with the configurations and cost of the exhaust system. The maximum performance (HP) would be with no exhaust system at all. Hence, I assume designing for “scavenging” has to do with the biggest pipe possible for the money and for practical design and production of the car, and not maximum horsepower as I indicated with a dragster that has no exhaust. The reason people go from single exhaust to dual exhaust, or from a 3 inch diesel exhaust to 4 or 5 inch, is because enlarging the exhaust pipe is a cheap way to get more horsepower.

I’m more about @Dirtman ‘s cool factor, but it will give a touch more horsepower, and it also enables the engine to run a hair cooler, but I don’t think either is a significant factor in practicality (to @97RangerXLT ‘s point about reality).

 

[Rick W]

Aside: how do I insert members names in the conversation??

 

[Dirtman]

Aside: how do I insert members names in the conversation??

Use the "@" symbol before their username.

@Rick W

 

[pjtoledo]

I'm still waiting for the most important part of scavenging to be discussed: getting the intake charge moving during overlap.

 

[19Walt93]

Hmmmm, As a ChemE myself, this is not my direct area of expertise, but I believe scavenging is a myth. It is the concept that exhaust leaving a cylinder creates a pulse down the tail pipe pulling a vacuum behind it (as if it were a piston in a cylinder ). Makes no sense. While there may be pressure pulses down the tail pipe, ALL the gases down the tail pipe are EXPANDING constantly. While there may be high pressure pulses coinciding with exhaust valves opening, there is never any time when the gases are drawing any kind of vacuum behind them. The “pulses” consist of a wave of very-high pressure peaks with lower high-pressure valleys between them, never a vacuum. When I put on my gear-head turn-wrenching hat, this is supported by the fact that dragsters and other ultra high power engines have little or no tail pipe present. One of the most important purposes of tail pipes when you consider everything, is that limiting the exhaust flow allows the exhaust fumes to act as a “coolant“ so you don’t burn your exhaust valves prematurely: longevity is not a consideration in quarter-mile cars. “Back pressure” is necessary for valve cooling, and it actually hurts horsepower.

But again, not my area of expertise, I may be confusing it with something I learned about cockroach farts.


I believe I have stated in almost every reference to Rick’s Ranger Rig that it has nothing to do with a need, common sense, smart spending or any other rational concept. It will be safe and functional, but it is all for therapy…




In summary, the entire project is akin to taping a sausage to your leg and going to the disco. The dual exhausts are simply a sweetener that makes my sausage hard.

and in closing:

i uważajmy na polskie żarty.

(let’s be careful with those polish cracks!)

No offense intended, I actually like kielbasa but my cholesterol doesn't. I have a French name and have been told more Canuck jokes than I can remember and never took them personally.

 

[Dirtman]

The only thing I know about gas laws, chemistry or thermal dynamics is this...

If you fart while blowing out a birthday candle you CAN travel through time.

 

[Rick W]

No offense intended

No worries, i was joking. I’m the most sarcastic guy on earth & if you dish you also have to eat, as long as it’s good intentioned!

 

[Rick W]

I'm still waiting for the most important part of scavenging to be discussed: getting the intake charge moving during overlap.

Sorry, not my area. I’m more an expert at expending hot gases (you may have noticed)....

 

[pjtoledo]

Sorry, not my area. I’m more an expert at expending hot gases (you may have noticed)....

that's OK, I'm fully loaded with BS myself.


anyway, scavenging in a nutshell:


critically timed pulses in the exhaust leave behind low pressure areas (not vacuum), they are extremely temporary as the pulses will look behind themselves and try to equalize the situation.

with both valves being open during overlap the intake gases see a lower pressure area left behind by the rapidly exiting exhaust and decide to fill in the void.

that gets the intake charge moving, remember the piston is at TDC and ain't enticing anybody to get moving.

the exhaust valve closes just before the returning exhaust gasses get there, they would really mess things up by pushing the intake gas back where it came from.

the piston has now moved and created a void and the already moving intake gasses are more than happy to fill the cylinder.


moral of the story?

the foolhardy exhaust gases in trying to get the hell out of there leave behind an opening for the intake gasses which would have otherwise been completely satisfied waiting for the retreating
piston to welcome them in.




back on topic, dual is cool

 

[Rick W]

Hmmmm. Not convinced, but I have to think about that one....