Why can't the catalytic converter be incorporated into the design of a header collector? I understand that it must not be possible, else the header manufacturers would have done it by now.

My guess is that there is a space limitation.

If space were available how would it affect performance? If it was the same diameter it couldn't flow enough. If it is larger to support the flow it would slow down velocity? And how does backpressure fit into this?

(Hopefully I don'tget sent back to the third gen board for this one...)

Jason

 

A few reasons I can think of:

The collector length is critical for tuning the exhaust pulses. Aftermarket
companies which aim for performance would lose their shirt if they stuck
such an obstruction in the collector.

Catalytic converters impede exhaust flow to some degree. Making the exhaust
work harder to get out of the chamber drops power significantly.

Since a fluid, or gas cannot flow on the surface of an object, the honey comb
becomes a restriction and creates back pressure. Even though the catalytic
is the same diameter, the surface area of the internals create drag.

We all know that back pressure is a killer when trying to make power.


Changing pipe size after the collector is not a good idea. Gas and fluids like
a smooth, continuous pipe to get the best flow (laminar). Any time there is
a bend, diamter change, or even change in the metal, the flow will be compromised.

 

Isn't the collector length based on the given of being a tube of specific ID and length with no obstructions?
Change the givens and an alternative should be possible.

Since catalytic Converts are a neccesary evil there has to be a way to optimize the exhaust event around it, correct?

It is understood that the best restriction is no restricition when it comes to making power, but everything has limits...

What actullay kills the power? The reduction in flow, velocity, both, or something else?

Since it gets heated more doesn't that create more pressure to keep up the velocity that has to be good for something, right?

 

Yes, to the best of my knowledge. Once the pressure pulse meets a surface ,
or the atmosphere, a reflection occurs.

Imagine incorporating the catalytic into the collector. YOu have now just
fixed the tuned length of the collector and it will be good for only one specific
application.

Those who want to tune the collector would normally be able to add an
intermediate pipe to change the secondary length.

With the catalytic built into the collector, that option is gone.

It's not always a necessary evil, but for emission cars that require converters,
the better cat. would have higher flow ratings to make power.

You can also install a cross-over pipe, such as an H-pipe before the cats.

That's behind my scope, but I'm sure all of the above as well as placement,
flow characteristics and temperature.

 

I suppose that this is what the OEMs do, or they put it where it fits, and adjust everything accordingly.

From what I understand most headers outside of a pure racing environment are a compromise anyway. Just like the collector size is more determined by the space available than the appropriate size for the engine this is just a different hurdle.

All I know is it would sure be nice to pass the mirror under the rocker panel test.

 

I'm not going to get sucked into this thread also I didn't even read it in its entirety, but i have to point a couple things out.

Flows involving air (exhaust gasses, whatever) is for all practical purposes always turbulent. Air simply does not have the viscousity for laminar flow to occur.

What about a gradual diameter change? That is a very broad statement you made.

After the system is run for about 10 minutes and carbon gets deposited everywhere will the different roughnesses of different metals really affect the drag in the exhaust system?

-brent

 

How much of a carbon deposit issue with OEM cats? Don'tthey have to last like 80k miles or something?

 

You confused me with that part..... if you keep the pipe diameter the same as the collector diameter, you are, in effect, continuing the collector. I thought the Vizard theory made a good case for a separation after the collector, to establish the actual collector length. The tests we did with some collector mufflers seemed to confirm this theory, with the larger diameter, more open design mufflers producing better results in some instances.

A "sudden enlargement" causes pressure loss... but I don't think you can tie it to a laminar transition. As noted above, I suspect the Reynolds number is well into the turbulent flow regime when you are consuming max air and fuel in the engine.

 

Somehow you have to offset the flow restriction with more volume. I think becuase the larger chamber has more restriciton (stuff in the way) the basic flow would be the same wouldn't it?

Jason

 

I seem to confuse many people Fred, don't take it personally.

Here I go debating with engineers :

When JRG asked if making the pipe larger would slow down velocity (assuming
he meant catayltic inlet), I stated that changing the pipe size after the collector
would not be a good idea.

I should have added, "changing the pipe diameter between the collector and
cataltyic would not be a good idea".

From the texts I have read, and exhaust systems I have seen, the connecting
pipe from collector to catalytic inlet are the same size; then reduced after the
catalytic.

My thought is reducing the pipe before the catalytic reduces the surface area
of the honey comb which will make it more difficult to flow the exhaust.

(I guess I'm playing a double edge blade misquoting DV vs. not supplying enough clarity in my reply).

Brent,

I understand the pipe joints, gaskets, and the like will make laminar flow for a
gas impossible. Isn't it our best interest to supply a path of the least turbulence
whenever possible?

There seems to be a great deal of emphasis on mandrel bent tubing, material
thermal retention, elimination of bends, etc.

 

Its not even the joints or gaskets causing the flow to go turbulent. The viscosity of air is sooooo low that even for a long straigh 3" pipe the flow is fully turbulent (Reynolds number ~4000, ) at approxiamately 25 cfm, or 3 m/s.

Note the first value 25 cfm is a volumetric flow rate and the second a linear velocity based on the 3" pipe. Also note that the transition from laminar to turbulent flow take place as the reynolds number changes from 2400-4000. If you don't understand Re number, sorry but you'll have to take my word. There are whole books on the topic and i'm not going to try to write my own here

The approximate values i listed above are so far from what would actually flow through an exhaust system that i won't even attempt to justify them by adjusting for exhaust composition, temperature or anything like that. For all practical purposes flow of air (gasses) is always turbulent.

Is it still very important to minimize drag/friction from bends, joints ect? Absolutely, but with the best exhaust system in the world your still not going to keep that flow laminar

back to work...

-brent

 

reynolds number = 1.00x10^5

Anyways, about headers with cats it would be application specific. But your right space is tight down there. Collector length is not crucial; unless your talking about merge collectors. The size of the collector, and transition is more important . If you could smooth the transition into the cat it could act like a venturi chocking the gases just right to improve velocity and then using megaphones perhaps after to help tune the car. It could be beneficial but would take a lot of time and money. Would be very interesting to see data on though…could be done but I don’t see it being very practical.

I’ve seen data that shows adding a muffler increased hp and tq vs. running a straight pipe so this could also be applied to this situation I believe. Even though adding the muffler increased overall backpressure it decreased it at that crucial point of TDC, increasing salvegeing.

And I apologize for my terrible handwriting it's late and I'm exhausted. No pun intended.
-b

 

yeeeeeaaaaah, ANYWAYS........... i gotta quote Hawk on this one:

hahahhahaha

 

When I thought of this I envisioned something that looked more like a cherry-bomb type muffler, trading some of the width for length. Part of my assumption is that you would have to increase the diameter of the collector now incorporating the converter to maintain the same effective diameter because now you have these bricks or screens or whatever obstructing the flow.

It is understood that the concept would be application specific, but at the same time with some experimentation I bet it could be simplified into a process. With a basic design arrangement the length is tuned same as the collector is/should be. Is it as unrestricted as an empty pipe? Absolutely not. But it shouldn't be as restrictive as a converter that fattens out to twice the diameter either.

Is there any information on how close or far away OEMs try to place the converter? I also may not completely understand what a converter looks like inside that affects its design and shape.

In my day car the first converter is connected to the manifold (transverse mounted i-4). There have been aftermarket headers designed that replace this converter, but they only pick up about 5-6 Hp with no other changes. It is far away from the 1hp/l but it doesn't rev to 8500 either.