Adding Catalytic converters to a Federal Bus

[69BahamaYellow]

While I'm repairing my heat exchangers, I'm thinking seriously about adding a couple of compact universal catalytic converters in place of the U pipes in my exhaust system. The flanges on my U-pipes are pretty rusty anyway, so just cutting them off and directly welding curved pipes to match up to the converters don't seem like a difficult job.

On paper, this seems like a pretty good idea, because:
-It's better for the environment
-A compact side inlet/exit 3 way converter only cost about $45
-New converters have almost zero back pressure, so theoretically they should cause no extra pumping losses or engine heating.
-There seems to be plenty of room for a cat where the U-pipes normally reside
-They would run the heat exchangers about 100deg F hotter, for better cabin heat
-The extra downstream exhaust temperature would help drive off moisture and acids in the exhaust system from short drives, so it would last longer
-A modern 3 way catalyst operates from 500F to 1600F, so placement at the U-pipe's current location (which I have measured around 700F, and it gets there within minutes) would provide rapid heat up and should run within the optimum range, unlike the factory California models, where the Catalyst is just ahead of the muffler (It's only about 400F there, so not really in ideal operating range)

Before I go too far down this rabbit trail, I thought the Samba community could chime in on what I'm not thinking about In my mind, the fuel system on these old buses is not the most efficient and would overheat the converters with all the extra CO, Unburned HC's, and NOx it would feed them, vs a modern engine, but my engine is fuel injected, and at least at idle, my CO readings are less than 1%...

[busdaddy]

Well it doesn't sound unreasonable to me, converter technology has come a long way in 40+ years. My worry would be the heat, would it need a heat shield to guard the front of the engine from the radiant heat? As to how the heat exchangers would handle the hotter inlet temps?, I guess time will tell.
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[Tram]

[busdaddy]

Yeah, interesting point, although the heat would be downstream so it may the same as any other F pipe system as far as what the heads see. But the sensor might be handy as well, cheap small converters likely aren't overbuilt like the OG converter so a spike may be all it takes to toast it. Hmmm...., getting into engineering territory here....
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[JaimeH]

Would an ECU for a catalyst bus make any difference? Were they programed to run any leaner? I have also considered adding a CAT to a Federal bus. I've wondered if it was just 'plug & play' or if it was part of a whole system.
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[raygreenwood]

Modern monolithic CATS are NOT "no restriction".

They are....very low restriction when compared to early pelletized cats. However....they have a TON of drag. They significantly slow exhaust velocity. Why?

Because with a straight through pipe....you had just the surface area of the inside of that pipe....with its own boundary layer above the surface....dragging on exhaust throughput. So with a straight pipe...you have some drag....but not huge unless the pipe is too small.

So you add a catalyst.....and even through its 2X to 2.5X the diameter....it has a hundred little ceramic straws in the block....each with its own little boundary layer. You have increased surface area drag by many times.

This is why.....catalysts on cars these days are rather large. The one on my 2012 golf five cylinder is on a 2.5" downpipe and is about 9" in diameter and 16" long.

Small 3", short cats are designed to be used one per cylinder....and thats still a lot of drag.

It can run well....but there will be "some" fuel tuning required.

And....each one of those cats will be a huge surface radiator of heat right under the back end.

As others have noted....you do not want to raise the heat exchanger temps. They are too hot as it is in some climates.

I would think you would be better off with a single large catalyst....larger than the stock bus cat. Ray

[69BahamaYellow]

Awesome feedback so far. So the negatives of adding cats are still many, but still worth examining:

1) Catalysts make extra heat, and extra heat kills type 4 heads. I'm still thinking most of the extra heat is downstream of the heads, so long as you control the radiant heat generated by the cat, which is significant. On a 1978 exhaust system, the U-pipes are well isolated from the heads by the engine tin (so long as all of it is in place), so that's not really a problem. What about conduction? Yes the extra heat from the catalyst will be conducted away by anything connected to it (like the F-Pipe from the heads), but the F-Pipe is right in the cooling air stream from the engine fan, so convection will take away most of that extra heat.

2) Can the heat exchangers take the extra heat from a catalyst - I think yes. When you have the tins off of them, you will notice the exhaust pipe that forms the heat exchanger is encased in a pretty large chunk of heavily fined aluminum. I didn't design it, so only VW knows for sure at what exhaust inlet temperature the aluminum brick would melt down, but just looking at it, I'd bet in could handle 1,500F all day long (assuming continuous cooling air flowing over the fins.

3) It's possible you need a different ECU to run a catalyst on a bus - This would make sense, since Catalysts need to be close to stoichiometric conditions for optimum efficiency. Modern cars have no issues with this, but the Federal bus has no O2 sensor for feedback, so no way to control this over all conditions. From a big picture perspective, I'm thinking a federal bus achieves stoichiometric fuel burn maybe 20% of the time, at best, which means that 80% of the time, the catalyst would be either over saturated with Oxygen (lean burn) or over saturated with CO, HC (rich burn). Under those conditions, would the catalyst work at all? or more importantly, would it be harmed? If it doesn't hurt it, then even if it's only effective 20% of the time, that's still better than nothing.

4) Catalysts are not zero back pressure - That's true, but they're close to no back pressure (unless perhaps a rich burn condition will clog them up). My F-Pipes are 1 3/4" in diameter, which gives a cross section of about 5.5 square inches. The Catalyst I have in mind is brick shaped and it's inlet opens up about 24 square inches, so even with all of the micro channels restricting the flow internally, the gas has plenty of room to overcome it.

5) The radiant heat of the catalyst will cook everything in front of the engine. - yep, also true. They get hot, so what if I design a sheet metal surround and pipe the outlet air from the heat exchanger into it, and connect the heater flapper valve on top of that. This would control the radiant heat, but also scavenge that heat to further boost the performance of the cabin heater system.

Haven't heard anything that's a show stopper yet, but PLEASE KEEP THE FEEDBACK COMMING! This is great stuff. I know there's an Achilles heel somewhere in this whole design process; just need to find it

[SGKent]

other than putting the cats nearer to the fuel lines, pump and gas tank I see no other issues with it. Can't imagine a malfunctioning plug sending raw fuel to a cherry red cat while sitting still in traffic would have any effect on those items I mentioned. BTW - I've see cats cherry red in that situation before - had to use a fire bottle once to put the grease under a car that caught fire from it while it was in my garage and I was trying to find the mis-fire on a friends car.
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[Tram]

Honestly I just wouldn't do it. If you really care for the environment just keeping what you have in tip top tune will do more.

Funny story- I did a brief stint as an assistant service manager at a Chevy store in the early 1980s. One day the shop took delivery on a brand new three band sniffer, and we were training the guys on how to use it. I had the bright idea to bring in my 1970 VW Westy that i had overhauled the engine on the winter before to see how it compared to an off the truck Chevette.

The non catalyst Westy slightly outperformed it in one of the 3 bands and was pretty in range even on the others. I can't remember the exact details but everyone including me expected it to sink like a stone on these tests.

Keeping the crankcase ventilation system clear is also crucial to low emissions.

I've never seen a catcon equipped Bus that didn't have a single wire O2 sensor and a Hall effect ignition. That said, I have worked in Ohio, Idaho, and Oregon so I'm not exactly an expert on them either.

We always think we can outsmart engineers and build a better mousetrap. Every so often that may happen, but there are usually engineering reasons for what they do that we never think of or comprehend so doing all this because we lack the training and theory. It's like the shop foreman said in my apprenticeship days "Just follow the repair procedure. Don't think- it's above your pay grade". I think has a better chance of an "aw, shit" negative impact than being any sort of improvement.

Melted engine and converter parts at the dump are likely less ecologically friendly that keeping it tip top stock.

Just my 2c, Confederate.
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[airschooled]

If the reasoning is environmental, consider the precautions VW went to in 1978 to keep the fuel tank cool. (Warmer fuel is quicker to evaporate and create hydrocarbon vapor pollution.) VW implemented 'heat dump tubes' to get the heat exchanger waste heat out the back of the engine instead of below the fuel tank, and the under-tank area was coated with a mat of insulation to keep radiant heat out of the fuel.

I'm thinking catalysts in the region by the U tubes would be significantly backwards compared to VW final designs.

Robbie
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[Tram]

The other piece of the "more environmentally friendly" puzzle is making sure the thermostat and flaps are all there and properly working to get you up to operating temp where engine efficiency is best as quickly as possible.
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[TomWesty]

[ivwshane]

Could one use exhaust wrap/blanket to keep temps in check?
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[69BahamaYellow]

[69BahamaYellow]

Sometimes a picture is worth a thousand words, so here's what I have in mind. This design uses an Eastern Catalytic model 70362 Universal Catalytic Converter or similar. I would need to fabricate a clamshell type casing that roughly mimics the dimensions of the converter, but with maybe a 1/2" air gap all around. The casing would then need an inlet and outlet to connect with the heat exchanger and the flapper valve respectively.

Any suggestions on fabrication methods for the clamshell converter casing welcome.

Also, I hear everyone's words of wisdom regarding the sanity of even attempting such a modification, but I'm still waiting on a local machine shop to finish some engine parts I need for a rebuild, so while I'm waiting, I've been way too focused on a catalytic converter, so If my parts are finished in the next few days, this cat project will probably take a back seat for a long time; otherwise, I may have a working prototype in a few days...

[SGKent]

just do it and don't ask. You got opinions and blew right thru them. Why ask if you don't listen to others? Personally I couldn't care one way or the other but since I HAVE seen bright red orange catalytic convertors I probably feel different about it than you do.
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[HerrFink]

Thanks for posting this topic. I have a 71 Westy I am finally getting to the part where I start putting it back together and also happen to have a 2002 Jetta catalytic converter in perfectly good shape. I was thinking, boy it sure would be nice if I didn't gas the neighborhood and figured out a way to install it assuming it doesn't kill power and burn down the bus or simply not work since there are no sensors and no computer to make any adjustments to it.
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[Hoody]

I think your making a big mistake. That set up is going to fry your heads. Rather than second guessing the German engineers that came up with this exhaust I would highly recommend sourcing some correct flanges and have an expert tig them onto your J pipes and U pipes using a jig. Scott at German Supply rebuilt mine and they are as good as new. The pipes are stainless so only the flanges get cooked. And both sets of these pipes see high temps. Thin walled steel is not going to like those exhaust temps.

[69BahamaYellow]

First off, I mean no offense in being ambitious. The feedback is greatly appreciated (good, bad, or indifferent). If I didn't care about it, I would not waste my time posting here. This exercise is all about knowledge sharing.

So top issues to address are:

1) Cylinder Head Temps - Under no circumstances do we want to increase these with either an exhaust restriction that raises back pressure, or with an extra heat source. I think the latter is not an issue, if you shield it properly. Radiant heat transfer will be the primary mode to control; Conduction and Convection heating are much less significant. I know that a restricted cat will typically create a 1-3 psi pressure rise before the cat, but I don't know how much that would raise head temps. On a normal cat, pressure rise is barely measureable, and since the Federal Government requires them to run that way for 8 years / 80,000 miles, today's cats are not nearly as prone to clogging as they were 20,30,40 years ago (in part because of their location in the much hotter exhaust area of the headers, but also due to better materials and design). Now this all assumes a systematic approach, with computer controlled fuel mixtures and ignition timing making sure the cat doesn't get fed too much unburned HC's or CO. When this happens, the cat will run much hotter. Under normal combustion conditions, a cat will only raise exhaust temps around 100F, but could the much less precise fuel system of a bus significantly change that? The noble metals inside a cat are themselves not prone to collecting soot, but what if a prolonged lean condition occurs in the cat that significantly lowers temperatures (think what happens to spark plugs when you change the heat range in an otherwise perfectly tuned engine) Would it collect soot and raise back pressure?

2) Overall Exhaust System Temps - Currently the hottest part of the VW system is the F-Pipes, which can get around 800F. The U-Pipe runs around 700F; now these are just my measurements with an infrared temperature gun on a properly tuned engine. If you have a rich running engine, exhaust leaks, aggressive cam timing with valve overlap, modified ignition timing, etc., these are all things that will greatly raise exhaust system temperatures. Steel is visibly red around 1200F and "cherry red" around 1400F, so it takes a big jump in temperature to go from stock temps to visibly red, but that kind of rise is completely possible. I've seen it on racing engines. On a bus or any other daily driver, that's a big problem, but a problem that's rooted upstream of the cat.

3) Clarification on rich/lean and heat - Most mechanics know that richer mixture = lower head temps, and leaner mixture = higher head temps, so why is the opposite true in the exhaust system? In the combustion chamber, extra fuel being vaporized, but not burned, has a significant cooling effect (keeping the heads cooler), but in the exhaust, the unburned portion will burn (especially in the presence of a catalyst) and raise temps. A lean condition gives complete combustion of the fuel and higher cylinder head temps, but leaves no extra fuel in the exhaust to burn and raise temps, so the exhaust will run cooler. Now catalysts need oxygen to convert CO and HC's to CO2 and Water, so they favor a lean condition (not good for head temps), but my thinking is that a bus produces enough lean burn conditions when coasting or off throttle to feed a cat enough oxygen (which they store) to do their job. Otherwise, air injection may be required just before the cat (some cats have air pipes in them for this purpose.)

Again, these are only my humble opinions, so take them for what they're worth. I'm no expert on anything, but probably pretty good at a lot of things automotive. Not bragging, just calling out that's a prerequisite if you're considering modifying OEM designs. I'm good at welding and metal fab, or I wouldn't even consider such a project. Too many ways to screw up, otherwise.

[aerosurfer]

I’d say your drawings no are ambitious with the amount of room you will need vs what’s there already. Moving the cat/exhaust upwards creates an unknown to temp rise, but trying to get 3 pipes, 2 exhaust and an airflow into a raised manifold like you depict is going to be tough.
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