I poured over Rehors book and one shot has a lineup of berks, one exhaust obviously shows a mini-exhaust coming out of the stack as if the air pump exhaust was just inside the stack. Later pics of the 800 class berks have a designed mini-stack just to the front of the regular stack with pipes going to it from the pumps. Getting the pump exhaust close to the stack aughta get some air current draw which is why it may be placed on 765 where it is. You guys on 765 maybe you should check on the 800 class maybe make a ministack.

Its probably a maintenance issue also, the smokebox is plenty full of the superheaters and piping, its probably one or 2 less pipes inside to concern about, pump pipe failures would be an easier fix than uncorking all the bolts on the smokebox. Usually when you plan to open the smokebox front its going to be uncorked for an extended time. And you have to align it back and so on.

Cylinder exhaust is channelled in the smokebox exactly so it drives smokebox air current up to get that draft. If a pump pipe fails inside I can see how that can cause a reverse airflow for the fire. It may be rare it would fail actually but you know rusting blasts of exhaust and so on do things like the auto car exhaust.
So it sounds like it was a good idea doing that. The detail is not on my 765 model, so they added that, and it looks like a custom job.

The air pump exhaust configuration is original to the locomotive. I'm not sure if it came from Lima that way or not, but it was definitely retired from the NKP with it. I'd have to search some pictures, but I tend to think this is original from Lima as other Lima locomotives, notably the Allegheney had the same separate pump exchaust with muffler. What appears to be a separate stack on some NKP berks is actually a bulge in the main stack where the the pump exhaust was indeed routed through the smoke box to the top front of the stack. I'm not sure what arrangement would be better. Some pro's and con's have been spelled out in this thread and it all comes down to the whims of the builder and the railroad and the maintenance staff. As a related note, I know the N&W 611 originally had both it's pumps and dynamo exaust into the smokebox. In 1992...after being around for some 40 years, the NS steam guys decided that re-routing the dynamo exhaust up to the top of the boiler would be a better alternative.

Also, I find it quite funny that you say your model doesn't have it so it can't be original. Generally we gauge the accuracy of a model by looking at the prototype, not vice versa.

I'm inclined to think that the air compressor exhaust muffler was an Advisory Mechanical Committee standard. Exhaust from the compressors came up the engineer's side feedwater heater piping ditch and then back to the muffler. The muffler separated steam and water, sending the water down a small pipe just in front of the builders plate. On engines with a jacketed smokebox, this line was under the jacket.

There are also auxiliary exhaust provisions in the stack casting. Behind the main circular stack there is a rectangular extension with two (or three?) cavities. On 1225, (if I recall correctly) the feedwater pumps exhaust into the cavity on the fireman's side. The others are blanked off. I would speculate that the engineers side might have been reserved for booster exhaust, thus requiring an independent air compressor exhaust. (Perhaps Les Beckman could tell us how 2789's booster is piped?)

I can think of a couple of possible reasons for not piping the exhaust up the main stack. They might have wished to maintain a finer control the draft with the blower alone while running and especially starting the compressors. Or they might have wanted to keep the smokebox interior arrangement as simple as possible.

Andrew Adams

I have been all over 765 in its restoration, the air pumps seem to elude me what what what but I have all my film to check. Just a curiousity youve got my whim going on to verify what was what. I don't believe the jacket ever came out to the smoke box, just to the end of the boiler section. Any piping is going to be exposed.
I slapped insulation right on the engine so I aughta know...8-P there might still be some insulation mud under my fingernails yet...8-D