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Yes, the 3912600 was the stock manifold for the 4bt with the P7100 and an HX30W turbo. Also came on engines with the H1C turbo. Probably about 3/4 of all the road 4bts had that manifold. The rest had either the top center mount version or one similar to yours only with a compound tilt 3906661. That one only came on CPL767 and Cummins discontinued it and changed to the one you have now. Only issue is finding the 3912600. Used to be as common as dirt but the one you have and the center version seems to be all we see. The price of a new one gets into the $300 range where the others can be found under $100 new. As far as down pipes, here's a photo of one on my workbench but don't know if it would clear your firewall even with the turbo moved over. This one has a section of 3" pipe attached.
 

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Discussion Starter #502
I began looking for a 3912600 exhaust manifold today. A few on eBay were sold out already. The only ones I could find were in the $300.00 range. I did see a China company was selling them for dirt cheap but didn't know if I would get ripped off in the process of purchase. I will keep my eyes peeled.

Here is a picture of Johnp77's downpipe from his build referenced on the previous page. Looks like he has the correct exhaust manifold.

Exhaust DownPipe.PNG
 

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Pictures are over 9 years ago. Setting the 105 HP 4bt in the engine compartment pointed the turbo exhaust directly at the glove box.

Took me a couple weeks and a couple of manifolds to find a solution.

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Left is the stock 1986 down pipe. Middle is the final down pipe with the stock flange welded on - it was built from two 45 degree bends, with the flange end shorted (cut off stub in picture). Got the bends at a local part store - This gives you the ability to rotate (before welding) the down pipe around obstacles. I was able to tuck my exhaust around the clutch housing and route it beside the transmission.

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I ended up using this manifold. 105 HP turbo shown.

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This test fit did not work well - the stock down pipe was too close to the ground. Put my exhaust below the chassis - Originally was going to have it shortened, but the "guesstimated" output ran into cross members.

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And it protruded into the cab a "little". Your Chevy floor board clearance will be different.

Regroup and go to the 45 degree pipe. I eventually had to make lower front motor mounts to get the driveline angles correct (motor was too high in the front). If I had got the engine mounts fixed first, the floor cutting would not be required (what was that old quote: " Measure twice and still cut the board too short...").

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Several years later with a 120 HP turbo (thanks scout4bta). I could not find a final installation picture of the 105 HP turbo.

Russ
 

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One big difference in those photos is those appear to be H1C turbos. That one is about 1.5" shorter than the HX30W because of the waste gate. Not sure why all those 3912600 manifolds suddenly went away. Used to see dozens of them on ebay. Of course, we used to see dozens of 4bts for sale and those have mostly disappeared. I've bought items out of China with no problem, but with this virus mess shipments might be long term. You will find prices out of China are super cheap but the killer can often be the freight.
 

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Discussion Starter #505 (Edited)
Thanks, Russ. Looked like the adventure that I am having now. I agree with char1355, the H1C is shorter but I am not deterred from using what I have just yet. Although I would like to find a good 3912600 manifold. That would make the job easier.
 

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The other possible manifold I mentioned was part 3906661. That one was the predecessor to what you have. It not only moved the turbo out from the block but also tilted the back end down. As far as I know, it was on used on CPL767. That manifold has been discontinued but they show up for sale for around $100. New ones are around $600. Cummins determined that downward tilt tended to cause premature wear on turbo seals. Here are a couple photos of that one.
 

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I have one of those manifolds. The reason I went away from that manifold was to prolong turbo life. As it is, I pulled the trigger on 3912600 manifold last night.
 

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One thing, that manifold that you have on the engine is a good one should twins be a possible idea. When flipped over, it puts the small turbo up front and has enough tilt to clear the #1 injector and lines. I bought one of those years ago with the intent of building a twin set but never got around to it. There is actually another manifold similar to 3912600 which is 3901919. This is an industrial manifold that has the turbo flange extended with bolt holes to mount a vertical exhaust pipe. It isn't exactly cheap either. Around $300.
 

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I looked the 3901919 up and the lowest I could find was $252.00. I have the 3912600 coming and will deal with that upon arrival. For now, I will start tacking the other projects around the engine. (Fuel, Hydraulic Hoses, Electrical, drive train and so on). I just hope the weather cooperates.
 

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After waiting a few days for shipping notification I called the parts company who stated they did not actually have the exhaust manifold and was contacting the supplier. I called around and found this to be the case all over for the 3912600 manifold. I began to look at the 3901919 manifold and the pictures I see show the outlet flange tilting the turbo in towards the engine. Not sure, you be the judge. Maybe an optical illusion? o_O

3901919 Cummins Exhaust Manifold3.jpeg 3901919 Cummins Exhaust Manifold2.jpeg 3901919 Cummins Exhaust Manifold1.jpeg 3901919 Cummins Exhaust Manifold.jpeg
 

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I
After waiting a few days for shipping notification I called the parts company who stated they did not actually have the exhaust manifold and was contacting the supplier. I called around and found this to be the case all over for the 3912600 manifold. I began to look at the 3901919 manifold and the pictures I see show the outlet flange tilting the turbo in towards the engine. Not sure, you be the judge. Maybe an optical illusion? o_O

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I haven't been on here for a bit and had a little catching up to do! It's good to see your project coming along well!👌🏼
BTW I've got a good used 3912600 manifold if you're still in search of one.

Best of luck!
Glendon
 
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You are probably correct about the 3901919 tilt. That manifold was designed as a top mount, not a low rear mount. The reason for the shortage of the 3912600 is likely they are coming out of China and with this virus mess that is a very slow boat. Also, I believe that unit is now on the discontinued list which makes it worse. Here's a company that shows it for a slightly higher price. Might check with them to see if they have it in stock. 3912600: Cummins® OEM Exhaust Manifold
 

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Yoder repower: I might be. The source to which char1355 is referring is who I ordered by manifold from. I called them on Friday, February 12, 2021, and they stated they would get back to me by Monday, February 15th. However, I was caught in the Texas winter storm that took out a few million folks' power, and subsequently the Internet. My power just came back on less than an hour ago. I will verify with them and then hit you up if needed. Thanks. Tim
 

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I've been looking in that mess across Texas and other states. That's a terrible situation, especially with this horrible weather and they say another storm system is coming. I give thanks that where I live is a corridor between that snowy deep freeze and the violent storms to the east of here.
 

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Discussion Starter #518 (Edited)
I've been looking in that mess across Texas and other states. That's a terrible situation, especially with this horrible weather, and they say another storm system is coming. I give thanks that where I live is a corridor between that snowy deep freeze and the violent storms to the east of here.
The problem was due to a lack of planning. Wind turbines and homes with private solar powers surround us. No one really thought we would ever ice over this far south in Corpus Christi. The turbines froze up, and the solar panels became coated with ice and unusable. The next thing we know is the state is short 19,000 megawatts of power, and the system could collapse the state as demand outweighed supply. 185 power plants were offline as of 2/16/21. Moreover, we lost water pressure, and the refineries went off-line (For that same reason: Lack of water pressure). Today we were in a local gas shortage. The domino effect was eye-opening. We had a run on drinking water and meat items in stores. While food was there, the shelves were becoming empty. I checked the local Walmart (supercenter) and found no dairy or meat in the store, save a few tidbits. Surprisingly, toilet paper was spared this time, and I saw a lot on shelves. The cell phone tower close to me was down as well, so communications halted. The answer is simple. We have to do a better job at planning and setting up the infrastructure to meet the end-users no matter what is happening. Whew! Okay, the rant is over; continue back to the build and regular posting.
 

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Yes, we human beings never seem to plan for a problem until it has already happened. I saw a video last night about some of this and people were asking why downtown areas of these major cities are lit up like a Christmas tree during the night while people are freezing to death at home. A problem of allocation of energy. Of course, this weather is something your state may not have seen in hundreds of years if ever. Like here in NC, we had a huge tornado hit Brunswick county. Tornados in February are not something you expect. Even Buffalo NY has issued state of emergency warning. Up there this kind of weather is normal but in just over a day they had 6 feet of snow with 2 more on the way. That is nuts. Back in the '70's, my first cousin worked on a government project in North Dakota. During the winter they didn't see the sun because the snow was above roof level. His sons walked to school in tunnels the army corps of engineers built. His job was very high paying but there are limits. He later moved to San Jose CA and then back to NC.
 

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Okay, a bit more of a rant. We were told by ERCOT that we would have rolling blackouts due to the shortage of electricity. I have a friend who lives across town and never lost power. I lost power for effectively 2.5 days with a twelve-minute restoration in the middle.
 
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