Rear Exhaust on an M923 Cargo Truck

Beerslayer

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The reduced cab noise in the M923 Cargo Truck is really enjoyable. But what if it could be reduced further?

Has anyone tried piping the exhaust on out the back? It looks like it would be easy. There might even be room for a muffler tucked up under the bed near the back.

It looks to me like it would be pretty simple to just cut the 4" pipe right where it comes out from under the cab and extend it on back to the rear of the truck.

Has anyone tried this or contemplated it?
 

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jrod66

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do it and report back your results. I would think this would be a good Idea, I plan on doing something similar to mine, cause i dont plan on having it in deep water anyways
 

LanceRobson

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It is worth noting the the military moved the exhaust on the deuces and 5 ton trucks from under the bed to above the cab to reduce carbon monoxide inhalation injuries to anyone in the cargo compartment. In fact, the small diameter of the original multi-fuel exhaust pipe was consciously chosen to result in a higher velocity exhaust flow at the cost of increased back pressure and reduced fuel economy in order to blow the exhaust even further from the bed. The bigger pipe on the turbocharged engines (which flow more air at all RPMs) is still undersized as compared to the civilian world for that very reason.

Unlike a school bus with an enclosed rear body your truck is going to get exhaust fumes into the cargo bed with a rear exhaust.

I would strongly advise doing this is there if any chance of someone ever riding in the bed at faster than parade speed.

Lance
 
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m816

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Looks like plenty of room to add a muffler between the cab and cargo area, why not just extend the stack up? Those fumes are rough on anyone in the back and the higher the exhaust pipe , the better the breathing for those behind the cab.
 

LanceRobson

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The extension of the M939 exhaust above the cab and cargo cover was kept somewhat shorter than the earlier trucks due to the cab height being well above the older trucks. That became more pronounced with the advent of the taller tires in the A1 and A2 series. If the pipe were as high above the cab as the earlier trucks that, combined with the much higher roof-line results in a lot of tree branch damage to exhaust pipes.

If you don't see that as a concern, raise away..... As long as the tip of the pipe is angled up, out and to the side of the truck you shouldn't have issues with the exhaust in the bed at either height anyway.

The trucks changed to exhaust flappers however can present a classic case of unintended consequences. I know of one National Guard unit that started changing exhaust pipes over to flappers with the thought of reducing exhaust system rusting as their trucks sometimes sat for months with snow and rainwater going down the stock pipes. They switched back when they found that the change resulted in exhaust fumes in the cargo bed.....

Best regards to all,

Lance
 

LanceRobson

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We have been making this modification for several years now on the Six Flags Theme Park Safari Trucks.
Hi Dave. Sorry for the late reply but your picture is apples and oranges to what happens to the bed passengers if the carg o cover sides are down. In that case the fumes would absolutely affect the bed passengers.

Best regards to all,


Lance
 

simp5782

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Straight piped mine out the side and its actually quieter then having the stack with the muffler. If someone was to ad a muffler it would be perfect
Hopefully no members in the US have that idea for their exhaust. It is against federal law to have your exhaust dump above or near the fuel tank opening.
 
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