No muffler

[daddyoften]

So I was looking over our new to us bus. It's a 4107 with a newer 8v71T with an auto. The p.o. installed the engine and tranny and was working on the conversion when he got sick and couldn't continue and since has passed. Back to my discovery..... I noticed it has no muffler. When I brought it home about a month ago it didn't sound loud at all, it had a nice tone to it. It only has a 4" pipe off the turbo bent around to come out the left rear corner. Is this ok, should I even consider putting one on? Is 4" big enough? It seems kinda small to me.
Thanks
Eric

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[TomC]

If it was a 6V-71 natural, then 4" would be alright. Everything right after the turbo should be 5". I have a 8V-71 turbo with 7G75 injectors. I have a Donaldson 5" oval muffler with the inlet and outlet on the same side that is rated for turbo. It only has one baffle in it and has a nice tone. I like it so much, I'm using the same kind of muffler on my truck conversion with Cat 3406B.
If you had a Jake brake on your engine, then you would definitely hear the exhaust and would be offensive to surrounding people. Good Luck, TomC

[daddyoften]

Thank you Tom. Yes it has Jake's but they aren't hooked yet but they will be. Is there any advantage to going bigger than 5? I know at some point you loose back pressure and that looses torque.
Thank you,
Eric

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[bevans6]

five inch out of the turbo is the correct diameter, so that Donaldson low restriction muffler (I would actually call it a resonator, you can roll a soft-ball through it) is a great choice.  It's what I have on my 8V-71T and it keeps the jake brake noise down to acceptable levels (still not using it in town).  The "engines need back pressure to make torque" thing is a myth.  Back pressure is never a good thing, and in particular back pressure after a turbo is never a good thing.  Where this gets confused is with the need for exhaust gas flow velocity.  Smaller exhaust tubes will have higher velocity with a given volume of gas flow, and often higher velocity is better than low velocity.  On my engine the tubes from the manifolds to the turbo are 3" diameter, to keep velocity of the gas flow higher.  On my 8V-71N those same tubes to the collector were 4" diameter - velocity was less important and lower back pressure was more important, so larger tubes on the natural engine.  Why is velocity important to a turbo engine?  Turbos work from gas volume, and the volume of a gas is larger with higher temperature.  Getting the gas to the turbo quickly and at maximum velocity retains the greatest amount of heat, and thus volume, so the turbo works more efficiently.  Once the turbo is done with the gas it could care less, it's used all the energy it can and wants to just get rid of it, so dumping straight to atmosphere is fine, and going through the least restrictive muffler is nice and polite.

In the race engine world tuning using header diameter and length is a common tool.  If we want a engine with a wide power band and a lower torque peak we will use very long, fairly small diameter tubes - higher velocity promotes scavenging and volume is lower because we are optimizing a lower RPM power peak.  If we want a higher HP engine, sacrificing peak torque and optimizing high rpm operation we will use larger diameter, shorter length tubes - larger volume of gas from the higher RPM, and thus keeping velocity high from increased volume keeping the scavenging working well.  All trade-offs not in any way relevant to two stroke diesel engines, but I was on a roll...

Brian

[krcevs]

Daddyoften,
I would not go past 5" on pipe size. You will start to loose torque and also run into an issue of fitment. Can you post a picture of your engine installation? I have a 4107 as well and have thought about doing a turbo conversion.

Ken

[daddyoften]

Thank you very much for the information Brian!
Ken I will see if I can get some pictures for you

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[luvrbus]

If there is no reducer at the turbo reducing it down to 4 inch then you are good with 4in the 300 hp 8v71T had a 4 in exhaust, if the turbo is 5in then you need 5in pipe 

[gg04]

I built mine with straight 5" exhaust..No muffler..Turbos are heat driven..the more heat the more pressure they put out. Like the way mine sounds...rdw