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I was wondering if anyone is knowledgable in exhaust tubing size as far as efficiency goes. Case in question my 720 diesel. It still has the factory set up which looks to be about 1 1/2 or so . Im going to replace it and dont know if going bigger will hurt me or not. I can say that when I split the tubing at one of the joints it seemed to loose some of its oomph. Does it need quite a bit of backpressure to work right?

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It's not back pressure really. Its exhaust gas speed. The faster it moves, the harder it is to stop. Like an out of control freight train. Even when the piston reaches TDC on the exhaust stroke and starts back down the cylinder the exhaust valve is still open briefly. The exhaust is traveling so fast is can't stop instantly and drags out the last of the old gasses in the cylinder like the caboose on the end of the train. At this point (TDC) the intake is also opening and this effect can actually help drag air into the cylinder.


More of the exhaust is removed and more intake air pulled in for basically free. The engine doesn't have to work any harder to do this, there is less exhaust left in the cylinder to dilute the intake air coming in and the engine efficiency goes up.


The only problem is that this works best at certain RPMs and certain pipe diameters. A small pipe increases gas speed but what works at highway speeds can be too restrictive at high RPMs. A larger pipe will drop gas speeds at the same RPMs but allow for more flow and speed at higher RPMs (on a gas engine) Diesels are not high revvers like gas engines and work through a reduced RPM range so the pipe diameter should not be increased or removed I would think. The exception would be if a turbo was added. The extra volume would allow a modest increase in pipe size yet keep the gas speed up.

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I guess I will keep the stock size then. The truck will never have a turbo. I guess datsun/nissan engineers must have made it that way for some reason. If it were a gas truck I would definately open it up. I guess im confused as to why it did not like being opened up. todays larger turbo diesels have way bigger pipes than gas counterparts right from the factory.

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The lack of pipe or going larger diameter reduces this 'scavenger effect' of the high gas speed. Newer diesels are larger displacement and likely turbo equipped and have more exhaust volume so a larger diameter pipe will still allow high gas speeds.


Maybe one day they will develop a reactive exhaust pipe that varies it's diameter in relation to exhaust volume. This way the gas speed is maximized at all RPMs.

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A big factor many people do not consider is the effects of sound waves in the exhaust pipe. The exhaust is not a constant flow, at least when it leaves the cylinder. It is a high pressure pulse. That is why open exhaust is so noisy.

I know you can "tune" the length of the exhaust pipe to help the exhaust flow. You can also combine the exhaust from different cylinders to aid exhaust flow.


I have had a 2" system on a 521 with a 1600, and later, a 1800. Used the stock L-16 manifold. I think it helped, but to be honest, I have not ran the setup for an extensive amount of time on a dynometer. I also have not done any testing on a race track.

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Maybe someone who has one of these engines will chime in with what they have tried. Wayno may have some ideas for me. Thanks for the input from everyone.


I have not really done much in this area, I just use the stock systems, the one that I have that seems to have the most power has the largest exhaust(not stock) that is also the shortest.

I put this system on from extra parts laying around, good sized head pipe(stock SD25), 2 1/4" flex tubing, and a giant muffler.




This is much larger exhaust tubing than stock, as the SD22 exhaust tubing is fairly small, mine has a 1 1/2" tail pipe.

My next system is likely to be 2 1/2" at least, as I am about to turbocharge the dually diesel.

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if its a turbo diesel whatever the size of the outlet of the turbo is, if n/a then whatever the size of the manifold outlet, going bigger than that wnt have much affect being that its choked down at the turbo or manifold outlet. think of a diesel (or any engine foir that matter) as an air pump the more you can get in and the more you can get out the more power it will make.

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I am going to go out on a limb here, and Wayno, or somebody else with more knowledge of diesel engines can correct me.

First of all, a diesel engine is always inhaling the same amount of air, at part power, or full power, assuming the RPM is the same. The power is controlled by injecting less fuel into the cylinder. You need the full charge of air for the temperature inside the cylinder to get hot enough under compression to ignite the fuel as it gets injected into the cylinder.


A gas engine does not use the engine compression to ignite the fuel. So, it chokes off the amount of air going into the engine, to control power.


Because a diesel engine flows more air, it needs a bigger exhaust pipe.


Now, lets put a turbocharger on the diesel engine. If you put any back pressure on the output side of the exhaust turbine, of the turbocharger, it really cuts down on the amount of energy the exhaust turbine can extract from the hot gasses passing through it.


So, long story short, a diesel engine responds much better to increases in exhaust flow than a gas engine.

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