exhaust pipe size vs boost pressure September 26, 2018 11:41AM
I've been watching diesel truck and tractor pulls. I noticed that the pickups run stove pipe size exhaust pipes and the smoke just bellows out but with no real force. The tractors push smoke out a small exhaust pipe and it goes straight up and then funnels out. Does exhaust pipe size affect boost pressure ? Also would a little back pressure on the turbo make it work a little harder ?
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Re: exhaust pipe size vs boost pressure September 26, 2018 02:04PM
I know nothing about these things but you have me thinking. If the truck has a small pipe at turbo that then emptys into a very large pipe that is much taller it seems it would then slow down the flow of the smoke plume.

S'no Farmer
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Re: exhaust pipe size vs boost pressure September 26, 2018 05:28PM
I think one of the main reasons is tractors have a lot more cubic inches than the trucks, therefore, more air flowing through.
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Re: exhaust pipe size vs boost pressure September 27, 2018 01:02AM
Just because an engine has larger cubic inch doesn't mean it moves more air. This is caused by large exhaust slowing down the velocity of the exhaust.
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Re: exhaust pipe size vs boost pressure September 27, 2018 12:05PM
Tractors and trucks have basically the same cubes from 414 to 680 right ? I was once told by an engine builder for Petty enterprises that a little back pressure is a good thing but he was talking gas not diesel. I would think it would make the turbo work a little harder to make more pressure. Kind of like running a tighter exhaust housing on the turbo. I wanted to hear what others think. I know both engine and turbo builders monitor this site. Maybe it's a trade secret.
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Re: exhaust pipe size vs boost pressure September 27, 2018 01:14PM
If you make 60 pounds of boost 70 80 100 or whatever on the intake you will have that much or close back pressure on the exaust dont matter if its got a 10 inch ex pipe
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Re: exhaust pipe size vs boost pressure September 28, 2018 10:31PM
My uncle always says it’s not how much smoke you have it’s how high can you blow that matters!
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Re: exhaust pipe size vs boost pressure September 28, 2018 01:56PM
The reason you have noticed that trucks have stove pipe sized exhausts is that we need to run a very short pipe off the turbo and then a much bigger pipe that is on the truck hood. That pipe has to be large enough in diameter to pivot over the pipe on the turbo when you lift the hood, which is why the turbo pipe needs to be short and the hood stack needs to be big. The tractors typically have longer pipes off the turbo which is why the smoke stands up more in many cases. No HP difference involved. You would need to have an extremely restricted exhaust pipe to actually make a boost pressure difference.
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