Tubro Down Pipe Sizing

Downpipe should have 7 degrees taper from turbo up to the maximum size tubing you can fit in your truck. It helps kill the harmful turbulence coming from the spinning turbo. Pipes splitted later to dual pipes used in many US trucks is not good for flow, single flows better even if it has smaller flow area. There's no use to have more area than you can have in downpipe. You can also lose flow if going so big that you must make very tight bends, a little bit smaller might give better flow if you can make smoother bends. Too tight bend and the flow separates from the inside turn and makes restriction. Other than that, bigger is always better, you don't lose torque.

 

I've often wondered about piping size versus the turbo outlet. The turbine wheel is smaller in diameter than the the 5" pipe by a considerable amount in a stock turbo. A BorgWarner OEM CAT replacement 550 turbo has a turbine wheel with a 3.58" if I remember correctly. The outlet of the turbine housing increases from the 3.58" turbine wheel diameter to 4.38" inside diameter of the outlet flange. It then increases to the 5" down pipe diameter. So the OEM truck manufacturer has increased the diameter of the down pipe a considerable amount above the turbine wheel diameter. I also felt that the dual exhaust system is more a consideration with the OEM truck builder in exhaust gas back pressure caused by the mufflers. Two mufflers will cancel the noise levels with less exhaust restriction compared to one.

The things I've noticed over the years is that large straight stacks seem to lose exhaust velocity when they go from 5" to the larger size. This can be seen by the fact that the chrome on the stack will turn blue in this area. The chrome turning blue is a sign of a higher temperature compared to other areas of the pipe. This tells me that the exhaust flow stagnates in this area. This leads me to think that the exhaust back pressure would rise in the 5" down pipe before the increase in pipe size occurred. The reason I say this is with the stagnate flow, the exhaust gases closer to the turbo have to work harder to to push through this stagnate area, the only way this happens is if the back pressure increases from the turbo to the stagnate gases. I've actually thought about changing my stack to a 6" diameter to see if Hp increases at the tires over the 8" pipes I have now.

 

Since my truck has dual 5" stacks using Walker Megaflow Mufflers that are straight thru, would the only thing different be the noise if I eliminated the "Y" and went to one stack/muffler?

I can't see where it would hurt the power or fuel mileage and in fact it might improve by eliminating the "Y" with the flex pipe and go with solid pipe from the turbo to the muffler elbow.

Getting rid of the weight of the extra muffler and exhaust pipe with the support brackets for the muffler should save alot of weight too.

 

that muffler will flow enough as a single and not create high back pressure. You will need to keep a piece of flex pipe between the engine and stack, otherwise the solid pipe will break over time from the truck and engine movement

 

The reason why people put the wraps on there exhaust and on the exhaust housing of a turbo is to contain the heat. It is not just fuel that spools a turbo but it is also heat. I would not be to worried about your EGTs unless you have massive amounts of fuel being injected.

 

Turbomanufacturers give the right taper, OEMs know it of course, 6 or 7 degrees is usually perfect but anything between 4 and 12 degrees will do.

I too think thats the reason for duals.

It's not the actual size that makes it but the abrupt change in diameter that hurts flow.