Back pressure

[CUMIN2pass]

I read someone talking about back pressure. " with lower back pressure = more smoke" What is the downside to having little back presure? I made my own 5"

system. All I can tell is lower EGT with faster boost PSI.

 

[cullyguy]

It is my understanding that the less backpressure that exists (ie:larger exhaust) the longer the turbo will take to spool up, thus resulting in more smoke. The bigger the injetcors will make this much more noticable than the 275's.

 

[Evan A. Beck]

I disagree, cully...



Pressure, as you know, is a resistance to flow. Therefore if there is more backpressure in your exhaust, how can the turbo spool up faster?



Think of it this way... For example, picture a turbine in a dam with a large inlet and outlet, say 4' diameter on each end. The flow of water through the turbine is good because the outlet is the right size, allowing the right amount of water to pass through. Shrink the outlet down, say 3 feet, and you'll notice the turbine speed will be slower, but the pressure will be higher due to the restriction. Make sense? Same conecpt applies to the turbo on the truck. I'm not an engineer by any means, but it is simple... The greater the pressure, the lower the flow in this case.



With larger exhaust, the speed of the hot gases from the engine will be greater because of low restriction on the outlet of the turbo. Because the speed of the gas is higher, the turbo will spool up slightly faster.



I'm sure there is a formula for figuring out all this stuff mathematically... Engineers, are you there?