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Exhaust Back Pressure

25K views 19 replies 7 participants last post by  WoodBoy 
#1 ·
I am currently running a canned tune, extreme race, with the SCT Livewire. I am having a hard time finding any information on exhaust back pressure. When purchasing my truck, the diesel tech told me not to add any exhaust modifications and specifically told me about the 6.4l and how it was designed to operate with back pressure. After reading the website for quit a while, I decided deleting the DPF seemed reasonable without causing any damage. I assume deleting the DPF affects the back pressure, so I would like to monitor the exhaust pressure but the numbers are way off when comparing desired to actual pressure when being monitored. At times the actual back pressure is 3 times higher then the indicated desired back pressure. Is this normal? Ive tried doing multiple searches on the topic with little results. Thanks in advance!
 
#2 ·
The sct reads different than psi if i recall correctly.. Your backpressure works mainly with your boost if i recall correctly so if you want to keep it id stay below 45psi to be safe.. Someone else should chime in
 
#3 ·
Thanks Woody! It makes sense that boost should reflect BP, I didn't put the two together. It almost seems like the desired exhaust back pressure reads at PSI and the actual BP uses a different measurement. The desired hovers around 40 PSI and the actual ranges 130-200 depending on throttle.
 
#16 ·
Exhaust back pressure reads in KPA and is also an absolute pressure, so you have to subtract atmosphere pressure from it. The reading should always be around 100 or so at idle, that is 14.5 psi basically Atmosphere pressure. Then have to calculate from there. Just use for every 100 change in kpa equals 14.5 psi. I don't know anything about the desired back pressure to see how it is figured.
 
#5 ·
BP in the motor and BP in the exhaust is two entirely different things. Motor BP gets up over 70 but if I am correct the BP in a diesel exhaust is not needed. More your turbo puts out the faster it exhausts the better. Well to an extent.
 
#6 ·
Not really sure which the mechanic was referring to. I assume the exhaust was responsible for creating the back pressure to the motor because he highly suggested leaving the stock exhaust on the truck because it was "specifically" designed for the motor and removing the exhaust would alter the back pressure. Not really sure if this statement was correct, so I figured I would ask the experts. Is it something to monitor, or not worry about?
 
#7 ·
Exhaust BP I wouldn't worry about. With the dbf deleted and some running up to 5" from the downpipe back I can't see where any bp is wanted. In the motor is another story. I am sure running big tunes and beating on it from time to time made my head gaskets blow. But some never have that problem. Big tunes, egr deletes w/waste gates are suggested and probably a necessity to relieve the bp created.
 
#9 ·
I do run SCT tunes, but the ones from Eric at Innovative. I used to run Spartans so I have a dash daq also. On there I monitor, egt's, trans temp, coolant temp, engine oil, fuel rail pressure and turbo boost. I would highly recommend running Eric's dbf delete tunes, I believe they are much better on your trans and way more power.
 
#13 ·
ok, you know what I mean. The damn big ass regen filter. LOL
 
#11 ·
exhaust back pressure is i the motor. its in the exhaust side of the plumbing. its not the exhaust everyone would think like downpipe and out..its in the exhaust sides of the turbos and up pipes. you definitely need to know what those pressures are.
 
#12 ·
ohhh ok, good to have that cleared up. I was thinking like a gasser. lol
 
#19 ·
I have been thinking about this subject as well but haven't found any solid answers. Subscribed.
 
#20 ·
I found this on another thread from another member..

You'll need an accurate pressure gauge for boost and exhaust back pressure. The dash daq provides these readings as such; manifold gauge pressure is the same as actual boost pressure. EBP (exhaust back pressure) is atmospheric pressure + exhaust back pressure. MAP (manifold absolute pressure) is boost pressure + atmospheric pressure. It is easiest to use MAP and EBP as readings, but if you like math, you can use MGP and EBP - atmospheric pressure. The ideal street ratio is approximately 10 psi actual peak exhaust pressure higher than actual boost pressure. For example, If your EBP reads 75 and your MAP reads 65, you're perfect. Assuming an atmospheric pressure of 13 psi, you'd also be perfect having EBP of 75, and MGP of 52. To know your actual atmospheric pressure, look at EBP or MAP with key on, engine off.

In a perfect world on the dyno you'd have a perfect 1:1 ratio of boost and back pressure. If this is your goal, expect street manners and drivability to suffer. Peak power could improve slightly, but it's usually not worth trying for a perfect 1:1 ratio. If you have any more questions please just give us a call as I'm not much of a novelist. I hope this helps.
 
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