back pressure questions

mopar or no car

mopar or no car

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Apr 9, 2008
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OK, so when I was in college I was taught that a turbo gave a diesel enough back pressure to run any length or diameter straight exhaust (hood stack, 5" straight, 4" duals). I have read some claims that running a good muffler gives a little better power and mpg. Recently talked to a guy bout how I was thinking of trying a dual 5" system to have a lower, deeper tone, but he told me that it wouldn't have enough back pressure. I am new to common rails, so not sure if it applies more to the new technology or is this just some people's opinions. I always thought that the manifold outlet was the main restriction and that you wanted to get the exhaust out and away ASAP to lower egts.

Can someone shed some light on this and explain where more or less back pressure is good?
 
jesus the **** people believe

appropriately sized mufflers have a negligible effect on a turbocharged engine
 
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overly large pipes with a splitter yields MORE backpressure than a properly sized pipe

how's your mind? blown?
 
And this is because a properly sized pipe will help with scavenging/drawing the exhaust out right?

Everything I was always taught was bigger, free-er, less restricted for efficiency, mpg, and lower egts. Drive pressure should provide more than enough back pressure.
 
Oh my . . .

Put whatever size you want with a muffler or without. You won't notice any difference except sound.


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So who has proof of scavenging effect on an exhaust system that has an interruption between the valves and the exhaust?
You have to have frequency/pulsation to scavenge.

Squeaky Wheel
 
correct, it's not a scavenging thing, it's the way the exhaust stalls and eddys in oversize pipe vs. maintaining good velocity

the divider hurts
 
Forrest Nearing said:
correct, it's not a scavenging thing, it's the way the exhaust stalls and eddys in oversize pipe vs. maintaining good velocity

the divider hurts
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I'll agree with most of what you said.......but, how does a divider that far down stream hurt things?
 
Forrest Nearing said:
correct, it's not a scavenging thing, it's the way the exhaust stalls and eddys in oversize pipe vs. maintaining good velocity

the divider hurts
Click to expand...
Oohh yes I understood what you were saying with that portion.

Squeaky Wheel
 
TX_5.9 said:
I'll agree with most of what you said.......but, how does a divider that far down stream hurt things?
Click to expand...
Any change in velocity is a change in pressure. Any change in direction is a change in velocity.

Squeaky Wheel
 
But here is the thing to remember. You will never even be able to notice the difference on your truck. Run whatever!


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A long while ago, I had a mostly stock common rail. One day the factory turbo let go, so in went a silver bullet 66. I drove it for awhile with the entirely stock exhaust attached to it, and then finally yielded to cutting the muffler out for more power. I don't understand how or why, but it most definitely spooled the turbo faster with the muffler in place vs that straight pipe. I never dynoed it to see if there was a difference, so it's entirely subjective. Call me crazy, but that was my experience.
 
Thanks big nasty-this is the stuff that I hear randomly and the fact vs bull reason for this thread. Main opinion seems to be that a 4" system is good till like 800 HP+, then 5" can benefit. While 5" won't hurt to run on a lesser HP truck. But then all these other stories or opinions show up, and like you said, you spooled faster. So.......are we wrong to be looking at these situations component wise or size wise vs velocity and pressure? For example, big nasty's spooling issue is related to a velocity and pressure change or am I just over thinking this whole deal?
 
mopar or no car said:
Thanks big nasty-this is the stuff that I hear randomly and the fact vs bull reason for this thread. Main opinion seems to be that a 4" system is good till like 800 HP+, then 5" can benefit. While 5" won't hurt to run on a lesser HP truck. But then all these other stories or opinions show up, and like you said, you spooled faster. So.......are we wrong to be looking at these situations component wise or size wise vs velocity and pressure? For example, big nasty's spooling issue is related to a velocity and pressure change or am I just over thinking this whole deal?
Click to expand...

It depends...Exhaust diameter will also cause back pressure on injectors causing the rifleing to reverse itself making the injector burp. That pressure has no place to go which could blow yore air filter upside down.
 
Best I can figure is, until sufficient drive pressure is built the muffler creates the back pressure that helps low rpm torque as it would in a n/a application. Extra low end gets things going and that increases the turbo response time.

Am I saying that muffler is better than no muffler? No. I'm sure once into the upper rpm there was a hp penalty for having the muffler. All I'm saying is, while minute, I've noticed differences in the affect an exhaust system has on a turbo engine.
 
TheBigNasty said:
Best I can figure is, until sufficient drive pressure is built the muffler creates the back pressure that helps low rpm torque as it would in a n/a application. Extra low end gets things going and that increases the turbo response time.
Click to expand...

I think you meant decreases turbo response time. Anyways, this is sort of my understanding as well. Some low end back pressure = more overall exhaust pressure = turbo spools sooner.
 
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