Ok! I am about to step into an ant mound but I have talked to SEVERAL people and found out several different things. BUT i have never gotten a straight answer from ANYONE why it is the way it is! The issue you ask? Exhaust back pressure on a diesel motor! OK I already know that too much back pressure is bad.....well if you want to stop, hence exhaust brakes. BUT i have talked to too many people and found from personal experience that bigger is not always better.

The most common issue i have been tiring to tackle is the straight pipe issue. I have been told that it is not a wise idea to run a straight pipe on a diesel truck. This would not offer enough back pressure to the turbo to spool up as fast. What i mean is there has to be some resistance to build pressure before the turbo to create a pressure drop which in turn spools the turbo. BUT i have been told that diesels are not like gas motors and don’t require back pressure due to valve overlap. I can maybe understand that a stock exhaust is soooo restrictive that a straight pipe may be the answer to lower back pressure somewhat but is it too much? I have also heard of 4" and 5" systems running no mufflers as well. I have talked to Bully Dog and they said not to even consider the 5" system unless you are running GOBS of power (375+ RWHP I believe). They said it will hurt performance with that big of a pipe. WELL, that seems to be being a back pressure issue then! I also know exhaust also has duty with heat evacuation to keep the motor and turbo happy temp wise. but isn’t heat also part of the equation of spooling a turbo? Loose too much heat, less spool, less power? I also called Magnaflow to ask why the had a 3.5" down pipe on their 4" system. They told me from dyno testing, a true 4" exhaust did not produce the power as a 3.5" down pipe 4" combo. When asked why, they said the turbo needed some back pressure to spool up quicker. I understand this setup is probably geared to a mild box and air intake. Big injectors, Blue Chips FMS box, Timing box, and big turbo would obviously require a bigger exhaust. BUT that is what I am talking about! Tuning your exhaust to what you have. Does a straight pipe reduce back pressure too much rather running a 4” performance muffler?? I have heard too may things either way and would like some good discussion on what works and what doesn’t. AND explain why it is the way it is! Haha….Also to add. I have tested some of my own theories and found that a smaller down pipe on moderately modified trucks works better then a full on 4” system. This is seat of the paints feel I am talking about. I wish I had access to a dyno to get this stuff on paper but my butt is the best I got to test with! Haha……Thanks!

 

Backpressure in any engine is a bad thing. The turbine wheel and housing on our engines generate way too much back pressure by themselves, so adding more with a restricitve exhaust system only hurts power. No theoretical model (The otto cycle, diesel cycle, carnot cycle, etc) has any input for backpressure that is needed for the cycle to work. The restriction to flow is only going to cause pumping losses and thus sap power.

From a spoolup point of view, you want the large pressure difference across the turbine to help it spool faster. This means straight through exhaust and properly sized pipe is a must. Why is 3 1/2 better than 4 and much better than 5 on a stock truck? It comes down to exhaust velocity. The large sewer pipes on a stock engine will cause the exhaust to cool too quickly and stagnate in the pipe. This stagnation causes the exhaust to slow down too fast and instead of flowing out it has to be pushed out. The 3.5" to 4" expansion will keep velocity up leaving the turbo. Higher speeds means less pressure, so the turbo works even better.

It becomes a balancing act between restriction and exhaust velocity when choosing an exhaust system. There is nothing at all wrong with straight piping a CTD!

John

 

Very intriguing. Never thought about the velocity aspect of it. That does make sense...but why the heck are there people preaching (and I am talking about Cummins Mechanics) that straight piping is a bad thing for the turbo/motor? Something had to put the bad taste in their mouth to make such a comment. Great response and very well backed up. Keep up the good info.

 

Dan,
I recently did a change over from a 4" to a 5" in anticipation of my twins. My Hx4016 reacted negatively to that change. Boost was slower reaching peak and it was even a tad laggier.

Someone else posted changing from a stock exhaust to a 5" and saw gains. I know they probably saw those gains but probably would have been better served going stock to 4" with their configuration.

Having said that I agree with the velocity statement as making sense. I don't have facts here just that limited experience.

James

 

With a large single turbo on a 12 vavle the 5 is not needed until past about 575HP. On my truck the 5" straight pipe picked me up 10 peak HP over the 4". The funny is...it is not as noisy or I am getting deafer Prolly the latter.

Don~

 

Here is a thought...can some mufflers flow even better then a straight pipe? They do have baffles cut in a spiral pattern and i would spinning the exhaust (far enough away from the turbo) would cause a scavenging affect.....thus pulling exhaust out of the pipe? Theory of putting a spin on water going through a funnel. It will go through faster with a rotation rather than none. Ponder that!

 

Velocity is the only factor that really matters. The restriction to flow posed by the stock exhaust is MINISCULE compared to the restriction caused by the 12cm housing.

Exhaust size is closely analogous to turbine housing size. True, a 16cm housing offers slower spooling than a 12cm housing (lower velocity), but less restriction as well.

The key is finding the point of diminishing returns. Ideally, you would run the smallest pipe that still allowed the flow you need. For example, say your engine flows 300cfm peak. The stock exhaust flows, say 250 cfm. upgrading to a 4" that (fictitiously) flows, say 400cfm would be a waste-- your ex velocity would go way down and you would lose performance.

Velocity is also affected by temperature and density, as Banshee mentioned.

Remember, that even small increases in the diameter of the pipe can mean LARGE increases in the cross-sectional area. Going from a 3" to a 4" pipe is a 33% increase in diameter. But the area goes from 7.07in^2 to 12.57in^2!! This is an increase of almost 78%!!

You can see how exhaust velocity would really decrease with a 78% increase in pipe area. Also, consider that as the exhaust expands to fill this larger area, it will become less dense and cool off. (like when an aerosol can gets cold after you use it). More importantly, it will SLOW DOWN.

This is why the very best exhaust system would feature a very gradual, tapered increase in exhaust diameter-- and NO sudden point of expansion. Any sudden transition from 3" to 4" or 4" to 5" is going to cause restriction. They key in airflow is GRADUAL! Every transition should be smooth and gradual.

The only way that a muffler can flow more than a straight pipe is if you have heavily pulsed flow like would come from a NA gasser exhaust. The turbo effectively dampens pulses, ensuring that a straight pipe will always flow the most.

BTW-- backpressure is NOT necessary on our engines, because they don't rely on NA breathing during overlap. Again, the turbo damps the only beneficial exhaust effect that "backpressure" provides. Even so, it's not the backpressure that's benficial at all! It's simply the higher exhaust velocity associated with higher back pressure.

Justin

 

i went from a 4" to a 5" on my truck[398hp] it felt better with a 4" system, and got better miliage. why i do not know. but i lost 1mpg from the switch and i drive 500 of so miles a week. i had heard that a 5" would do more harm than good[up to 450 hp], but i didn't belive it. i do now, i will keep my 5" on just because i am going to bump the power here in a little bit and will see what happens. mike

 

GREAT INFO GUYS! Thanks for finaly putting a long battle to rest here with a few of us. This all makes very good sence. Anyone else got their 2 cents??

 

I think (ponder, theoreticize ;D ) that one of the key factors will be the way the expansion is handled at the exit of the turbine housing. We had lots of gasser turbos change performance due to changing the place where we widened up from the turbine's outlet which is a venturi to keep speeds and therefore delta P across the axial part of the turbine wheel up. What we tested seemed to indicate that the "tuning" of the exhaust pipe to the turbo makes most sense for the first 10" with the small gassers we worked. A major factor was whether there was a bend on the first 4" or not.(measured from the plane that is made by the trailing edge of the turbine axial part)- I think it's consistent with my observations that higher hp trucks will benefit from the bigger exhaust size since they necessarily have bigger turbos.
The main factor for backpressure seen by the engine is the combo of exhaust manifold and turbo. Here we do win or lose scavenging that keeps our engines cool. Too much scavenging can also lead to a false sense of security, simply because even at the temperatures where your pistons are happily melting away there is enough cool air scavenged through the head during overlap to keep EGTs down in the "safe" zone. I know that I am talking about extreme cams now.

I think that I may conclude that in some cases the radical expansion from a 3" turbine outlet to let's say a 5" downpipe will cause enough of a turbulent vortex to disturb the flow over the turbine blades and actually add backpressure and reduce the potential deltaP across the turbine.

Just my 2c ( € ;D )

AlpineRAM

 

I want to join in and bring up another contributing factor to the discussion. Let me start by saying that velocity matters in the exhaust housing of the turbo. To create velocity you need pressure and to create pressure you need a restriction. To move something past a restriction it takes force or power. In a turbo the power used create the velocity; caused by the restriction of exhaust housing and the turbo blades, is more than offset by the power gained by compressing the intake air. However in the exhaust system, post turbo, there can only be gains in efficiency by reducing the effort needed to push the exhaust out the end of the pipe. This means as little restriction to flow as possible. What are some things that can cause a restriction to flow? The number of bends, how sharp those bends are, restrictions in the mufflers or resonators, the size of the pipe, temperature as well as turbulence of the exhaust gasses. Any hindrance to flow that can be eliminated should be, and most of the after market exhaust vendors have done an excellent job of removing those physical power robbing restrictions. We tend to look at the hardware and the physical construction of the system more than how the exhaust actually flows inside that system.
I would like to focus on exhaust flow. To make the exhaust system efficient it needs to have smooth and unrestricted flow. I tend to believe the major reason why a 5-inch exhaust system may cause a decrease in power on some trucks is turbulence caused by eddy currents. Think of turbulence inside the exhaust pipe like water flowing in a river. Many times the water after leaving a narrow section in the river (the turbo) will form eddy currents as it enters a wider area (bigger pipe). As the exhaust gas enters the larger pipe it creates negative pressure in the area where the flow stream has not yet made contact with the inside walls of the larger pipe. This pressure differential causes the outside edge of the flow stream to slow rapidly, circle around and move in the opposite direction back toward the oncoming exhaust flow. A greater jump in pipe size say three- inch to five-inch only means a bigger area for negative pressure and a larger eddy current. Another factor that can cause eddy currents is when the exhaust gasses cool to quickly along the inside walls of the pipe. A bigger pipe means more surface area to cool the gasses. As exhaust gasses along the inside walls of the pipe cool they take up less space and slow down due to the loss in their volume. However the gasses in the center of the pipe are still hotter and moving faster. Now you have pressure differentials in the flow stream that again create eddy currents. When this happens in an exhaust system you loose efficiency, think of it as hundreds of small head on collisions in the exhaust flow. Eddy currents or turbulence is a restriction just as much as to tight of a bend would be in the exhaust tubing. The design of the system should be balanced to the amount of exhaust the engine is producing. To those who posted previously and said that the size of the exhaust system needs to be matched to power level of the engine, I have to agree completely. A person running 500+ HP needs a larger exhaust than someone running 350 HP. Bigger is better only if it works.

Edward

 

What Ed said. Or was it what i said??
yeah..eddy currents...lack of laminar flow....preventing shear in the flow stream....yeah, that's the ticket.

HOHN

 

Edward, I follow your thoughts on the Eddy's

I learned some of this with air flow as well.

Certain bends can inprove upon flow as long as that flow is not forced to chsnge in another curve very soon after.

The Scotty II has one big curve in the turbo inlet hose. If it had another opposing curve to get to the turbo, it would not flow and give the results it does.

I also wonder why the stock air box is square, then goes to the round turbo inlet hose, then squashed at the restriction to the intercooler, then it opens up again coming out and then again gets reduced down and then last but not least goes into a right angle pipe before the intake!

Adding a smooth bore round inner diameter at that point could work as long as its shaped to flow the air, not to create eddy's before the manifold. Banks' hi Ram needs a bit of tweaking for it to flow and benefit like it really should.

Whoops I digressed...oh well

Back to exhaust...factors that dictate what size exhaust should be used are scavenging, flow, volume and the pressure that the volume is be pressed out the pipe and the exit and its angle. Count the curves in the pipe over the rear diff...and how many angles there are.

If your pipe is too big for the volume, it can slow down the flow. It does need some balancing to the power you have.

Back to the shop to work on the new [censored] that is taking advantage of the shape of the truck. ;D

 

[quote author=HOHN link=board=7;threadid=22079;start=0#msg206844 date=1068180509]
What Ed said. Or was it what i said??
yeah..eddy currents...lack of laminar flow....preventing shear in the flow stream....yeah, that's the ticket.

HOHN
[/quote]

HOHN
Hey I agree with what you said even though you said what you said before I said what I said. Does that make sense or is there a lack of laminar flow in here someplace?

Edward

 

Good posts! Back pressure only aplies to engines that are not forced induction. When you force air through a motor then back pressure is no longer a factor for performance gains. Why? Just that...you are forcing the air through so you need no other help from "velocity", which people like to call "back pressure". Velocity, depending on intake or exhaust, helps draw the air in and push the air out on naturally aspirated engines. Forced induction motors and diesels work on a totally different set of rules.