Discussion on exhaust size's and related items...

[BushWakr - RIP]

Hey guys, how about kicking this around a bit...

Just for the sake of this discussion lets diregard the very cool sound you can get with upgrade exhaust systems ok...



The larger exhaust systems are used to reduce or eliminate all the kinks and bottle necks in the piping/exhuast flow (as it expands and passes) so that the exhaust moves quickly and efficiently out with minimal difficulty...

Now my thinking (and my understanding) is this... as long as the volume of exhaust being produced or, expected to be produced, is not an "overload" in terms of volume, the 'overly large' exhaust is not needed.

All you need is the ability to move the exhaust out and without restrictions or bottle necks... yes... ???



Now, throw into this the idea of exhaust velocity.....



Take the inlet to a turbo exhaust housing here... particularly when you port it's inlet... . you don't want to over-do this process because you can and, actually do, reduce the exhaust gas velocity as it enters the 'hot end'. Yes, you derive your turbo spoolup from the thermal energy but you also benefit from the velocity of those hot gases right

We often do this porting or at least gasket match the inlet to our exhaust housings to reduce/eliminate the "backup" or delaying of the exhaust passing thru the housing... .



Ok, so, if you port your hot end just right and, you have an exhaust sytem that provides just a hint of backpressure (to hold the hot gases just enough to transer their heat/velocity) you stand to benefit from this "tuned and/or complimentary" combination... . ???



BTW, if HDM48 is around maybe he can jump in here to address the issue of 'loss of velocity' as it pertains to diesel/turbo charged engines... HEY DAVE. . !!!!!!.....



Right now, many of the guys running twin turbo systems and lots of fuel are using a 5" exhaust system on their trucks... . for many reasons... . one is the outlet of the final turbo happens to be 5"..... kinda a practical reason for the larger diameter pipe I'd say... right. .

Keep in mind that these trucks (including the 1st gen guys) are usually very heavily fueled and, they run higher boost pressures as well...





So, the question is this (and this goes along with the question asked about exhuast diameter(s).....

"... Is there a "good" or "bad" size of piping to use on our trucks based on fueling/performance setups etc. ?????... . "

 

[BHiggins]

In regards to diesel exhaust, I often compare it to a two-stroke dirt bike engine. On a two stroke, the engine relies on the gasses expanding in "the pipe" to create a vacuum which evacuates the cylinder. Different sizes and shapes in "the pipe" change the end result... some enhance the bottom end, resulting in a gain in tourque, and some boost the top end resulting in more top-end horse power. Most good moto-cross pipes result in a trade off, you can't have both. I believe this is the point that PB is trying to make, we can, and do "over do it" believing bigger is better, but not for all applications.

 

[Greenleaf]

The longer the system/the more bends and turns your system requires, the bigger the pipe will need to be.



If you have a pipe off the turbo and up through the hood, you'd only need three inch or so. If you have it out the back of the truck with several bends you'd need a larger system. Each bend slows the velocity.



It would benefit to use an increasingly larger system such as the three inch down pipe, then 3. 5 inch past the transmission then into four inch over the rear axle.

 

[JLEONARD]

Has anyone actually measured backpressure? That is how Cummins specs out their exhaust requirements.

All the marine apps spec out 3 in Hg (or 1. 5 psi) as a MAX. I suspect that this would apply to us as well.

I have some numbers from various 6 BT marine apps for post turbo exhaust flow.

220 hp... . 1150 CFM

270 hp... . 1454

330 hp... . 1531

370 hp... . 1919

Someone who's into air flow can possibly use these numbers to calculate diameter required???

I'll do a little research tonight in my diesel engineering reference book and see what shakes out on the subject.

I always thought that diesels want no restriction and bigger exhaust is better... to a point that is.

I always thought that after the exhaust gasses have done their duty and spooled the turbine they should be escorted out the door as quickly as possible... . walking not running, just like in a fire drill.

Jay

 

[tugboatphil]

Disclaimer (You all know me as the Village Idiot, so keep it in mind before reading)



When I was in the Navy, I noticed that the elevated steam pipes that ran across the base has 90 degree bends every so often. There appeared to be no logical explanation for this. It wasn't as if they bent the pipe to go around an obstruction, it would just go straight, 90right, 90 left, 90 left, 90 right and continue straight for a while. This would be repeated throughout it's length.



I finally asked one of our Boiler Techs and he told me that steam loses velocity if it travels in a stright pipe for too long a period. The bends built pressure in that area, which eliminated the need for booster pumps to maintain the pressure.



Don't know if he was pulling my leg or not. But if you increased the size of the exhaust pipe a lot, eliminating the sharp bends, would you be decreasing the effectiveness of the exhaust on the turbo?



I'll go back to making smart aleck comments now.

 

[keimmmo]

Actually, as the exhaust cools as it gets farther from the motor it has less volumn, and therefore can actually use a SMALLER pipe diameter! That is why many factory exhaust systems have a larger head pipe than tail pipe. However, excesive numbers of bends or very tight bends do slow the flow and a larger diameter pipe helps compensate for the restriction of the bends.



In a basically stock truck, the stock size exhaust is fine... although it helps to do something about the restrictive stock baffled muffler. As the power level goes up, so does the need to move more air... and therefore the need for a larger exhaust.



In theory, too big an exhaust shouldn't hurt anything behind a turbo because they work best with zero back pressure. However, some have reported that with big exhaust on a stock motor they have lost low end torque. My theory on that is that the lack of velocity in the long exhaust system CREATES back pressure. When you want to up the velocity to accelerate, there is more volumn of air to push in the bigger pipe, so it takes more time and pressure to get it moving... hence the loss of low end power.



Exhaust modifications are one of the best buys for improving power and economy... on ANY vehicle. However, the "bigger is better" theory doesn't always prove true with exhaust. I recommend keeping the tube size on the conservative side... unless you are looking for nothing but top end power and are willing to sacrifice the low end power.



Steve Keim

 

[JSchlachta]

TugboatPhil



I have seen the same jog in long runs of cw pipe. This is usually to account for thermal expantios or contraction of the pipe.

 

[PToombs]

TugboatPhil



I have seen the same jog in long runs of cw pipe. This is usually to account for thermal expantios or contraction of the pipe.

This is what I was told about steam pipe also.



Bends add a lot of restriction, the manual for my pellet stove states " a 90* bend = 4 feet of straight pipe. " This is on a 3" stove pipe. Just something else to keep in mind, less bends = better flow.

 

[JLEONARD]

I didn't find anything directly related to exhaust system size in my research last night. I did read a lot about pulse turbocharger systems and heat energy transferred to the turbine... nothing at at that indicated we would want any restriction or velocity reduction there however.

The valve overlap and the manifold arrangement is supposed to cover that.

(and sets of 3 is the best as in our set up).

So my conclusion from what I inferred from reading last night and what I have read in the past is that we want as little restriction as possible in an exhaust system... therefore I believe a little bigger is a little better up to the point where you gain no benefit. Or perhaps better said that removing any restrictions possible is better... . that would cover removing the bends as mentioned above.

It also mentioned in one paragraph that insulating the exhaust manifold to transfer more heat to the turbine would yield better results in terms of transferred energy and turbo boost.

Anyway, that's the result of about an hour of hittin' the book. The rest was a lot of formulas and charts.

Jay

 

[Greenleaf]

I have dtudied quite a lot about the installation of wood stoves and the related stove pipe. It's best to have the pipe straight up through the ceiling. No bends. Bends slow the draft.



Also if the pipe is too big that's bad too. Lost velocity. It also helps to move the gases the hotter they remain. Insulated stove pipe. Think about that... ...



One more thing. The down pipe on the school buses are wrapped with this kevlar type insulation. It holds ROAD SALT and MOISTURE. It rots the exhaust pipe away faster'n you can say "tugboatphil"

 

[tugboatphil]

It rots the exhaust pipe away faster'n you can say "tugboatphil"

HEY!! Wait a minute, that's me! :-laf :-laf

 

[ptmoore]

many good points to ponder in these posts, starting with the ported hot side inlet to allow the turbine to spool faster. once the gasses have done their job on the turbine blades there is no further use for them in on a turbocharged engine, so the problem is to get rid of them as effiecintly as possible. hence the down pipe which is there to convert the swirling gass to a linear flow(swirling gas travels much farther to reach the end of the pipe than does gas traveling in a straight line). then to the rest of the exhaust system which should be large enough to expel the gas with as minimal back pressure as possible(backpressure limits the ability for more hot gasses to enter into the turbine from the engine). 4in pipe seems to be a proven diameter for out trucks until one gets to the hotrod stages some of u guys are with the twins and or super big singles. thanks for letting me in with my 2cents worth

 

[NIsaacs]

I have never figured that exhaust mods made much difference, good or bad. The stock system seems to work just fine, with bigger taking up more space and adding weight. This is based more on full length straight pipe. Some mufflers and or cats may cause some performance issues. I also feel the stock intake works just fine. Keep in mind I am talking about our turbo diesels not naturally asperated motors. I will take it one step further and say that intercoolers/aftercoolers will not give cooler EGT's under full power. However, I have no numbers to prove any of this, just theory.





"NICK"

 

[keimmmo]

I feel that the stock diameter is fine on a virtually stock truck. But, that stock muffler is a chamber of horrors for the exhaust to pass through!!! Gutting my muffler made a MAJOR difference in part throttle responce... less turbo lag. And, it picked up about 2 mpg on the economy! The stock exhaust system DOES NOT work for me!!!



Steve Keim

 

[JLEONARD]

The stock exhaust system DOES NOT work for me

Agreed. I saw some pretty significant gains (3 psi boost) by uping to 4 inch... even with a muffler in the system.

 

[RailroadJim]

This appears to be the right moment to ask about the effects of scavenging, whatever that is?