Engine/Transmission (1998.5 - 2002) Losing HP with 4" exhaust

[Lube Man]

This subject came up on another thread but I need wider response. Some people are saying that going from a stock to a 4" exhaust can reduce low end HP and mileage. Some say it depends on the truck. Some disagree.



Does anyone have any sound engineering analysis to back up the claim for reduced HP. Any dyno testing? What about you guys that sell and install exhaust systems, what kind of feed back do you get?



What do they mean by low end? How often does one drive at low end? How would you ever check low end mileage unless you drove around all day at 1500 RPM?

 

[BTowler]

The mileage reduction is a factor of to little power at take off requiring more fuel to get moving which brings overall mileage down, the reduction in low end power, i. e. , taking off from a stop can be a problem if you are towing, you see improvements at highway speeds due to the increased flow, so if you do mostly highway driving your mileage should go up. I got the info I posted on the other thread direct from Magnaflow, they said they did testing with a stock truck and that was the results they came up with. Also, a friend of mine who also owns a shop has seen the same results on a few trucks he has done exhaust on. Anybody else wanna chime in?

 

[mcoleman]

I'm running a 5 inch exhaust and lost no hp and actually gained a tad on mileage. Plus I dropped EGTs over the stock exhaust

 

[BTowler]

Originally posted by mcoleman

I'm running a 5 inch exhaust and lost no hp and actually gained a tad on mileage. Plus I dropped EGTs over the stock exhaust

That is the plus on bigger exhaust, lower egt. As Far as hp loss, it isn't really a loss, it is shifted to higher rpms. Plus you are running a comp, I am referencing a basically stock truck.

 

[Lube Man]

I appreciate response but still don't see an explanation for less power at take off due to a bigger exhaust.

 

[bmoeller]

I never lost any power on the bottom end with the 5". Nothing noticable, if there was any. The turbo spools up faster with a straight-through exhaust system.

 

[SnoKing]

I believe that Magnaflow feels that a 3. 5 down pipe is better for a stock truck. My SS 4" came with a 3. 5 down pipe. Should I call it a 4-" system. Snoking

 

[BTowler]

Originally posted by Lube Man

I appreciate response but still don't see an explanation for less power at take off due to a bigger exhaust.

If you flow too much too fast, the turbo won't spool up as quick. Plus free flowing exhaust works better at higher rpms where the extra flow is needed, you aren't pumping as much air through the engine at lower speeds so you don't need that much flow capacity. It is a balancing act, too much on one end hurts the other. Thats why I recommend the Magnaflow, they are coming out with a 4" retrofit down pipe so you can change later as you need too.

 

[fox]

I think the theory goes like this.

A large pipe right after the turbo allows the velocity to slow and the air to cool, this will creat a standing wave or wall of pressure. The next air needs to push thru this.

Of course with lots of heat the bigger pipe is a must to control and move this heat.

The biggest obstruction to flow is the exhaust housing, thus stock motors don't benefit from bigger pipes.

The restiction to flow is why the turbo works.

 

[Lube Man]

I appreciate the last couple of comments but they don't make a lot of sense. Are there any true engineering types out there who can comment on this? Someone schooled and/or experienced in thermodynamics?

 

[RobbyRam]

I'm schooled in engineering, but,... alas... flunked out!... But I know this from my personal experience: The 4 inch exhaust with the 5 inch tip that I had installed sounds and looks cool! Isn't that what we are all striving for?

Bob in Sacto

 

[Sticks]

Originally posted by fox

I think the theory goes like this.

A large pipe right after the turbo allows the velocity to slow and the air to cool, this will creat a standing wave or wall of pressure. The next air needs to push thru this.

Of course with lots of heat the bigger pipe is a must to control and move this heat.

The biggest obstruction to flow is the exhaust housing, thus stock motors don't benefit from bigger pipes.

The restiction to flow is why the turbo works.

I also remember reading a thread about this. I think KArts was the one having the issue. Someone had mentioned the same thing as the above. Has any of the companies out there come out with a 3. 5" turbo end and a gradual increase to the 4 or 5" (cone shaped) down pipe, or are they all either 3. 5 down pipe, or 3. 5 with an expansion to 4 right away.

 

[Cooker]

These comments are compliments of "Diesel Freak"



"I agree that untill you are over 350 RWHP you probably do not need a complete 4" exhaust system (a straight piped 3" works VERY well), unless you just want the cool sound. Four inch ID pipe has a cross sectional area of 12. 56in^3, three inch ID pipe has an area of 7. 07in^3. As a gas expands, it cools, so if you do not have enough exhaust volume to fill a 4"pipe it could cool so much that just the action of moving the cooler more dense gas out of the pipe could raise backpressure to more than it would be if a 3" straight pipe were used. Keep the Gas hot and moving! Why do you think header wrap came into existance?



As for turbo spool up and back pressure... ... Turbines abhore backpressure!



All turbines whether they are in a Turbo, Gas Turbine, Steam Turbine, and even a Hydroelectric Power Plant require a differential pressure to work, and in some instances a temperature difference (condensing turbines).



Lets do an analysis on the gas flow through the exhaust side of the turbo.

As we add fuel to the engine, it speeds up and exhaust temperature gets hotter ... Higher mass flow rate and kinetic energy (flow work)



the effective nozzle area of the turbo housing is conatant 9, 12, 14, 16 (these numbers are completely arbitrary for this analysis) as we all know them.



As the mass flow rate out of the engine rises exhaust manifold pressure rises(potential energy) and consequently so does the mass flow rate into the turbine.

The velocity of the gas rises through the nozzle due to a differential pressure. The high velocity gas impinges on the turbine blades and the kinetic energy of the gas decreases and the rotational energy of the turbine increases (it accelerates)



If the differential pressure across the turbine nozzle is raised the gas velocity at the nozzle outlet will be faster therefore the turbine wheel will accelerate faster.



How can the differential pressure be raised?

Add more fuel to raise manifold pressure or change compressor wheels to raise boost and lower intake charge temperature and consequently raise mass flow rate through the engine which raises exhaust manifold pressure.



The easiest thing to do to raise the dP across the turbine is to put a low restriction exhaust on the engine... the closer you can get to atmospheric pressure at the turbine outlet the better!

Be careful with this (see my first paragraph)



The temperature drop across the turbine is mainly due to the drop in pressure across the turbine nozzle. As a gas expands it cools.



The reason that some people have had trouble with larger exhaust systems at high altitudes is due more to the efficiency of the compressor wheel and the air density at the intake manifold than due to exhaust modifications.



The key to optimum engine performance is air density at the intake manifold and the ability of the engine to breathe. Raise the volumeteric efficiency of the engine and turbo as a whole will make the engine run like it should.



Lower the back pressure on the turbine as much as possible, put a "properly" modified turbo on the engine to match the power output, and rpm range you want. "



There are a couple of very good 4" exhaust threads started by Ted Jannetty.



Here they are:



https://www.turbodieselregister.com/forums/showthread.php?s=&threadid=29611&highlight=exhaust



https://www.turbodieselregister.com/forums/showthread.php?s=&threadid=29612&highlight=exhaust



https://www.turbodieselregister.com/forums/showthread.php?s=&threadid=29610&highlight=exhaust