4" exhaust or 3" performance muffler?

[JGort]

Stacks

Can't do the stacks because I have a Lance 11' 3", but thanks for the idea!



Think I'm lookin' at doin' the cat-back system. I know I don't have a Cat so I'm guessing mine will actually be a Resonator back, but without the Resonator. Will it help at all to go 4" from where I remove the resonator back? I've seen 4" turbo back systems (Jardine, JRE) are there any 4" cat-back or "get rid of the resonator"-back systems? Are you tired of my questions yet? Sorry!

 

[Fred Swanson]

BFC, add more fuel, then you'll hear it even more. The sound is awesome with some serious HP. That way driving normal it stays quieter, get on it, and it howls.

 

[FFutch]

Catback

Hey J, I'll probably have your 4" cat back after this weekend after I install the stacks. It is Bully Dog stuff. Muffler and tailpipe. I'll see how the stacks go and probably sell the stuff reasonable. All you would probably need is an adapter. Just a thought. Doug

 

[JTroiano]

Mike, I have never heard that a 3" has quicker spool up than a 4". Can you expalin why, so I don't waste my money on a 4" system.

Jon

 

[MikeR]

Troiano,



In my post above, I explained why it happens. Please re-read it.



See, the thing most guys get confused about when they change the stock exhaust is flow. The factory muffler system is very restricted in flow, and that's why the turbo seems slow to perform. So when they install a 4" free-flowing exhaust system, they now have turbo performance. In actuality, you can get the same performance with the stock 3" pipe by just installing a free flowing muffler, or no muffler at all. Of course this is with stock to moderate hp levels.



If you want a demonstration of back pressure and force, do this test on yourself... .

Put your hand 3" in front of your face and with your lips open, blow hard and feel how much air pressure is against your hand.

Now close your lips to about the size of a nickel and blow hard against your hand again. Did you notice how much more pressure there was then?



Your stock sized lungs moved the same amount of air both times, but made more pressure with the smaller opening. To me this is a good example of stock or moderate power.

Now if your lugs were twice as big moving twice the volume, then the bigger opening becomes the better choice. Right? Do I seem off in my example?



Of course I'm not here to tell anyone how to spend their money. But I'll try and give the best explanation on why something works or doesn't.



-Mike

 

[JMarkos]

One thing to consider if you are planning on going with bigger injectors the smoke will be very very very noticeable, with the 4" my smoke was almost cut in half.

 

[Diesel Freak]

hmmmm

This has always been a hot topic.



I agree that untill you are over 350 RWHP you probably do not need a complete 4" exhaust system, 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" 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.

 

[Cooker]

Diesel Freak,



Excellent info, although I will have to read through it a few more times to understand it completely

 

[JTroiano]

My head hurts trying to understand. It sounds like Diesel Freak and MikeR are saying two different things? I think I'll just get the 4'' system and see if it feels different. I think If I put in DD2's with the Comp I sould be around 350 HP.

Jon

 

[Ramafid]

FYI when I replaced the stock exhaust with the Jardine 4" I gained a solid 3 lbs of boost. I don't know how much performance this translates to but I was happy with the end result. Everything I have read about turbo's says less back-pressure equals more power and better fuel economy.