What's the truth about exhaust and back pressure?

September 21, 201113 yr

Author

September 21, 201113 yr

Some great info in this thread, but i'll throw my .02 in there. The stock turbo on your 07 5.9 is so restrictive itself that unless there is a major malfunction with your exhaust you really will not tell a difference in performance with adding or removing components. The stock cat, in good working order, is very high flow and dyno testing has shown it to only rob 2-4 hp, and if it flows that good it will have no effect on low flow data, as in just off idle. In fact some dyno testing has shown that too much exhaust restriction will actually hurt bottom end performance becuase the power required to push the exhaust out is greater, thus slower moving exhaust and slower exhaust means a slower spinning turbine. The stock turbine wheel is only 2.36" across, so the exhaust back pressure has to really stack up to effect a 2.36" hole with a 4" one! It takes added feet of exhaust. The stock 4" exhaust on 3rd gen's is good for 600-700 rwhp without sacrificing performance. A 4" exhaust is really the best size for nearly all 5.9's, it matches the flow very well. A 5" exhaust will likely not effect performance or EGT's in any way, but will be louder and have the possibility of droning easier.So I would say stick with 4", and as far as the cat.. It's probably more of your butt meter wanting to see a change, and not seeing one so the results feel negative.

October 23, 201113 yr

Actually, the only place you want back pressure is between your exhaust and your turbo, once it exits the turbine side you want NO back pressure. And to the OP, the reason you have a sluggish low end is because it is a common rail, most of the older mechanical diesel had something "like" on demand torque being they spool up quickly, ' Add a smarty and add fuel on the low end and you will notice a huge difference, the stock programming was designed to save your transmission. I drive a large international 4400 around at work, and with a DT560 inside it i found the best ondemand torque you get out of that CR's is when you have a load on it

Not to call you out on anything but you can't really compare the DT570 fuel system to a common rail. While there is a common fuel supply to all the injectors in the electronic DT engines, they are a HEUI (Hydraulicaly actuated Electronically controlled Unit Injector) fuel system. Once again I'm not trying to step on your toes, but not an honest comparison IMHO.

October 24, 201113 yr

The only comparison was a general one because even my 08 acts the same way as the international doe. I was not going to get to indepth on the finer points

October 24, 201113 yr

Interesting read, but I would like to ask;Why does the manufactures of big rig diesel trucks (like Kenworth and such) put on exhaust like 6" stacks for a full exhaust of the gases ? The turbos outlets are not that large of an opening the last time I looked, yet they are over size exhaust ?Just wondering.

October 24, 201113 yr

Interesting read, but I would like to ask; Why does the manufactures of big rig diesel trucks (like Kenworth and such) put on exhaust like 6" stacks for a full exhaust of the gases ? The turbos outlets are not that large of an opening the last time I looked, yet they are over size exhaust ? Just wondering.

The pressure pre and post turbo outlet is very different. Our truck may have 30-50psi in the exhaust manifold and 1-3psi in the exhaust. The venturi drops the pressure and temp as it comes out the turbine wheel, but increases the velocity.

October 24, 201113 yr

The pressure pre and post turbo outlet is very different. Our truck may have 30-50psi in the exhaust manifold and 1-3psi in the exhaust. The divergent duct drops the pressure and temp as it comes out the turbine wheel.

I think he was asking why are there bigger exhaust pipes on the big rig trucks when there is no need as the guy stated with the dyno results personal I think there is a need for a larger exhaust about 4 inch for our trucks because it allows for better spool up of the turbo and cooler exhaust

October 24, 201113 yr

I think he was asking why are there bigger exhaust pipes on the big rig trucks when there is no need as the guy stated with the dyno results personal I think there is a need for a larger exhaust about 4 inch for our trucks because it allows for better spool up of the turbo and cooler exhaust

A 6" exhaust is not too big on a 900 cu in motor, even if the turbine wheel is only 3-4" across, like I said its allows for a pressure/temperature drop as the gasses leave the turbine. Too big and you do loose flow. A 4" is perfect for our motor, 5" is overkill until the 700+ hp range. An exhaust should have little to no effect on spool as that is all from the turbine housing and turbine wheel. Exhaust will have effect once spooled on the ability to move all the gasses.

October 24, 201113 yr

For the heck of it I went to Kenworth and looked up an engine 'somewhat' close to ours:

The PX-6:

6.7 liter

200 - 750 hp

520 - 750 # torque

here's something different:

VGT Turbocharger: PACCAR’s variable geometry turbocharger is both simple and precise. Electric actuation allows infinite adjustment, providing the exact amount of boost necessary for superior response, time after time.

High Pressure Common Rail Fuel System: The high pressure common rail fuel system supplies a precise mixture of fuel and air at a constant pressure, regardless of engine rpm. This timing flexibility allows multiple injection events per cycle, which maximizes fuel economy and performance while decreasing noise and exhaust emissions.

Cooled EGR: This next-generation exhaust gas recirculation system lowers combustion temperatures for reduced emissions and optimized fuel economy.

October 25, 201113 yr

A 6" exhaust is not too big on a 900 cu in motor, even if the turbine wheel is only 3-4" across, like I said its allows for a pressure/temperature drop as the gasses leave the turbine. Too big and you do loose flow. A 4" is perfect for our motor, 5" is overkill until the 700+ hp range. An exhaust should have little to no effect on spool as that is all from the turbine housing and turbine wheel. Exhaust will have effect once spooled on the ability to move all the gasses.

Yea but like th guy pointed out in the other three there is no difference between a 3 and a 4 inch exhaust. Why would a 700 up motor be different

October 25, 201113 yr

There are many people that have great benefits from a 4" on a 5.9 over the 3" on a 2nd gen and 3.5/4" on a 03-04. OEM is now 4" for a reason. The reason 700+ makes a difference is flow, more hp means more flow. It's why a 235hp 2nd gen has a 3" and a 325/350 hp 3rd-4th gen has a 4", hp takes fuel and fuel takes air and air needs to be able to escape in a decent fashion. On a stock 2nd gen or 1st gen a 4" may not make a noticeable difference but it makes a nice benefit on a 03-04.

October 25, 201113 yr

Owner

But even with my straight pipe 3" it still very manageable with Edge Comp and the RV275 Injectors which puts me right there with a CR Engine truck... :shrug:

October 25, 201113 yr

But even with my straight pipe 3" it still very manageable with Edge Comp and the RV275 Injectors which puts me right there with a CR Engine truck...

I bet you would benefit from a 4", its a great upgrade for a 2nd Gen like yours. Personally I beleive they went to a 4" on the 04.5 for 2 reasons. The first and main reason was the added size allows for a more rapid post-turbo EGT drop based on the 1450°+ stock EGT so that you aren't cooking the underside of the truck. The other reason is the CR's move a lot of air (despite the lousy cam/turbo) and the power is better with the 4".

October 26, 201113 yr

On cummins quickserve if I put in my CPL it shows no rating for exhaust size but the same engine in generator configuration is supposed to have 4" ran from it OEM. It's not like we don't see WOT just like a generator so somewhere it got reduced to 3", maybe just because of the service factor (when do you run WOT 24/7...). They could easily detune the engine so it wouldn't melt at WOT. The 3" would also cut down on drone (like 4" even has any but still). They do a lot of things as a selling point. Not everyone wants a truck with stacks and a turbo out the hood.. It's much better to make something for the common man who just wants a truck to pull his fishing boat to the lake and relax. For those wanting more, buy a cutting torch.

October 26, 201113 yr

Interesting read, but I would like to ask; Why does the manufactures of big rig diesel trucks (like Kenworth and such) put on exhaust like 6" stacks for a full exhaust of the gases ? The turbos outlets are not that large of an opening the last time I looked, yet they are over size exhaust ? Just wondering.

Depending on the manufacturer, most of the big(6 in. or bigger) stacks are just pretty much for looks. Most standar euipment stacks are either 4 or 5 in. I know of one particular semi that has 3 different sizes of exhaust pipe between the engine and the stack. It is 4 in. at the turbo and about 2 feet back it jumps to 5 in. It is 5 in. into the muffler and 6 in. out of the muffler, then splits into two 4 in. pipes then goes back to 5 in. for the actual stack. I honestly think that most manufacturers just use what works, meaning what they can get to fit and whatever is the cheapest. There is obviously a minimum that they have to use due to heat but as far as semis go, it is more a money game than anything else. Thos big shiny chrome stacks sell trucks, and about 9 times out of 10, if you see a truck with stacks bigger than 5 or possibly 6 inch, they were put on after the truck was bought and it is an owner/operator's truck and not a company owned rig.

October 26, 201113 yr

For years I have heard so many stories on this topic and as many personal testimonies, (mine included) on this topic. (I personally have shown an increase of mpg since installation of a larger exhaust, but have 'NO' actual technical data as to proof And, I could have done something else, like a slight change in driving habit I didn't notice.) And I have yet heard any actual documented engineering reports that can quantify the advantage of increasing or not increasing the stock size of the exhaust. I have Google this on the net, looked in the library, read many website pertaining to diesel engines, discussed this with 2 engineers I use to work with ... and nothing.I do know for a fact, from engineers, that they are told to figure out the 'minimum' amount of anything to save money, so a company can increase profit. Add to the fact that the EPA wants 'perfect air quality' that is NOT achievable in real life makes this more confusing. Does anyone have ACTUAL data on this topic ?

October 26, 201113 yr

I have never found these numbers for land enignes, but a 5.9 marine isn't moving a crazy amount more/less air for a given boost/rpm. 230 hp 3000 rpms 22 psi of boost has 575 CFM in 1275 CFM out380 hp 3000 rpms 31 psi of boost has 724 CFM in 1677 CFM out425 hp 3000 rpms 35 psi of boost has 793 CFM in 1990 CFM out480 hp 3400 rpms 35 psi of boost has 932 CFM in 2174 CFM outSo now look at flow rates of mufflers and see what it takes to not have a restriction. I run a low restriction 40" Donaldson an it's has a 1800 CFM rating at .5 psi restriction and 2546 at 1 psi of restriction. I figure I move about 1300-1800 under normal towing.

October 30, 201113 yr

The nice thing about a 4 in on a near stock truck is that you don't have to 'manage' your temps. It just won't get very hot.The original question was about back pressure. You don't need or want back pressure with a diesel because the fuel dosn't have to be drawn into the cylinders. On gas engines without direct injection back pressure pulses are used to draw the mix into the cylinders. It is really effective on two stroke race engines.

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