exhaust question...

[94black6.5]

howdy everyone i thought i was being a pioneer by putting a 5in exhaust on my truck but i think it may be too big anyone whink im losing sound maybe losing a lil power too? also in another post i saw a guys sig that has a custom 6setting tune any info on how the build was or how u did it and if so pics? i have been taking my truck out to pulls for fun because i have the only 6.5 for miles around here and people are surprised to see one i only pulled 176 last night bounced around and popped a tire im looking for a lil more oomf...input greatly apreciated thanx all

 

[Crankme69]

I do not believe the exhaust on a turbo diesel can ever be too large. Back pressure is supplied via the turbo.

What kind of air box are you running? Do you have boost & EGT gauges?

Where are ya located in ILL?

 

[buddy]

I have the normal 3" turbo downpipe to a 4" pipe, and once it gets past the crossmembers its 5" but immediatelly turns out before the rear wheel with a 6" tip. In my 2wd 1500 5" doesnt fit between the body and the xmembers, the 4" barely does.

It works great, like Crankme said, its hard to have too big because the turbo works off a differential pressure.

 

[94black6.5]

buddy ur the one with the multi chip how did u make it and info or u keeping it a secret? and thnx for the info i have stock flat airbox and im from flat rock il out in the middle of nowhere 0o i have a boost gauge no egt yet soon though when i get the att

 

[buddy]

I guess how I made the multi program chip it is not a real secret, I custom designed a PCB, selected the components and built them, and I custom program them with larger memory pieces. I sell them on ebay occassionally, you can PM me an email address if you want more information about them.

 

[saturntech95]

I wouldnt be so sure on the 5" exhaust. I know you want free flowing exhaust in a diesel but with a exhaust as large as 5" you are gonna loose velosity, that loss in velocity is gonna cause the preasure waves of the exhaust to bounce back inward and actually create more backpreasure. Now Im no sure if 5" is big enough to cause that but I know in a 6.0 ford 4" is all you want unless your getting to the 1000hp mark for that reason. So I would have to assume the same idea for our motors. Maybe porting and polishing the manifolds would increase your pre turbo velosity wich would help with post turbo velosity and give you lower EGTs and faster spooling?

As far as more power for pulling I would say play with your TDC offset for some more fuel and put a turbomaster on it to get a bit more boost. Deffinently get some videos of you pulling and add them to the pull section seeing as I am pretty much the only 6.5 owner I have ever seen post there. Im kinda curious on how you set up your truck and such.

 

[buddy]

Since about half of mine is 5" I can say it does really well.

Exhaust velocity would be irrelevant with a well laid out pipe. Like tight bends or how and where it exits and vehicle aerodynamics would be a factor. If you exit in a high pressure area then you want the exhaust to exit with the flow straight back or even create a venturi like effect with the tip slightly angled into the stream so it hits it and in the act of flowing around it and accelerating it creates a vacuum at the tip. We dont all have a cool diagram of our vehicle's aerodynamics and air flow around them at high speed, but you can make some general observations, like right behind a tire should be alright, but I dont know why a lot of systems point near perpendicular to the truck. I exit before the rear tire because I wanted to eliminate as much pipe as possible, because pipe itself creates backpressure, and the bending to get over the axle. velocity is a factor for just being able to exit the pipe, but if there is no velocity that should mean there is no pressure, so its not a problem in and of itself unless there is pressure because outside air flow is pushing back into the exhaust tip.

 

[EWC]

Sorry Buddy but I'm with saturntech on the pipe issue . Too big can be an issue even with a turbo . I'd like to see some tests on your system with a smaller pipe installed . If you extrapolate your reasoning , no pipe on the turbo would be best . Velocity means flow , flow means more energy through the turbo . Just look at the differences between the exhaust housings on the GM turboes . A correctly designed system with produce better results . This is not one area of bigger is better .

 

[buddy]

Well, you are indeed correct, no pipe on the turbo would be absolutely the best possible scenario for turbo performance.

You must realize for there to be velocity there has to be pressure. The absence of velocity simply means there is no backpressure. And I can tell you there is still plenty of velocity out of the 5" pipe. A turbo works off of pressure differentials, the lower the pressure on the outlet the better it will perform. Pulses in the exhaust are more of an issue with engine thats have a good amount of valve overlap, but the reason the exhaust being too big for them is for another reason where the velocity is used to create a pull on exahust in the cylinder when the valves are open. We have no overlap, and most turbo diesels will not, or at least not a whole lot.

 

[EWC]

Exhaust still has weight , that means energy to move it . A properly designed system might be able to help move the exhaust quicker . Your system actually does the exact opposite of what it should do . As the exhaust moves , it cools down and slows . Going from a smaller opening to a much larger opening , as the system lengthens , only slows the exhaust more . The engine now has to work against the turbo and the exhaust still in the pipe ie your pressure differential . How much does the pressure increase from the rear of the tailpipe , larger opening , to the turbo , smaller opening , over a standard straight pipe or a two step design ? This is atmospheric pressure that is basically working on a funnel and doing it the wrong way for exhaust flow . If you could decrease the exhaust drive pressure in a turbo system and still maintain boost , what are the consequences ? A properly designed system will help the turbo and you do need a pipe on the turbo for best performance imho .

 

[tanman_2006]

Exhaust still has weight , that means energy to move it . A properly designed system might be able to help move the exhaust quicker . Your system actually does the exact opposite of what it should do . As the exhaust moves , it cools down and slows . Going from a smaller opening to a much larger opening , as the system lengthens , only slows the exhaust more . The engine now has to work against the turbo and the exhaust still in the pipe ie your pressure differential . How much does the pressure increase from the rear of the tailpipe , larger opening , to the turbo , smaller opening , over a standard striaght pipe or a two step design ? This is atmospheric pressure that is basically working on a funnel and doing it the wrong way for exhaust flow . If you could decrease the exhaust drive pressure in a turbo system and still maintain boost , what are the conequences ? A properly designed system will help the turbo and you do need a pipe on the turbo for best performance imho .

I am not an expert but after reading your post it seems a turbo would work better w/o a pipe because it wouldnt have to use energy to push exhaust through a pipe. This would negate the need for velosity. Getting exhaust gas out from the engine bay would be the problem there (obviously).

94Black6.5: The member above that does these tunes does a great job. I am about 90% happy with mine and its nothing he did that i am unhappy with. Some of the options I requested are no longer needed and have gotten annoying. I will be getting a reflash after i get some mods done, IE: bigger tires, walbro LP, exhaust, possible turbo upgrade, as well as a boat load of cooling mods. I also plan to do some testing and will use a few settings for that. (i dont mention his name when talking about the custom tune because we know who it is and it just gets edited out anyways)

You should look up Buddy's recommended mods/ upgrade list. It is probably in a sticky or you can look up the thread.

 

[buddy]

Im sorry but that is incorrect, the larger pipe allows the same volume to flow at lower pressure. There is a direct corrlation between pressure and velocity. I can push way more volume, flow, out of larger pipe at lower pressure. You have to understand fluid dynamics. Velocity is of no consequence, pressure is, unless the system is designed so that velocity is required to create a scavenging, vavuum effect. The overlap on gas valve timing sometimes does require a certain velocity to perform correct scavenging.

My system is exactly how I want it. I push over 20 psi on my ATT and I can fairly readily reach 130mph, I think its doing quite alright. Granted you can do that with a 4" exhaust, but by this theory of velocity is the main factor we should all just have straight 3" pipes and that would be ideal, and its not.

edit: I will agree that velocity could have an effect if you have none or just so little that flow becomes turbulent, but I'm letting you know even at idle there is plenty of flow out for high velocity exhaust, becuause we have a high displacement engine, and it only gets faster with RPMs and boost as you flow more and more mass per minute. There is no time when the exhaust will be stagnant.

I tested my truck with varying exhaust setups and it got better with each step I took to this one.

 

[EWC]

My point is that there is a certain diameter for the exhaust pipe to be an optimal size for good performance and not just the bigger is better theory . If you have tested your truck and found that going to the larger size works for you and you are happy with the results , stick with the results . I do know a little about fluid flow , not alot but better than some .

 

[buddy]

94black, the stack probably is not ideal, with the tight bend and the climb. Kind of like an implementation where bigger is not necessarily better. You might do a little better just dumping it out under the truck during your pulls. The exhaust is overkill for a GMx turbo, and you could likely pull better if you got a bit bigger turbo that would take more advantage of the larger exhaust. Like an HX40, there are some of those with T3 flanges like an inexpensive knockoff on ebay called HX40WII.

How much boost do you run now?

 

[tanman_2006]

Buddy, my Case 8920 has a wastegated HX40 T3, Might look into that for future mod options. I have thought of stealing it off of my tractor (putting it on a truck) and upgrading the tractor when I tune the tractor for 250+ HP as it will be the bottle neck for cooling/egt's in this application anyways.

 

[94black6.5]

i run about 12 wide open boost wise and yah i was thinking about dumping the stack and putting a 6in tip exiting right before the rear axle and tanman ill try to get some vids of the pulls up i have only done one so far and got a shady 176 0_o but i did only weigh 6,000 pounds and my baby really didnt wanna build boost in 3rd to take off with the sled so i pulled in 2nd bouncing around and popped a front tire or she was going for alot more :/

 

[94black6.5]

sorry edit lol i ment saturn ill get some vids up not tanman :/

 

[buddy]

If youre still using the stock PCM program then 12psi is enough, but you can do way better with quite a bit more fuel output and boost at 15psi and up. Even with a larger turbo you could get to 15+ psi.

 

[Gunar]

Hello friends,
I have been driving for many years without a complete exhaust ... My exhaust consists of the Y-pipe which comes from the turbo and then goes into a pipe that shows behind the gear holder down ..needs the 6.5td a backlog so the engine runs better, or can a good exhaust system get realy something more out of power from the engine? You'll think now crazy German-but it´s nice to her the engine sounds ;-) ....

Greats
Gunar