6.7 Twin Turbo Setup

Wayne · Feb 3, 2014

also keep in mind that the further you have to push the exhaust, the greater back pressure you will have, especially when it comes to bends. An example might be my 6.4 vs. JD's. he has a 5" exhaust, which goes from 4-5 after the trans cross member. mine is 4 all the way back. I plan on doing a similar 5" system on my 6.4 excursion conversion to reduce the back pressure as it goes over the rear axle, and out the side of the vehicle. After seeing the velocity of my truck's exhaust on the dyno, I'm convinced it will be better in the power department going this way. On VGT turbos, a little back pressure is a good thing after the turbo(s) to help stabilize the control ring inside the turbine housing. On big power trucks, it almost seems like you want to do whatever you can to reduce back pressure. One example of this might be when I stopped in at the bully dog HQ for some stuff back in '07 when one of the employees had is truck ('05ish cummins) on the dyno. He just switched back from dual 7" bed stacks (5" bends) to a single 5" out the back. His peak power went from about 670 to @820 rwhp with just the exhaust change. The power curves were near identical until about 2600 rpms when the stack setup began to fall off. The reason he suspected was turbulence/back pressure in the 3 different 90* stack bends.

4EverBoosted1 · Feb 3, 2014

So it looks like there is going to be a lot of back pressure in that custom bed exhaust with all those bends and direction changes.

Six_Sloww · Feb 3, 2014

Wait.. What? He gained 150 hp going from 7" stacks to a 5" over the axle? Did I read that completely wrong or is that actually what you meant?

WHY NOT · Feb 3, 2014

Six_Sloww said:
Wait.. What? He gained 150 hp going from 7" stacks to a 5" over the axle? Did I read that completely wrong or is that actually what you meant?

it was 5" threw the bed and 5" bends

Six_Sloww · Feb 3, 2014

Essentially 5" vs 5" just bent differently, I understand that... 150hp?

97stoker350 · Feb 3, 2014

So if what ur saying is right then a hood stack should be worth 200hp because it's hardly no bends?

ChattyCathy · Feb 3, 2014

Wayne said:
also keep in mind that the further you have to push the exhaust, the greater back pressure you will have, especially when it comes to bends. An example might be my 6.4 vs. JD's. he has a 5" exhaust, which goes from 4-5 after the trans cross member. mine is 4 all the way back. I plan on doing a similar 5" system on my 6.4 excursion conversion to reduce the back pressure as it goes over the rear axle, and out the side of the vehicle. After seeing the velocity of my truck's exhaust on the dyno, I'm convinced it will be better in the power department going this way. On VGT turbos, a little back pressure is a good thing after the turbo(s) to help stabilize the control ring inside the turbine housing. On big power trucks, it almost seems like you want to do whatever you can to reduce back pressure. One example of this might be when I stopped in at the bully dog HQ for some stuff back in '07 when one of the employees had is truck ('05ish cummins) on the dyno. He just switched back from dual 7" bed stacks (5" bends) to a single 5" out the back. His peak power went from about 670 to @820 rwhp with just the exhaust change. The power curves were near identical until about 2600 rpms when the stack setup began to fall off. The reason he suspected was turbulence/back pressure in the 3 different 90* stack bends.

no ***in way wayne

bigrpowr · Feb 3, 2014

Wayne said:
also keep in mind that the further you have to push the exhaust, the greater back pressure you will have, especially when it comes to bends. An example might be my 6.4 vs. JD's. he has a 5" exhaust, which goes from 4-5 after the trans cross member. mine is 4 all the way back. I plan on doing a similar 5" system on my 6.4 excursion conversion to reduce the back pressure as it goes over the rear axle, and out the side of the vehicle. After seeing the velocity of my truck's exhaust on the dyno, I'm convinced it will be better in the power department going this way. On VGT turbos, a little back pressure is a good thing after the turbo(s) to help stabilize the control ring inside the turbine housing. On big power trucks, it almost seems like you want to do whatever you can to reduce back pressure. One example of this might be when I stopped in at the bully dog HQ for some stuff back in '07 when one of the employees had is truck ('05ish cummins) on the dyno. He just switched back from dual 7" bed stacks (5" bends) to a single 5" out the back. His peak power went from about 670 to @820 rwhp with just the exhaust change. The power curves were near identical until about 2600 rpms when the stack setup began to fall off. The reason he suspected was turbulence/back pressure in the 3 different 90* stack bends.

sorry wayne, but this is some of the dumbest crap i have ever read, in my life.

Six_Sloww · Feb 3, 2014

It's cummins math... It all makes sense now

Powerstroke Cowboy · Feb 3, 2014

agreed.

WHY NOT · Feb 3, 2014

He left out the part about adding 3 Giant "C" stickers between runs too, maybe even flipped the mirrors out

Josh@DirtyDiesels · Feb 3, 2014

Wayne said:
also keep in mind that the further you have to push the exhaust, the greater back pressure you will have, especially when it comes to bends. An example might be my 6.4 vs. JD's. he has a 5" exhaust, which goes from 4-5 after the trans cross member. mine is 4 all the way back. I plan on doing a similar 5" system on my 6.4 excursion conversion to reduce the back pressure as it goes over the rear axle, and out the side of the vehicle. After seeing the velocity of my truck's exhaust on the dyno, I'm convinced it will be better in the power department going this way. On VGT turbos, a little back pressure is a good thing after the turbo(s) to help stabilize the control ring inside the turbine housing. On big power trucks, it almost seems like you want to do whatever you can to reduce back pressure. One example of this might be when I stopped in at the bully dog HQ for some stuff back in '07 when one of the employees had is truck ('05ish cummins) on the dyno. He just switched back from dual 7" bed stacks (5" bends) to a single 5" out the back. His peak power went from about 670 to @820 rwhp with just the exhaust change. The power curves were near identical until about 2600 rpms when the stack setup began to fall off. The reason he suspected was turbulence/back pressure in the 3 different 90* stack bends.

That seems a bit wild of a claim, but I can definitely feel a difference going from my hoodstack to a 3.5" dp with 4" exhaust...

Dan · Feb 3, 2014

I agree gains from 4"-5" on high hp rigs is noticeable, and from what I have read, the down pipe is, say, the most important part.

We'll be doing a 5" down pipe into the situation that's in the bed which is all 5" piping

Exhaust restriction IMO will be at a minimum

Wayne · Feb 3, 2014

dzlpwr said:
That seems a bit wild of a claim, but I can definitely feel a difference going from my hoodstack to a 3.5" dp with 4" exhaust...

I figured it was a bit wild too until I got the guy to show me the dyno graphs from earlier that day. At the higher velocity/cfm/HP, the turbulence caused a lot of restriction though. 5 sharp 90* bends added a lot of extra turbulence to the mix. It would have been interesting to throw a pressure gauge after the turbo with before & after results to see how it worked out.

Powerstroke Cowboy · Feb 3, 2014

So you are saying there were five 90* bends in the exhaust??

Jbenso127 · Feb 3, 2014

David@BSDP said:
So you are saying there were five 90* bends in the exhaust??

One at the 90* under the bed, 2 at the Y pipe and 2 going into the actual stacks... I'm guessing?

Powerstroke Cowboy · Feb 3, 2014

Jbenso127 said:
One at the 90* under the bed, 2 at the Y pipe and 2 going into the actual stacks... I'm guessing?

Should only need two 90* bends for a stack kit. That is if it done right.

Five I could see where it messes things up.

Have you ever watched a truck with twin stacks start up in could weather? The stack closest to the engine will eject most of the exhaust and the other one does not do much. That is because of poor design. I am talking semi trucks not a pickup. Or even see one on a hill or taking off and most of the smoke goes out just one pipe?? Poor design. That must have been the case here.

Jbenso127 · Feb 3, 2014

David@BSDP said:
Should only need two 90* bends for a stack kit. That is if it done right.

Five I could see where it messes things up.

Have you ever watched a truck with twin stacks start up in could weather? The stack closest to the engine will eject most of the exhaust and the other one does not do much. That is because of poor design. I am talking semi trucks not a pickup. Or even see one on a hill or taking off and most of the smoke goes out just one pipe?? Poor design. That must have been the case here.

Something like this is what I was referring to... I count 5 :shrug:

mustube · Feb 3, 2014

Any single exhaust particle would only take 3 in that picture, so I'd say 3 not 5.

Powerstroke Cowboy · Feb 4, 2014

Jbenso127 said:
Something like this is what I was referring to... I count 5 :shrug:

yea one more good reason to not like them..

In that picture I could count 5 to.. That is a poor design. Very easy to install and clean but for flow, poor design. As the air leaves the smaller pipe and enters the big pipe, That in itself will help make the expand to the outside then swirl towards each other in the middle and cause loss of flow. Then as it doing that it hits the "Y" pipe and really messes up the flow path. I could see it taking the already swirling air and speeding up the swirl when it hits the "Y" pipe. Air that swirls will slow down the air behind it and will cause a restriction. That "Y" pipe looks like it meant to swirl the air. If you put a Aero Turbine muffler in each side it would help it a lot. Would help "suck" the air out and smooth air flow back out plus make it quieter.

That make a trucks exhaust look nice and smooth!