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LB7 Turbo Downpipe

HoytBows

HoytBows

Member
Messages
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Ex-Michigander, now Elizabeth Colorado
Its my understanding that nobody makes a larger downpipe for the turbo on my truck. First question is why. Second question is somebody going to in the future or has somebody heard of anybody fabricating their own. If the answer to all these questions is no, is the only answer a bigger turbo?

The whole idea of running a 4" or bigger exhaust behind a dinky down pipe bothers me. Seems a little pointless
 
Diamond Eye's downpipe is only 3".

Part Number: 321056

The problem with the down pipe is simply the space. There isn't enough clearance the run a 4" downpipe (at least without it being a little custom) in the stock location.

Even swapping the down pipe without removing the turbo is an act of contorsion.
 
i honestly dont see it making much of a power gain to justify 700 dollars. you could buy efi live for that and get a butt load more power even on the stock exhaust then with the down pipe.
 
I believe they said that an upgraded downpipe won't make much of a difference so long as you're running the stock turbo. The stock downpipe can easily support whatever it can flow.
 
Here's the problem with doing that.....

Your exhaust is only as efficient as the smallest restriction.

The exhaust needs to be the largest where it's the hottest and moving the fastest, which is right at the turbo.

After that, the exhaust cools and condenses extremely fast so stepping up to the 4" beyond that is kind of a waste. It actually needs to get smaller to maintain efficiency. If it was 4" off the turbo and then stepping to a 3.5" after the muffler, you'd be better off.

Without having it as a true 4" (or at least 3.5") you're not making any significant gain for the money you spend.

And yes, a TRUE 4" system will make an impact. If you look at the voume of air that a 6.6L, forced induction motor can move, it FAR exceeds what a 3.5" exhaust can support on the upper end.

Even my 5.9 is over the volume of what a 4" can flow on the top end, and that's with a stock turbo.
 
Cumminalong said:
Here's the problem with doing that.....

Your exhaust is only as efficient as the smallest restriction.

The exhaust needs to be the largest where it's the hottest and moving the fastest, which is right at the turbo.

After that, the exhaust cools and condenses extremely fast so stepping up to the 4" beyond that is kind of a waste. It actually needs to get smaller to maintain efficiency. If it was 4" off the turbo and then stepping to a 3.5" after the muffler, you'd be better off.

Without having it as a true 4" (or at least 3.5") you're not making any significant gain for the money you spend.

And yes, a TRUE 4" system will make an impact. If you look at the voume of air that a 6.6L, forced induction motor can move, it FAR exceeds what a 3.5" exhaust can support on the upper end.

Even my 5.9 is over the volume of what a 4" can flow on the top end, and that's with a stock turbo.
Click to expand...
not bustin balls, but show me some dyno numbers on an otherwise stock truck to support that theory. and justify 700 bucks. the diamond eye pipe is about as big as a dmax will hold without major modification and most every report is it helps temps, but doesnt really effect power until upper rpms. rpms most guys on stock internals will never see. this is on tuned or non tuned trucks.
 
stacks04 said:
not bustin balls, but show me some dyno numbers on an otherwise stock truck to support that theory. and justify 700 bucks. the diamond eye pipe is about as big as a dmax will hold without major modification and most every report is it helps temps, but doesnt really effect power until upper rpms. rpms most guys on stock internals will never see. this is on tuned or non tuned trucks.
Click to expand...


On an otherwise stock truck, you'll never notice it.

But yes, that is the theory.

Here's a little redneck thermodynamics for you, tell me where I'm flawed.

Hot air under pressure = expanded volume and high velocity, which means a necessity for increased pipe diameter so as not to increase backpressure.

Cooler air under decreasing pressure = condensed and lower velocity, which means you need a smaller diameter pipe to maintain the same velocity.

The key to the exhaust is maintaining the velocity and not creating so much space that the air tumbles in the exhaust, causing backpressure. At the same time you need to have enough pipe to keep from causing a restriction.

Dyno numbers....got 'em.....742 HP / 1456 lbs of torque. My exhaust goes from 5" from the turbo to 4" at the muffler outlet.

A 4" exhaust at the turbo on my truck would choke it. 4" pipe can support 1700 - 1900 CFM of air, depending on the temp. At wide open throttle and full boost, I'm pushing 2400 - 2600 CFM of air.

The shorter the exhaust, the less you need to step down the pipe.

1200 degrees at the turbo is only 600 - 700 degrees at the tip.

And $700?....you never asked me for a price.
 
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So what im kinda getting is that it would help to have a larger downpipe if my truck was really hopped up. But since it isnt anything crazy I wont be able to notice much of a difference. Which makes me wonder how much better a 4" system will be than my stock straight pipe....
 
Couple the 4" Exhaust with the Downpipe and you WILL Feel a Difference.

...and I'm not Kockin "Cumminalong" but a Straight 6 compared to a V8 is a Totally Different Comparison.

Not only will you Feel a Difference but you'll See an EGT Drop as well, and a Sound Difference too.
 
Cumminalong said:
On an otherwise stock truck, you'll never notice it.

But yes, that is the theory.

Here's a little redneck thermodynamics for you, tell me where I'm flawed.

Hot air under pressure = expanded volume and high velocity, which means a necessity for increased pipe diameter so as not to increase backpressure.

Cooler air under decreasing pressure = condensed and lower velocity, which means you need a smaller diameter pipe to maintain the same velocity.

The key to the exhaust is maintaining the velocity and not creating so much space that the air tumbles in the exhaust, causing backpressure. At the same time you need to have enough pipe to keep from causing a restriction.

Dyno numbers....got 'em.....742 HP / 1456 lbs of torque. My exhaust goes from 5" from the turbo to 4" at the muffler outlet.

A 4" exhaust at the turbo on my truck would choke it. 4" pipe can support 1700 - 1900 CFM of air, depending on the temp. At wide open throttle and full boost, I'm pushing 2400 - 2600 CFM of air.

The shorter the exhaust, the less you need to step down the pipe.

1200 degrees at the turbo is only 600 - 700 degrees at the tip.

And $700?....you never asked me for a price.
Click to expand...

those numbers mean absolutely nothing to me. i might be just a tad crazy in saying this, buttttttttttttt i am fairly certain your NOT using the oem turbo in that truck. i am not knockin the performance your getting out of your truck, it is very impressive, but to ever think of reaching those numbers on a stock cummins with stock turbo..... you can put a 10" down pipe on the lb7 turbo, the power to price ratio will never justify 700 bucks. (the link steve cole posted).


here are the numbers from a tuned dmax. on a newer vvt truck.

"Made same peak HP as stock pipe on Wendy. But at 3100 rpm starting to show gains. The higher you spin it, the more the gains.

At 3350rpm the Diamond Eye makes 519rwhp
At the same RPM, the stocker makes 503rwhp

So up top, the Diamond Eye will make over 15rwhp gain.

Move your shift points up to about 3400-3500 to get the most advantage."

like i said, unless your turning past the rev limiter regularly, the down pipe does not do much on a stock truck by way of power. temps all around do get reduced so for a tow truck it is a diamond eye for 350 is a worthy investment.
 
Mad Maxx said:
...and I'm not Kockin "Cumminalong" but a Straight 6 compared to a V8 is a Totally Different Comparison.
Click to expand...

Engine configuration makes ZERO difference to an exhaust pipe. After your exhaust goes into the pedestal and spins the turbo, how many places does it exit the turbo? 1. After mine goes into the turbo and exits, how many places does it come out? 1.

Pipe is pipe, there is only so many ways to make, bend or shape it.

Engine volume and total CFM are what dictate exhaust size. I don't care if it's a 12L truck engine or a 2L 4 cylinder, an engine must move a given amount of air. That's all it is, an air pump.

That air pump is only as efficient as the smallest restriction. If that restriction is right at the turbine discharge, the rest of the system has no impact, unless there is more of a restriction farther down stream (i.e. a muffler)

Perfect example is a funnel. Take a gallon of water in a bucket and pour it out....it dumps right out. Pour that same gallon of water into a funnel....what happens? It slows down because it has to go through the restriction. Take that same funnel and attach a 4" diameter hose at the end and pour the water into it......Did the water pass through the funnel any faster? Nope.

Same concept with your exhaust. If you make a restriction, the only way to move the same volume through the smaller opening is to increase pressure (i.e. back pressure).

Figuring out the RPM range, total CFM, expected exhaust temperature is how you figure out what size piping you need.

And no, my truck is FAR from stock. What is on my truck is sized for the anticipated air flow requirements.

On a stock 5.9, a full 4" exhaust works for 3/4 of the RPM range, but at the top (even on a stock truck) it's at it limits. Increase the size of the engine (i.e. 6.6L) or increase the turbo size to a 66 or larger and you're far exceeding what a even a full 4" system can handle on the top end.

Bottom line is you can make A SECTION of an exhaust larger, but if a previous section is smaller, it's of no benefit. Unless you can get rid of that restriction in the top of that pipe, adding a 4" exhaust is pointless. The best thing to get rid of is the muffler.

Could this go on and on? Absolutely. The only way to see what it really does is to do a before and after dyno. I think you'll be highly dissapointed in what you get for that amount of money.
 
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Cumminalong said:
Engine configuration makes ZERO difference to an exhaust pipe. After your exhaust goes into the pedestal and spins the turbo, how many places does it exit the turbo? 1. After mine goes into the turbo and exits, how many places does it come out? 1.

Pipe is pipe, there is only so many ways to make, bend or shape it.

Engine volume and total CFM are what dictate exhaust size. I don't care if it's a 12L truck engine or a 2L 4 cylinder, an engine must move a given amount of air. That's all it is, an air pump.

That air pump is only as efficient as the smallest restriction. If that restriction is right at the turbine discharge, the rest of the system has no impact, unless there is more of a restriction farther down stream (i.e. a muffler)

Perfect example is a funnel. Take a gallon of water in a bucket and pour it out....it dumps right out. Pour that same gallon of water into a funnel....what happens? It slows down because it has to go through the restriction. Take that same funnel and attach a 4" diameter hose at the end and pour the water into it......Did the water pass through the funnel any faster? Nope.

Same concept with your exhaust. If you make a restriction, the only way to move the same volume through the smaller opening is to increase pressure (i.e. back pressure).

Figuring out the RPM range, total CFM, expected exhaust temperature is how you figure out what size piping you need.

And no, my truck is FAR from stock. What is on my truck is sized for the anticipated air flow requirements.

On a stock 5.9, a full 4" exhaust works for 3/4 of the RPM range, but at the top (even on a stock truck) it's at it limits. Increase the size of the engine (i.e. 6.6L) or increase the turbo size to a 66 or larger and you're far exceeding what a even a full 4" system can handle on the top end.

Bottom line is you can make A SECTION of an exhaust larger, but if a previous section is smaller, it's of no benefit. Unless you can get rid of that restriction in the top of that pipe, adding a 4" exhaust is pointless. The best thing to get rid of is the muffler.

Could this go on and on? Absolutely. The only way to see what it really does is to do a before and after dyno. I think you'll be highly dissapointed in what you get for that amount of money.
Click to expand...

I'm sure you're right, and know alot more about it than I do, as proved by your Truck, but it just doesn't make Sense to me that it would all be equal across the Board now matter the Engine Size.

Could be having a Brain Fart and not seeing the Obvious, or just don't "get it".
 
It's not that it's equal across the board.

What I meant was that a given engine size, pushing a given CFM of air, needs a given size pipe.

Whether it's 1.5" on a 2L four cylinder or 7" for a 15L truck engine.

Now, because all of our stuff is turbo'd you have to add that to the calculation and have to consider EGT's as the exhaust is hot AND under pressure.

The hotter the gas, the more it expands and requires greater volume. If the pipe can't handle the volume, then you've created back pressure.

Here's a project we have in the shop right now.....

This is a '70 Chevelle with a 454 stroked to 496 that we're dropping a Garrett 4718 turbo into. We've custom fabricated everything on the the exhaust from the flanges, to the primaries, to the collectors, pedestal, intercooler piping, exhaust, etc.

How big do you think the exhaust needs to be on this engine?

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Believe me, the math that goes into calculating an efficient exhaust is rediculous. Throwing a set of pipes at something is easy, getting it right takes math.
 
Well I hate to disagree with you but the gas cannot expand in a fix size pipe so what happens is the pressure rises as does the velocity. Now as the exhaust cools it becomes denser and also becomes harder to move which again raises the pressure required to push it along. So now according to you were to make the pipe large in the front where you have high velocity (at the turbine exit)which would allow the air to expand and slow down then make it smaller to push the denser exhaust into. Sorry but that's not going to work properly at all. The drive pressure in the turbo is going to rise due to the exhaust pressure rise your going to create if you were to do it your way. You need to look at the whole thing just not one part of the math or it doesn't work. Its a give and take of what you can do and what you cannot. As far as the downpipe goes when installed on a stock motor with tuning at 450 RwHp the TTS Kit gained 10 Hp and dropped 100 degrees EGT. So for something that you say will not work, something is really wrong with the way your trying to use the math.

As far as the price you need to look at what your getting as the two parts are not near the same. The cheap one is just a mild steel downpipe. It still uses the stock pipe connection which is about 2 1/2" ID. The TTS Kit is Stainless Steel and includes a 4" Stainless front pipe, a 4 1/2" stainless vband clamp and the heatshield for the down pipe. So your not comparing apples to apples at all.
 
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