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femno

Rb26 Low Mount Tubular Manifolds - Do They Make Any Difference?

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23 minutes ago, Lithium said:

So what benefit do you think there may be in having a lower flow rate?

And yeah, very interesting stuff.   Hoping we can try some stuff out with a design on a car we're doing, but the problem is there won't be any comparisons - it will just be part of an overall setup so hard to know what will contribute to what.  

I don't think there would be a benefit from lower flow rate, although im not sure. However lengthening the runners may have a benefit via timing exhaust pulses and scavenging?

That is the trouble most of the time right, we try different modifications but dont always have the time or money to do direct back to back comparisons! It doesn't work comparing different cars with different setup, because there are far to many variables. 

Ah well, hopefully one day someone will on here and post the results for all to see!

Edited by Ben26

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35 minutes ago, Ben26 said:

Yep, understood and agreed with both of you, diameter of the runners has more influence on the pipe velocity, however flow rate varies dramatically with length of the pipe. Half the length of a pipe and expect double the flow rate. 

All very interesting, now who has the budget and time to test all of this with different length runners and pipe diameters on the same turbo setup and see what happens!

I haven't found in any of my engineering texts how halving the length doubles the flow. 

It doesn't change flow rate at all funnily except for drag on the walls, and temperature drop and some other minor parts. 

Flow rate is in metres/second, and then you multiply by the cross sectional area, and you have the volumetric flow rate.  Note how NONE of it looks at the LENGTH? 

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This is the hagen-poiseuille equation. Am I interpreting wrong?

 

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10 minutes ago, Ben26 said:

This is the hagen-poiseuille equation. Am I interpreting wrong?

 

That's for calculating a pressure differential. 

Do you know what the pressure difference is From exhaust valve to turbo? 

2/5ths of nothing. Which means that equation won't apply for altering the flow rate. 

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I disagree mate, if you rearrange for Q image.png.aab03af4077865e77465912e9942eba5.png. Ie they are inversely proportional if all else stays the same.

Maybe I'm missing something but I just typed pipe length vs flow rate into google and this was the answer as well "Flow rate varies inversely to length, so if you double the length of the pipe while keeping the diameter constant, you'll get roughly half as much water through it per unit of time at constant pressure and temperature."

 

Edited by Ben26

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23 minutes ago, MBS206 said:

That's for calculating a pressure differential. 

Do you know what the pressure difference is From exhaust valve to turbo? 

2/5ths of nothing. Which means that equation won't apply for altering the flow rate. 

Yep.

Sure, double the length will double the loss due to length, but the changes in direction, interference etc would be a far more significant cause of losses than these relatively short lengths. Or so my intuition (and only intuition, no actual data) tells me.

 

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21 minutes ago, Ben26 said:

I disagree mate, if you rearrange for Q image.png.aab03af4077865e77465912e9942eba5.png. Ie they are inversely proportional if all else stays the same.

Maybe I'm missing something but I just typed pipe length vs flow rate into google and this was the answer as well "Flow rate varies inversely to length, so if you double the length of the pipe while keeping the diameter constant, you'll get roughly half as much water through it per unit of time at constant pressure and temperature."

 

Yes, but pressure differential can be regarded as 0, there flow flow rate regarding pressure change = 0 change in the runner. 

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That last part talking about water in the pipe, has a pressure differential that is quite dramatic to calc it's flow. 

Exhaust gas flow doesn't At that point in the system. 

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1 hour ago, Ben26 said:

Half the length of a pipe and expect double the flow rate

Oh heck! I didn't even read that.

It's absolutely not true. Not even close. The length of the manifold runner contributes a TINY percentage of the total resistance to flow in the inlet-engine-exhaust-turbo-exhaust flow path. Even if you ignore everything upstream of the exhaust valve, the fraction of the resistance coming from the runner length is almost nil.

The only time that the quoted statement is remotely close to being true is when there is the pipe and nothing but the pipe. Like a pipe on the outlet of a fan. And even then it's not true, because there are fixed entry and exit pressure drops associated with any pipe. Time for a thought experiment. Blow through a 6" long drinking straw. Measure the flow. Now do it with a 12" long straw. You won't have half the flow rate.

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7 minutes ago, MBS206 said:

Yes, but pressure differential can be regarded as 0, there flow flow rate regarding pressure change = 0 change in the runner. 

If the pressure differential was 0, wouldn't that also mean the flow rate is 0? That's what that equation says. 

I'm happy to be wrong and learn something new, but going by that equation and researching pipe flow vs length, thats what im reading!

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4 minutes ago, GTSBoy said:

Oh heck! I didn't even read that.

It's absolutely not true. Not even close. The length of the manifold runner contributes a TINY percentage of the total resistance to flow in the inlet-engine-exhaust-turbo-exhaust flow path. Even if you ignore everything upstream of the exhaust valve, the fraction of the resistance coming from the runner length is almost nil.

The only time that the quoted statement is remotely close to being true is when there is the pipe and nothing but the pipe. Like a pipe on the outlet of a fan. And even then it's not true, because there are fixed entry and exit pressure drops associated with any pipe. Time for a thought experiment. Blow through a 6" long drinking straw. Measure the flow. Now do it with a 12" long straw. You won't have half the flow rate.

Hey gtsboy. Im not talking about resistance, only what the hagen-poiseuille equation says regarding flow rate and length of a pipe. 

image.png.aab03af4077865e77465912e9942eba5.png

Of course I was assuming all else is remaing constant, as I previously mentioned, so if there are other variables in the formula changing than that would affect the relationship between flow rate and length of the pipe. 

Hmm thats an interesting example but something that to me cant be quantified without measurement in my mind. 

Awesome that we can have this discussion!

 

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4 minutes ago, Ben26 said:

If the pressure differential was 0, wouldn't that also mean the flow rate is 0? That's what that equation says. 

I'm happy to be wrong and learn something new, but going by that equation and researching pipe flow vs length, thats what im reading!

Read my reply above, then stop. You are trying to work with just one element in the middle of a system. If you don't consider the whole system, you come to the wrong conclusions.

Pressure drop (loss of energy) in pipe flow stems from internal friction (viscous effects in the fluid flow) and from wall friction. Add fittings (bend, tees, orifices, changes in section, etc) and you get more of each of those. All governed by some ridiculous equations (Bernoullis, etc) that are effectively insoluble for real people with calculators. We don't even solve the real equations when we're doing CFD unless it's a simple problem and you have an enormous computer available. In CFD we use dramatically simplified models to do the turbulence calculations

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3 minutes ago, Ben26 said:

Im not talking about resistance

Yes you are. The Hagen-Poiseville equation ignores entry and exit effects, etc, as I said before.

Pressure and pipe resistance are basically equivalent to voltage and electrical resistance. You get a reasonably simple relationship where the flow is directly proportional to the driving force and inversely proportional to the resistance. The H-P equation just wraps up the diameter to area relationship along with the other factors in one equation. It actually hides a lot of thinking. R^4 for example, is actually R^2, just mentioned twice. One of them is the pipe area considering its effect on the flow capacity (the bigger it is the more it will flow). The 8 comes from a number of divide by 2s. The L is only the bottom line of the equation because the longer the pipe, the higher the resistance to flow. But as I said, the pipe does not exist in isolation.

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13 minutes ago, GTSBoy said:

Read my reply above, then stop. You are trying to work with just one element in the middle of a system. If you don't consider the whole system, you come to the wrong conclusions.

Pressure drop (loss of energy) in pipe flow stems from internal friction (viscous effects in the fluid flow) and from wall friction. Add fittings (bend, tees, orifices, changes in section, etc) and you get more of each of those. All governed by some ridiculous equations (Bernoullis, etc) that are effectively insoluble for real people with calculators. We don't even solve the real equations when we're doing CFD unless it's a simple problem and you have an enormous computer available. In CFD we use dramatically simplified models to do the turbulence calculations

I am considering the whole system. I understand what you are saying and I do not think that in the circumstance of an exhaust manifold, halving the length would double the flow rate, because the other variables in the equation of fluid flow are also changing. My point was that length does have an impact on fluid flow, and that if all else stays that same, there is an inverse relationship. 

Yep I have done a little CFD and understand what you are saying!

Edited by Ben26

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6 minutes ago, GTSBoy said:

Yes you are. The Hagen-Poiseville equation ignores entry and exit effects, etc, as I said before.

Pressure and pipe resistance are basically equivalent to voltage and electrical resistance. You get a reasonably simple relationship where the flow is directly proportional to the driving force and inversely proportional to the resistance. The H-P equation just wraps up the diameter to area relationship along with the other factors in one equation. It actually hides a lot of thinking. R^4 for example, is actually R^2, just mentioned twice. One of them is the pipe area considering its effect on the flow capacity (the bigger it is the more it will flow). The 8 comes from a number of divide by 2s. The L is only the bottom line of the equation because the longer the pipe, the higher the resistance to flow. But as I said, the pipe does not exist in isolation.

Ok yea, Im not sure on the entry and exit effects so I don't know how that would impact flow in a manifold. Yep that makes sense what you are saying, thanks for explaining!

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I read all that and my conclusion is that a proper equal length twin scroll manifold, single turbo with a divided rear housing and a pair of external waste gates will be better.

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1 minute ago, Dose Pipe Sutututu said:

I read all that and my conclusion is that a proper equal length twin scroll manifold, single turbo with a divided rear housing and a pair of external waste gates will be better.

haha agreed Dose Pipe!

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8 minutes ago, Dose Pipe Sutututu said:

I read all that and my conclusion is that a proper equal length twin scroll manifold, single turbo with a divided rear housing and a pair of external waste gates will be better.

Yeah. Even though twins and twin scroll both address the exhaust pulse interaction issue, no-ones doing modern bolt on twins (AFAIK, tiny market by comparison) so you're stuck with old & inferior turbo tech if you go that way.

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