Automotive How long until vacuum (Hg/In) drops off? (turbocharger)

[Forcibly-Induced_232]

I've been searching but i can not find the answer. This is for my turbocharged car. I just fabricated a rear turbocharger at the back.

Really not sure how to word this,sorry.

Question 1:
So my car produces 26Hg/in vacuum at the engine. About how much pressure drop per foot is expected?
The factors involved are a 2" inner diameter pipe that is 12foot long.

I think i can get an approximate guesstimate on Boost (positive pressure).
Thanks , people. I saw some charts online but they only dealt with water and CFM. I figured a smarty pant or two would know something about this here.
Thanks again. :)

 

[sophiecentaur]

Welcome to PF
Are you suggesting active extraction of the exhaust gases? What would be the power source for this? If it is the exhaust gases themselves (as with a normal turbo) then the overall result would be worse than without your system. It seems to me that you are effectively expecting a pump to 'pump itself', which is a bit like a (forbidden on PF) perpetual motion machine.
The turbocharger itself uses a power source in the form of the fuel that the engine burns and the exhaust gases that pass through the turbo are going slower than they would if they went straight out of the back but what power source do the exhaust gases provide for your system?
Design of a good (passive) exhaust path is how motor cars optimise their fuel use.

 

[Forcibly-Induced_232]

Welcome to PF
Are you suggesting active extraction of the exhaust gases? What would be the power source for this? If it is the exhaust gases themselves (as with a normal turbo) then the overall result would be worse than without your system. It seems to me that you are effectively expecting a pump to 'pump itself', which is a bit like a (forbidden on PF) perpetual motion machine.
The turbocharger itself uses a power source in the form of the fuel that the engine burns and the exhaust gases that pass through the turbo are going slower than they would if they went straight out of the back but what power source do the exhaust gases provide for your system?
Design of a good (passive) exhaust path is how motor cars optimise their fuel use.

I may just have an air leak as i have not yet clamped(sealed) the silicone couplers down yet. A boost/vacuum gauge connected to the turbocharger only showed .5 hg in . I will find out soon if a leak is the culprit but at the same time i am wondering how far a negative pressure can travel before it stops registering on a meter.

In this case i am using about 12 feet of 2" pipe from front, side, to back. With a max vacuum of 27 at the engine.

Thank you for responding.

 

[sophiecentaur]

I may just have an air leak as i have not yet clamped(sealed) the silicone couplers down yet. A boost/vacuum gauge connected to the turbocharger only showed .5 hg in . I will find out soon if a leak is the culprit but at the same time i am wondering how far a negative pressure can travel before it stops registering on a meter.

In this case i am using about 12 feet of 2" pipe from front, side, to back. With a max vacuum of 27 at the engine.
View attachment 221317

Thank you for responding.

You have not confirmed or denied what I posted. Are you, in fact, trying to use the exhaust gases to power a turbine which you hope will help to extract them better?

 

[Forcibly-Induced_232]

You have not confirmed or denied what I posted. Are you, in fact, trying to use the exhaust gases to power a turbine which you hope will help to extract them better?

Powered by exhaust gases. Used to create positive pressure.
I just need an idea of how far a vacuum pressure can travel. It will give me a general idea of how far positive pressure can go and how long.

I'll just get a vacuum and attach some pipe to it. Wont be as strong a vacuum as an engine and i can't really measure it but it's another general idea.

 

[sophiecentaur]

Actually, you “just need” to know some very basic Thermodynamics. You cannot get more out than you put in. The only loophole you can exploit could be to put a massive cooler at the end to salvage the dregs of Energy. But you still couldn’t improve on using the cooler directly on the gases.
Feel free to prove for yourself that PM doesn’t work or save the effort and read around a bit.

 

[Ketch22]

Knowing more about the system you are attempting to build is the key here. It is a trick used by some show car and sleeper car builders to mount the turbo in the rear. These can be effective and have the advantage of usually not altering the outward appearance of the car.
A working installation uses the exhaust to drive the turbo and produces a positive pressure which is ducted back to the intake. The air uptake is in the trunk or a rear body panel. There is no place that a vacuum is being ducted. At least not for a distance that needs to be calculated. Conventional dynamic head calculations for pipe length and bends do apply for the air flow which depends on rate and volume which you will also need to provide.
If as previously asked you are trying to "suck" the exhaust with an exhaust powered device, well you can't squeeze blood out of a turnip.

 

[Forcibly-Induced_232]

I'm not always great with words and i don't think we are on the same wave lengths here.

Obviously drinking from a straw requires vacuum and a 6 inch straw doesn't lose any with such a narrow diameter and short length.
If one was to suck or blow on a 2" inner diameter straw that is 12foot long with 28 psi or 28vacuum how far would until the pressure drops. There has to be some chart or something to lead me the right direction.

I rather read vacuum/boost from the turbo, which is where my gauge will is routed to.

 

[Ketch22]

I rather read vacuum/boost from the turbo, which is where my gauge will is routed to.

I would start with a good lesson on how to read a compressor map. You will then need to come up with some numbers on the air required for your motor. This will enable you to choose a turbo that best suits your engine needs. In the region where the compressor is producing more air than your engine needs you will be boosting. In those areas where the engine is taking more air you will be in a vacuum state.

If this is a modern engine or it has a modern ECU controlling electronic fuel injection just make sure it has sensors that can sense both vacuum and pressure. The ECU will adjust the fuel air ratio automatically according to the flow entering the engine. Make sure you put your sensor near the intake manifold. The challenge is self correcting and follows your pedal actions.
If however you are running carburetor or mechanical injection get a broad band O² sensor and get ready for an extended dyno or street tuning process.
The changes in flow by throttle position, RPM, engine speed, air temperature, and many other variables make what you are looking at difficult at best to calculate. Even more difficult to achieve without lots of tinkering. You can however get an approximate baseline using dynamic head tables from the plumbing world and a couple ballpark flow rates from your selection process notes,

 

[jim hardy]

There has to be some chart or something to lead me the right direction.

I rather read vacuum/boost from the turbo, which is where my gauge will is routed to.

Pressure drop along a pipe is in proportion to the square of the volume flow rate . That's why your charts deal with CFM.
Water is one way to measure pressure. That's why your charts deal with water.
So is mercury.
An inch of mercury is 13.6 inches of water.
27.7 inches of water is one PSI..

You need to measure pressure at both ends of your pipe and subtract one from the other.

The answer to this question .

I just need an idea of how far a vacuum pressure can travel.

is ":Forever" ..

So my car produces 26Hg/in vacuum at the engine.

Can I assume that's at idle on the manifold side of the throttle plate?

To be meaningful you'll need readings at wide open throttle.

 

[sophiecentaur]

i don't think we are on the same wave lengths here.

I think this is true. You haven't confirmed or denied my impression that you are trying to 'suck' the exhaust by using a turbo. It this actually what you are trying to do? (Simple yes / no would do)

 

[Forcibly-Induced_232]

Pressure drop along a pipe is in proportion to the square of the volume flow rate . That's why your charts deal with CFM.
Water is one way to measure pressure. That's why your charts deal with water.
So is mercury.
An inch of mercury is 13.6 inches of water.
27.7 inches of water is one PSI..

You need to measure pressure at both ends of your pipe and subtract one from the other.

The answer to this question .

is ":Forever" ..Can I assume that's at idle on the manifold side of the throttle plate?

To be meaningful you'll need readings at wide open throttle.

I missed your post, sorry about the delayed response. Your assumption is correct, the vacuum is being measured at the engine at idle and i was trying to measure from the turbo at the end of the 12ftx2" pipe(it has a port for the gauge hose). I may either have air leaks at the couplers(where the pipes join and have a bend) and/or i haven't had a chance to let the pressure lag catch up to the gauge. When i "blip" the throttle the gauge reads 1in vacuum. I may need to test while driving.

Side-note from me:
i just watched a video of a guy blowing through a 1,000ft hose. on the other end was a balloon. Very intriguingly enough when he blew through the balloon there was a pressure lag(he would quit blowing but air was still traveling) . Let me just link a video demonstrating. I believe this is worth a watch for anyone on the Physics Forums :

 

[jim hardy]

the vacuum is being measured at the engine at idle and i was trying to measure from the turbo at the end of the 12ftx2" pipe(it has a port for the gauge hose).

There's a throttle plate in your throttle body. On one side of it is atmosphere(or output port of turbocharger ), on other side of it is a plenum, manifold and intake valves.

Point is - there's a lot of pressure drop across that throttle plate. That is its purpose, to control how much air gets into manifold.

Imagine yourself very tiny and able to hitch a ride on a molecule of air.
You pass easily through the air filter(we hope), enter turbocharger impeller vanes where the air molecules get hurled against a diffuser,
squeezing them closer together thereby raising air pressure,
Then you move through the 12 foot pipe experiencing hopefully only a small loss of pressure
and arrive at the throttle plate which you say is closed (because you're at idle.)
So very little air squeezes around the closed throttle plate into the manifold.
Every time an intake valve opens air gets sucked out of the intake manifold by a piston. That's why you have 26 inches of vacuum in there.

Beware of "Word Salad", words that sound good but are unrelated to what's really going on. The mind will believe Word Salad that's plain foolishness.
Always start your analytical thinking by figuring out what really IS going on.
Immersing yourself in the process as described above is the first step.
Drawing a picture will help you describe it in words that aren't just word salad .. Attention to precise wording is the price of straight thinking.
Next you figure out how to describe it with numbers. That's when understanding commences.

old jim

 

[sophiecentaur]

I think this is true. You haven't confirmed or denied my impression that you are trying to 'suck' the exhaust by using a turbo. It this actually what you are trying to do? (Simple yes / no would do)

I'm bumping this post because the answer is highly relevant. - even if the OP may not be aware. The nuts and bolts of this are not relevant if, as it seems, he is proposing PPM.

 

[Forcibly-Induced_232]

Just wanted to say i put a shop vac in the end of the 12ft pipe.. (also tested the shop vac and it showed about 10HG/In)
the result: the lag is virtually non existent! It even spools the turbo!
Amazing to me. I may be one of the very few who can actually prove rear-mount turbo lag is a MYTH. If you get the right size A/R and trim turbocharger of course.

 

[jim hardy]

it seems, he is proposing PPM.

Could be.
I'd assumed a conventional turbocharger using exhaust flow to boost intake pressure. Never occurred to me to 'suck' exhaust gas out, though 'tuned exhaust' popular in a friend's 1950's go-kart circles achieved something similar with standing waves...

This question implied to me abject lack of thought about fluid flow.

i am wondering how far a negative pressure can travel before it stops registering on a meter.

i wasn't being a wiseguy when i replied "Forever" . And it's why i implored him to think about the air molecules.

Here's the picture i was working from, top left corner of his

Vacuum for all practical purposes ends at the throttle body butterfly valve. Which i assume is at engine end of his 12 foot pipe not the turbo end.

A question well stated is half answered. Maybe he'll clarify his question.

old jim

 

[sophiecentaur]

A question well stated is half answered. Maybe he'll clarify his question

I hope he will.
I cannot even understand where his long pipe goes or what the turbo is supposed to be doing. He has refused three times to answer my simple question and I really suspect there is some PM process that's planned here. It appears that it is not working for him and he is trying to explain it in terms of a 'leak'. I suspect it's just loss in the system which you always get when you try to pull yourself up by your thermodynamic bootstraps.

Vacuum for all practical purposes ends at the throttle body butterfly valve.

But that applies to a normally aspirated engine, surely. A turbo provides positive pressure at the inlet and you can't have blow and suck at the same time(?). I can't see how a 12ft pipe can be placed at the inlet side of an engine . Totally confusing.

 

[Averagesupernova]

I don't really think it's that complicated. First thing to address is the 26 inches of vacuum. That has to be manifold vacuum. If that is vacuum on the turbo side of the throttle body, something is seriously blocked. So to me the 26 inches of vacuum versus the .5 inches is totally meaningless.
-
Next is the OP wonders how much vacuum loss exists on 12 feet of pipe. I think this is where he has a few things wrong. There will be absolutely NO VACCUUM/PRESSURE LOSS as long as there is no flow. Not very helpful I suppose. So the next question is how much flow? We don't know because we don't know the displacement of the engine to determine CFM. The higher the flow, the more loss.
-
I don't see this as some perpetual motion scheme.

Powered by exhaust gases. Used to create positive pressure.
I just need an idea of how far a vacuum pressure can travel. It will give me a general idea of how far positive pressure can go and how long.

I'll just get a vacuum and attach some pipe to it. Wont be as strong a vacuum as an engine and i can't really measure it but it's another general idea.

What I placed in bold/italics simply tells me the OP wants to use a turbo in the conventional manner aside from the obvious of mounting it at the rear of the vehicle.
-

I hope he will.
I cannot even understand where his long pipe goes or what the turbo is supposed to be doing. He has refused three times to answer my simple question and I really suspect there is some PM process that's planned here. It appears that it is not working for him and he is trying to explain it in terms of a 'leak'. I suspect it's just loss in the system which you always get when you try to pull yourself up by your thermodynamic bootstraps.

But that applies to a normally aspirated engine, surely. A turbo provides positive pressure at the inlet and you can't have blow and suck at the same time(?). I can't see how a 12ft pipe can be placed at the inlet side of an engine . Totally confusing.

Pretty sure the OP is not answering the question about perpetual motion since he views it as a stupid question. Frankly I would tend agree. A poster comes on here asking about pressure/flow losses with what does appear to be some confusion about the amount of flow since engine size is not mentioned and right off the bat folks jump on the YOU CAN'T TALK ABOUT PERPETUAL MOTION ON THIS FORUM bandwagon. We are better than that here I would hope.

 

[jim hardy]

It appears that it is not working for him and he is trying to explain it in terms of a 'leak'.

@sophiecentaur May use your questions to try and hep OP?

To make any sense of his questions i have to assume it's a plain vanilla turbocharger, exhaust driven turbine , with centrifugal compressor on other end of same shaft to raise pressure of air en route to throttle body.

I think he doesn't think in terms of absolute pressure, furthermore he expects same pressure on both sides of throttle butterfly.
He reported manifold is 26 inches of vacuum and that's 4 inches absolute pressure. For some reason he expects that same pressure at turbocharger. . Has he moved throttle butterfly clear back to there ? I doubt it but who knows...
Considering the engine as a positive displacement pump there's not much mass flow through that engine with only 4" of manifold pressure.
That's because the air is rarefied to 4/30 of standard density as it squeezes past the butterfly.
With so little mass flow through the turbine it doesn't deliver much power to the compressor so there's hardly any boost present at the throttle body butterfly.
Pressure at inlet side of butterfly will be 30 inches absolute, 1 atmosphere,, and on other side of butterfly in the manifold it'll be 4 inches absolute. .

When throttle butterfly gets opened wide it no longer is a restriction. Manifold pressure rises to one atmosphere, 7.5X what it was at idle so mass flow through the engine goes up by same ratio.
Now the engine is making considerable power. I think he's not thought that through yet.
Increased mass flow through the turbine speeds it up so the compressor starts really raising its output pressure.
That raises manifold pressure further, again raising mass flowrate and it's positive feedback thereafter with all of positive feedback's attendant dynamics. That's the root of "Turbo Lag".
That positive feedback can bring about the kind of failures in that recent thread 'engine explodesduring dyno test" https://www.physicsforums.com/threads/engine-explodes-during-dyno-test.940991/#post-5954125

A turbo provides positive pressure at the inlet and you can't have blow and suck at the same time(?).

Yes, to the INLET (Upstream) side of throttle butterfly.
Pressure downstream of butterfly depends on butterfly position and air flow rate,
... and can be less or more than atmospheric.. That's what he's missed.
He asks how far along the pipe he should read vacuum. Answer is he shouldn't read vacuum anyplace upwind of the butterfly. Except maybe for a brief instant after slamming throttle wide open before turbo speeds up.

That's my take. I hope it helps him phrase his question.

Old Jim

 

[sophiecentaur]

@sophiecentaur May use your questions to try and hep OP?

To make any sense of his questions i have to assume it's a plain vanilla turbocharger, exhaust driven turbine , with centrifugal compressor on other end of same shaft to raise pressure of air en route to throttle body.

I think he doesn't think in terms of absolute pressure, furthermore he expects same pressure on both sides of throttle butterfly.
He reported manifold is 26 inches of vacuum and that's 4 inches absolute pressure. For some reason he expects that same pressure at turbocharger. . Has he moved throttle butterfly clear back to there ? I doubt it but who knows...
Considering the engine as a positive displacement pump there's not much mass flow through that engine with only 4" of manifold pressure.
That's because the air is rarefied to 4/30 of standard density as it squeezes past the butterfly.
With so little mass flow through the turbine it doesn't deliver much power to the compressor so there's hardly any boost present at the throttle body butterfly.
Pressure at inlet side of butterfly will be 30 inches absolute, 1 atmosphere,, and on other side of butterfly in the manifold it'll be 4 inches absolute. .

When throttle butterfly gets opened wide it no longer is a restriction. Manifold pressure rises to one atmosphere, 7.5X what it was at idle so mass flow through the engine goes up by same ratio.
Now the engine is making considerable power. I think he's not thought that through yet.
Increased mass flow through the turbine speeds it up so the compressor starts really raising its output pressure.
That raises manifold pressure further, again raising mass flowrate and it's positive feedback thereafter with all of positive feedback's attendant dynamics. That's the root of "Turbo Lag".
That positive feedback can bring about the kind of failures in that recent thread 'engine explodesduring dyno test" https://www.physicsforums.com/threads/engine-explodes-during-dyno-test.940991/#post-5954125
Yes, to the INLET (Upstream) side of throttle butterfly.
Pressure downstream of butterfly depends on butterfly position and air flow rate,
... and can be less or more than atmospheric.. That's what he's missed.
He asks how far along the pipe he should read vacuum. Answer is he shouldn't read vacuum anyplace upwind of the butterfly. Except maybe for a brief instant after slamming throttle wide open before turbo speeds up.

That's my take. I hope it helps him phrase his question.

Old Jim

If he's just using the turbo in a conventional way, to increase manifold pressure when needed, why not use the conventional layout? What does the long pipe do for him?
I take your point about low pressure with the butterfly closed but that is not "at the same time" as the turbo is being used (nothing to drive it with).

Pretty sure the OP is not answering the question about perpetual motion since he views it as a stupid question. Frankly I would tend agree. A poster comes on here asking about pressure/flow losses with what does appear to be some confusion about the amount of flow since engine size is not mentioned and right off the bat folks jump on the YOU CAN'T TALK ABOUT PERPETUAL MOTION ON THIS FORUM bandwagon. We are better than that here I would hope.

If the OP had said that, for some reason, he's putting a turbo right at the back of the car and wants to know why it's not working as well as if it was right on the manifold then there would have been no problem. He has never (affaics) made that clear - or the reason for the odd siting. That's why I was asking random questions in order to decode what he was writing. I'm really not sure whether any of the replies he has been given will actually help significantly and there has been no feedback about
There is enough turbo delay in the conventional position and there can only be a further delay due to the delay in pressure build up (and drop in temperature?) in the volume and length of the pipe. And then there is pressure drop for the fresh air going back to the inlet. This was such a strange scenario that I assumed there must be more to it. There is no reason why that confusion couldn't have been sorted out after my initial question.
But, as Jim so often points out - a question well posed is half way to being anwered.

 

[jim hardy]

That's why I was asking random questions in order to decode what he was writing.

i too was confused and just answered what i assumed was the question. I all too often answer a question that wasn't asked that way..

for some reason, he's putting a turbo right at the back of the car

I've heard of that. With today's crowded engine compartments it makes for an easier amateur installation.
Here's the better way to do it but as you see it's non-trivial.

 

[Averagesupernova]

I think the OP confused manifold vacuum with the ever so slight vacuum that would exist between the throttle body and turbo. That is my assumption anyway. Other than the confusion of 26 inches of vacuum at the engine and .5 inches of vacuum at the turbo I don't think the OP was wondering why it 'didn't work'. I don't think anyone can say that it doesn't work. While I had not heard of people putting turbochargers at the back of the vehicle, this does not surprise me. So what if there is 12 feet of pipe on the intake? So what if exhaust has cooled some by the time it reaches the turbo thus yielding less energy? I think it can be assumed that it is a compromise but it is not in my place to point out any more than that.

 

[sophiecentaur]

So what if there is 12 feet of pipe on the intake? So what if exhaust has cooled some by the time it reaches the turbo thus yielding less energy?

The turbo takes power away from the engine and will have been designed for maximum efficiency when placed right at the exhaust manifold. 12ft of pipe length must affect it performance. As for the inlet path, the 12 ft would cool down the compressed air before entering the engine. That could perhaps be good. Inter-coolers improve efficiency but they are proper heat exchangers and not just a straight pipe so how much improvement would the pipe provide? A turbo for use with an intercooler would be designed for different operating conditions so we don't know that a regular turbo would benefit. Also, any advantages for the inlet air would be a disadvantage for getting energy from the exhaust gases.

I don't think the OP was wondering why it 'didn't work'

The OP would need to know whether or not the system was providing any real advantage in order to know whether or not it 'worked'. I have no doubt that the engine would run OK with all sorts of modifications. But we have been kept in ignorance about the performance figures of his system. Normally, we get details of systems and that helps to elicit useful and better targeted answers. It's a shame that we are so much in the dark about this - even down to the actual reason behind putting the turbo at the back.

 

[Ketch22]

Forcibly-induced, a note from a tuner of engines to someone who is trying to recreate a complex installation. I like what your trying to do here. The space used and the clean lines make good custom rods. However from the engineering side of things one needs to, as some of the previous post state, measure your vacuum in equivalent areas. The post throttle plate manifold will never equate to the intake track. Also the lag you expressed concern of can be explained and demonstrated. When a remote turbo is running it is using the exhaust as provided in the current throttle state. At the opening of the throttle the manifold pressure will move closer to neutral (lower vacuum if naturally aspirated or closer to 0 bar relative boost) as the engine draws more air. As the combustion process increases it produces more exhaust flow which drives more boost. A long transport in either direction can increase "lag time." This is the delay you also stated discovery about.
Keep in mind that exhaust is a thermal product. With a long metal transport, cooling and resultant lower volume is a reality. To this I would say that Thermal wrap is your friend, without it all bets are off as to whether or not the turbine will operate as expected. Also with a longer transport lag is a given but sufficient volume in the transport allow quicker recovery from throttle position changes. However, too much volume will cause increased lag as the turbo must pressurize a larger volume to get effective pressure at the distal end. One last consideration is that the long transport will increase the cooling of the induction air. This will reduce the load on an intercooler but also lower the pressure. exposed pipe is usually your friend here.
If you are going for a show car. it will be easy to make it run. If you are looking for a real sweet sleeper plan on lots of tuning and that will need to be on the street, a dyno cannot simulate the full air flow induced cooling that you will find on the road.

 

[sophiecentaur]

However, too much volume will cause increased lag

I find it annoying enough that there's turbo lag in a runabout diesel 4X4. How would any extra be acceptable in a 'high performance' special?