Cam + Turbo Questions about LSA, ICL and DCR
07-20-2006, 02:35 PM
#21
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I would think the mass of the turbine spinning at 10,000+ RPM along with
any exhaust energy could easily overcome compressing intake charge for
a period of about 200 milliseconds.
any exhaust energy could easily overcome compressing intake charge for
a period of about 200 milliseconds.
Last edited by Adrenaline_Z; 07-20-2006 at 04:05 PM.
07-20-2006, 04:01 PM
#22
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Originally Posted by INTMD8
"The turbo is still spinning and trying to pull the air out"
So, the turbo has enough stored energy to cross over between being driven by the exhaust and actually pulling it out of the cylinder, all while continually compressing the inlet charge? (and in most cases with measured exhaust pressure much higher than that of intake manifold pressure)
It would seem to me that the resistance of compressing the inlet charge would easily overcome any inertia of the rotating turbocharger assembly.
So, the turbo has enough stored energy to cross over between being driven by the exhaust and actually pulling it out of the cylinder, all while continually compressing the inlet charge? (and in most cases with measured exhaust pressure much higher than that of intake manifold pressure)
It would seem to me that the resistance of compressing the inlet charge would easily overcome any inertia of the rotating turbocharger assembly.
07-20-2006, 04:46 PM
#23
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Originally Posted by CamKing
You seem to be forgeting the other 7 cylinders. When one cylinder is not pushing on the turbo, the others are. On a V8 the turbo sees 4 pulses every 360 crank degrees. You've always got 3 or 4 exhaust ports open and feeding the turbo, and 4 or 5 exhaust valves closed.
07-20-2006, 05:05 PM
#24
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Originally Posted by 87calais
if you have no pressure in the other cylinder, shouldn't logic and physics dictate that the exhaust gasses will always go the path of least resistance?
worry about than reversion.
An intake cycle will always contain more intake charge than residual exhaust
gas.
The most opportune time for exhaust gas to sneak back into the cylinder is
near EVC where the piston is close to TDC and the intake valve is just starting
to open.
During overlap, with a positive intake manifold pressure and proper pulse tuning,
reversion is not a huge concern.
07-20-2006, 06:09 PM
#25
FormerVendor
Originally Posted by CamKing
You seem to be forgeting the other 7 cylinders. When one cylinder is not pushing on the turbo, the others are. On a V8 the turbo sees 4 pulses every 360 crank degrees. You've always got 3 or 4 exhaust ports open and feeding the turbo, and 4 or 5 exhaust valves closed.
And fwiw, I don't believe reversion is as much of a problem as everyone makes it out to be. Through experimentation of my own I've consistently seen the most power and extended rpm range with relatively tight LSA's and high overlap. (As compared to the current turbo cam trends) and if reversion was a problem that would not be the case.
For an example, lets say you are operating at 25psi intake manifold pressure and 40psi turbine inlet pressure. We have a cylinder approaching TDC on the exhaust stroke and there is a measured 40psi at the turbo collector. How at this point, with other cylinders driving the turbo through the same collector, will the turbine act upon that cylinder in some sort of scavenging effect?
.
07-20-2006, 07:21 PM
#26
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Originally Posted by INTMD8
I just don't understand how the turbine at any point could help pull exhaust gas out of the cylinder.
Air has mass. It wants to keep moving straight, and it doesn't like to change direction.
Have you ever opened the side window on a car at a high rate of speed and have a piece of paper fly out of the car?
With no other window open, the velocity of the air passing past the open window will change the pressure inside the car.
07-20-2006, 08:05 PM
#27
FormerVendor
Yes, I understand that concept but that is not factoring in an impeller or the pressure differential but I will try to reword my example to your example.
Lets say your car is operating at 25psi cabin pressure and there is 40psi atmospheric pressure. We have a drivers window leaving TDC on the down stroke and there is a measured 40psi atmospheric pressure. How at this point will the paper be pulled out of the vehicle by some sort of scavenging effect?
Lets say your car is operating at 25psi cabin pressure and there is 40psi atmospheric pressure. We have a drivers window leaving TDC on the down stroke and there is a measured 40psi atmospheric pressure. How at this point will the paper be pulled out of the vehicle by some sort of scavenging effect?
07-20-2006, 08:30 PM
#28
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25psi intake manifold pressure and 40psi turbine inlet pressure.
At what point in time are these values taken? What is the pressure at
the exhausting port during this time?
How is the intake manifold pressure connected to the exhaust manifold pressure? Through overlap only?
What are the other cylinders doing when cylinder X is overlapping? How
much time do the valves stay open together at 3000, 4000, 5000, 6000 RPM?
How are the pulses mixing and which way are they moving in relation to the valve events?
How do these values change as RPM increases? How do these values change
upon acceleration?
07-20-2006, 09:01 PM
#29
FormerVendor
Would those questions not be more relevant to the question of pulse scavenging ?(which I believe to be dulled by the pressure differential caused by turbo restriction).
My confusion only lies with-
"As the piston slows down toward TDC, the pressure in the cyl drops. The turbo is still spinning and trying to pull the air out. This drops the pressure in the port which dropps the pressure in the cyl."
That is not referring to pulse scavenging, it is specifically claiming that the turbocharger is pulling air out of the cylinder at TDC. (which it would somehow have to do through a collector that as CamKing mentioned, all 7 other cylinders are firing through).
My confusion only lies with-
"As the piston slows down toward TDC, the pressure in the cyl drops. The turbo is still spinning and trying to pull the air out. This drops the pressure in the port which dropps the pressure in the cyl."
That is not referring to pulse scavenging, it is specifically claiming that the turbocharger is pulling air out of the cylinder at TDC. (which it would somehow have to do through a collector that as CamKing mentioned, all 7 other cylinders are firing through).
07-20-2006, 09:16 PM
#30
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I would concentrate more on his statement about a mass in motion.
Grab your buddy and do this experiment:
Run toward a brick wall as fast as you can. At any point in time, your friend
can yell 'stop'.
Do you stop dead in your tracks, or do you have forward momentum and continue
a few more paces?
How does this effect the pressure at the port?
Similar to the paper in the car analogy:
A car speeding by a pile of leaves does not physically touch the leaves,
however, as the car passes the leaves will get pulled in behind the car.
Much like a mass of exhaust gas leaving the cylinder, it will continue to pull
charge out of the cylinder as it creates a low pressure region in its path.
Grab your buddy and do this experiment:
Run toward a brick wall as fast as you can. At any point in time, your friend
can yell 'stop'.
Do you stop dead in your tracks, or do you have forward momentum and continue
a few more paces?
How does this effect the pressure at the port?
Similar to the paper in the car analogy:
A car speeding by a pile of leaves does not physically touch the leaves,
however, as the car passes the leaves will get pulled in behind the car.
Much like a mass of exhaust gas leaving the cylinder, it will continue to pull
charge out of the cylinder as it creates a low pressure region in its path.
07-20-2006, 09:34 PM
#31
FormerVendor
Yes, what your explaining is inertia and scavenging which I completely understand and am not disputing.
I am only disagreeing with the statement that the turbine impeller sucks exhaust out of the cylinder at TDC. (which would be a completely different condition).
I am only disagreeing with the statement that the turbine impeller sucks exhaust out of the cylinder at TDC. (which would be a completely different condition).
07-20-2006, 11:09 PM
#32
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Originally Posted by Adrenaline_Z
I would concentrate more on his statement about a mass in motion.
Grab your buddy and do this experiment:
Run toward a brick wall as fast as you can. At any point in time, your friend
can yell 'stop'.
Do you stop dead in your tracks, or do you have forward momentum and continue
a few more paces?
How does this effect the pressure at the port?
Similar to the paper in the car analogy:
A car speeding by a pile of leaves does not physically touch the leaves,
however, as the car passes the leaves will get pulled in behind the car.
Much like a mass of exhaust gas leaving the cylinder, it will continue to pull
charge out of the cylinder as it creates a low pressure region in its path.
Grab your buddy and do this experiment:
Run toward a brick wall as fast as you can. At any point in time, your friend
can yell 'stop'.
Do you stop dead in your tracks, or do you have forward momentum and continue
a few more paces?
How does this effect the pressure at the port?
Similar to the paper in the car analogy:
A car speeding by a pile of leaves does not physically touch the leaves,
however, as the car passes the leaves will get pulled in behind the car.
Much like a mass of exhaust gas leaving the cylinder, it will continue to pull
charge out of the cylinder as it creates a low pressure region in its path.
On my rotary setup I had a boost gauge and backpressure gauge.
I saw nearly 2:1 pressure difference between the backpresure in the exhaust manifold and the intake manifold.
Backpressure being 50% higher than intake pressure.
My O2 readings were giving me some crazy numbers very quickly in data logging, lean/rich rapidly changing depending on throttle openings.
My Haltech E6-K data recordings of intake manifold pressure/vaccume every 500 rps showed fluctuations when it was trying to go into boost, intake manifold pressure would try and go positive under full throttle, but it showed an up and down "struggle" with vaccume if you will.
It would make some boost, but the data showed it was "two steps forward and one step back" as far as making boost and goin vaccume under load per 500 rpms.
>>>>>>>>>>
it will continue to pull
charge out of the cylinder as it creates a low pressure region in its path.
How can it "pull" and create "low" pressure in the exhaust manifold unless the throttle is almost/ all the way closed?
btw, I tried a 1.15 housing and i had less boost pressure/more lag and about the same back pressure, and the rpm "pickup" was way less on acceleration and i still had teh same problems pressent in data logging.
Nice thread!
carry on....
BT
07-21-2006, 08:34 AM
#33
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How can it "pull" and create "low" pressure in the exhaust manifold unless the throttle is almost/ all the way closed?
The valves are preventing interaction between the intake port and exhaust
port until the overlap period.
Ask yourself how much pressure is at the exhaust port when the piston is moving
xxx feet per second toward TDC and squeezing out exhaust gas.
Think of the velocity of the pulse leaving the cylinder.
Now the intake valve begins to open. Combine the intake pressure with the
forward moving exhaust pulse.
Before any of the backpressure has a chance to revert the charge, the exhaust
valve closes and the intake stroke has already begun.
You may want to look up Bernoulli at this link:
http://www.lmnoeng.com/Flow/bernoulli.htm
Read the section about mid way down the page:
Circular pipe diameter change, Non-circular duct area change
Venturi flow meter (C=0.98), Nozzle flow meter (C=0.96), and Orifice flow meter (C=0.6)
Is your gauge measuring a differential pressure before and after the turbo?
Is it measuring the difference between the cylinder and exhaust runner, or
is it reading the average pressure in the exhaust?
Last edited by Adrenaline_Z; 07-21-2006 at 09:52 AM.
07-21-2006, 10:00 AM
#34
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Originally Posted by INTMD8
I am only disagreeing with the statement that the turbine impeller sucks exhaust out of the cylinder at TDC. (which would be a completely different condition).
The turbo is always being run by 3 or 4 cyl's. It is not a constant pressure, it is 4 pulses per 360 degrees. The velocity of the impeller wants to stay constant. As the piston reaches TDC, the velocity drops at the valve side of the exhaust port. Thanks to the velocity of the impeller and the velocity of the exhaust pulse feeding the impeller, you get a big drop in pressure at the exhaust valve. That pressure drops below the pressure of the cyl and the pressure of the air in the intake port.
Remember that the pressure at the intake valve is much higher then the pressure in the plenom.
07-22-2006, 02:26 AM
#35
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Originally Posted by Adrenaline_Z
There is no direct link between intake pressure and exhaust pressure.
Is it measuring the difference between the cylinder and exhaust runner, or
is it reading the average pressure in the exhaust?
Is it measuring the difference between the cylinder and exhaust runner, or
is it reading the average pressure in the exhaust?
xxx feet per second toward TDC and squeezing out exhaust gas.
Think of the velocity of the pulse leaving the cylinder.
Now the intake valve begins to open. Combine the intake pressure with the
forward moving exhaust pulse.
Before any of the backpressure has a chance to revert the charge, the exhaust
valve closes and the intake stroke has already begun.
You may want to look up Bernoulli at this link:
http://www.lmnoeng.com/Flow/bernoulli.htm
Read the section about mid way down the page:
Circular pipe diameter change, Non-circular duct area change
Venturi flow meter (C=0.98), Nozzle flow meter (C=0.96), and Orifice flow meter (C=0.6)
Is your gauge measuring a differential pressure before and after the turbo?
Is it measuring the difference between the cylinder and exhaust runner, or
is it reading the average pressure in the exhaust?[/QUOTE]
Is your gauge measuring a differential pressure before and after the turbo?
>>>>
before and after the turbo?
no, and "before and after the turbo" i don't understand what you mean.
Is it measuring the difference between the cylinder and exhaust runner, or
is it reading the average pressure in the exhaust?[/QUOTE]
>>>>
It would be the "absolute" pressure, not average pressure in the exhaust at the inlet for the turbine.
Is it measuring the difference between the cylinder and exhaust runner, or
is it reading the average pressure in the exhaust?[/QUOTE]
>>>
I would think this would allow enough mix from the exhaust backpressure in the turbine to case reversion back into the exhaust valve entering into the new intake charge before TDC, being as intake boost would be lower at any given rate with overlap between intake and exhaust pressure.
07-23-2006, 11:20 AM
#36
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What is that you are contesting?
Are you trying to imply that reversion is occuring at all times because your
exhaust gauge is showing 40 PSI and your intake boost is showing 25 PSI?
The exhaust is an average pressure of all eight cylinders. At the point you
are measuring, you are reading the result of 8 pressure pulses not just one.
Each port is also at a different pressure at any point in time. Pressure waves
are sinusoidal. The pressure along the runners and exhaust is also changing
at any given length at a particular time.
You would have to take a snap shot of the pressure at each exhaust port, each intake port,
each cylinder, the collector pre-turbo, the collector post-turbo and then
view the position of each intake and exhaust valve to know which waves are
reverting and which are carrying forward.
Are you trying to imply that reversion is occuring at all times because your
exhaust gauge is showing 40 PSI and your intake boost is showing 25 PSI?
The exhaust is an average pressure of all eight cylinders. At the point you
are measuring, you are reading the result of 8 pressure pulses not just one.
Each port is also at a different pressure at any point in time. Pressure waves
are sinusoidal. The pressure along the runners and exhaust is also changing
at any given length at a particular time.
You would have to take a snap shot of the pressure at each exhaust port, each intake port,
each cylinder, the collector pre-turbo, the collector post-turbo and then
view the position of each intake and exhaust valve to know which waves are
reverting and which are carrying forward.
07-23-2006, 05:26 PM
#37
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ok who has a handle on all of this and how much will the specs cost? 
events are a better way to look at things but most people talk cams in durations and centerlines
I'm using a mech roller @.050 255/262 .600/.600 114LSA 110ICL
not a turbo cam by any stretch, but it works past 7500
my rwhp/tq @5200 was 550/550 at 20psi. 350 cubes 4" bore TH400/9"
I am definately getting reversion based on the soot in my intake
my backpresure with twin P-trim .81A/R is 1:1
the ford guys are making big power on smallish cams by running large compressors with small turbines. high backpressure gets the thing spooling fast, and low exhaust duration keeps the reversion down.
I'd settle for less torque to get more (over 6500) topend. torque kills tires
rpm needs duration and duration makes overlap
seems like you would want at least 240 on the exhaust to make good topend.
I have a SBC which means I probably shouldnt be in here but this is some of the best cam discussion I can find
events are a better way to look at things but most people talk cams in durations and centerlines
I'm using a mech roller @.050 255/262 .600/.600 114LSA 110ICL
not a turbo cam by any stretch, but it works past 7500
my rwhp/tq @5200 was 550/550 at 20psi. 350 cubes 4" bore TH400/9"
I am definately getting reversion based on the soot in my intake
my backpresure with twin P-trim .81A/R is 1:1
the ford guys are making big power on smallish cams by running large compressors with small turbines. high backpressure gets the thing spooling fast, and low exhaust duration keeps the reversion down.
I'd settle for less torque to get more (over 6500) topend. torque kills tires
rpm needs duration and duration makes overlap
seems like you would want at least 240 on the exhaust to make good topend.
I have a SBC which means I probably shouldnt be in here but this is some of the best cam discussion I can find
07-23-2006, 09:22 PM
#38
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Originally Posted by Rusted40
ok who has a handle on all of this and how much will the specs cost?
events are a better way to look at things but most people talk cams in durations and centerlines
I'm using a mech roller @.050 255/262 .600/.600 114LSA 110ICL
not a turbo cam by any stretch, but it works past 7500
my rwhp/tq @5200 was 550/550 at 20psi. 350 cubes 4" bore TH400/9"
I am definately getting reversion based on the soot in my intake
my backpresure with twin P-trim .81A/R is 1:1
the ford guys are making big power on smallish cams by running large compressors with small turbines. high backpressure gets the thing spooling fast, and low exhaust duration keeps the reversion down.
I'd settle for less torque to get more (over 6500) topend. torque kills tires
rpm needs duration and duration makes overlap
seems like you would want at least 240 on the exhaust to make good topend.
I have a SBC which means I probably shouldnt be in here but this is some of the best cam discussion I can find
events are a better way to look at things but most people talk cams in durations and centerlines
I'm using a mech roller @.050 255/262 .600/.600 114LSA 110ICL
not a turbo cam by any stretch, but it works past 7500
my rwhp/tq @5200 was 550/550 at 20psi. 350 cubes 4" bore TH400/9"
I am definately getting reversion based on the soot in my intake
my backpresure with twin P-trim .81A/R is 1:1
the ford guys are making big power on smallish cams by running large compressors with small turbines. high backpressure gets the thing spooling fast, and low exhaust duration keeps the reversion down.
I'd settle for less torque to get more (over 6500) topend. torque kills tires
rpm needs duration and duration makes overlap
seems like you would want at least 240 on the exhaust to make good topend.
I have a SBC which means I probably shouldnt be in here but this is some of the best cam discussion I can find
Since you're in NC, I'll PM you, and see if I can help.
07-24-2006, 03:26 PM
#39
6600 rpm clutch dump of death Administrator
From a very old thread on this topic... (Camshaft discussion II)
Originally on norcal-ls1
http://www.norcal-ls1.com/forum/show...0&postcount=12
Turbos are limited in exhaust duration. Let me clarify... For a STREET driven turbo car where low-end is important, the exhaust lobe is limited.
Because of the pressure in the exhaust manifold, turbo motors have a longer 'power stroke' then N/A, SC, & Nitrous cars. These motor's power stroke is from 0 to 90 degrees. Turbo motors continue to push for another 45 degrees of crank rotation. Hard one to follow, but true.
Stock cams work so well with turbos becuase of the lack of overlap & small lobes. Aftermarket cams are setup for N/A & SC's for the most part & open the exhaust inside the power stroke of a turbo motor. Average spot to open the exhaust valve is ~45 degrees BBDC @ .050 lift, which is ~ 70-80 BBDC @ .006 lift (depending on ramp-rate). So, the exhaust lobe on a turbo motor *should* be opened late to avoid bleeding off power. At roughly 60 degrees BBDC is were you can open the exhaust valve. Add this to the lack of overlap on a turbo motor, because exhaust manifold pressure is ALWAYS higher then the incoming intake charge (by 1.5-2 times depending on compressor efficiency) & you can see why large cams don't work so well with these motors.
So, if you're following me, a street turbo motor is limited to ~ 260-270 total duration on the exhaust side. & that's WITH ~40 degrees of overlap @ .006. (nominal stock overlap)
Track turbo motors use more traditional N/A grinds as they stay in the upper RPM band were overlap will actually help the turbo make power. But it KILLS the low-end.
Turbos make full boost by ~3000rpm & therefore make tons of torque, so most don't notice that the N/A cam they're using is bleeding off power.
I'm going for the most efficient combo as possible.... This new cam will open the exhaust @ 60.5 degrees BBDC. As opose to 69 BBDC w/the stock cam in there now.
Originally on norcal-ls1
http://www.norcal-ls1.com/forum/show...0&postcount=12
Originally Posted by 93PONY
Turbos are limited in exhaust duration. Let me clarify... For a STREET driven turbo car where low-end is important, the exhaust lobe is limited.
Because of the pressure in the exhaust manifold, turbo motors have a longer 'power stroke' then N/A, SC, & Nitrous cars. These motor's power stroke is from 0 to 90 degrees. Turbo motors continue to push for another 45 degrees of crank rotation. Hard one to follow, but true.
Stock cams work so well with turbos becuase of the lack of overlap & small lobes. Aftermarket cams are setup for N/A & SC's for the most part & open the exhaust inside the power stroke of a turbo motor. Average spot to open the exhaust valve is ~45 degrees BBDC @ .050 lift, which is ~ 70-80 BBDC @ .006 lift (depending on ramp-rate). So, the exhaust lobe on a turbo motor *should* be opened late to avoid bleeding off power. At roughly 60 degrees BBDC is were you can open the exhaust valve. Add this to the lack of overlap on a turbo motor, because exhaust manifold pressure is ALWAYS higher then the incoming intake charge (by 1.5-2 times depending on compressor efficiency) & you can see why large cams don't work so well with these motors.
So, if you're following me, a street turbo motor is limited to ~ 260-270 total duration on the exhaust side. & that's WITH ~40 degrees of overlap @ .006. (nominal stock overlap)
Track turbo motors use more traditional N/A grinds as they stay in the upper RPM band were overlap will actually help the turbo make power. But it KILLS the low-end.
Turbos make full boost by ~3000rpm & therefore make tons of torque, so most don't notice that the N/A cam they're using is bleeding off power.
I'm going for the most efficient combo as possible.... This new cam will open the exhaust @ 60.5 degrees BBDC. As opose to 69 BBDC w/the stock cam in there now.
07-24-2006, 07:57 PM
#40
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Originally Posted by J-Rod
From a very old thread on this topic... (Camshaft discussion II)
Originally on norcal-ls1
http://www.norcal-ls1.com/forum/show...0&postcount=12
Originally on norcal-ls1
http://www.norcal-ls1.com/forum/show...0&postcount=12
