Cam + Turbo Questions about LSA, ICL and DCR
07-24-2006, 09:20 PM
#41
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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.
>>>>>>>>>>
Actually, you would need a snapshot of the exhaust flow from inside the combustion chamber "trying to exit the exhaust valve" to understand how much of the exhaust is not being let out of the combustion chamber due to backpresure in the exhaust manifold with overlap, which would contaminate the next "fresh" intake charge with exhaust from the prior stroke.
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.
>>>>>>>>>>
Actually, you would need a snapshot of the exhaust flow from inside the combustion chamber "trying to exit the exhaust valve" to understand how much of the exhaust is not being let out of the combustion chamber due to backpresure in the exhaust manifold with overlap, which would contaminate the next "fresh" intake charge with exhaust from the prior stroke.
Last edited by B T; 07-25-2006 at 08:39 PM.
07-26-2006, 10:36 PM
#43
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Originally Posted by Adrenaline_Z
Got you covered my friend
I Just had to ask, thanks.
BT
07-26-2006, 10:53 PM
#44
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Agree.
The flow of charge through the motor (intake stroke) is dictated by high pressure
at the intake, lower pressure in the cylinder...and then high pressure in the
cylinder, lower pressure in the exhaust (exhaust stroke).
For scavenging: Intake pressure high > cylinder lower > exhaust pressure lower
The flow of charge through the motor (intake stroke) is dictated by high pressure
at the intake, lower pressure in the cylinder...and then high pressure in the
cylinder, lower pressure in the exhaust (exhaust stroke).
For scavenging: Intake pressure high > cylinder lower > exhaust pressure lower
07-26-2006, 11:48 PM
#45
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lower pressure in the cylinder...and then high pressure in the
cylinder, lower pressure in the exhaust (exhaust stroke).
>>>>
At what point would the intake charge be at more PSI than the exhaust manifold PSI with overlap between the valves?
The engine is just a pump, correct?
For scavenging: Intake pressure high > cylinder lower > exhaust pressure lower
>>>>>
For some reason I don't understand how this could happen with overlap between the exhaust valve and the intake valve.
Could you explain this for me please?
Thanks.
cylinder, lower pressure in the exhaust (exhaust stroke).
>>>>
At what point would the intake charge be at more PSI than the exhaust manifold PSI with overlap between the valves?
The engine is just a pump, correct?
For scavenging: Intake pressure high > cylinder lower > exhaust pressure lower
>>>>>
For some reason I don't understand how this could happen with overlap between the exhaust valve and the intake valve.
Could you explain this for me please?
Thanks.
07-27-2006, 07:56 AM
#46
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Originally Posted by B T
lower pressure in the cylinder...and then high pressure in the
cylinder, lower pressure in the exhaust (exhaust stroke).
>>>>
At what point would the intake charge be at more PSI than the exhaust manifold PSI with overlap between the valves?
cylinder, lower pressure in the exhaust (exhaust stroke).
>>>>
At what point would the intake charge be at more PSI than the exhaust manifold PSI with overlap between the valves?
At overlap, you look at the pressure in the cyl, the pressure right in front of the intake valve, and the pressure right behind the exhaust valve.
The air leaving the cyl into the exhaust port causes a vacuum behind it. The air stacking up in front of the closed intake valve causes an increase in pressure. As the Intake valve opens on overlap, the pressure at the intake valve is greater then the pressure in the cyl, and the pressure right behind the exhaust valve.
The pressures are greater then those in an NA engine, but the amount of pressure doesn't matter. The only thing that matters is the pressure differentials.
07-27-2006, 08:11 AM
#47
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At what point would the intake charge be at more PSI than the exhaust manifold PSI with overlap between the valves?
there will be a high pressure pulse at the intake valve, and a negative pulse
at the exhaust port.
The pressure differential along with the momentum of the exhaust and pressure
in the cylinder about the time of EVC and IVO.
It's the piston moving upward during EVO to EVC that is creating a very high
pressure in the cylinder allowing the exhaust gas to move out.
If the pressure waves are timed well with the valve events, the high pressure
pulses will add, and the negative pulse will occur at the appropriate time to
create the highest differential.
The concept has been touched on from post #26 forward, but it might be
easier to draw pressure diagrams to illustrate what is happening. That way,
you can see how the pressure changes over time instead of just seeing a
number on your gauges.
08-01-2006, 09:12 PM
#49
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Originally Posted by stang90gt50
All this discussion and you guys completely missed the point of replying to help the posters question and answers...
I answered it in the very first reply.
You don't want a cam with more exhaust then Intake duration, and you don't want to run it 4 degrees advanced.
09-14-2006, 11:20 AM
#53
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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.
Originally Posted by CamKing
I think that guy needs to go back and check his numbers 
I know this is an old post, but I'm curious what you mean (i.e. what numbers need to be checked). Also, if you disagree, what specifically do you not agree with, or how do you see it differently?
I'm not calling you wrong or disputing your input, I simply like to hear from people on how they see things working, and why. I would simply appreciate your input in that regard
09-14-2006, 09:58 PM
#54
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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.
>>>>
He says, "bleeding off power", I think he might mean the exhaust manifold pressure is not high enough to turn the turbine fast enough to allow the compressor to make up the difference.?
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.
>>>>
How do you bleed off power using a N/A cam?
Thanks.
>>>>
He says, "bleeding off power", I think he might mean the exhaust manifold pressure is not high enough to turn the turbine fast enough to allow the compressor to make up the difference.?
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.
>>>>
How do you bleed off power using a N/A cam?
Thanks.
Last edited by B T; 09-14-2006 at 10:08 PM.
02-04-2007, 10:57 PM
#55
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These theories are great but does anyone has any actual test between a tight lsa and a loose lsa on a forced induction engine? Which works out better in a dcr calc assuming the same engine variables? What effect does the dcr have on a forced induction application?
02-20-2007, 07:16 PM
#56
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Originally Posted by Lythropus
These theories are great but does anyone has any actual test between a tight lsa and a loose lsa on a forced induction engine? Which works out better in a dcr calc assuming the same engine variables? What effect does the dcr have on a forced induction application?
Frank
02-24-2007, 05:07 AM
#57
Originally Posted by Lythropus
These theories are great but does anyone has any actual test between a tight lsa and a loose lsa on a forced induction engine?
The closest I've found is INTMD8's testing, which showed that more overlap = more power, even on a turbo motor with 2/1 backpressure ratio. However, he never got over ~0 deg overlap at .050.
02-24-2007, 05:09 AM
#58
Originally Posted by CamKing
You don't want a cam with more exhaust then Intake duration, and you don't want to run it 4 degrees advanced.
Several here seem to agree that the momentum of the exhaust leaving will pull down the pressure in the chamber, which will help clear it of exhaust gas and the intake air can start coming in. However, most any good NA guy can tell you that the intake momentum and exhaust momentum doesn't change from atmospheric pressure by more than a few psi. If you have 40 psi in the exaust header and 20 psi boost, then you can't expect the momentum affect to help you more than about 37 psi on the exhaust and 23 psi on the intake. The baseline pressure has a much greater affect than the pressure caused by changes in momentum.
02-24-2007, 07:43 AM
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I have to agree with Mike on the exhaust duration point. I don't understand
how more duration would end up hurting since the exhaust valve is smaller.
As for the pressures, I'm not sure what you are trying to explain? Are you
speaking about pressure during the overlap period? If so, how would
2-3 PSI higher at the intake port hurt if the piston cannot provide much
pressure change at this point in time? Afterall, it's the 'net' differential that
dictates flow correct?
how more duration would end up hurting since the exhaust valve is smaller.
As for the pressures, I'm not sure what you are trying to explain? Are you
speaking about pressure during the overlap period? If so, how would
2-3 PSI higher at the intake port hurt if the piston cannot provide much
pressure change at this point in time? Afterall, it's the 'net' differential that
dictates flow correct?
02-24-2007, 08:28 PM
#60
FormerVendor
Originally Posted by engineermike
However, he never got over ~0 deg overlap at .050.