Double Checking: Head milling
Oct 30, 2006 | 05:09 PM
#31
Originally Posted by FarmerBeau
So a lower LSA/ICL puts our Exhaust valve at a closer position to the piston when it is coming up on the exhaust stroke?
1)265*-112LSA/112ICL = 64.5* EVO, 20.5* EVC
2)265*-112LSA/108ICL = 68.5* EVO, 16.5* EVC
3)265*-110LSA/108ICL = 64.5* EVO, 20.5* EVC
4)265*-108LSA/108ICL = 60.5* EVO, 24.5* EVC
5)265*-108LSA/104ICL = 64.5* EVO, 20.5* EVC
First, remember that the EVOpens before bottom dead center (BBDC) and the EVCloses after top dead center (ATDC). So, if we were to add 64.5* + 180*(up the bore) + 20.5*, what do we get? 265* of duration.
From 1 to 2, the 4* of advance opened/closed the exhaust valve 4* earlier.
From 2 to 3, tightening the LSA 2* causes the exhaust valve to open/close 4* later.
From 3 to 4, tightening the LSA another 2* causes the exhaust valve to open/close 4* later.
From 4 to 5, 4* of advance puts us at the same clearances as cam #1.
Oct 30, 2006 | 07:41 PM
#33
Originally Posted by FarmerBeau
Wait! Holy hell so how the hell are is 1 and 5 so similar when the LSA is dropped 4* then is advanced another 4*? WTF? They are the same distance?
We'll use .050'' specs.
216/216-112LSA/112ICL vs. 216/216-108LSA/104ICL
1)216*-112LSA/112ICL: IVO = 4* ATDC, IVC = 40* ABDC
2)216*-108LSA/104ICL = IVO = 4* BTDC, IVC = 32* ABDC
Cam 1 opens/shuts 8* later than camshaft 2.
Oct 30, 2006 | 07:57 PM
#34
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I understood but i always thought that a lower lsa decreases p to v clearance. By "BTDC" means BEFORE TOP DEAD CENTER right? So would the 108/104 cam be "retarted" rather than advanced thus increasing p to v clearance?
Oct 30, 2006 | 08:07 PM
#35
With the intake. Earlier events reduce P2V due to the valve opening up as the piston reaches TDC. The sooner the valve opens, the closer it is going to be to knicking the piston.
With the exhaust. Later events reduce P2V due to the valve closing as the piston reaches TDC. If the valve hangs open too long it may bump into the piston.
If you take Josh's setup: Ptv minimum clearance should be .080 on the intake and .100 on the exhaust.
and I think I had .080 and .090, you can see that the exhaust valve is already .010 closer than 'minimal' clearance. So, you'd focus more so on the exhaust VE's, adjusting duration, LSA, and ICL to clear.
With the exhaust. Later events reduce P2V due to the valve closing as the piston reaches TDC. If the valve hangs open too long it may bump into the piston.
If you take Josh's setup: Ptv minimum clearance should be .080 on the intake and .100 on the exhaust.
and I think I had .080 and .090, you can see that the exhaust valve is already .010 closer than 'minimal' clearance. So, you'd focus more so on the exhaust VE's, adjusting duration, LSA, and ICL to clear.
Oct 30, 2006 | 08:10 PM
#36
Originally Posted by FarmerBeau
I understood but i always thought that a lower lsa decreases p to v clearance. By "BTDC" means BEFORE TOP DEAD CENTER right? So would the 108/104 cam be "retarted" rather than advanced thus increasing p to v clearance?
ATDC : after top dead center
ABDC : after bottom dead center
BBDC : before bottom dead center
You are correct. Tightening the LSA will push the exhaust VE's later, reducing P2V clearance.
Oct 30, 2006 | 08:17 PM
#37
To help...
How to calculate VE's given duration, LSA, & ICL:
Example cam 230/236 112LSA 108ICL
IVO = ((intake duration/2)-ICL)
Ex: ((230/2)-108)= 7 BTDC (negatives indicate ATDC)
IVC = ((-IVO) + intake duration - 180)
Ex: ((-7) + 230 - 180) = 43 ABDC
ECL = ((2 * LSA) - ICL)
Ex: ((2 * 112) - 108) = 116
EVC = ((exhaust duration / 2)- ECL)
Ex: ((236 / 2) - 116) = 2 ATDC (negative indicates BTDC)
EVO = ((-EVC) + exhaust duration - 180)
Ex: ((-2) +236 - 180) = 54 BBDC
Overlap = ((intake duration + exhaust duration)/2) - (2 * LSA)
Ex: ((230+236)/2) - (2 * 112) = 9
How to calculate VE's given duration, LSA, & ICL:
Example cam 230/236 112LSA 108ICL
IVO = ((intake duration/2)-ICL)
Ex: ((230/2)-108)= 7 BTDC (negatives indicate ATDC)
IVC = ((-IVO) + intake duration - 180)
Ex: ((-7) + 230 - 180) = 43 ABDC
ECL = ((2 * LSA) - ICL)
Ex: ((2 * 112) - 108) = 116
EVC = ((exhaust duration / 2)- ECL)
Ex: ((236 / 2) - 116) = 2 ATDC (negative indicates BTDC)
EVO = ((-EVC) + exhaust duration - 180)
Ex: ((-2) +236 - 180) = 54 BBDC
Overlap = ((intake duration + exhaust duration)/2) - (2 * LSA)
Ex: ((230+236)/2) - (2 * 112) = 9
Oct 30, 2006 | 08:34 PM
#39
I just attempted to explain it about 2 threads down.
Actually, you were correct with your first comment. Given so much clearance due to piston height, dome/dish, reliefs, head milling, valve angle, and valve size leaves one other major factor - The valve events.
Think about where the piston is when the valve is reaching max lift. The intake valve hits peak lift when the piston is headed down the bore. The exhaust valve reaches max lift while the piston is coming up, but it is way down in the bore................... so, what does that leave us with? When the valves are opening and shutting... Valve events...... Duration, LSA, ICL
Think about where the piston is when the valve is reaching max lift. The intake valve hits peak lift when the piston is headed down the bore. The exhaust valve reaches max lift while the piston is coming up, but it is way down in the bore................... so, what does that leave us with? When the valves are opening and shutting... Valve events...... Duration, LSA, ICL
Let me see if I can explain it like this.
We'll only look at the intake valve relative to the piston.
cam: 220* of duration @ .050'', 112 LSA, 0* of advance
VE's: -2* ATDC, 42* ABDC
VE's: IVO/IVC - Intake Valve opens, closes
With 220* of duration and a 112* LSA, with 0* of advance the intake valve will open 2* after the piston has reached top dead center (TDC) and the intake valve will close 42* after the piston has reached bottom dead center.
The piston came up, pushed all of the exhaust out and 2* after it started heading back down the intake valve opened. The piston went all the way back down and 42* after it reached the bottom, it closed.(the beginning of dynamic compression.... )
Time out. Read it again. All of it.
Okay. What do we have? The valve opened 2* after the piston reached the top. The piston then went all the way down and became coming back up. All of this time by the way, you are bringing in the intake charge. 42* after the piston reaches the bottom, the valve closes.
This means that somewhere inbetween all of this, peak lift was met. It happened at the very middle if it's a symetrical lobe (equal up the lobe and down). The valve has opened near the top and closed after the piston started coming up. The middle of all that is somewhere while the piston has started traveling down the bore.... Pretty far down the bore I might add.
If this cam has .550'' of lift or 600'' of lift, it's probably not going to matter because the piston is so far down the bore. What is going to matter tho is the valve events.
We'll only look at the intake valve relative to the piston.
cam: 220* of duration @ .050'', 112 LSA, 0* of advance
VE's: -2* ATDC, 42* ABDC
VE's: IVO/IVC - Intake Valve opens, closes
With 220* of duration and a 112* LSA, with 0* of advance the intake valve will open 2* after the piston has reached top dead center (TDC) and the intake valve will close 42* after the piston has reached bottom dead center.
The piston came up, pushed all of the exhaust out and 2* after it started heading back down the intake valve opened. The piston went all the way back down and 42* after it reached the bottom, it closed.(the beginning of dynamic compression.... )
Time out. Read it again. All of it.
Okay. What do we have? The valve opened 2* after the piston reached the top. The piston then went all the way down and became coming back up. All of this time by the way, you are bringing in the intake charge. 42* after the piston reaches the bottom, the valve closes.
This means that somewhere inbetween all of this, peak lift was met. It happened at the very middle if it's a symetrical lobe (equal up the lobe and down). The valve has opened near the top and closed after the piston started coming up. The middle of all that is somewhere while the piston has started traveling down the bore.... Pretty far down the bore I might add.
If this cam has .550'' of lift or 600'' of lift, it's probably not going to matter because the piston is so far down the bore. What is going to matter tho is the valve events.