Re: Why is a cams intake center line usually around 102 to 112?
[Re: 451Mopar]
#2167582
10/04/16 10:21 AM
10/04/16 10:21 AM
|
Joined: Jan 2003
Posts: 10,081 Frostbitefalls MN (Rocky&Bullw...
gregsdart
I Live Here
|
I Live Here
Joined: Jan 2003
Posts: 10,081
Frostbitefalls MN (Rocky&Bullw...
|
David Vizzards' book How to Make Horsepower, 2nd edition, goes into great detail about centerlines and where to install a cam, as well as every other aspect of cam design. Basically the lobe separation angle is about how well a motor breathes at overlap. A 318 with great heads on it, with large valves can start to fill a cylinder much better than a stroked 440 with old iron heads and small valves for the cubes they feed. The 318 will want a smaller amount of overlap, so a wide LSA is better. The stroker 440 would want a very large overlap, so the LSA would be much narrower to produce that overlap needed. There is a limited time that the exhaust pulse produces a partial vacuum at the exhaust valve, and having both valves open at the same time the right amount of time and the right amount of lift for the rpm range you want to run is critical. That overlap period puts a huge draw on the incoming air and fuel, much greater than most would think. The length of overlap needed grows at a rate about the same as the total duration does for any given rpm range you want to run, so a certain engine combo will want about the same LSA regardless of rpm range. Notice that split pattern cams with more exhaust duration have a wider lsa. BUT, the overlap for that motor and head combo will be close to the same as a single pattern cam, and the only opening and closing event that really changes for the cam is the exhaust opening event. The higher the compression ratio, the quicker the cylinder pressure drops, and then the exhaust valve can open earlier to help the motor breath better. My 528 race motor with 15/1 compression can open the valve very early, and the cam is 283 @.050 intake, 296 @ .050 exhaust, but the lsa has been spread to 114 to keep that overlap in about the same range of a single pattern cam. Put this cam in a low compression engine, and the exaust valve would open WAY before the work was done, because of the low compression keeps cylinder pressure high later in the power stroke, providing more work closer to BDC.
8.582, 160.18 mph best, 2905 lbs 549, indy 572-13, alky
|
|
|
Re: Why is a cams intake center line usually around 102 to 112?
[Re: 451Mopar]
#2167715
10/04/16 02:14 PM
10/04/16 02:14 PM
|
Joined: Oct 2007
Posts: 4,219 New York
polyspheric
master
|
master
Joined: Oct 2007
Posts: 4,219
New York
|
If you set it at 90 degres after TDC then it would be at max lift halfway down the cyl
90 degrees isn't ever halfway down the cylinder.
Boffin Emeritus
|
|
|
Re: Why is a cams intake center line usually around 102 to 112?
[Re: 451Mopar]
#2167755
10/04/16 03:18 PM
10/04/16 03:18 PM
|
Joined: Oct 2007
Posts: 4,219 New York
polyspheric
master
|
master
Joined: Oct 2007
Posts: 4,219
New York
|
Repeating it doesn't make it true. 90 degrees isn't ever halfway down the cylinder.
Boffin Emeritus
|
|
|
Re: Why is a cams intake center line usually around 102 to 112?
[Re: 451Mopar]
#2167758
10/04/16 03:21 PM
10/04/16 03:21 PM
|
Joined: Jan 2003
Posts: 19,537 Las Vegas
Al_Alguire
I Live Here
|
I Live Here
Joined: Jan 2003
Posts: 19,537
Las Vegas
|
Damn I am an NA guy and all my junk is well over 108...Apparently been doing it wrong..IMO the reason you see all the narrow LSA's is you are looking at old school Mopar grinds(my guess)and that is an old way of thinking
"I am not ashamed to confess I am ignorant of what I do not know."
"It's never wrong to do the right thing"
|
|
|
Re: Why is a cams intake center line usually around 102 to 112?
[Re: polyspheric]
#2167784
10/04/16 03:48 PM
10/04/16 03:48 PM
|
Joined: Nov 2006
Posts: 2,261 IL
furious70
top fuel
|
top fuel
Joined: Nov 2006
Posts: 2,261
IL
|
Repeating it doesn't make it true. 90 degrees isn't ever halfway down the cylinder.
Should that be applied to your post as well?
70 Sport Fury 68 Charger 69 Coronet 72 RR
|
|
|
Re: Why is a cams intake center line usually around 102 to 112?
[Re: Al_Alguire]
#2167793
10/04/16 04:13 PM
10/04/16 04:13 PM
|
Joined: Sep 2014
Posts: 4,457 Washington
madscientist
master
|
master
Joined: Sep 2014
Posts: 4,457
Washington
|
Damn I am an NA guy and all my junk is well over 108...Apparently been doing it wrong..IMO the reason you see all the narrow LSA's is you are looking at old school Mopar grinds(my guess)and that is an old way of thinking LSA has absolutely NOTHING to do with how agressive (or how soft) a lobe is. LSA should be based on head flow at overlap. The better the head, the wider the LSA can be. That is where Comp has failed. They decided 110 is the de facto LSA. Not a good way to decide LSA.
Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
|
|
|
Re: Why is a cams intake center line usually around 102 to 112?
[Re: polyspheric]
#2167809
10/04/16 04:37 PM
10/04/16 04:37 PM
|
Joined: Jan 2003
Posts: 27,421 Balt. Md
383man
Too Many Posts
|
Too Many Posts
Joined: Jan 2003
Posts: 27,421
Balt. Md
|
Repeating it doesn't make it true. 90 degrees isn't ever halfway down the cylinder. You know what I mean. If the piston is 90 degrees from TDC its halfway through its travel of its stroke. At 90 degrees from TDC the piston should have gone the distance of the crank throw. At 180 it would have done a full stroke and be at the bottom. Dont you agree if the pistons travels 180 degrees it has traveled through its stroke from TDC to BDC ? Ron
|
|
|
Re: Why is a cams intake center line usually around 102 to 112?
[Re: 383man]
#2167817
10/04/16 04:48 PM
10/04/16 04:48 PM
|
Joined: Feb 2008
Posts: 724 Central TEXAS!!!!
sr4440
super stock
|
super stock
Joined: Feb 2008
Posts: 724
Central TEXAS!!!!
|
Repeating it doesn't make it true. 90 degrees isn't ever halfway down the cylinder. You know what I mean. If the piston is 90 degrees from TDC its halfway through its travel of its stroke. At 90 degrees from TDC the piston should have gone the distance of the crank throw. At 180 it would have done a full stroke and be at the bottom. Dont you agree if the pistons travels 180 degrees it has traveled through its stroke from TDC to BDC ? Ron doesn't work that way, punch in some numbers and see for yourself. http://dansmc.com/mc_software2.htm
Without Data, you’re just another guy with an opinion.
|
|
|
Re: Why is a cams intake center line usually around 102 to 112?
[Re: 451Mopar]
#2167844
10/04/16 05:22 PM
10/04/16 05:22 PM
|
Joined: Jan 2003
Posts: 9,924 Weddington, N.C.
Streetwize
master
|
master
Joined: Jan 2003
Posts: 9,924
Weddington, N.C.
|
I think we need to visualize the fulcrum effect, the Trig, if you will of the connecting rod and piston motion relative to the stroke, 90 degrees after TDC does not put the piston 1/2 way down the bore....not with a connecting rod greater than 0 in length anyway, lol
But it is interesting to note that when the ROD is 90 degrees to the Crank throw this is the point of maximum piston VELOCITY in the bore. Think of your leg pushing on a bike pedal.
Last edited by Streetwize; 10/04/16 05:23 PM.
|
|
|
Re: Why is a cams intake center line usually around 102 to 112?
[Re: 451Mopar]
#2167846
10/04/16 05:25 PM
10/04/16 05:25 PM
|
Joined: Sep 2014
Posts: 4,457 Washington
madscientist
master
|
master
Joined: Sep 2014
Posts: 4,457
Washington
|
At 90-degrees crank rotation the crank pin is 1/2 around, but the piston rod is 1/2 way down and 1/2 way over so the piston is more than 1/2 way down the bore at 90-degrees.
Anyhow, I was talking about intake lobe centerline, not lobe separation angle. LSA is usually nearly the same number resulting in an equal split on overlap. The intake centerline is wherever you put it. It can have a 114 LSA and you can install it at 102. It would be 12 degrees advanced. But you could still do it.
Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
|
|
|
Re: Why is a cams intake center line usually around 102 to 112?
[Re: madscientist]
#2167847
10/04/16 05:25 PM
10/04/16 05:25 PM
|
Joined: Jan 2005
Posts: 176 Sterling Heights, MI
John_T_Brown
member
|
member
Joined: Jan 2005
Posts: 176
Sterling Heights, MI
|
LSA should be based on head flow at overlap. The better the head, the wider the LSA can be. This is a true statement.
If it ain't broke fix it anyway!
|
|
|
Re: Why is a cams intake center line usually around 102 to 112?
[Re: John_T_Brown]
#2167852
10/04/16 05:40 PM
10/04/16 05:40 PM
|
Joined: Jan 2003
Posts: 9,924 Weddington, N.C.
Streetwize
master
|
master
Joined: Jan 2003
Posts: 9,924
Weddington, N.C.
|
Actually (also) the better the port, the wider the lobe centerline can be. A good port and combustion chamber with little to no valve shrouding doesn't have to rely on "fifth cycle" scavenging (overlap) to keep the cylinder charged and the VE up, also if the port moves air and fuel efficiently as a charged mass, you can delay the Intake centerline (and more importantly the Intake closing event) which can effectively RAM more air and fuel into the hole before the valve closes then the natural "effective displacement" ie < than the static displacement which gets smaller for every degree past Bottom Dead Center.
Last edited by Streetwize; 10/04/16 05:40 PM.
|
|
|
|
|