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Discussion Starter #1
Sup guys ,

Almost every N/A cam i've used or seen have a 8* duration split .. does this balance out the difference in flow number ratio between Intake and Ex ..
 

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Sup guys ,

Almost every N/A cam i've used or seen have a 8* duration split .. does this balance out the difference in flow number ratio between Intake and Ex ..
mine is 280/284
 

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Discussion Starter #4
Was that the 1st cam that was specd for the motor ? It just seems that most use the 8* split as a start and go from there ..
 

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vetteDr if you have not bought a intake yet look at ad#2855792 racingjunk.
 

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One often overlooked area is the exhaust lobe's closing side. The intake valve opening STARTS the overlap period XX° BTDC and the exhaust valve closing XX° ATDC ends the overlap period. Here is why the closing side is so important, IMO and when I do a camshaft. When the intake valve starts to open XX° BTDC starting the overlap period, the exhaust valve's lifter is already on the closing side of the lobe, and when the exhaust valve closes XX° ATDC obviously the exhaust valve's lifter is on the closing side
 

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Mine is 281/295 SBF
 

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278/285 @.050
 

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the @ .050" lifter lift numbers do in fact tell some...


but,

there can be GIANT amounts of duration gain at other lifter lifts ..as say .100 to .200' lifter lift this is where the engine "sees" a BUNCH....
 

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Split is a fallacy.... These 2 numbers have nothing to do with each other, except in over lap... And these 2 numbers don't indicate that.
 

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I tend to think it may be a function of a intake tract to exhaust tract relationship and valve layout.

I know of a 295/280 split N/A motor that held an NHRA record back in the day.
 

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I think duration splits are directly related to intake/ex flow ratio.
Would they not also be related to rod ratio and piston velocity? In my thinking.....on the backside of an asymmetric lobe.....by the time the ev actually seats, the piston is roughly 1/3 of the way to it's greatest velocity. As it is the pressure differential between the cylinder and intake port that fills the seat, you wind up in a very fine balance of getting the ev on it's seat quick enough to inhibit reversion, bit not so quick as to bounce the valve. I think a lot of the split that we relate to crossflow actually comes from the balance necessary on the closing flank.
 

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agree with post # 8 as well as # 11

there are probably ten cams that can have the very same exact @ .050" lifterlift numbers that will run different with the same other components in same exact engine

this is because the OTHER lift numbers of duration are different

along with this, what about all of the various inverted/ inverse cam shapes that were developed by ultra-dyne back in the day...
same @ .050' lifter lift duration numbers..but BIG everywhere else...
faster rate with same lift over the nose along with having ability to controll overlap that contains more compression essentially.......
 

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You guys need to start paying attention to opening and closing events of the valve. These 'duration numbers' mean nothing.... it's a sales number for a catalog. And again, the duration numbers of the intake and exhaust have nothing to do with each other... .there is NO RELATION.

If you really want to start learning about how an engine operates, you need to start focusing on what the opening and closing are doing.... and then once you establish the where's and why's..... you have a duration, and a lobe separation.


Duration and LS are results of your desired open and close events. The duration numbers themselves are not the design.
 

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exh open.... depends how fast you can burn fuel, how good and large the port is,, and how high are you going to rpm it

exh closing... usually try to make overlap somewhat symetrical about tdc depending on head characteristics,, rod ratio,, header style, desired idle and driveability

int open... driveability,, size of port respective to displacement,, effeciency of int port velocity

int close... effeciency of port flow/velocity,, length of int runner,, dynamics of manifold to generate inertia,, desired rpm,, mim port cross section


from those 4 numbers you get duration,, then you get sep from there

example,,,,,

i have a decent motor i want to make 950hp with,,, run to 9000 rpm,, high comp,, so ill open exh valve at 85bbdc,,, close at 27atdc,, ill open intake at 32 btdc, its a race motor,, idle at 1500,,, ramp on exh is slower than intake so 50 number is always longer on int since overlap is really looked at 20 numbers,,, int closing at 66,, should peak at 8500 there,, due to port cross section,, length of int runner(strength of inertia),, and cylinder fill properties in int stroke...

so then you say..

exh 85 27 292 at .050 119 center
int 32 66 278 at .050 107 center
(107+119)/2= 113 seperation

thats how "normal" people figure out a cam,, there are more parameteres that can be considered for figuering each event,, but in general,, thats how its done,, so you can see nobody will just say 8 degrees split is good,, if they do they are basing it off a similiar engine,, or have no clue about cams


jeff
 
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