[Fot] Kastner F cam

[Duncan Charlton]

Jerry,

This hits the high points but leaves out some complicating details:

If you add advance, you add low end torque but take away a bit of top end power; retard it and the opposite happens.  So how you time the cam depends largely on how the engine will be used — in general terms it makes sense to advance the cam timing in a street car.  When you advance the cam, the intake valve closes earlier while on its upstroke, meaning the engine now is compressing more volume.  However the exhaust valve is also closing earlier, so there is less scavenging effect to help draw in the intake charge, which comes into play in a big way at high rpm.

This also interacts with your static compression ratio.  If your intake valve closes 90 degrees after bottom dead center the piston is basically halfway up its stroke when the intake valve closes (which is what defines the engine's dynamic compression ratio), so it’s only compressing half the mathematical volume of the cylinder, meaning you need a very high static compression ratio to produce a reasonable amount of pressure in the combustion chamber at the time of ignition.
 
When I’m starting from scratch, I find true TDC (there are a few well-publicized methods of doing this) and put a degree wheel on the nose of the crank, noting the number of degrees when the cam lift (or valve lift; if using valve lift you may as well set your lash to zero for simplicity’s sake) reaches a given dimension while opening — 0.010” or 0.040” or 0.050”, etc. — and noting when it reaches the same dimension while the valve is closing. This must be done on both intake and exhaust.  To set the cam in the “straight up” position, ie: zero degrees of advance, the number of degrees at intake opening should match the number of degrees at exhaust closing.  But — I’m only stating this in general terms since some cams are not "symmetrical.”

One more note:  valve to piston clearance decreases when you retard the cam timing.  The point at which the piston crown and valve head come closest most likely occurs between 8 and 12 degrees after TDC.  You can check this (easier if you are using only one valve spring on each valve at this point) quickly by turning the engine to 10˚ ATDC (later trying a few degrees before and after this figure) and, after setting a zeroed dial indicator on the spring retainer, force the valve open until it touches the piston.  Your dial indicator will tell you how much clearance you have.  Some are willing to go as tight as 0.050” but most people prefer about 0.080”.  It is best to check this with zero lash to give a safety margin.

Duncan Charlton
Elgin, Texas USA



> On Dec 23, 2016, at 11:33 AM, Tony Drews <tony at tonydrews.com> wrote:
> 
> If you are putting in 4 degrees of advance, you want the valve opening 4 degrees of crankshaft rotation earlier than whatever spec you're following (0.010 lift or 0.050 lift or whatever).  The degrees are confusing because you have the "Before" and "After" degrees (BTDC - Before Top Dead Center and ATDC - After Top Dead Center) - for one you add the 4 degrees, for the other you subtract it.  I always spend about 5 minutes figuring out which way to go.  I'm pretty sure that an intake normally opens before top dead center, so if it was supposed to open to 0.050 lift at 20 degrees before top dead center, 4 degrees of advance would have it opening at 24 degrees before top dead center.  It would also close 4 degrees earlier.  If close is supposed to be 50 degrees ATDC, 4 degrees early would be 46 degrees ATDC (no one would close the intake that early I'm pretty sure).  I guess you add the advance to the "before" and subtract from the "after" settings.
> 
> When I degree a cam, I measure both the open and close events.  If the cam isn't ground PERFECTLY to spec, if you only measure opening you may end up with more or less advance than you expected.  So average the open / close to get the centerline in the right spot.  If close is 4 degrees early when open is on spec, you really have 2 degrees of advance already.  I have a hard time explaining it so it's clear - but it's clear in my head... :)
> 
> Tony Drews
> 
> At 08:46 AM 12/23/2016, Jerry Van Vlack via Fot wrote:
>> Friends, can someone explain installing a cam with xx advance, somewhere I saw a reference to doing a cam installation with a 4 degree advance. Does that mean that if the valve opens at xx degrees that it should be set to open at xx minus 4 degrees and what purpose does that serve? Is this the same as using .010 or some other checking clearance. Seems that these two variables are related and one should use one or the other method but not both. Thanks, I'd like to learn more about this stuff. JVV
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