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Greg Reimer 7376 · 07-03-2010, 07:34 PM

Several contradictory terms collide in a discussion of camshaft timing and event nomenclature.The actual reference point that determines where the cam'is' in an engine is actually the #1 cylinder intake valve lobe centerline. If a cam card specifies that the cam is "in" at say, 104 degrees, that is the 'straight up' position. Straight up doesn't mean that the two OEM timing marks are pointing at each other.That could mean anything.Such factors as error in the position of the crankshaft keyway,error in the crank sprocket keyway location,wear in the chain, error in the position of the cam sprocket, etc.etc. are all variables that could stack up in a variety of ways to result in many different outcomes.That is why a thorough understanding of how to degree a camshaft according to the manufacturer's instructions is essential.The figure given at the manufacturer's spec is the equivalent of "straight up",no matter how you get it. That might require an advanced key, a retarded key, or a lot of different things. The 104 degrees ATDC figure, if you put the cam in at, let's say,102 degrees ATDC,would be 2 degrees advanced. If it checked at 108 degrees ATDC,it would be 4 degrees retarded. The dial indicator and the degree wheel are what matter, and all this is for naught if true #1 cyl.TDC isn't accurate or unknown. This has an awful lot to do with the reason one man's car flat out flies, yet another car with identical parts doesn't run nearly as well. Back in the day, I built a 409 Chevy, and it only went 14.80's at about 100. It just didn't pull. After much tuning, i observed that fuel clouds could be seen hovering above both carburetors at wide open throttle when going through the gears.I got lucky, set the engine up at TDC, removed the front cover and the cam was one tooth retarded.Re-installing the cam, pulling the distributor and installing it where it belongs,(40 degrees total) resulted in an instant low 13 second car at 107 mph with no other changes. Cam timing means a lot and it has to be right. Good luck making it work. Just start with the cam grinder's reccommendation printed on the cam card,then experiment from there as a starting point.

07-03-2010, 07:34 PM	#3
Greg Reimer 7376 VIP Member Join Date: Nov 2008 Location: Glendora,Calif. Posts: 1,137 Likes: 172 Liked 719 Times in 220 Posts	Re: cam timing ? Several contradictory terms collide in a discussion of camshaft timing and event nomenclature.The actual reference point that determines where the cam'is' in an engine is actually the #1 cylinder intake valve lobe centerline. If a cam card specifies that the cam is "in" at say, 104 degrees, that is the 'straight up' position. Straight up doesn't mean that the two OEM timing marks are pointing at each other.That could mean anything.Such factors as error in the position of the crankshaft keyway,error in the crank sprocket keyway location,wear in the chain, error in the position of the cam sprocket, etc.etc. are all variables that could stack up in a variety of ways to result in many different outcomes.That is why a thorough understanding of how to degree a camshaft according to the manufacturer's instructions is essential.The figure given at the manufacturer's spec is the equivalent of "straight up",no matter how you get it. That might require an advanced key, a retarded key, or a lot of different things. The 104 degrees ATDC figure, if you put the cam in at, let's say,102 degrees ATDC,would be 2 degrees advanced. If it checked at 108 degrees ATDC,it would be 4 degrees retarded. The dial indicator and the degree wheel are what matter, and all this is for naught if true #1 cyl.TDC isn't accurate or unknown. This has an awful lot to do with the reason one man's car flat out flies, yet another car with identical parts doesn't run nearly as well. Back in the day, I built a 409 Chevy, and it only went 14.80's at about 100. It just didn't pull. After much tuning, i observed that fuel clouds could be seen hovering above both carburetors at wide open throttle when going through the gears.I got lucky, set the engine up at TDC, removed the front cover and the cam was one tooth retarded.Re-installing the cam, pulling the distributor and installing it where it belongs,(40 degrees total) resulted in an instant low 13 second car at 107 mph with no other changes. Cam timing means a lot and it has to be right. Good luck making it work. Just start with the cam grinder's reccommendation printed on the cam card,then experiment from there as a starting point.