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Discussion Starter #1
A buddy is rebuilding his 1974 brand X motor and came to me to ask how much cam I thought he could get away with. The truck must be smogged here in Ca. every 2 years. It has an air pump but no cats. I looked at the Summit t catalog and reviewed the Crane cams that are built for computer controled cars and some of these have Carb #s so they are at least a good benchmark. I would like to suggest the Comp Cams Extreem 4x4 210-218 cam ....Whadddaaa ya think??
 

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There's a old rule of thumb that will usually get you close. Using the .050 cam timing numbers, there should be "0" or less (negative) overlap if meeting emission standard is important. More overlap than that makes meeting emission standards difficult and of course less will be easier. The biggest hydraulic off the shelf cam I know of that gives "0" overlap at .050 is the Crane Powermax H-278-2 grind (222-234 114 l/s) which is available for most engine families. The lobe seperation angle makes a big difference. For every 2 degrees that overlap is reduced, the total degrees of BOTH the intake and exhaust lobes must be reduced by 8 degrees to keep the overlap the same. As an example a 212-220 108 l/s cam will also have "0" overlap, but has a total of 24 less degrees of total valve timing on both lobes. Modern cam designs with faster ramps will also produce less emissions as compared to older slower profiles. I hope that helps.
 

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Discussion Starter #3
Mach,Excellent info! So based on that rule of thumb a 210-218 at 110 lsa will work out ok. Thank You!
 
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