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[QUOTE]Originally posted by 93PONY: [QB] The XE274HR is: Intake lobe #3632 Exhaust lobe #3634 Intake lobe specs: .006/.050/.200/lobe lift 273/224/148/.347 Exhaust lobe specs: .006/.050/.200/lobe lift 281/232/155/.353 The XE274HR comes ground on a 112LSA & 108ICL. This means the cam has 53 degrees of overlap @.006, & 4 degrees of overlap @.050. Typically a camshaft will pass smog on a 3" stroke with 1 or less overlap @.050 lift. On a 3.25"-3.4" stroke you can get away with more overlap & still pass smog. The XE274HR has been proven to pass the sniffer when in good tune with good cats. As you can see the exhaust lobe is larger....both duration & lift. 7-8 degrees across the entire range of lift. Valve events @.050 when installed as ground (108ICL): IVO: 4 BTDC IVC: 40 ABDC EVO: 52 BBDC EVC: 0 (TDC) By the events you can see that the camshaft is actually [b]12[/b] degrees exhaust bias relative to TDC. (52-40) This means relative to TDC there is 12 more degrees of exhaust duration than intake duration. Although the intake on a boosted application is under pressure, it's still under FAR less pressure than when the exhaust valve cracks open. Overscavanging the cylinder can be a problem. (depending on your exhaust system as a whole). AFR heads are notoriously high flowing on the exhaust side. Couple the high flow exhaust port with longtubes & off-road exhaust & you have WAY more exhaust system (as a whole) then needed. Given this, an easy solution is to retard the camshaft 4 degrees (to 'straight up') which reduces the exhaust bias of the cam by 8 degrees. (add for to the intake closing event & subract 4 from the exhaust opening event) The results would be better low/midrange performance (due to less overscavanging of the cylinder, or 'exhaust reversion') & slightly higher power at RPM (due to the later intake valve closing event). [/QB][/QUOTE]
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