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techtalk:evo:engmech02e [2025/09/16 06:08] – [Cam Grind Comparisons] hippysmacktechtalk:evo:engmech02e [2025/09/16 06:12] (current) – [TDC Lift and Overlap] hippysmack
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 ====== TDC Lift and Overlap ====== ====== TDC Lift and Overlap ======
 +The operative cam spec is the "TDC Lift" for both the intake and exhaust valve. That's how much the valves are open at top dead center between the exhaust and intake strokes, which is the overlap period (the exhaust cycle is ending as the intake cycle begins, and the two cycles overlap each other). \\
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 "Bolt-in" for 1200 heads and "bolt-in" for 883 heads are two different things. ((aswracing of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-motor-engine/sportster-motorcycle-engine-conversions/203708-883-to-1200-cam-question#post4564105)) \\ "Bolt-in" for 1200 heads and "bolt-in" for 883 heads are two different things. ((aswracing of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-motor-engine/sportster-motorcycle-engine-conversions/203708-883-to-1200-cam-question#post4564105)) \\
 883's have longer valves than 1200's have. They do that so the valve heads will be closer together, to fit within the 883's 3 inch diameter chamber. The 883 chamber is shallower to match. That means the valves start out closer to the pistons to begin with, as compared to 1200 heads. \\ 883's have longer valves than 1200's have. They do that so the valve heads will be closer together, to fit within the 883's 3 inch diameter chamber. The 883 chamber is shallower to match. That means the valves start out closer to the pistons to begin with, as compared to 1200 heads. \\
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 The critical period is during __overlap__, when the piston passes through TDC with both valves open. That's when you can get into trouble with piston to valve clearance. P-V clearance has nothing to do with max lift as many people mistakenly assume, because max lift always occurs with the piston well down the bore. It's the lift during overlap that you need to be concerned about. \\ The critical period is during __overlap__, when the piston passes through TDC with both valves open. That's when you can get into trouble with piston to valve clearance. P-V clearance has nothing to do with max lift as many people mistakenly assume, because max lift always occurs with the piston well down the bore. It's the lift during overlap that you need to be concerned about. \\
  
-The operative cam spec is the "TDC Lift" for both the intake and exhaust valve. That's how much the valves are open at top dead center between the exhaust and intake strokes, which is the overlap period (the exhaust cycle is ending as the intake cycle begins, and the two cycles overlap each other). +Hammer Performance tells customers you're taking a chance when you run a TDC lift of over about .200" on either valve, when using stock 883 heads. I have seen many times when the N4's have caused contact when used with stock 883 heads. Generally it's slight, only enough to mark the valve pockets, not enough to bend the valves, but enough to make some noise. Not all do it though. It's roughly 50-50. Depends a lot on the pistons used. But if you're that close to making contact, a little bit of float is going to get you in trouble in a hurry. The piston chases the exhaust valve closed, so that's where you get contact during float. The intake valve chases the piston down, so float doesn't put you at risk on the intake side. \\
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-Hammer Performance tells customers you're taking a chance when you run a TDC lift of over about .200" on either valve, when using stock 883 heads. I have seen many times when the N4's have caused contact when used with stock 883 heads. Generally it's slight, only enough to mark the valve pockets, not enough to bend the valves, but enough to make some noise. Not all do it though. It's roughly 50-50. Depends a lot on the pistons used. But if you're that close to making contact, a little bit of float is going to get you in trouble in a hurry. The piston chases the exhaust valve closed, so that's where you get contact during float. The intake valve chases the piston down, so float doesn't put you at risk on the intake side. +
  
 If you want a set of cams that gives an N4 type power band, but are totally safe for 883 heads, look at the Feuling 505/515 grind. Compare the specs and you'll see that they're a lower overlap version of the N4's, with correspondingly lower TDC lifts. I suspect they designed them that way specifically to make them safe with 883 heads. \\ If you want a set of cams that gives an N4 type power band, but are totally safe for 883 heads, look at the Feuling 505/515 grind. Compare the specs and you'll see that they're a lower overlap version of the N4's, with correspondingly lower TDC lifts. I suspect they designed them that way specifically to make them safe with 883 heads. \\

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