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--- techtalk:evo:engmech02e [2025/09/16 18:23] – [TDC Lift and Overlap] hippysmack
+++ techtalk:evo:engmech02e [2025/10/01 07:14] (current) – [TDC Lift and Overlap] hippysmack
@@ Line 5: / Line 5: @@
 ====== TDC Lift and Overlap ======
 At piston overlap TDC (exhaust stroke), the cams are positioned such that both valves are slightly open.
-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). \\
+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). Overlap allows the exhaust to affect intake flow. A poorly working exhaust really should have as little overlap as possible. This is why the D cams have so little. But an exhaust that's pulling hard during overlap can utilize a larger overlap window to pull on the intake and help cylinder fill. ((aswracing of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-motor-engine/sportster-motorcycle-motor-top-end/55471-questions#post1151157)) \\
 "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)) \\
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 .226/.212 are not overly high TDC lifts and at stock valve sizes (1.810/1.575), it's not going to cause any valve to valve clearance concerns. Just as a point of reference, SE .536's are similar at .212/.236 and these are routinely run with larger than stock valves (1.850/1.615) without sinking and we still get .045-.060 of valve to valve clearance. ((aswracing of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/rubber-mount-sportster-motorcycle-talk-2004-2006/48756-great-problem-with-n8-cams/page7#post2546706)) \\
+**Piston to Valve Clearance is all about TDC lift**: \\
+The cam's base circle size doesn't have anything to do with piston to valve clearance. The valve lifts the same amount and with the same timing regardless of the base circle of the cam. Valve to piston clearance has nothing to do with max lift either. At cam's max lift, it's piston is well down the bore, nowhere near the valves. The piston chases the exhaust valve closed. The intake valve chases the piston down. Piston to valve contact can be subtle and it can bend the valve just enough to mess up the valve seal. ((aswracing of the XLFORUM https://www.xlforum.net/forum/buell-motorcycle-forum/buell-motorcycle-top-end/131895-not-exactly-a-conversion#post2810574)) \\

Trace: • EVO: Engine Mechanicals - Sub-02E • REF: Engine Mechanicals - Sub-04L

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