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| techtalk:evo:engmech02e [2025/09/16 18:18] – [TDC Lift and Overlap] hippysmack | techtalk:evo:engmech02e [2025/09/16 18:28] (current) – [TDC Lift and Overlap] hippysmack |
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| ====== TDC Lift and Overlap ====== | ====== TDC Lift and Overlap ====== |
| At piston overlap TDC (exhaust stroke), the cams are positioned such that both valves are slightly open. | 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)) \\ |
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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)) \\ |
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| The main driver of the rpm range of the cams is the intake close point. At 48 degrees for the 505/515's, that's really close to the 46 degrees of the N4's. So the rpm range will be similar. The downside to reducing overlap, however, is you lose exhaust augmentation of the intake flow. The N4's will be more sensitive to the pipe, which is a good thing if you've got a good pipe. \\ | The main driver of the rpm range of the cams is the intake close point. At 48 degrees for the 505/515's, that's really close to the 46 degrees of the N4's. So the rpm range will be similar. The downside to reducing overlap, however, is you lose exhaust augmentation of the intake flow. The N4's will be more sensitive to the pipe, which is a good thing if you've got a good pipe. \\ |
| Having more overlap in the cams gives more control of the powerband to the exhaust pipe. \\ | However, having more overlap in the cams gives more control of the powerband to the exhaust pipe. Exhaust augmented intake flow is an extremely effective way to fill the cylinder. I suggest running some overlap and getting a pipe that utilizes that overlap to give you the powerband you want. When you have overlap in the cams, the pipe has a profound effect on the motor, use it to your advantage. Really, the only time I would ever want to limit the overlap in the cams is to mitigate the negative effects of a lousy pipe. But if you've got a good pipe, overlap is your friend. ((aswracing of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-motor-engine/sportster-motorcycle-motor-top-end/55471-questions#post1151157)) \\ |
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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)) \\ | .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)) \\ |
