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| techtalk:evo:engmech02e [2025/09/16 06:08] – [Cam Grind Comparisons] 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. | ||
| + | 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:// | ||
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| 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__, | The critical period is during __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). | + | 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' |
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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' | + | |
| 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' | 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' | ||
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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/ | The main driver of the rpm range of the cams is the intake close point. At 48 degrees for the 505/ | ||
| + | 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:// | ||
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| + | .226/.212 are not overly high TDC lifts and at stock valve sizes (1.810/ | ||
