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Sportster Motorcycle Motor - Top End Discuss Sportster Motorcycle Top End issues. Rockerboxes, Valves, Cylinders, Pistons, Rings, Lift Rods, etc...

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  #1  
Old 22nd August 2019
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Default Bigger valves

Is there any advantage to using bigger valves than already in the xb heads? Especially on stock 1200 pistons/cylinders?
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Old 22nd August 2019
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Default

this is a good question.

On automotive heads, bore size and head chamber size limits the effectiveness of larger valve after a point. If the valve is getting close to the edge of either it hurts airflow and CD. Some compensation is steeper valve seat angle, the actual seat and the top cuts into the chamber. Altering valve inclined angle can help. (Gets the valve away from the edge, but much more work) A bigger valve without the port work rarely is beneficial either. (Only if the port and throats are already too big for the valve)
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Old 22nd August 2019
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ole oilfield saying
"if you can't fix it, get a bigger hammer"
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Old 22nd August 2019
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Default

I was talking to someone about this the other day when we were discussing cleaning up the heads and blending the seats on the intake to improve flow and he was wondering about bigger valves.
He has stock 1200 pistons/cylinders and I think 551 cams. Maybe N4s.
Sorry for the double threads. Not sure what happened.

My thinking is you would lose velocity.
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Old 22nd August 2019
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AFAIK, using larger valves depends on the head design. If the head was designed for street use, even with porting, larger valves will probably decrease air flow. Going to 3-4 valves per cyl. will increase flow. Again, head design is the major factor.
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I think the valves on these heads are plenty big enough. Especially on 1200 flat tops. You would want to increase velocity not lose it. That's my amateur thinking.
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I just want to steer him down the right path because he's not happy. He rode my bike yesterday and I thought he was coming off the back. Now mine is nothing spectacular but it is a spunky little street bike.
I didn't ride his. He came in his car but he said mine has way more power which just made him more determined to make improvements and asked about headwork and bigger valves.
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