high ratio rockers question
02-15-2010, 11:25 PM
#1
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high ratio rockers question I know someone way smarter than I has figured all this stuff out already, but so I know, anyone want to explain about high ratio rockers. Seem like there would be a balance to be achieved, but higher ratio rocker could open valves faster and wider without the need for extremely harsh cam ramps. Thoughts? thanks
02-16-2010, 08:54 AM
#4
I know someone way smarter than I has figured all this stuff out already, but so I know, anyone want to explain about high ratio rockers. Seem like there would be a balance to be achieved, but higher ratio rocker could open valves faster and wider without the need for extremely harsh cam ramps. Thoughts? thanks
When the ratio increases, it's usually the pushrod cup that's moved in closer to the fulcrum, and the spring force on the rocker tip is amplified by the rocker ratio by the time it gets to the pushrod. Say a given spring provides 400lbs of pressure. With a 2:1 rocker, that would be roughly 800lbs of force translated to the pushrod.
Typically, besides the guys that don't really know what in the hell they are doing, the only time a professional engine builder would elect to use a high ratio rocker is when the cam cannot physically be designed with any more lobe lift, or, as in the case of a NASCAR Nextel Cup engine, flat tappet lifters are used, in which case the ramp rates and consequently lift, are limited by the lifter diameter. In those instances, the amplification chracteristic of the rocker makes up for what the camshaft cannot do.
02-16-2010, 04:19 PM
#5
TECH Fanatic
They do, but the draw back is extremely high loads on the pushrod. It's an example of one of the earliest mechanical devices in the world: the lever.
When the ratio increases, it's usually the pushrod cup that's moved in closer to the fulcrum, and the spring force on the rocker tip is amplified by the rocker ratio by the time it gets to the pushrod. Say a given spring provides 400lbs of pressure. With a 2:1 rocker, that would be roughly 800lbs of force translated to the pushrod.
Typically, besides the guys that don't really know what in the hell they are doing, the only time a professional engine builder would elect to use a high ratio rocker is when the cam cannot physically be designed with any more lobe lift, or, as in the case of a NASCAR Nextel Cup engine, flat tappet lifters are used, in which case the ramp rates and consequently lift, are limited by the lifter diameter. In those instances, the amplification chracteristic of the rocker makes up for what the camshaft cannot do.
When the ratio increases, it's usually the pushrod cup that's moved in closer to the fulcrum, and the spring force on the rocker tip is amplified by the rocker ratio by the time it gets to the pushrod. Say a given spring provides 400lbs of pressure. With a 2:1 rocker, that would be roughly 800lbs of force translated to the pushrod.
Typically, besides the guys that don't really know what in the hell they are doing, the only time a professional engine builder would elect to use a high ratio rocker is when the cam cannot physically be designed with any more lobe lift, or, as in the case of a NASCAR Nextel Cup engine, flat tappet lifters are used, in which case the ramp rates and consequently lift, are limited by the lifter diameter. In those instances, the amplification chracteristic of the rocker makes up for what the camshaft cannot do.
Big ****** pushrods don't really hurt anything, so you really don't have to worry much about mass (and loads) on the lifter side.
Not everyone sees it this way, of course.
Jon
02-16-2010, 05:04 PM
#6
It's a little more involved than that. Think valvetrain stability. That may be more important in a Cup engine than lift. Heck, it may be more important in any engine that runs more than a few seconds at a time in anger. Even those engines benefit from better valvetrain stability at the extreme rpms.
I agree, just except for the unit loading on the pushrod-pushrod cup interface. I've heard of guys running solid billet pushrods and running oil passages through the stands, but at some point, with those 1,000+lbs spring pressures (I know, not Cup springs), those pushrod tips burn up. I think I read that the friction the Cup guys see between the pushrod and pushrod cup in the rocker, have been a major focus to reduce.
