does anyone know what effect going from 1.6 to 1.7 rockers would have on a cam with 216/228 duration at .050 & .484/.512 lift? the new lift i can compute just fine (.514/.544). i dont know what to think about the duration change. i also wonder if my current springs (288#) will handle the new rockers? thanx in advance.
the only thing that changing the rockers does is change the lift. the duration stays the same on the cam. thats the sepration of the lobes. by going to 1.7's you are letting your engine breath better and enhancing cam perfermance. your cam specs will be the same except the lift of course. the springs im not too sure about.......anybody?
Hmmm.... I would think the duration would change also (dunno to what degree). Even though the cam's geometry stays the same, the 'lever' change, in effect, makes the valve stay open for a longer duration (hard to convey the image I have in my punkin'). Eureka! Found an article: http://www.moparmusclemagazine.com/howto/38620/ "The benefit of increased rocker arm ratio is the valve is opened a little further and faster, allowing more air in and out of the engine, thus creating more power. According to the guys at Crane Cams, changing from a rocker ratio of 1.5 to 1.6 generally adds about 3 degrees of valve duration, and changing from 1.6 to 1.7 rockers adds another 1-2 degrees. The net result is you are effectively changing your cam specs without changing your cam." (from the 'what it's worth' department)
i do know the duration increases. its a question of how much. at .050 lift the valve opens sooner due to the "longer lever". i'm guessing less than 10 degrees probably (hopefully). just want to get a little more cam in the car & i need some rockers anyway since my current ones are old (bought in 1970) & used a lot. that lift increase might be a bit much for my current springs (268#, not 288 as thought before). maybe i'll just try the rockers & see if the springs survive. thanx guys.
Duration will only change a couple degrees, with the higher lift .050" valve lift happens a little sooner. I have 1.7:1 rockers on my GT40Ps, had to make the same calculations.
I had 1.7 rockers on GT40 heads with the stock cam. I got a much better result with a special grind cam with a .494 lift with a 108 lobe sep. & 1.6 roller rockers. 1.7 rockers were not sitting well in my mind with the pistons I used. Like everyone else, the duration changes, but just slightly. More lift is the main change.
