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Hi to all of you,, I have a serious question about duration changes when installing 1.6 roller rockers from the factory 1.5 to 1 ratio,, and the effects it will have on my computer,, 1 - a cam with a 112 LSA & 202 intake 210 exhaust and is Computer compatable at 1.5 to 1 rocker ratio, when changing to 1.6 to 1 rockers it will change the duration of this cam to 215 intake & 221 exhaust, WILL this cam still be compatable with the stock COMPUTER ? My stock computer will support a cam with 204 intake and 216 exhaust, it shows really good results on the Dyno 2000 program switching to the roller rockers, this is on a modded high compression 88 2.8 , somewhere I read that switching to 1.6 rockers with the stock 2.8 HO cam was a good mod for these early 2.8 underdog motors,, thus using a performance cam and then swapping to the 1.6 rockers yields good results but I am not sure if the computer would work with the new durations after the change,, thanks Bill...
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How did you come up with that amount of duration increase? Usually the increase in by a degree or two, not that much. -
Duration calculation....
Hi Raven --- dur.@ .050 divide by advertised dur.
that figure divided by 1.5 rocker ratio
that figure times 1.6
that figure times advertised duration again
202 / 250 = .808
.808 / 1.5 = .359
.359 X 1.6 = .861
.574 X 250 = 215.46 intake duration @ .050
Bill...Comment
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I thought you'd be doing that and it doesn't work that way.Originally posted by rvbilly View Post
The way the rocker lifts off the seat is still determined by the cam profile, seat to seat duration will alwasy stay the same, this can not be changed. What the increased ratio does though, is accelerate the valve's lift from a seated postion, so the .050 to .050 duration will increase, by a very little bit, usually a degree or two as I mentioned earlier.Comment
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Duration
Great - okay then If I use my performance cam then it will still be within my computers limits, so a 202 & 210 will actually only work out to some where around 204 & 212 in that ballpark ? thankyou , Bill..Comment
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I wouldn't worry about your computer with those small changes, it'll compensate for them easially.Originally posted by rvbilly View PostComment
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So what your saying is it doesn't change the effective duration... Doesn't make sense. yes the camshaft always happens at so the camshaft never gains duration when you change the ratio up top why??? I know you can't make the cam gain duration but you effectively lift the valve away from the seat quicker and with this change ratio your valve lift at .050 is shifted thus shifting where your lift point. Why wouldn't it shift the duration as much as it shifts the lift? Is it because of the grind profile? and you only see the result on the other side of the lifter? Because its only a 9.3% gain you would only shift the cam to a lift of .0546 and thus the reason you don't get a ratio increase on the cam duration.? So effectively your shifting it to a lift of .0457 for the same. This would happen on both sides of the cam why wouldn't it translate into more duration? Even that type of mathmatics results in a 5-6 degrees total duration gain. Say the cam is rated at .004 272 and you take the .050 at 216 and you then minus the difference in degrees which is 56 but you have to split that because its half that number meaning 28 degrees on onside and 28 on the other This is if the cam is even profile on both sides of the lobe. Most are. So it takes from .004 to .050 28 degrees of duration divide that and you get .0016 lift per degree it might be a little less or more depending on the profile. That means taking the .0457 which would be the difference of 9.3% at .050 you gain about 3 degrees. thats just one side now you got to do it on the down profile too cuss it passes .050 twice once up once down. so another 3 degrees. So it would be at a gain of 5 degrees on that camshaft lobe from 216 to 221
Sorry I told him that equation and i was not really thinking about it That is my fault. Sorry I guess I was just thinking about rocker ratio and lift and used the wrong math and just thought it looked right.Comment
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Trust me, it just doesn'y make that much of a difference in duration, usually less than 3 degrees overal, yes due to grind profile If lobes were square, you might be able to use that kind of math, but lobes are not square, and need a soft ramp, to keep the valve train in control. Yes, roller cams generally have a steeper ramp, but the basic lift off point whihc is where essentially that .050" lift measurement comes into play, is very similar on all cam shafts, especiallty when it comes to stock or street cams. If that point is too sharp, the valve itself can bounce, and/or lose control over the valve train, but actually having the rocker or lifter or other components actually lose contact with the next component in the valve train.
Yes, there will be a duration increase, but it's so minute of a change that it may as well not effect it.Comment
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