whats the differance between 1.5 and 1.6 rocker arms?

 

0.1
Rocker arms are a lift multiplier. Let's say I had a cam with a .500 lift (which I do). Those advertised numbers are relating to the lift at the valves based on using stock ratio 1.5 rockers. The actual cam lobe lift would be around .334. (.334 x 1.5 = .500).
Now install 1.6 rockers (.334 x 1.6 = .534). You've just added more valve lift with the 1.6 rockers.

 

and a wee bit more duration because the valve opens sooner and closes later.

But, unless you have new valve springs, I would not suggest 1.6's in your stock valve train. If you raise your right hand and promise not to rev beyond 5k rpm's then you might be ok with stock springs

 

I'll have to respectfully disagree with that there then. Duration refers to how long a valve is open in relation to the degrees of rotation on the crank. For instance, a cam with a duration of 230 means the valves will be open a total of 230 degrees of the cranks rotation (one full rotation is 360 degrees). Changing the lifters makes the valves lift higher yes, but it doesn't alter the grind of the cam (sooner-where the valve starts to lift, and later-where it's fully dropped), which is what determines the cam to crank degree relationship (duration).
I do agree that it's a good idea to make sure your springs and clearances can handle the extra lift. "Oops" never goes over very well when it comes to the valve train.

 

I know why you say that. I also simply look at the lift increase however, basic geometry comes into play as long as when the valve opens and closes. In fact, this has been discussed in a tech article that I'll find for you. I too thought the question of increased duration would be answered as "nope" but the article shows a minimal increase.

 

here it be

http://www.carcraft.com/techarticles...est/index.html

 

quick question about this:
in 1.5 rockers, is the 1.5 a result of the ratio between the distance between the pushrod contact point and the rocker bolt and the distance from the rocker bolt to the spring cap contact point?
(pushrod to center/center to spring). If so then your calculation makes sense. But i was under the impression that the rocker was a direct measurement either of the arm distance or something like that. If so then the actual lobe lift couldnt be calculated from the numbers given. I have seen similar calcs done, but never calling the .334 number the actual lobe lift. usually they refer to a stock setup with the 1.5 rocker, and then do a ratio calculation to find new lift. i.e. if lift was .500 with a stock 1.5 rocker, the lift would be .500*(1.6/1.5)=.534 with a 1.6. but this calculation uses the ratio in a way that skips any actual cam measurements.

I honestly dont know either way. I just wanted to bring up how I have seen this done in the past
I gotta agree with 69 here.... kinda... we are kind of getting into technicalities here.

to be specific, the rocker arm ratio wont affect duration. an easier way to think of it is to imagine a theoretical cam. Now take this cam and just put a spike on the lobes at its max lift to add another 25% lift (and we will just pretend that this wont grenade an engine... in this imaginary engine it can handle a crazy spike on the cam profile). the parts of the lobe that control when it starts lifting or is finished lifting are unaffected so duration isnt affected - we just made it so that for a portion of its travel it is moving faster than it would have otherwise - but it does this faster movement within the time frame it was previously moving.
apply this to a real cam with a multiplier. a lift of 0 multiplied by anything is still 0. So any time the lift was 0 before the multiplier it HAS to be 0 after.

however, on a technicality most people don't start talking cam figures on anything under .050 lift. so, if we are talking this point then TIG is right. while the 0 points remain the same, the rocker arm multiplier will get us to .050 sooner than it would have before, resulting in a small gian in duration as measuered from .050"

ok, im done talking... the lack of sleep and surplus of coffee is kicking in and i am sick of going back to correct spelling errors due to my jittery fingers....

 

I read the article on the rockers but I dont see the math on the duration. I see the math on the lift, and I understand the math with the flow of the heads but I cant see how it will effect duration. The valve will still start to open at the exact same time in the cam's rotation. it will open faster and farther due to the ratio of the rocker. It will also close faster, thus leaving the duration the same as with the 1.5 rockers. There will be a gain in performance so long as the heads can flow more air, but that is not because of duration but more from less restriction. IMO duration does not change, lift time and lift amount is all that changes.

Massey

 

this is an awesome discussion where it's going. Even at .050, one of the articles reported about 3 degrees. I'll look for it later. Again, it isn't significant to get excited about however it is there. I too used to think it was just lift but they had a graph. I'll find it.

but, please note that i said a "wee bit"

these are the kind of discussions I enjoy in these forums. It's a lot more mind challenging than "which cat back system"

 

^ i will give it to you if we are discussing .050, but the nature of the mechanics says it MUST be a 0 degree change for total duration. it is absolutely impossible for total duration to be affected by a rocker change. if they had a graph claiming that then they are wrong