MotorHead

I know the 1.6 : 1 rockers change the lift of the cam but it also must change a few other things like duration etc. For example if I had a 270 H cam, put the 1.6:1 rockers on would it then have more characteristics of a 280 H cam ?

Fevre

I don't think it will change durations since it is open the same amount of time but opens higher so the valve speed is increased but no duration increase. The valve speed has to increase since it is traveling a farther distance in the same amount of time.

Pete79L82

Measured at full lobe profile a 1.6 rocker will not change the duration of a hydraulic cam. Measured at .050 lift it will change the duration because the nose (valve side) of the rocker will be moving faster with a 1.6 rocker so it will see .050 lift on the leading ramp quicker. It will also hold the valve open longer on the trailing ramp of the cam before it closes to .050. You would have to plot out the valve events to be sure but I doubt it would be more than 2 or 3 degrees at most.

Fevre

Gotta quick lessons in '.050 lift'? I have heard/seen that term but never heard/see an explianation of what is means. Thanks

Aaron-74

At .050 lift your cam is putting out XYZ durration.

Aaron-74

For example, my lift is .460 (lift with standard 1.52 rockers) / 1.52 = .302 (cam lift) .302 X 1.6 = .484. (your lift with 1.6 rockers)

It doesn't change duration. Higher ratio rockers open the valve faster though.


[Modified by Aaron-74, 4:19 PM 7/19/2002]

Pete79L82

Duration is measured from the point that the lifter moves off of the cam base circle, (say the first .001 of lift) until the the lifter goes through full lift and returns to the cam base circle, (say the last .001 of lift). The problem in comparing cams is the first .050 lift can be very gradual (giving you a very long duration number cam) or very quick (giving you a very short duration number cam) even though the lobe profile from .050 are the same between the two. Comparing duration numbers from different cams at .050 will give you a more accurate comparison.

Pete79L82

With 1.5 rockers it takes .033 lifter movement to open the valve .050. With a 1.6 rocker it takes .031 lifter movement to open the valve .050, which means the valve opens sooner and closes later. So a 1.6 rocker will increase valve duration even though it is minimal.

Fevre

I have to say I am with Aaron on this one. If you look at the lifter indepedant of the rocker it stays open a set amount of time since it predetermined by the cam and lifter profile. Changing the rocker arm ratio would only change the lift height and lift speed. I guess if you used a calc equation and used nul and infinety you could get some duration difference but it would be so small as to be for all pratical purposes zero. When I say small I mean like .00000000000000001 of a degree small not a degree or two small. Why did I have to think of calc? :confused:

Pete79L82

If you measured at the lifter it would make no difference but lift and duration are measured at the valve not the lifter. If you measured your duration at the lifter it would be totally different than the advertised duration of the cam.

I plotted the valve events of a Comp Cam 270H 224° at .050 with .470 lift and the difference between a 1.5 and a 1.6 rocker is 1.341 degrees. So with a 1.6 rocker the duration at .050 is 225.341°


[Modified by Pete79L82, 9:02 PM 7/19/2002]

Fevre

Are you sure? Then how do you determine the your valve duration with a specific cam if it is not directly related to the cam duration? Guess I just see the valve train is one unit from the lifter to the valve. In my mind as soon as the cam started the lifter up, the valve would open and as soon as the valve let the lifter seat, the valve would be closed.

I would like to know how valve duration is calculated if you don't mind. Trying to learn as much as possible.

Thanks

Chris

Pete79L82

Duration is related to the cam but it is measured at the valve. Because of this you have to take into consideration everything in the valve train.


Advertised duration (270° on a 270H) is the point that the valve first moves off the seat until the valve returns to the seat. As I said earlier, useing that number it doesn't matter what rocker arm is used the duration will be the same because everything as far as that valve is concerned has stopped moving.

Once the cam starts to move the lifter then it makes a difference because the higher ratio rocker arm will get the valve to a specific opening height faster (earlier). At 0.050 the 270H cam with a 1.5 rocker is opening the valve at a rate of .00311 at each degree of rotation. With a 1.6 rocker the lifter has to move the rocker arm .00208 less to still get a valve lift of .050. That calculates to .6705° on the opening ramp and the same on the closing ramp for a total of 1.341°

Again the duration at the cam is the same but the duration at the valve (which is where cam specs are measured) is different because of the speed of the nose of the rocker arm.

You have something similar happening with a solid lifter cam. If you tighten the lash from .030 to .015 the cam has less lash to take up so it will start to open the valve sooner and hold the valve open longer (more duration) and also give you .015 more lift.

All these changes are very small but on a dyno these changes will tell you what direction a motor will like. On some motors installing 1.6 rocker or tightening the lash will help the motor make horsepower telling you the motor would like more cam, on other motors those changes will drop horsepower telling you the motor wants less cam.

MotorHead

I want specifics ! :lol:

All kidding aside I was looking for a technical answer and I got exactly what I was looking for Thanks Pete79L82 ! :D

Taijutsu

The increased lift usually gives 10-15 hp depending. Side benifits are much cooler running "that's why I got them" and the sweet spot on my engine went up about 200 rpm. Can't comment on mpg.

Fevre

Thanks Pete

I must say I am still not sold on the idea but I can offer no hard evidence to contridict it. Thanks for taking the time to explain it.

Chris