I know how to find a cam's lift with 1.6 rockers compared to 1.5's. My question is, do the 1.6 rockers also increase the duration like they do the lift ? If so, do you figure it the same way ? Thanks......

72Demon

 

No, the opening and closing points of the valves do not change with different ratio rockers.

 

take you lift numbers at 1.5 and divide them by 1.5 and then multiply by 1.6 or what ever your new rocker ratio is

 

68Bird....Thank you for the information

59Apache....Like I said before, I know how to find the lift from 1.5's to 1.6's, but thank you for reconfirming that I have been right.

 

Actualy the will change the lift because the higher ratio will have your valves reach .05 lift 1/3 of a thousands earlier than 1.5 at true 1.6 and 1.5 ratio. Not a significant amount but yes it does increase duration most likley (guesstimate) less than one degree.

 

sorry about that, i should read the questions more carefully from now on

 

Turb0racing is correct. With a 1.6 rocker the valve will reach 0.050 mils of lift before that of one with a 1.5 rocker. However, the increase in duration is very small and depends on the cam lobe shape. Also, when a cam manufacturer states duration at 0.050 mils, it is tappet lift not valve lift. So the duration they state is independent of the rocker size you use. That's what I was getting at.

 

Clarification:

0.050 inches
which is equal to
50 mils

Got a little mixed up, sorry.

 

I'm confused. What is mils?

 

Yeah, it can get confusing.

1 mil is defined as 1/1000 of an inch.

1 mil is the same as 1 thousandth or "thou" of an inch.

So,

50 mils = 50 thou = 0.050 of an inch.

Hope, it makes sense now.

 

Well, it's a little misleading. Most people talk about duration at 0.050" lift. Higher ratio rockers will increase duration at 0.050". Seat-to-seat timing does not change, as has been stated.

Rich Krause

 

OK, I guess I learn things no matter how long in the trade. BTW, that's about 35 years for me, with this being the 1st time I recall, it being presented this way. Maybe it's a regional 'thing'.

 

I agree that the ".050" thing is misleading. Fairly standard industry practice rates cam lobe duration at .050 of lobe lift, not valve lift. So, with 1.5's that's .075 at the valve and with 1.6's it's .080 at the valve.

Using Cam Dr. data from one of (y)our favorite hydraullic roller lobes, at .050 lobe lift the duration is 224 (crankshaft) degrees. If, however you looked at duration with the valve at .050, the "valve duration" becomes 240 degrees with 1.5's and about 241.5 degrees with 1.6's. At these low lobe lifts (.031-.033) you are still in the ramp area. Of course that's why .050 lobe lift was chosen for comparisons.

We do calculations and buy cams based on .050 lobe lifts. Well, if you want to do very accurate calculations, you use actual measured lift/angle numbers like those from Cam Doc.

So, my interpretation is that duration at the valve only increases a degree or two with a change from 1.5's to 1.6's but that duration is about 15-16 degrees more than the ".050 duration" we banter about.

What really happens is the area under the lift curve increases, with the greatest increase at the higher lifts. Of course that's where the valve is open for the least amount of time.

In the lobe cited above, area under the curve increases 3.26% with 1.6's over 1.5's (320.0 deg*sq-in vs. 309.9 deg*sq-in).

Because airflow thru the valve is approximately proportional to the area of the lift curve, my feeling is that the flow increase is about the square root of the ratio of the Rocker Ratio. 1.6/1.5 = 1.0667. The sq root of 1.0667 is 1.0328 or about 3.28% more.

 

I mis-stated the lobe lift thing and again manged to confuse it with valve lift. In fact, I have made this mistake many times. Thanks for clarifying. What I was trying to say was that the valve motion with higher rocker ratios will simulate a longer duration lobe at intermediate points on the lift curve. Obviously, the lifter is not effected.

Sorry if I confused things.

Rich Krause

 

That's what a forum of this nature is about, sharing thoughts and information, as well as second and third views on topics. It's good to see individuals sharing their knowledge. Thank you to all.