[All4Fun]

I know how to calculate how much lift is gained by changing from a 1.5 ratio to another ratio ie. [(lift/1.5)X1.65] Is there a formula to calculate exactly the additional duration gained from the change?
John

[All4Fun]

I know how to calculate how much lift is gained by changing from a 1.5 ratio to another ratio ie. [(lift/1.5)X1.65] Is there a formula to calculate exactly the additional duration gained from the change?
John

[Rocky Rotella]

When I spoke with Nunzi he said that jumping from 1.5 to a true 1.6 my cam gained about 3 degrees duartion and .030" lift. 1.5 to a true 1.65 should get you about 4 degrees duration and .040" lift.

[All4Fun]

Thanks Rocky!
I should have been a little more specific. I am adding the Harland Sharpe 1.65's which I believe are a true 1.70 ?
I am trying to get an idea of what the change will be in duration for input into the Desktop-Dyno. I am using the OEM 068 .408/.407 215/225 dur @.050 New lift.462/.461 dur. ???/???
John

[Chambers]

Look at your cam card and multiply the lobe lift by the rocker ratio, it will give you the EXACT valve lift. Don't know about how to determine exact duration increase, if any!

[Will]

There really is no duration increase since the cam itself has not changed and the valves are open the same total amount of time.

Since .050" duration is measured at .050" tappet lift, not valve lift, then with a higher ratio rocker arm, the valve will be open further at .050" tappet lift.

You can think of the duration increase with high ratio rockers as the number of degrees before .050" tappet lift that the valve is open to the same point it would be open at .050" tappet lift with lower ratio rockers.

Unfortunately, without knowing the exact amount of lift for each degree of cam rotation, you cannot accurately calculate the effective duration increase. A rule of thumb of 3-4 degrees is probably a good bet. Always best to bet conservative on this stuff anyway.

[Cliff R]

Although the actual opening and closing events do not change when adding the higher ratio rockers there is an "effective" duration increase. The cam will act bigger since everything it was doing before is now multiplied by the percentage increase of the rockers. I was at KRE several weeks back breaking in a 455 on the dyno and we made the change from 1.5 rockers to 1.65 rockers. Nearly as I can remember the horsepower increase was around 12. Another interesting note though a bit off subject is that changing from three tube to 4 tubes headers netted an additional 38hp!

[PONTIAC DUDE]

Sorry Will, not trying to be a jerk, but:
Yes, there is a duration increase. your rocker will open faster thus creating a longer duration for the lift quoted. I.E. .050/.200 etc, etc. It's what the Engine will see and use as horsepower. You will obtain .200 quicker thus achieving greater duration and more lift sooner. Put the 1.5's on your engine and take it to .050 and read the degrees. Now take the 1.65's and do the same, then go to .200 with both. It will change the location sooner, creating more duration. Only zero stays the same. Comp Cams has a chart to use for their cams to show the difference. Going from 1.5 to 1.7 in my application has given me a theoretical 6 degree increase. Later.

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NOS is like American Express.....You don't leave home without it.
http://www.angelfire.com/fl4/pontiacdude428

[All4Fun]

Let me get this right. When I degree this in, I should be able to tell the increase in duration on the degree wheel. Yes?No? Or is this increase what the engine sees and I cannot. Should I degree it first with the 1.5's and then check the 1.65's and then figure the change? I could try this and let you all know the results some time next week. If anyone cares that is.
Thanks guys!
John

[Tim Corcoran]

Duration is the number of cam shaft degrees of rotation that the valve is open, off it's seat. This is controlled by the cam lobe. Changing the rocker arm ratio can make the valve open faster, and open further, but can't change the actual duration. Cam shaft manufacturers measured duration differently. Since actual duration is difficult to measure (seat to seat) it is usually measured with the valve partially open. Comp Cams measures it at .006 and .050. That means when if you measure how many degrees of rotation from when the valve opens to .050" to when it is .050" from closing you will get the duration at 050. Here is where the confusion is, since the valve opens faster with a higher ratio rocker arm if you measure the duration at .050 or .006 the valve will reach .050 or .006 with fewer degrees of rotation since it is opening and closing faster. But, the actual number of cam shaft degrees that the valve is open (duration) can not change without changing the cam. So you don't get more duration by changing the rocker arms, it only appears that you do if you measure the duration when the valve is off it's seat.

Tim Corcoran

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65 LeMans 11.13@120

[Jim Hand]

There is information on Eric D's site that clarifies the effects of higher ratio rocker. And it clearly illustrates the added duration that will be obtained at .050 with higher ratio rockers. Jim Hand
http://www.wauknet.com/douthitt/rocker_arms.htm

[PONTIAC DUDE]

To put it in perspective. The cam is just the focal point in relation to crankshaft degrees. What the motor sees is in the chamber. If the valve opens sooner (and faster off the seat) in relation to the piston coming to TDC, then there is a duration (and lift)change. Whether it's at .050,.200 or even .350. Degreeing a cam is done at the lobe (.020 to .050) to set it in relationship to the manufactures grind and basically what you required for your application. Same applies to tightening down or loosing the lash to see if you need more or less camshaft. You are changing the spec from the manufactures recommendation. If you tighten lash, say, .010, then you have effectively change the duration as the valve will open sooner. The cam at the lobe hasn't changed but you changed timing.
All that is really important is what is happening in the cylinder, as this is where horsepower is made and the results show. Hope this helps and isn't to confusing. Rocker ratio can make or break horsepower.

------------------
NOS is like American Express.....You don't leave home without it.
http://www.angelfire.com/fl4/pontiacdude428

[Torment]

I think it comes down to showing more "area under the curve" as cam manufactures call it. Whether you look at this as synonymous with duration or not is just perspective. Like Pdude said zero to zero doesn't change with respect to the cam lobe but it will effectively "act" larger/smaller depending on valve lash and rocker ratio.

[Skip Fix]

Tim, you said it in your post, duration is the time the VALVE is off the seat. Higher ratio rockers do this, when you look at the "equivalent" duration of 0.050 and 0.200 duration at the valve. No they can't change the duration of the cam as that is ground in and when you degree cams as you said it is referenced off tappet lift how you measure.

But the "equivalent" increase in duration(actual at the valve duration) at 0.050 is add 2 degrees per 0.1 ratio increase over 1.5.(1.7 add 4 degrees @ 0.050) Since the "area under the curve" (i.e. the amount the valve is off the seat as shown in the graph at the site mentioned) is increased with the higher ratio the motor "feels" the same as if it had a bigger duration cam with 1.5 rockers.

Seat duration and seat overlap do have a large effect idle but the motor still feels the extra "equivalent" duration increase also. Throw a vacuum gauge on before and after a rocker change, they do affect idle. Not to the extent that a cam with bigger seat duration does but they do affect it. When I was running NMCA Top Stock and they factored motor hp based on idle vacuum I tried alot of different things and increased rockers do change that. I still believe in them but the motor does "feel" the increase also.

[Will]

Boy, I really didn't write that very well did I...

Pontiac Dude, you're definitely not being a jerk, just correcting what on a second reading appears to be very misleading information.

What I was trying to say is the fact that the duration of the cam only appears to have increased (the effective duration) because the valve is open further sooner, not because the cam has actually gotten larger. In truth, the lifter will still be up .050" off the base-circle of the cam lobe at the same point, the valve will just be open further at that point. This does have the same effect as installing a slightly larger cam. I was just arguing semantics.

Note to self: Don't argue semantics anymore.

If you measure the valve lift at the valve with the degree wheel installed then you should be able to easily measure the effective duration increase.

Cheers,
-Will

[All4Fun]

Thanks for the help guys!
I will definitely play around with the degree wheel and see what I come up with. I was just having trouble understanding how the duration could be increased when the opening and closing points don't change. I had heard that they increased "effective" duration, but the idea was so abstract to me I could'nt understand it. The lift gain and the accelerated opening and closing I understand, that was what I was after by going to these rollers. Also I'm running Total Seal rings so I wanted to keep valve guide wear to a minimum.

Never too old or too smart to learn!!
John

[Bud Lyons]

Unless your Gach. That poor boy will never learn.