hello- i understand how 1.6 rockers affect lift but , do they affect duration also? is there a formula for this too?

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86 camaro 405 sm. blk. th 350 tranny 3.42 gears

 

I have thought about this alot.Most people will tell you no,but the way I see it is if you open the valve farther than the lifter is riding on the lobe longer which would mean more duration.I think Vader mentioned something about this a while back.

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We need Perry Mason

 

i was jus getting ready to ask this same ques i jus installed 1.65 rr

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1992 camaro RS
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If you read the info in some of the major camshaft company catalogs, like Crane and CompCams you'll see that going from 1.5s to 1.6s will give you a duration increase of about 2/3 degrees.

Many guys like to argue that there is NO duration increase, but I've seen that it does for many, many years. I proved it recently with my camshaft.

Recently I had my ZZ9 hydraulic roller checked on a computerized camshaft checker at Dennis Wells' Racing and it, too, reported that there was a duration increase with an increase of rocker ratio.

The duration change is so small that it's hardly noticable though. The BIG change is in the lift which will increase about .030 or so.

Jake

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1986 Corvette Coupe, 415 CID, Edelbrock 6073s, ZZ9

[This message has been edited by JakeJr (edited February 17, 2001).]

 

Duration is measured at the lifter, not the rocker arm. Therefore camshaft duration can not be changed by the rocker arms.

The advertised duration is when the lifter just starts to be pushed up by the ramp of the lobe. Different ratio rocker arms do not change this point.

A higher ratio rocker arm gives the camshaft the appearance of a taller lift lobe as seen from the valve train.

 

Yes the duration at .050 valve lift is increased slightly with increased rocker ratio. The valve opening event also happens slightly sooner giving the effect of a slight advance in camshaft timing.

What the camshaft does is not the issue. The important thing is the valve opening and closing events, and the rocker controls that.

ODB

 

I dont see how it could increase duration. Although it may increase the amount of duration you are able to measure, it was still there before, it was just too little lift to notice. Theoretically, the valve is open for the same amount of time, only it is open higher.

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"A four cylinder is only half an engine"

 

Ward,

You are correct. The advertised duration of a given cam profile cannot be changed by varying the ratio of the rocker arms. However, if you research the specifications for most camshafts, you will notice the advertised duration, and usually the effective duration, measured at 0.050" lobe lift, or 0.075" valve lifts.

The advertised duration of a camshaft profile is a larger number, but since there is little meaningful flow past a valve at anything less than 0.075" valve lift, and many valve trains have that much lash, the effective duration numbers are more significant. The effective duration IS increased with higher ratio rockers, since the valve reaches the 0.075" lifts SOONER than with a lower ratio rocker, and is subsequently held open LATER than with standard rockers. This creates a slight increase in effective duration, because the area "under the curve" of a given cam profile is larger/longer than with lower ratio rockers.

The advertised duration - the point at which the lobe ramps begin and end, can't be changed with the rocker, but the meaningful lift duration of the valves is increased slightly. The exact number of degrees depends on the profile, and should be expressed more as a percentage than a number of degrees. It is most accurate to plot the cam profile and measure the extra effective duration.

The addition of higher ratio rockers is similar to but opposite of the effect of fast-bleed hydraulic lifters.

This is a plot of a typical TPI "LA" cam with 1.5:1 and 1.6:1 rockers:



The graph plots may not be exact to the numbers, but demonstrate the effect pretty clearly.

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Later,
Vader
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"No matter how hard you try you can't stop us now..."
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[This message has been edited by Vader (edited February 18, 2001).]