I have .48/.48 lift and 210/220 @ .05 duration right now with 1.5 roller rockers. This is a roller cam and has 112 deg. seperation. What kinda duration and lift will I have with 1.6 rockers?

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'89 Formula 383 TPI

 

you get 6.66% more lift with 1.6:1 ratio rockers, or about .512" lift with your cam. The duration difference will not be any more than 3 degrees.

 

Here is the way to calculate it. Take your .480" lift and divide it by the stock rr ratio. Example .480/1.5 This will equal .32. Now multiply .32 x 1.6, which equals .512" lift.

 

or .480 lift divided by 1.5, then times(X) 1.6

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350 4 bolt main, Speed-O-Motive 383 stroker kit, re-sized rods ground for clearance,TRW Forged pistons,10.7:1 compression,HPC Coated RPM intake, Speed Demon 750 cfm carb,World Products Sportsman II heads,2.02/1.60 valves, Comp Cam 1.6 roller rockers, Comp Xtreme 284 cam,240/246@.050 with .540 lift,Crane pushrods,ATI damper,Proform HEI and MSD6A box,HPC coated 1 5/8 headers, 2.5" exhaust with 40 series mufflers,TH350 with B&M 3000 stall, Powertrax NoSlip Locker, 4.11 gears.

 

I was under the impression that changing rocker arm ratios does not affect duration, only lift. How does a 1.6 rocker give you more duration than a 1.5?

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L98, 3.27 9-bolt, Hooker shorty headers, custom 2.5inch Y-pipe, no cat, 3inch 2chamber flowmaster, JET AFPR, Ported MAF, Best ET: 13.86 @100mph. 1.99 60'
17 inch ROH "ZS" wheels. 17x8.5(front) and 17x9.5 (rear). Firestone Firehawk SZ50s. 245/45/zr17s and 275/40/zr17s. T56

On the way... Radar Blue 89 Formula, T56, Ram Jet 430, Ram Air, 17inch ROHs.

 

Rockers do not affect duration.

 

Duration should not change at all. The only way you can play with duration is by increasing or decreasing valve lash on a solid cam.

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87 IROC-Z Pro ET Bracket Race Car
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87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

The .050" duration will change very slightly, it will increase to what used to be the .047" duration. Very tiny, a degree or 2 in most cases, but still there. The steeper the cam's ramps are, the less of a change it will be.

For all practical purposes though, the increased lift is the main thing. Also: if you are changing from stock rockers as opposed to something quality, you'll be getting rid of pieces with more or less random mechanical ratios between 1.4 and 1.45 and with all sorts of flex that gives away even more lift, and putting in something stable and predictable.

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"So many Mustangs, so little time..."
ICON Motorsports

 

Correct. Rockers do not affect advertised duration of the cam profile. However, since most of us measure effective duration, or duration at 0.050" cam lift (0.075" valve lift) the increased ratio of the rockers will cause the valve to reach the 0.075" lift sooner than the standard ratio rockers. This increases the effective duration, or duration of the valve at or above 0.075" lifts.

On an engine with the "LA" (LameAss) or "peanut" cam, this equates to an increase in effective duration of almost 4° on the intake lobes (the difference is actually 3.68°, but closer to 4° than 3°). If you plot the valve lift curve of the cam profile with standard rockers versus increased ratio rockers, you can measure the difference.

But the real cam profile duration cannot be changed by rockers alone.

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Later,
Vader
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