[slowbird]

Since i see more cam discussion in this section, thought id post this hear. Is there way to calculate the .050 and .200 duration increases from a rocker ratio increase?

[Steve C.]

Rocker ratio's effect on duration

https://www.speed-talk.com/forum/viewtopic.php?t=11349

More here:

http://www.superchevy.com/how-to/eng...cker-arm-ratio


.

[1968GTO421]

Originally Posted by Steve C.

Rocker ratio's effect on duration

https://www.speed-talk.com/forum/viewtopic.php?t=11349

More here:

http://www.superchevy.com/how-to/eng...cker-arm-ratio


.

Thanks, Steve. It sounds like he would gain a minimal amount of duration, gain some "under the curve", but most of the gain would be in the increased lift. (JMO)

[Steve C.]

Valve lash also affects the duration. But it will depend on the design of the lobe itself with different velocities. And more often than not different on the opening side vs the closing side.

Example UltraDyne solid roller lobe R10 rated 288 degrees at .020" tappet lift with a 1.5 rocker ratio and .026" lash. I'm using 1.65 rocker arms with it and at that ratio it has 292.9 degrees with a .026" lash. However with the 1.65 ratio and .022" lash it has 300.7 degrees duration.

Those duration numbers are the total between the opening side and the closing side of the lobe. UltraDyne cams the closing side is slower and therefore longer than the opening side. Example on Harold's tighter lash cams, about .006" tighter changes the duration about 5 degrees, two on the opening side and 3 degrees on the closing. With real slow ramps, the ratio may be 1 to 2


.

[Stan Weiss]

Brian,
I don't know if this will be of any value to you The more aggressive the cam is the greater the increase will be. Do you have a cam that you looking at or is this just a general question?

Stan

Originally Posted by Stan Weiss

Jay,
I don't know if this will be of any value to you. The theoretical valve lift curves with those rocker arm ratios.

Stan

Rocker_Arm_Ratio_=_1.650
VALVE_____Lift______Opens___Closes__Duration
_________________Deg_BTDC__Deg_ABDC_____________Ar ea
_________0.00600____32.23_|__94.87_|_307.11_|__42. 08
_________0.02000____22.97_|__74.67_|_277.64_|__41. 90
_________0.05000____11.55_|__58.06_|_249.61_|__41. 45
_________0.08250_____3.00_|__48.00_|_231.00_|__40. 88 <- 0.050" lifter raise
_________0.10000____-0.82_|__43.82_|_223.00_|__40.47
_________0.15000___-10.36_|__33.74_|_203.38_|__39.22
_________0.20000___-18.97_|__24.81_|_185.85_|__37.73
_________0.25000___-27.43_|__16.05_|_168.62_|__35.83
_________0.30000___-36.32_|___6.83_|_150.51_|__33.21
_________0.35000___-45.98_|__-3.04_|_130.98_|__30.11
_________0.40000___-56.94_|_-13.97_|_109.10_|__26.18
_________0.45000___-70.54_|_-27.44_|__82.02_|__20.23
_________0.50000___-91.93_|_-48.72_|__39.35_|__10.20
_________0.51000__-100.31_|_-57.07_|__22.62_|___5.65

========================

Rocker_Arm_Ratio_=_1.7
VALVE_____Lift______Opens___Closes__Duration
_________________Deg_BTDC__Deg_ABDC_____________Ar ea
_________0.00600____32.41_|__95.27_|_307.67_|__43. 36
_________0.02000____23.28_|__75.19_|_278.47_|__43. 19
_________0.05000____11.99_|__58.64_|_250.63_|__42. 71
_________0.08500_____3.00_|__48.00_|_231.00_|__42. 11 <- 0.050" lifter raise
_________0.10000____-0.21_|__44.49_|_224.28_|__41.75
_________0.15000____-9.57_|__34.57_|_205.00_|__40.56
_________0.20000___-17.98_|__25.84_|_187.87_|__39.08
_________0.25000___-26.17_|__17.36_|_171.19_|__37.17
_________0.30000___-34.71_|___8.51_|_153.80_|__34.83
_________0.35000___-43.91_|__-0.94_|_135.15_|__31.91
_________0.40000___-54.19_|_-11.25_|_114.56_|__27.79
_________0.45000___-66.54_|_-23.48_|__89.99_|__22.67
_________0.50000___-83.87_|_-40.69_|__55.43_|__14.56
_________0.51000___-88.87_|_-45.67_|__45.46_|__12.04
_________0.52000___-95.33_|_-52.10_|__32.57_|___8.43
_________0.53000__-107.95_|_-64.72_|___7.34_|___2.12

========================

Rocker_Arm_Ratio_=_1.8
VALVE_____Lift______Opens___Closes__Duration
_________________Deg_BTDC__Deg_ABDC_____________Ar ea
_________0.00600____32.72_|__95.99_|_308.71_|__45. 91
_________0.02000____23.83_|__76.19_|_280.03_|__45. 74
_________0.05000____12.84_|__59.72_|_252.56_|__45. 27
_________0.09000_____3.00_|__48.00_|_231.00_|__44. 59 <- 0.050" lifter raise
_________0.10000_____0.95_|__45.74_|_226.69_|__44. 31
_________0.15000____-8.11_|__36.10_|_208.00_|__43.18
_________0.20000___-16.15_|__27.73_|_191.59_|__41.69
_________0.25000___-23.88_|__19.73_|_175.85_|__40.00
_________0.30000___-31.80_|__11.52_|_159.72_|__37.80
_________0.35000___-40.23_|___2.81_|_142.58_|__34.87
_________0.40000___-49.45_|__-6.52_|_124.03_|__31.48
_________0.45000___-60.00_|_-17.00_|_103.00_|__27.24
_________0.50000___-73.27_|_-30.15_|__76.58_|__20.58
_________0.51000___-76.51_|_-33.38_|__70.11_|__19.06
_________0.52000___-80.09_|_-36.93_|__62.98_|__17.26
_________0.53000___-84.13_|_-40.94_|__54.93_|__15.16
_________0.54000___-88.87_|_-45.67_|__45.46_|__12.75
_________0.55000___-94.90_|_-51.68_|__33.41_|___9.48
_________0.55000___-94.90_|_-51.68_|__33.41_|___9.48
_________0.56000__-105.19_|_-61.93_|__12.88_|___3.65

[Stan Weiss]

Brian,
Maybe this will show it better. The red line is an after market 041 lobe for use with a 1.65:1 rocker arm. The green line is a lobe when used with a 1.5:1 rocker arm will produce the same valve lift curve.

Stan

[Steve C.]

Your going to get different answers and opinions as you do your Google searches. Another:

Changing Rocker Ratios

"Let’s say to replace the stock 1.5 ratio rocker arms with higher lift 1.6 ratio rocker arms. The same camshaft will now produce .512˝ of lift at the valve (.320˝ times 1.6). So by simply swapping the stock rocker arms for higher lift rocker arms, you increase total lift 6.7% and probably gain 15 to 20 horsepower. How does changing rocker ratios affect the duration of the camshaft? Because the cam lobe is still the same, the point where the cam starts to move the lifter is still the same. The same is true for the closing side of the ramp. But, the rate at which the valve opens is now somewhat faster because of the higher ratio of the rocker arm, so the effective duration of the camshaft is increased slightly – maybe a couple of degrees in the above example."

And not it says EFFECTIVE duration.

This follows right along with 1968GTO421 comment above.

And Jim Hand talks about it:
http://www.pontiacstreetperformance....ockerArms.html


.

[tom s]

OK so Mike says the lash needs to change.Now what about hyd cams?Tom

[Steve C.]

To complicate matters a hydraulic lifter acts like a shock absorber, the lifter "gives" just a little as you load it under under running conditions. From their Spintron research engineers at Comp Cams actually see running valve duration decrease as rpm increases due to this increased inertial loading. The faster the cam spins, the harder it hits the "shock absorber", and the lifter loses some valuable duration as a result. Their data indicated a "real world" dimension for a running engine with hydraulic lifter tappets.

Example the popular XE hydraulic roller lobe 3315 rated 230 degrees at 0.050".
With a 1.5 ratio rocker ratio the static duration measured 'at the valve' is 243 degrees at .050" tappet lift. However at that same .050" tappet lift under actual running condition it was measured with 240 degrees 'at the valve', or a loss of three degrees duration. Further under actual running conditions there can be as much as 10 degrees less seat duration.

.

[Skip Fix]

Comp used to have a chart and I think the 0.050 on it was about 2 degrees per .1 ratio change.

My pump gas motor made about 5-8 hp per bump starting at 1.5s to 1.72 HS and then the 1.8 Crowers. UD 239/247 @ 0.050 HFT-broken in with Crower SBF 1.33 break in rockers but no dyno pull with them.

[slowbird]

Originally Posted by Stan Weiss

Brian,
I don't know if this will be of any value to you The more aggressive the cam is the greater the increase will be. Do you have a cam that you looking at or is this just a general question?

Stan

Lobes are 294-242-164 (adv-.050-.200) .360lobe lift and 318-252-164 .360

[pastry_chef]

Originally Posted by slowbird

Lobes are 294-242-164 (adv-.050-.200) .360lobe lift and 318-252-164 .360

Comp lobes?
If yes and you specify lobe numbers, the two rocker ratios and valve lash I can provide good numbers.

[Steve C.]

There are computer programs that deal with the related subject.
Not specific but of interest:

http://www.originalho.com/Calculators/Calculators.html

http://www.audietech.com/DynomationFeatureFocus3.htm


pastry_chef can attest to this.

Now as I 'sometimes' do, add another realted topic ..... bring this to the coffee table for interest: Pushrod flex and how it might affect duration.

From Manton....

Higher ratio rockers will increase the load on the pushrod because the rocker is moving the valve side faster. This means more pushrod flex as the valve starts to open. To get the most performance from higher ratio rockers a rigid pushrod is a must. Do not be overly concerned about pushrod weight. The pushrod is on the slow moving side of the valve train. The additional weight of a heavy wall pushrod usually provides a much needed increase in valve train stability.

Typical .080-.082 wall or use 0.118" with little cost difference ?


.

[slowbird]

Originally Posted by Steve C.

There are computer programs that deal with the related subject.
Not specific but of interest:

http://www.originalho.com/Calculators/Calculators.html

http://www.audietech.com/DynomationFeatureFocus3.htm


pastry_chef can attest to this.

Now as I 'sometimes' do, add another realted topic ..... bring this to the coffee table for interest: Pushrod flex and how it might affect duration.

From Manton....

Higher ratio rockers will increase the load on the pushrod because the rocker is moving the valve side faster. This means more pushrod flex as the valve starts to open. To get the most performance from higher ratio rockers a rigid pushrod is a must. Do not be overly concerned about pushrod weight. The pushrod is on the slow moving side of the valve train. The additional weight of a heavy wall pushrod usually provides a much needed increase in valve train stability.

Typical .080-.082 wall or use 0.118" with little cost difference ?


.

3/8 pushrods are handy for situations like this

[Jay S]

Ball park rocker ratio duration change estimate at .050”.

Beginning ratio = 1.5

New ratio = 1.65

.050 X 1.5 / 1.65 = .0455

.050 - .0455 = .0045. Net change in tappet position

.0045 X 1000 = 4.5 *. Hydraulic cam


For a solid cam the lash is removed. Say .018” lash with 1.5s. Now lash is adjusted to .0198” ... or .020”

Subtract .0198 - .018 = .0018”

.0045 - .0018” = net change at valve from rocker change. = .0027 x 1000= 2.7* removing lash.

.050” changes are fairly consistent. .2” changes more with the cam design. The 1000 factor gets fairly close estimate. If you know what the lifter rise is you can adjust that accordingly.

[slowbird]

Slight change if subject, when putting a solid lift on a hyd cam (roller) with the tight lash and zero shock adsorption (like a hyd lifter has) is there any loss in duration/lift on a running engine?

[Stan Weiss]

Originally Posted by slowbird

Lobes are 294-242-164 (adv-.050-.200) .360lobe lift and 318-252-164 .360

I am going to guess Comp Lobe 3317

1.6:1 244.3 170.3
1.65:1 245.3 173.1
1.7:1 246.3 175.7

Stan

[Jack Gifford]

BY DEFINITION:
.050" duration & .200" duration are characteristics of a camshaft lobe profile.

Carry on...

[slowbird]

Originally Posted by Jack Gifford

BY DEFINITION:
.050" duration & .200" duration are characteristics of a camshaft lobe profile.

Carry on...

You are correct, i should have been more specific. What is going on at the valve duration wise with the higher ratios

[70GS455]

To answer the OP, you would need to know the velocities (slope) to determine the effective change in duration.

The equation then becomes: delta_duration = (2/velocity)*lift_ point*((ratio2/ratio1) - 1)

Sent from my SM-T817V using Tapatalk