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Cam and rocker ratio info.
While Thumbing through a DV book on BBC motors once again I came across info I had failed to recall.
DV gave very detailed info that I will try and condense. If the Cam is of a solid type and has been designed to work with 1.5 rockers and is working well as part of the overall combo then if a change to higher ratio rockers is implemented these changes are needed . To get the full benefit of that rocker change because the change itself may not provide any benefit, and indeed could reduce both torque and Hp. When I higher ratio rocker is used the motor thinks that the Cam has been advanced. The faster opening rate changed the intake part of the overlap triangle, it has increased in area. This needs to be compensated for by retarding the Cam 1 to 1.5 degrees especially if a ratio increase has gone from 1.5 to 1.7 or more. The lash needs to change also with the higher ratio to keep the ramp on the lobe working as intended. In going from a 1.5 rocker to a 1.65 the lash ( minus expansion) needs to be increased by some .004”. If this is not done DV then states that the advantage of a faster opening rate can be lost. I found this very enlightening and this all assumes that in each case the geometry was adjusted very well. |
X2 Very enlightening, thanks Steve!
Frank |
Valve Lash with a change to higher ratio rockers
Divide the Old Lash by the Old Rocker Ratio, then multiply by the New Rocker Ratio to get the New Lash. How does the ramp design*factor in ? This from Harold Brookshire ..... The way I design solid lifter ramps you can loosen them up at least .004", and tighten them down at least .008", on the normal, ie--.017" ramps. On tighter lash cams, you can run them about .006" looser, and .006" tighter, because they generally have lower opening velocities, because of the tight lash. On either type of ramp, the closing side is slower and therefore longer than the opening side. About .006" tighter changes the duration about 5 degrees, 2 degrees on the opening side and 3 degrees on the closing. With real slow ramps, the ratio may be 1 to 2 . |
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Steve not sure I agree totally the increased ratio adds duration on opening as well as closing. Comp has a nice graph of this.
On my 455 in my 78 right now it is a HFT so the lash goes away as a debate factor but UD 296/312 we picked up HP with each bump of ratio from 1.5 -1.65(1.7 actual) and 1.8. We did not make a pull with the 1.33 break in rockers. My 470" 409 single pattern SR no difference on dyno between 1.7 and 1.8 rockers Since lash changes alone can make differences on the same ratio-we found that out on my IA 2 with the same ratio shaft system SR. I think that would be a hard A to B comparison to actually see on the dyno comparison. Tightened exhaust and loosened intake made the best HP and a lobe designed for higher ratio rockers. |
"An increase in rocker arm ratio nets more than additional lift. It will also change the cam's duration characteristics. Because the increased ratio effectively speeds up valve movement, that means the valve will reach any opening height sooner than it would with a lower-ratio rocker arm. Higher ratios open the valves quicker and close the valves a little later. Since the increase is symmetrical on either side of the cam lobe centerline, a higher ratio will lengthen the overall valve timing, making your cam act bigger. The higher ratio also causes valve timing to increase proportionally as the valve opens farther."
Source: Rocker Arm Ratio - Power Multipliers https://www.motortrend.com/how-to/su...20rocker%20arm. . |
Going from a 1.65 to 1.75 rocker ratio 5- 6 degrees more duration thats what, comp cam came up with on my combo. Not sure how accurate that is but definitely incase duration
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But you also have to make sure there’s enough install hight as well as seat and open pressure, And check for coil bine, so be very careful if your thinking of incasing rocker ratio.
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Rocker ratio's effect on duration
https://www.speed-talk.com/forum/viewtopic.php?t=11349 Note Mike Jones example provided and his comments in this thread. . |
This is a comp cams solid roller cam for a SBF that I have a Cam Dr file for. It is 267.84 @ 0.050" lifter raise.
It was designed to use 1.65:1 rocker arms and I also show it with 1.8:1 rocker arms. _______I__N__T__A__K__E Rocker_Arm_Ratio_=_1.650_________Valve_Lash_=_0.02 100________Valve_Angle_=_20.0 VALVE_____Lift______Opens___Closes__Duration _________________Deg_BTDC__Deg_ABDC_____________Ar ea _________0.00000____47.69_|__84.37_|_312.05_|__61. 01 _________0.00600____44.55_|__80.80_|_305.35_|__60. 99 _________0.01000____42.73_|__78.67_|_301.39_|__60. 98 _________0.02000____38.71_|__73.99_|_292.70_|__60. 91 _________0.04000____32.32_|__66.84_|_279.16_|__60. 69 _________0.05000____29.68_|__63.94_|_273.61_|__60. 56 _________0.10000____18.96_|__52.59_|_251.56_|__59. 70 _________0.15000____10.52_|__43.84_|_234.35_|__58. 68 _________0.20000_____3.06_|__36.19_|_219.26_|__57. 27 _________0.25000____-3.97_|__29.07_|_205.10_|__55.41 _________0.30000___-10.85_|__22.20_|_191.36_|__53.71 _________0.35000___-17.79_|__15.20_|_177.41_|__51.04 _________0.40000___-24.96_|___7.98_|_163.03_|__48.74 _________0.45000___-32.52_|___0.38_|_147.86_|__45.30 _________0.50000___-40.80_|__-7.88_|_131.31_|__41.46 _________0.55000___-50.19_|_-17.26_|_112.55_|__36.16 _________0.60000___-61.45_|_-28.50_|__90.05_|__29.76 _________0.65000___-76.72_|_-43.55_|__59.73_|__20.32 _______I__N__T__A__K__E Rocker_Arm_Ratio_=_1.800_________Valve_Lash_=_0.02 290________Valve_Angle_=_20.0 VALVE_____Lift______Opens___Closes__Duration _________________Deg_BTDC__Deg_ABDC_____________Ar ea _________0.00000____47.69_|__84.37_|_312.06_|__66. 56 _________0.00600____44.80_|__81.08_|_305.88_|__66. 54 _________0.01000____43.10_|__79.10_|_302.20_|__66. 52 _________0.02000____39.34_|__74.72_|_294.07_|__66. 45 _________0.04000____33.29_|__67.87_|_281.16_|__66. 26 _________0.05000____30.76_|__65.12_|_275.87_|__66. 11 _________0.10000____20.53_|__54.24_|_254.76_|__65. 34 _________0.15000____12.51_|__45.89_|_238.39_|__64. 34 _________0.20000_____5.49_|__38.67_|_224.16_|__62. 90 _________0.25000____-1.07_|__31.99_|_210.92_|__61.52 _________0.30000____-7.41_|__25.64_|_198.23_|__59.54 _________0.35000___-13.72_|__19.31_|_185.59_|__57.54 _________0.40000___-20.15_|__12.82_|_172.67_|__54.88 _________0.45000___-26.79_|___6.13_|_159.33_|__52.28 _________0.50000___-33.83_|__-0.94_|_145.22_|__48.41 _________0.55000___-41.53_|__-8.61_|_129.86_|__44.67 _________0.60000___-50.19_|_-17.26_|_112.55_|__39.44 _________0.65000___-60.42_|_-27.46_|__92.12_|__33.13 _________0.70000___-73.66_|_-40.54_|__65.79_|__24.28 _________0.75000__-100.71_|_-66.52_|__12.77_|___4.51 Stan |
i seem to recall seeing someone running old faithful with 1.65 rockers ... isn't that an awful lot of lift? will a valvetrain live a long life with that cam and 1.65 rockers?
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This whole thing about increasing rocker ratio after cam has been designed for you combo. Kind of makes me wonder. After my conversation just recently with Ultra Dyne, I call to compare what spec cam he recommended just to compare with Comp cam, it was very interesting to say the least. As we talked he was punching information into his cam program. One of the big questions was installed hight In relation to my 1.75 rocker.
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That's good accurate information, but don't forget it only applies if the cam timing is correct to begin with.... That is a BIG if.
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Lol..... You going full Biden on us Gach? Ultradyne shut down years and the owner passed quite a while ago. |
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278/288 on a 110 with int 310/4485 lobe and exh 322/ 4455 lobe. $650.00 smackers…Its cracking me up.
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I kept asking him if 310/4485 if the 4485 was the lobe lift. He didn’t really answer me. With 1.65 rocker if my math is correct is .740/735 lift. I said no, it’s 1.75 rocker not 1.65. The 1.75 is what I’m running now, he said no you don’t have enough install height for 1.75. I said oh really?
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He wouldn’t really answer me, because I think he saw the controversy over the Comp cam 4470/4360 lobe.
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