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rocker ratio pro & con
I been thinking about what is the advantage of a 1.7:1 rocker ratio verses the factory 1.6:1.
At first glance, I thought that you can just grind the cam with more lift and do the exact same thing, but then I realized that ramp rates limit how quickly you can open and close the valves. Therefore the 1.7 would have an advantage. On the other hand, the higher ratio puts more load on the lifter & pushrod side of things. The Boss 302 used 1.73:1, so it must be a workable load. I would assume that it is no problem keeping the rocker centered on the valve stem, which brings me to a question. What changes: from the pivot point to the push rod or to the valve or both? Anyways what ratio do you run? What good or bad things have you seen or heard? Give the pro's and con's. |
I suggest keeping the length from valve stem to pivot the same and shorten the rocker arm on the pushrod side. That way your rocker arm does not "hang over" the valve tip, creating all sorts of funny loads on the stem or valve tip. Just keep an eye on pushrod to guide clearance.
In theory, you would gain 6.25% in lift and duration (yes, also duration, because it's listed for a given value: .050" from where the valve lifts). That would also call for a 6.25% harder spring because the force on the lever (rocker arm) has been increased. Also, your valve train geometry would be affected slightly. The pushrods would be a tad too short. For the Chev SB you can buy the 1.6 ratio to replace the 1.5. I never checked how the new ration has been achieved. My supplier was Scorpion in Florida, but any manufacturer can answer that question. Dom |
The dimension from the valve guide to the Rocker Pivot Shaft/Rocker stud is fixed. The dimension that is changed is the distance from the Push Rod seat in the Rocker Arm to the Rocker Arm stud. The later is shortened to increase the ratio, and increase valve lift.
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Changing rocker arm ratios is a quick way to pick up some power .... however , I don`t know what your piston to valve clearance is . If you are already at the acceptable minimum , then you might have a problem . You might want to check that .
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push rods
OLDDOG
Be sure to check your push rod length. I changed heads and used the old 1.7 rockers and the stock lenght push rods. The valve guides wore out quickly. Push rods were .100" short. Cost $400 to replace the guides. Dwight |
Actually we are concidering the 1.7 for a new stroker for my sons Mustang. I guess my biggest question is: are the 1.7:1 rockers hard on the valve train or is it just a matter of setting them up correctly?
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you need to setup the geometry correctly: pushrod length, valve spring travel and compressed height. verify lift at cam/lobe and again at valve. -- what is your sons setup >?
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So I am asking do you think building a new engine with a blank sheet of paper that 1.7 rockers are a good idea, both from a power and a reliability point of view. |
Are you planning on running a flat tappet or a hydraulic roller camshaft?
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Then it's even less relevant. A 408 with a fair amount of duration running a hydraulic roller will still have a pretty smooth idle. For example a 408 with 10.0:1 compression with an AFR 205 head and an OTS hydraulic cam .544/.573 238/248 @.050 will make around 500 HP and 500 ft lbs of torque. Adding the lift from a 1.7 cam might add a few HP...but nothing you'd feel. |
If you order a new cam, surely you can get the lift and duration you desire without looking into an increased rocker ratio.
Dom |
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Now if you are using a roller lifter cam, I do not know if the ramp rate is limited by lobe and lifter geometry or if the ramp rate is limited by valve train wear or what ever it may be. If it is still in the lobe lifter area then the higher ratio rocker may still make since. If it is limited by valve train wear then it would be a bad idea. |
I see what you mean. However, that applies only to a very radical cam profile.
Any reputable cam manufacturer should be able to assist you here. Remember, duration is advertised from the point when the lifter moves 0.050" inch up. The same cam with a 1.7 ratio would cause the lifter to raise earlier, resulting in more duration. A roller cam's profile features a much bigger area under the curve, because the valves open more quicker. A roller cam with 260 deg advertised duration would fill the cylinders much faster at low lift than a flat tappet. It would also sound more radical as a flat tappet, "sounding" like more duration. Generally, I would not use a higher ratio because it results in more force on the valve shafts and stems. I consider it more a "saturday afternoon" hot-tip than anything else. Especially since you build a complete new engine, including camshaft. Camshaft ramp rates have improved dramatically in the last years due to better material that I wouldn't hesitate to specify the ramp I need - in discussion with the cam manufacturer. Dom |
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