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Old 08-22-2010, 06:55 PM
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Valve springs

Why is it that on some cams example xe268 the recommended spring is a 981-16with a 370lbs/inch spring rate on a single spring,with a 1.254 in, yet on a xs 272s they recommend a double spring with 322lbs/inch,and a1.430in diameter,could they not do th e same job or would the single higher rate spring wipe out a lobe on the solid cam. i'm just trying to understand the basics of spring rate etc before I switch cams ,I do want to swap the xs 272s for my xe268,any advice would be appreciated.Thak you to all.
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Old 08-22-2010, 08:32 PM
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Valve Springs

Anyone out there please. I dont want to make a mistake and watch my hard earned money as all of ours is go up in the air,Due to a lack of knowledge.
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Old 08-22-2010, 08:38 PM
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Some of it has to do with the amount of lift each cam has. I'm not an expert but it really goes without saying, you have to use the recommended springs for the cam or the cam manufacturer won't warranty the cam. Same goes for lifters. Anytime you change cams, you have to change lifters.
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Old 08-22-2010, 09:04 PM
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Some of it has to do with the amount of lift, and some of it has to do with the difference in lift rates between cam types, and some of it has to do with seat pressure and open pressure.

The other spring you are referring to at 322lbs rate is the 986-16, correct??

The 981 single is 105 lbs @ 1.700" installed height and 273lbs @ 1.250". The 986 double is 132lbs at 1.750", 147lbs @1.700" and 293lbs @ 1.250". While the 981 has a higher rate, it has insufficient seat and open pressures to control a solid lifter valvetrain, the solid is moving the valve faster to peak lift,holding it there, and then setting it back down on the seat faster. The 981 spring would not have enough seat pressure to keep the valve from bouncing off the seat at the faster valve closing velocity that the solid cam has, and the valve would bounce off the seat, which absolutely kills power. This is what valve float actually is, not the lifter launching off the nose of the lobe like so many uninformed people seem to think valve float is.
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Old 08-23-2010, 01:04 PM
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Valve springs

Thank you all, especially you Eric Nova now it's crystal clear to me,986-16 it is,asper comps recommendation and for warranty ,Thanks a bunch guys.
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Old 08-23-2010, 01:15 PM
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FYI - The comp tech guys are usually really good and could break this down for you on the phone as well
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Old 08-23-2010, 03:56 PM
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Quote:
Originally Posted by sapsz28
Anyone out there please. I dont want to make a mistake and watch my hard earned money as all of ours is go up in the air,Due to a lack of knowledge.
Spring pressure is only one part of a complex relationship. The difficult part is how a spring reacts to rate of acceleration put upon it and the dynamic weights it has to move, this applies to both opening and closing and the additive feature of RPMs which is a frequency. These forces interact with the springs natural frequency. So in addition to supplying enough force to keep the valves assembly tracking with the lobe, the frequency response of the spring which is independent of its strength, needs to be kept out of the anticipated rev range of the engine. This can and does lead to some odd combinations when you're looking at this from the outside without the knowledge of any given systems harmonic frequency responses.

All sorts of damping features have been tried, we're currently in the beehive and ovate wire shape period, but in the past, and even now, flat wound dampers have been used to change a spring's frequency response, multiple springs with reverse direction windings to not only increase pressure but to frustrate frequency responses within the desired operating range.

This is a constant battle as you need to use a s little spring pressure as possible to get the job done because of component wear, secondary part bending, and reliability issues but at the same time have enough force to overcome events like lofting the valve mechanism off the cam lobe as it goes over the top. There are three big variable control issues, one I mentioned is lofting where the spring can't control the inertia of the valve system as the valve is opened which allows those parts to continue lifting the valve after the peak of the lobe has passed out from under the lifter. When the spring gains control of this situation it slams everything back into the lobe with a lot of impact. Adding to this situation is the bending of parts under load. The push rod and studs do most of this but the rocker also has a contribution as they try to escape the applied forces by bending. All this uncommanded motion is why top racing classes use a solid rather than hydraulic lifter. The problem with hydraulics is that they chase any looseness they see in this motion. They see this lost motion space as lash to take up which results in what's called pump-up. So the lifter essentially gets taller and finally reaches a point where it won't let the valve back on its seat till the lifter bleeds down which requires a reduction in RPM which it achieves for itself by blowing off the compression by holding the valves open for a few moments, thus slowing the engine RPMs.

Stiff push rods, large studs and stud girdles as well as stainless steel rockers are all attempts to eliminate these extraneous motions from the valve train. Needless to say the stiffer the valve springs and the heavier the valve train components the worse these bending moments become. So this becomes a circular argument as to how to maintain valve control with ever heavier parts and more spring pressure, obviously more engineering needs to go into the problem than simply bigger is better.

Bogie
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Old 08-23-2010, 06:43 PM
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Leave it to Bogie to make things crystal clear!!!

Beagle
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