Cams that eat Valve Springs
11-26-2009, 03:40 PM
#1
Cams that eat Valve Springs What size or type cams are the hardest on valve springs? How long will a typical set of springs last? Some of these street cams that are on the market today that snap the valves open look like they would be hard on the springs.
11-26-2009, 05:16 PM
#6
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I would say improper installation would be more to blame then anything. I'm sure if the truth be told about alot of premature spring failures you would find no installed height measurements taken or the springs were used in the wrong application. Lobe ramp rates do play a part also but correctly setup springs will always have more longivety then incorrect setups no matter what the ramp rate. IMO anything .625 or higher is what I call a large lift.
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11-26-2009, 10:10 PM
#10
ok, so how long can a set of springs be expected to last before they need to be replaced? Say we're talking the average street LS1 with a hi-performance 600 lift cam. Would they last 10,000 or 20,000 miles? assuming they are set up right
11-26-2009, 10:17 PM
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Anybody that answers that question is guessing to many variables. I would say pull a couple every 10k or 15k and have them checked to see how much seat pressure they have lost.
11-26-2009, 10:54 PM
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Always run enough seat pressure to control the valve action as it returns to the seat. Heavier valves require more seat pressure. Strong, lightweight valves require less seat pressure. When in doubt, run slightly more seat pressure . . . not less.
What is Valve Spring Open Pressure and Why is it Important?
Open pressure is the pressure against the retainer when the valve is at its maximum open point. Adequate open pressure is necessary to control the valve lifter as it first accelerates up the opening flank of the cam lobe and then quickly decelerates to pass over the nose of the cam which causes the valve to change direction. Inadequate open pressure will allow the lifter to "loft" or "jump" over the nose of the cam (referred to as "valve train separation", or "valve float"). When the lifter strikes the closing flank with a severe impact, camshaft life is drastically shortened.
Open pressure is a function of seat pressure, net valve lift, and spring rate. It must be sufficient to control the valve action at the highest expected engine speed without being excessive. Excessive open pressure aggravates pushrod flexing which in itself aggravates "lofting" of the valve and valve train separation. Selecting a spring to give the proper open pressure, while minimizing pushrod flexing, provides many opportunities for developing a unique, horsepower-enhancing combination. Obviously, lightweight valves require lower open pressures and tend to reduce pushrod flexing and valve train separation.
One final point: Excessive valve spring open pressure will result in reduced camshaft and lifter life.
What is Valve Spring Open Pressure and Why is it Important?
Open pressure is the pressure against the retainer when the valve is at its maximum open point. Adequate open pressure is necessary to control the valve lifter as it first accelerates up the opening flank of the cam lobe and then quickly decelerates to pass over the nose of the cam which causes the valve to change direction. Inadequate open pressure will allow the lifter to "loft" or "jump" over the nose of the cam (referred to as "valve train separation", or "valve float"). When the lifter strikes the closing flank with a severe impact, camshaft life is drastically shortened.
Open pressure is a function of seat pressure, net valve lift, and spring rate. It must be sufficient to control the valve action at the highest expected engine speed without being excessive. Excessive open pressure aggravates pushrod flexing which in itself aggravates "lofting" of the valve and valve train separation. Selecting a spring to give the proper open pressure, while minimizing pushrod flexing, provides many opportunities for developing a unique, horsepower-enhancing combination. Obviously, lightweight valves require lower open pressures and tend to reduce pushrod flexing and valve train separation.
One final point: Excessive valve spring open pressure will result in reduced camshaft and lifter life.
https://ls1tech.com/forums/generatio...cam-guide.html
Last edited by BaddBird; 11-26-2009 at 11:00 PM.
11-27-2009, 09:29 AM
#17
It comes down to cost, wear on components, and maintenance. A very stiff spring needs a very stiff pushrod, heavy duty rockers with high quality studs or even shaft mount rockers, better than OEM lifters, retainers, and keepers. And even when the high quality, more expensive components are used spring life for stiff, high lift springs is diminished and more heat is generated during operation. So, you want enough spring, but not too much.
Flat Tappet Cams
How do you determine the right spring then? First, you need to decide what type of lifter you will be using. For flat tappet cams a seat pressure of 105-125lbs for small blocks and 115-135lbs for big blocks is appropriate. Open pressures should be in the 220-250lb range for low rpm street use and 250-350lbs for hi-performance or racing use. Go on the high side for a big block motor, though since these are not usually revved as high as a small block the need for added spring pressure is not necessarily large. In each case, the lighter the valves, the less spring is needed. Avoid the use of press fit rocker studs as open pressures approach 300lbs. For full-out race use, stiffer springs are often needed. However, unless the highest quality parts are used with careful assembly and break in the life of the cam and lifters may be short.
Hydraulic Roller Cams
Hydraulic roller (HR) cams require higher pressures to control the inertia of the heavier roller lifters and the faster acceleration of valve train components allowed by the use of the roller follower. Pure street small blocks should have 260-300lbs open pressure. For performance use, aim for 300-350lbs open. Racing small blocks that regularly see 6,000+rpm need over 400lbs open pressure. At these pressures, premium valve train components including a "billet" type cam are required. Even with these components, there will be reduced service life and the consequent need for more frequent parts inspection and replacement. Big blocks need closer to 300lbs open pressure for street driving and 350-375lbs is preferred for performance use. A racing big block needs 450lbs. As with small blocks, premium components including lifters are needed at higher pressures and rpm. As with solid lifter cams, seat pressures should be in the range of 105-125lbs for small blocks and 115-130lbs for big blocks for performance street cars. Blower cars and race cars will need higher seat pressures.
Solid Roller Cams
Solid roller (SR) cams were once considered very exotic. They are coming into more and more common use, first on race cars, and most recently on street-strip cars. These cams are typically designed with very steep lobes which produce very high rates of valve acceleration. To prevent the valves from bouncing on the seat, elevated seat pressures are required. Street-strip cars should aim for ~175-200lbs seat pressure. Mild race applications need 225-250lbs on the seat. Professional level racing require ~350lbs seat pressure or more. Obviously, for these last categories only the finest components should be used and they will need frequent replacement. In blown fuel cars, springs may last just one 1/4 mile run. It is difficult to give guidelines for open pressures, since application vary so much. But assuming that most of our readers are interested in street-strip use, we recommend a minimum open pressure of at least 400lbs. High rpm race engines will need a lot more. Professional race engines may require open pressures exceeding 750lbs.
Valve Lift and Spring Length Selection
Once you have determined the pressures you need, you can select a valve spring with the appropriate length by taking into account the amount of valve lift in you setup. Start with the installed height needed to get the required seat pressure. Subtract the maximum valve lift plus at least 0.050-60" for coil clearance. If the installed height minus the sum of the valve lift plus 0.050" is more than the coil bind height, the spring has enough lift. Of course, you will still need to check for interference between the retainer and the valve guide, the rocker and the retainer, etc. If the numbers indicate the spring is too short, you will need to pick one with a similar rate but a higher installed height. Special valve retainers or longer stemmed valve may be needed to accommodate higher lift. Some engine builders prefer to keep coil clearance at a minimum. This tends to have a dampening effect on the coils, potentially preventing harmful harmonic vibration. If this approach is chosen, each spring must be carefully checked for adequate clearance.
Example
Let's say you are putting together a small block with an aggressive HR cam for street-strip use. The maximum lift is .575". You know you will need seat pressure in the 125lb range and want ~350lbs open. Go to a spring catalog and look at the springs in a diameter to fit your heads. Find the springs that have at least the desired maximum lift and simply pick one with a rate that closest to your requirements. It's possible that to get the right rate you will need to shim a longer spring or to install at a higher than normal installed (closed) height using special retainers and/or a taller valve.
http://www.kennedysdynotune.com/Valv...ing%20Tech.htm
11-28-2009, 09:25 AM
#18
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Gotta match camshaft "lobe family" with the right valve spring family, and set the stuff up properly. Oh, and stay away from the IR cams, they eat lifters and springs up real fast to.
