Calculating Max lift potential of valvesprings
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Registered User
Joined: Nov 1998
Posts: 746
From: Columbia, SC, USA
Calculating Max lift potential of valvesprings
If I am doing the math right, to calculate max lift capability of a valvespring it should be
Installed height - (coilbind + .060).
Is that right? What affect does the seat and open spring pressures have on it? For example, I have a set of CC985 valvesprings, open pressure is 333, seat is 164. If the installed height is 1.8 (isn't that stock LT1?), and coil bind is 1.15, then I would estimate that I could run .590 lift. I've also heard some people say you can run a tighter clearance, of .040, so that would make them good to .610.
I'm asking because I saw a post Lloyd did about a set of heads with the CM612s. He said open pressure was 350 and seat was 140. He said they were limited to .570 lift. Is that due to seat pressure, or installed height, or what?
Basically, a "how to choose valvesprings based on lift" would be nice. Please don't just post, "Go with these cause they work. I want to know the how and why behind it, not just that it does." Thanks!
Installed height - (coilbind + .060).
Is that right? What affect does the seat and open spring pressures have on it? For example, I have a set of CC985 valvesprings, open pressure is 333, seat is 164. If the installed height is 1.8 (isn't that stock LT1?), and coil bind is 1.15, then I would estimate that I could run .590 lift. I've also heard some people say you can run a tighter clearance, of .040, so that would make them good to .610.
I'm asking because I saw a post Lloyd did about a set of heads with the CM612s. He said open pressure was 350 and seat was 140. He said they were limited to .570 lift. Is that due to seat pressure, or installed height, or what?
Basically, a "how to choose valvesprings based on lift" would be nice. Please don't just post, "Go with these cause they work. I want to know the how and why behind it, not just that it does." Thanks!
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Joined: Oct 2002
Posts: 2,931
From: Upstate NY
Correct valvespring choice is a lot more than "valve lift". It controls the dynamics of the valvetrain. IMO, more valvetrains have been lost or damaged by the wrong valvesprings than most other factors combined.
Some of the factors which determine spring requirements are:
lift, cam profile, mass (or weight) of lifters, pushrods, rockers, valves, springs, retainers. Also stiffness of the lifters, pushrods and rockers is important. Max rpm is a biggy, as is rpm at which the engine spends most of it's time (Daytona being a good example with a few hundred rpm range).
My advice is have your valvetrain professionally designed from the cam up, including the springs. At the least, have a pro spec a spring based on the exact parts (and cam profiles) you want to use. The correct springs are rarely the least expensive ones if you have fairly stout cam and rpm expectations.
Obviously the desired seated load determines the installed height for a certain spring, and the spring rate then determines the max. load at max lift.
Some of the factors which determine spring requirements are:
lift, cam profile, mass (or weight) of lifters, pushrods, rockers, valves, springs, retainers. Also stiffness of the lifters, pushrods and rockers is important. Max rpm is a biggy, as is rpm at which the engine spends most of it's time (Daytona being a good example with a few hundred rpm range).
My advice is have your valvetrain professionally designed from the cam up, including the springs. At the least, have a pro spec a spring based on the exact parts (and cam profiles) you want to use. The correct springs are rarely the least expensive ones if you have fairly stout cam and rpm expectations.
Obviously the desired seated load determines the installed height for a certain spring, and the spring rate then determines the max. load at max lift.
Thread Starter
Registered User
Joined: Nov 1998
Posts: 746
From: Columbia, SC, USA
So you would want to base your installed height on the seat load. How would you determine the seat load on a spring at a higher or lower installed height, without measuring, if possible? In other words, the 985s are rated at 164 @ 1.75 installed height. Would the seat load increase if I installed them at 1.8? Also, does a higher seat load prevent valve float at higher rpms and higher lift? I would presume so, as my friends Mustang has .650 lift and he is running 236 seat pressure. Not sure of installed height, but it is a solid roller, so he can get away with stiff springs. I understand its best to have it professionally designed, but sometimes even a professional will work around parts they have. For example, would I HAVE to get new valvesprings to run a GM847 cam with 1.6 rockers? If so, where am I doing my math wrong? I see that at a 1.75 installed height, coil bind would be at .540, so that would be way too small. If I installed them at 1.8, would there be an issue? I just hate to order a new set of valvesprings, cause everyone said, these worked for me, so you should use them.
Registered User
Joined: Oct 2002
Posts: 2,931
From: Upstate NY
Originally posted by brain
So you would want to base your installed height on the seat load.
Sure. A given spring has a given load at a certain height. The load changes by the spring rate. A 400 lb/in spring rate changes load by 40 lbs for every .100 it is compressed. (400 lb/in x .100 in = 40 lbs). For example if the 400 lb/in spring was installed at 150 lbs seat load then compressed .600 (the valve lift) the max load would be 150 +(400 x .600) = 150 + 240 = 390 lbs.
How would you determine the seat load on a spring at a higher or lower installed height, without measuring, if possible? In other words, the 985s are rated at 164 @ 1.75 installed height. Would the seat load increase if I installed them at 1.8?
You should be able to do the math from the example above if you know the spring rate. Hint: seat load would decrease installing them @ 1.80 vs 1.75. Let's assume they are 2.15 long in the free state where they have zero installed load.
Also, does a higher seat load prevent valve float at higher rpms and higher lift? I would presume so, as my friends Mustang has .650 lift and he is running 236 seat pressure. Not sure of installed height, but it is a solid roller, so he can get away with stiff springs.
Well, it's probably the load at or near max lift that keeps the valve toss (float) minimized.
I understand its best to have it professionally designed, but sometimes even a professional will work around parts they have. For example, would I HAVE to get new valvesprings to run a GM847 cam with 1.6 rockers? If so, where am I doing my math wrong? I see that at a 1.75 installed height, coil bind would be at .540, so that would be way too small. If I installed them at 1.8, would there be an issue? I just hate to order a new set of valvesprings, cause everyone said, these worked for me, so you should use them.
I guess you need to know whose judgement you trust. Sure you could run any given spring. It might work and it might not. The only thing you have to lose, other than power (which is a good possibility) are parts of your engine. I'm not making a call on your choice, because I haven't studied you entire engine and it's intended use. Remember that free advice is worth everything you paid for it. If you asked a good engine builder to spec a spring and then bought the springs from him you have a good chance of getting springs that will work. Of course it depends on the engine builder!
So you would want to base your installed height on the seat load.
Sure. A given spring has a given load at a certain height. The load changes by the spring rate. A 400 lb/in spring rate changes load by 40 lbs for every .100 it is compressed. (400 lb/in x .100 in = 40 lbs). For example if the 400 lb/in spring was installed at 150 lbs seat load then compressed .600 (the valve lift) the max load would be 150 +(400 x .600) = 150 + 240 = 390 lbs.
How would you determine the seat load on a spring at a higher or lower installed height, without measuring, if possible? In other words, the 985s are rated at 164 @ 1.75 installed height. Would the seat load increase if I installed them at 1.8?
You should be able to do the math from the example above if you know the spring rate. Hint: seat load would decrease installing them @ 1.80 vs 1.75. Let's assume they are 2.15 long in the free state where they have zero installed load.
Also, does a higher seat load prevent valve float at higher rpms and higher lift? I would presume so, as my friends Mustang has .650 lift and he is running 236 seat pressure. Not sure of installed height, but it is a solid roller, so he can get away with stiff springs.
Well, it's probably the load at or near max lift that keeps the valve toss (float) minimized.
I understand its best to have it professionally designed, but sometimes even a professional will work around parts they have. For example, would I HAVE to get new valvesprings to run a GM847 cam with 1.6 rockers? If so, where am I doing my math wrong? I see that at a 1.75 installed height, coil bind would be at .540, so that would be way too small. If I installed them at 1.8, would there be an issue? I just hate to order a new set of valvesprings, cause everyone said, these worked for me, so you should use them.
I guess you need to know whose judgement you trust. Sure you could run any given spring. It might work and it might not. The only thing you have to lose, other than power (which is a good possibility) are parts of your engine. I'm not making a call on your choice, because I haven't studied you entire engine and it's intended use. Remember that free advice is worth everything you paid for it. If you asked a good engine builder to spec a spring and then bought the springs from him you have a good chance of getting springs that will work. Of course it depends on the engine builder!
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Joined: Aug 2002
Posts: 1,509
From: Red Oak Texas
The CM 612's can handle more lift before coil bind (depending on set up ht.) but odds are, if thje cam has over a .570 lift, you would be MUCH better off with a dbl springs set up.
The CM 612's (when set up at 140 lbs on the seat) will work fine with any of the cc 305, Hot Cam, XE 218/224, 224/230, 230/236 (Not the high lift ones), etc. and be fine with stock lifters and handle the 6000 to 6400 RPM that these cams would red line at.
The "right" way is to pick a cam and then get the springs and set them up to work right, just like OldSStroker said. Some of the above mentioned cams (other than the XE cams) require less than 140 seat/350 open but would be fine with this kinda pressure.
Lloyd
NightTrain66@msn.com
The CM 612's (when set up at 140 lbs on the seat) will work fine with any of the cc 305, Hot Cam, XE 218/224, 224/230, 230/236 (Not the high lift ones), etc. and be fine with stock lifters and handle the 6000 to 6400 RPM that these cams would red line at.
The "right" way is to pick a cam and then get the springs and set them up to work right, just like OldSStroker said. Some of the above mentioned cams (other than the XE cams) require less than 140 seat/350 open but would be fine with this kinda pressure.
Lloyd
NightTrain66@msn.com
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