im unsure about shimming valve springs, or even WHY its done??
in my head and sort of following, measureing installed hieght, understand that
now if its installed and the total lift leaves more than or at least 0.060-0.070" its all good -right
if theres excess is this when i would shim
so in my head assuming i want full lift at 0.060-0.070" before bind
total installed height would equal 0.070" + total lift + shims? and to shim 0.070 more the spring would bind at total lift
is this so that the springs are as tensioned as possible , or is there another reason
or have i missed the point altogeather
why & how to shim valve springs
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jrandy
- Posts: 498
- Joined: Wed Oct 03, 2007 10:39 pm
You may want to ask the question on the racing forum.
Basically, you shim the springs to increase and/or adjust spring seat pressure.
If you need to shim your springs you are building a race/performance engine.
High rpm and radical cams mean valvetrain harmonics, valve float, etc that spring seat pressure is adjusted to compensate for.
Higher lift than stock cams mean checking for coil bind at full lift.
A bit of clearance between the coils must be there or the spring will go solid and lots of bad things happen.
What are you building?
What cam & what are the specs?
Single springs or duals?
What ratio rocker arms?
Stock pushrods or aftermarket?
etc, etc, etc...
Basically, you shim the springs to increase and/or adjust spring seat pressure.
If you need to shim your springs you are building a race/performance engine.
High rpm and radical cams mean valvetrain harmonics, valve float, etc that spring seat pressure is adjusted to compensate for.
Higher lift than stock cams mean checking for coil bind at full lift.
A bit of clearance between the coils must be there or the spring will go solid and lots of bad things happen.
What are you building?
What cam & what are the specs?
Single springs or duals?
What ratio rocker arms?
Stock pushrods or aftermarket?
etc, etc, etc...
-
Scott Novak
- Posts: 522
- Joined: Mon Nov 08, 2004 1:31 pm
Gene Berg had always maintained that you need to use at least one shim under a valve spring to prevent the spring from chafing the aluminum cylinder head.
He also claimed that insufficient valve spring pressure would cause more damage to the engine than excessive spring pressure. Better to cause a little extra wear on the valve train than to have valve float that could break your valves. Once your valves start to float, valves come slamming down on their seats, lifters come crashing down on the cam lobes and all sorts of nasty things happen.
Even with a stock engine, Berg suggested a 0.060" shim underneath the intake valve springs and a 0.030" shim underneath the exhaust valve springs, and that would be close to correct. He also recommended that you measure the actual spring pressure and check for coil bind. Berg recommended at least 0.100" travel beyond maximum valve lift before coil bind. 0.060" to 0.070" clearance before coil bind might be insufficient.
As jrandy said, "Basically, you shim the springs to increase and/or adjust spring seat pressure." That's quite true.
By selecting the uncompressed spring length and spring stiffness, and by shimming the valve spring, you can adjust the minimum spring force when the valve is closed and the maximum spring force when the valve is at maximum lift.
If the valve spring was not compressed at all when the valve was closed, you would have zero spring pressure. When the valve opens, the valve spring goes through a large percentage of change in spring pressure, which can also be considered a change in stress on the spring.
By shimming the valve spring so it is compressed when the valve is closed, there will always be pressure on the valve, and the total change in the percentage of spring pressure (stress on the spring) between open and closed will be much less.
This may result in longer spring life as well. ALL springs will eventually fail due to stress fatigue. Stress fatigue is the phenomenon where if you bend something back and forth, like a wire, eventually it will break. The further you bend the wire each time, the faster it will break. If you have matched your valve train components and setup your valve train properly, hopefully you won't have spring breakage during the life of your engine, or at least springs that will last the racing season before tear down and rebuilding of the engine. Valve spring life in race engines is sometimes measured in the number of trips down the drag strip.
As an example of stress fatigue in bolts, in one test a bolt that was rated for 180,000 psi tensile strength was torqued until it was prestressed to 40% of it's ultimate tensile strength, and subjected to a cyclical load of 12,000 lbs and it failed after 4,900 cycles. An identical bolt was then pre stressed to 60% of it's ultimate tensile strength, and it lasted 6,000,000 cycles before failure!!!!! By prestressing the bolt more, the total change in stress that the cycilcal load applies will be less, and the longer the bolt survives.
Better that you tighten more than less, as long as the prestress plus the load stress does not exceed the ultimate tensile strength.
I believe that the same effect happens in valve springs. Your springs may last longer if they are prestressed by shimming, so they go through a smaller percentage of stress change.
The actual amount of valve spring pressure required is dependent on your particular engine configuration. Hopefully you can find some recommendations from someone with a similar engine configuration.
BTW, pick up a copy of Caroll Smith's "Nuts, Bolts, Fasteners and Plumbing Handbook". Lots of very practical information nicely complied in one book.
Scott Novak
He also claimed that insufficient valve spring pressure would cause more damage to the engine than excessive spring pressure. Better to cause a little extra wear on the valve train than to have valve float that could break your valves. Once your valves start to float, valves come slamming down on their seats, lifters come crashing down on the cam lobes and all sorts of nasty things happen.
Even with a stock engine, Berg suggested a 0.060" shim underneath the intake valve springs and a 0.030" shim underneath the exhaust valve springs, and that would be close to correct. He also recommended that you measure the actual spring pressure and check for coil bind. Berg recommended at least 0.100" travel beyond maximum valve lift before coil bind. 0.060" to 0.070" clearance before coil bind might be insufficient.
As jrandy said, "Basically, you shim the springs to increase and/or adjust spring seat pressure." That's quite true.
By selecting the uncompressed spring length and spring stiffness, and by shimming the valve spring, you can adjust the minimum spring force when the valve is closed and the maximum spring force when the valve is at maximum lift.
If the valve spring was not compressed at all when the valve was closed, you would have zero spring pressure. When the valve opens, the valve spring goes through a large percentage of change in spring pressure, which can also be considered a change in stress on the spring.
By shimming the valve spring so it is compressed when the valve is closed, there will always be pressure on the valve, and the total change in the percentage of spring pressure (stress on the spring) between open and closed will be much less.
This may result in longer spring life as well. ALL springs will eventually fail due to stress fatigue. Stress fatigue is the phenomenon where if you bend something back and forth, like a wire, eventually it will break. The further you bend the wire each time, the faster it will break. If you have matched your valve train components and setup your valve train properly, hopefully you won't have spring breakage during the life of your engine, or at least springs that will last the racing season before tear down and rebuilding of the engine. Valve spring life in race engines is sometimes measured in the number of trips down the drag strip.
As an example of stress fatigue in bolts, in one test a bolt that was rated for 180,000 psi tensile strength was torqued until it was prestressed to 40% of it's ultimate tensile strength, and subjected to a cyclical load of 12,000 lbs and it failed after 4,900 cycles. An identical bolt was then pre stressed to 60% of it's ultimate tensile strength, and it lasted 6,000,000 cycles before failure!!!!! By prestressing the bolt more, the total change in stress that the cycilcal load applies will be less, and the longer the bolt survives.
Better that you tighten more than less, as long as the prestress plus the load stress does not exceed the ultimate tensile strength.
I believe that the same effect happens in valve springs. Your springs may last longer if they are prestressed by shimming, so they go through a smaller percentage of stress change.
The actual amount of valve spring pressure required is dependent on your particular engine configuration. Hopefully you can find some recommendations from someone with a similar engine configuration.
BTW, pick up a copy of Caroll Smith's "Nuts, Bolts, Fasteners and Plumbing Handbook". Lots of very practical information nicely complied in one book.
Scott Novak