Gt40p Spring Rate With Flat Tappet Cam

As is widely known, the most important aspect of selecting a valve spring is to get a spring with the correct seat pressure, open pressure, and spring rate for the cam in the engine and with the rev limit that will be used kept in mind. Too little seat pressure robs power and impairs idle quality and vacuum. Too little open pressure can lead to valve float with resultant power loss and even damage to the valve train. So why not simply use the baddest, stiffest spring you can find?



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 engines will need a lot more. Professional race engines may require open pressures exceeding 1000lbs.



Triple Versus Double Springs

Triple springs were the way to go on high rpm solid roller race setups in the past. These were required to provide the needed pressure. There are now double springs available that provide over 1000#open pressure. These have at least the theoretical advantage of lower mass of the spring itself. The lower mass allows more of the spring pressure to be available to control the valve train, rather than the spring itself.



What spring should I use?

The above are just general statements about valve spring requirements. The best source of information is your camshaft supplier. We can select and provide springs at a competitive price if we know your cam specs or part/grind number.



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.

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