In the previous post we specified the head to run to 6500 rpm using a mechanical lifter cam with an approximate .550” valve lift.
Previous CI heads used the standard small six 4.260” long 5/16” stem valves with an installed spring height of 1.600”
Since there are no other length valves available near that spec I suspect the new heads will have the same valves with the possible exception of a 1.80” intake instead of the 1.84”.
In order to run 6500 rpm with .550 lift, the spring seat pressure needs to be at least 110 lbs with a spring rate of 400 lbs/in.
A spring with that rate needs a hardened steel locator under it to prevent damage to the aluminum head surface and are typically .060” to .062” thick.
If the .060” thickness is subtracted from the available 1.600” retainer height, that leaves a 1.540” installed spring height.
If the valve lift is .550” and another .050” margin is needed before spring coil bind for a total of .600", then the spring must have a 1.540” - .600” or .940” coil bind height.
We are now looking for a spring with a 1.540” installed height with an installed pressure of approximately 110 lbs, spring rate about 400 lbs/in and a coil bind height of .940” max.
I couldn’t find anything close so I called SI and they confirmed my findings.
They are also the valve and guide suppliers for the aluminum head and are familiar with the problem.
One of the solutions is to get a custom valve. You need an installed height of at least 1.700” to find a variety of springs that will work for high lift and rpm applications so the minimum valve length needs to be the stock 4.260” plus the difference between 1.540” and 1.700” or .160” which is a 4.420” minimum valve length.
Manley and SI are the only valve companies that will do a single custom set run. Manleys price is $32.10 per valve.
One possible less expensive solution is to use the GM 3.8 V6 valves, shorten the valve stem, recut the keeper groove and use lash caps.
The new heads are being completed at a different shop than the original CI heads.
My suggestion is to see if a bare head can be purchased and finish the head at a trusted performance shop using the longer valves, the correct springs, retainers, locators and shimmed for the proper installed spring height. Viton seals included.
You will want to use the 1.65 ratio Yella Terra adjustable roller rockers.
The base plate will need to shimmed up to get the geometry correct with the longer valves.
Longer pushrods will also be needed.
In summary: The rpm of the valve train for this head has been limited by the lack of valve spring pressure due to the short valve length which limits the selection of valves springs.
This is one of the reasons why the full power potential of the small six aluminum head has not been utilized.
Other insights?
Comments?
Previous CI heads used the standard small six 4.260” long 5/16” stem valves with an installed spring height of 1.600”
Since there are no other length valves available near that spec I suspect the new heads will have the same valves with the possible exception of a 1.80” intake instead of the 1.84”.
In order to run 6500 rpm with .550 lift, the spring seat pressure needs to be at least 110 lbs with a spring rate of 400 lbs/in.
A spring with that rate needs a hardened steel locator under it to prevent damage to the aluminum head surface and are typically .060” to .062” thick.
If the .060” thickness is subtracted from the available 1.600” retainer height, that leaves a 1.540” installed spring height.
If the valve lift is .550” and another .050” margin is needed before spring coil bind for a total of .600", then the spring must have a 1.540” - .600” or .940” coil bind height.
We are now looking for a spring with a 1.540” installed height with an installed pressure of approximately 110 lbs, spring rate about 400 lbs/in and a coil bind height of .940” max.
I couldn’t find anything close so I called SI and they confirmed my findings.
They are also the valve and guide suppliers for the aluminum head and are familiar with the problem.
One of the solutions is to get a custom valve. You need an installed height of at least 1.700” to find a variety of springs that will work for high lift and rpm applications so the minimum valve length needs to be the stock 4.260” plus the difference between 1.540” and 1.700” or .160” which is a 4.420” minimum valve length.
Manley and SI are the only valve companies that will do a single custom set run. Manleys price is $32.10 per valve.
One possible less expensive solution is to use the GM 3.8 V6 valves, shorten the valve stem, recut the keeper groove and use lash caps.
The new heads are being completed at a different shop than the original CI heads.
My suggestion is to see if a bare head can be purchased and finish the head at a trusted performance shop using the longer valves, the correct springs, retainers, locators and shimmed for the proper installed spring height. Viton seals included.
You will want to use the 1.65 ratio Yella Terra adjustable roller rockers.
The base plate will need to shimmed up to get the geometry correct with the longer valves.
Longer pushrods will also be needed.
In summary: The rpm of the valve train for this head has been limited by the lack of valve spring pressure due to the short valve length which limits the selection of valves springs.
This is one of the reasons why the full power potential of the small six aluminum head has not been utilized.
Other insights?
Comments?
