LKN Speed
06-22-2009, 02:21 PM
story time fellas, pull up a chair LOL
At least a thousand times, I have been asked by customers- "why cant I run the stock springs with my new cam? my dad/uncle/buddy did it and it was fine!" After being active in another thread on here concerning springs, I do know MOST of you are aware of the importance of the proper spring...but for the rest of you guys, heres the WHY........
"I have stock springs and my valves arent floating"........yes they are!!!!
lets skip the whole part about retainer to guide issues, coil bind, etc....
this one is about spring pressures!
wayyyy back in the day, when I was learning the basics, I swapped springs on my 66 olds (396BBC) to the recommended spring for my cam, but knew nothing about 'installed height'............ the engine lost a ton of power above 4000rpms, but NO valve float until 6000rpms, and at 6200 it straightened out and would spin to 6500 easily. When I called the cam company, they asked what my installed height was............... whats that?
In basic terms:
A valvespring has a spring 'rate' which determines how much pressure you have at open versus closed and cannot be altered. It also will say "XX# @XX height" lets say 125# at 1.75 installed height. this is with your spring at rest, valve on seat, closed, and is changed by using shims. If this is the spring you need for your cam, but your installed height is 1.85, guess what? youre now 30lbs too light on the seat, which also means youre at least 30lbs (probably more) light 'over the nose' (at full lift on your cam) HELLO valve bounce, hello valve float!!!!
this is exactly what was wrong on the olds.....I needed 125# on the seat and 325 open, and had 80# on the seat and about 250 open.
again- there was NO noticeable valve float until 6000rpms, but the engine was very 'flat' from 4000rpms-up. Shimming until I was at 125# on the seat gave me 3mph and .2 at the track!!!! experimenting on the dyno at the new shop, I have seen 500-600rpms more rpms, and 25+hp by correcting spring pressures only! Its a balancing act..too light and you have problems, too much and you have parasitic drag and/or destroyed cam lobes and lifters!!!
On a very mild cammed engine, you might be ok with stock springs, but anything more, and if you want to see anything over 5500rpms, a spring change and installed height setting is a MUST if you want your engine to scream! Dont just drop those new springs in- at minimum, check your installed heights on one intake and one exhaust, and shim correctly-- if you REALLY want to do it right, have every spring checked (they will have a 5-10lb variation) and match accordingly! Unless your heads have been machined to make every spring pocket, valve seat and retainer match, you will have a slight difference in installed heights from valve to valve. Match the weaker spring to the shorter height, stronger spring to taller height, and shim accordingly. After some running, your springs will lose a good 10% pressure, and not always evenly, but setting them up correctly will give you the best chance of keeping things 'liveable', unless you enjoy changing springs every 5k-10k miles or less.............
there you have it! Hope you guys enjoyed this, and to those who already knew these things, im sorry to have wasted your time:lol1:
At least a thousand times, I have been asked by customers- "why cant I run the stock springs with my new cam? my dad/uncle/buddy did it and it was fine!" After being active in another thread on here concerning springs, I do know MOST of you are aware of the importance of the proper spring...but for the rest of you guys, heres the WHY........
"I have stock springs and my valves arent floating"........yes they are!!!!
lets skip the whole part about retainer to guide issues, coil bind, etc....
this one is about spring pressures!
wayyyy back in the day, when I was learning the basics, I swapped springs on my 66 olds (396BBC) to the recommended spring for my cam, but knew nothing about 'installed height'............ the engine lost a ton of power above 4000rpms, but NO valve float until 6000rpms, and at 6200 it straightened out and would spin to 6500 easily. When I called the cam company, they asked what my installed height was............... whats that?
In basic terms:
A valvespring has a spring 'rate' which determines how much pressure you have at open versus closed and cannot be altered. It also will say "XX# @XX height" lets say 125# at 1.75 installed height. this is with your spring at rest, valve on seat, closed, and is changed by using shims. If this is the spring you need for your cam, but your installed height is 1.85, guess what? youre now 30lbs too light on the seat, which also means youre at least 30lbs (probably more) light 'over the nose' (at full lift on your cam) HELLO valve bounce, hello valve float!!!!
this is exactly what was wrong on the olds.....I needed 125# on the seat and 325 open, and had 80# on the seat and about 250 open.
again- there was NO noticeable valve float until 6000rpms, but the engine was very 'flat' from 4000rpms-up. Shimming until I was at 125# on the seat gave me 3mph and .2 at the track!!!! experimenting on the dyno at the new shop, I have seen 500-600rpms more rpms, and 25+hp by correcting spring pressures only! Its a balancing act..too light and you have problems, too much and you have parasitic drag and/or destroyed cam lobes and lifters!!!
On a very mild cammed engine, you might be ok with stock springs, but anything more, and if you want to see anything over 5500rpms, a spring change and installed height setting is a MUST if you want your engine to scream! Dont just drop those new springs in- at minimum, check your installed heights on one intake and one exhaust, and shim correctly-- if you REALLY want to do it right, have every spring checked (they will have a 5-10lb variation) and match accordingly! Unless your heads have been machined to make every spring pocket, valve seat and retainer match, you will have a slight difference in installed heights from valve to valve. Match the weaker spring to the shorter height, stronger spring to taller height, and shim accordingly. After some running, your springs will lose a good 10% pressure, and not always evenly, but setting them up correctly will give you the best chance of keeping things 'liveable', unless you enjoy changing springs every 5k-10k miles or less.............
there you have it! Hope you guys enjoyed this, and to those who already knew these things, im sorry to have wasted your time:lol1: