Boost and valve springs?
Thread Starter
Staging Lane
Joined: Jan 2008
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Boost and valve springs? I have a set of L92 heads and picked up a turbo cam with around .550 lift. (hyd). Now my question is what valve springs to run since its not too radical of lift.. would comp 918s work? Or would the boost (15-20) make them not enough? How does boost factor in with spring selection? How much more spring do you need if so to make up for the extra pressure on the valve? Any? I dont want to excessively over spring it and dont want to under spring it.
FormerVendor
Joined: Dec 2001
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From: WWW.SPEEDINC.COM
I have a set of L92 heads and picked up a turbo cam with around .550 lift. (hyd). Now my question is what valve springs to run since its not too radical of lift.. would comp 918s work? Or would the boost (15-20) make them not enough? How does boost factor in with spring selection? How much more spring do you need if so to make up for the extra pressure on the valve? Any? I dont want to excessively over spring it and dont want to under spring it.
As an example 20lbs of boost going through the intake isn't going to effect anything when the valve is on the seat. When the intake valve opens you have flow into the cylinder and pressure is trying to equalize between the intake and the cylinder. As you reach intake valve closing the compression cycle starts and pressure in the cylinder immediately eclipses pressure behind the intake valve.
On the exhaust side however you may as an example have 40psi backpressure. As the exhaust valve is closing your intake valve starts opening (overlap) and there is still 40psi on the back of the valve but little pressure in the cylinder to balance it. At this point you can bounce the exhaust valve causing a drastic loss in rpm/power.
For the hyd roller stuff that's going to see 20psi boost I usually run a spring with around 400lb/in rate so I can get 200lbs on the seat and 425 or so over the nose. Seat pressure is what's important here.
That is very generalized and like anything else should be decided on when selecting the rest of the parts for the combination. I can say with certainty that 918's will not be enough for a turbo LS1 that will see 20psi boost.
Thread Starter
Staging Lane
Joined: Jan 2008
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TECH Apprentice
Joined: Mar 2006
Posts: 361
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From: australia
firstly,
i would rather over spring than under spring..
if it takes an extra 10 hp to run the engine, and all the parts live, so be it...
the exhaust valve / push rod cops the most load..
i have seen the exhaust push rod have rub marks from cylinder head / intake runner casting.
the push rod bends, untill the cylinder pressure drops enough to allow the valve to open...
this was with 3/8 dia , and .140 wall thickness
consult a rep engine builder...
ash
i would rather over spring than under spring..
if it takes an extra 10 hp to run the engine, and all the parts live, so be it...
the exhaust valve / push rod cops the most load..
i have seen the exhaust push rod have rub marks from cylinder head / intake runner casting.
the push rod bends, untill the cylinder pressure drops enough to allow the valve to open...
this was with 3/8 dia , and .140 wall thickness
consult a rep engine builder...
ash
Thread Starter
Staging Lane
Joined: Jan 2008
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TECH Apprentice
Joined: Mar 2006
Posts: 361
Likes: 1
From: australia
I would never run a single spring nomatter what... (918's) are a single spring...... no back up if ya ever break a spring.... ive seen the inner spring hold the valve and not drop it into the motor and destroy the motor....
I ran the patriots with no problem... made 1100rwhp with them...
Had two different shops compare valve springs unknowing what brand was what for open and closed seat preasure....
Patriots VS Com 921's...
Patriots were better at both shops.......
Kyle
I ran the patriots with no problem... made 1100rwhp with them...
Had two different shops compare valve springs unknowing what brand was what for open and closed seat preasure....
Patriots VS Com 921's...
Patriots were better at both shops.......
Kyle
Thread Starter
Staging Lane
Joined: Jan 2008
Posts: 58
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