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help with valve springs for boost 

 

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 Post subject: help with valve springs for boost
Posted: Wed May 05, 2010 1:59 pm 
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me and my friends are having an arguement over why a spring with a higher pressure is needed for turbo application's as opposed to na. I know it's due to the boost pressure on the valve but i can't really get them to understand. I keep getting responses like 'well my stock engine is now turbo and it runs fine'. I'm getting frustrated with trying to get it through they're thick skull's. It's a 347 windsor with track heat heads. Running 12 lbs boost. 9:1 comp
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 Post subject: Re: help with valve springs for boost
Posted: Wed May 05, 2010 5:02 pm 
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The back side of the intake valve has boost pressure (psi, pounds per square inch) applied. The back side of the intake valve has an area (x.xx square inches). When you multiply pounds per square inch by area (square inches) you get an answer in pounds of force applied to the back side of the valve which offsets some spring pressure. eg. 2 square inch valve back side area and 10lbs boost pressure. 2 x 10 = 20lb of force trying to open the valve. So you would run intake springs with an extra 20lbs seat pressure to compensate.

The engine would still run with stock springs, but valve bounce becomes much more likely as the spring is effectively much weaker under boost conditions. Under normal driving conditions there isn't any boost in the intake, so stock springs are just fine.

 

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 Post subject: Re: help with valve springs for boost
Posted: Wed May 05, 2010 6:56 pm 
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thanks^. I worked out my valve area is 3.14 in. And with a max of 15 lbs boost will have approx 45 lbs holding the valve open. Which is 20 kg each valve. My springs are rated at 125 lbs @1.78 in, so i should step up to a 165~170lbs spring.

So when i tell my friends that my spring will have a pressure of 80 lbs or 36 kg at max lift instead of 77 kg , which is 50 percent off what it should be.
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 Post subject: Re: help with valve springs for boost
Posted: Wed May 05, 2010 9:00 pm 
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eboost wrote:
thanks^. I worked out my valve area is 3.14 in. And with a max of 15 lbs boost will have approx 45 lbs holding the valve open. Which is 20 kg each valve. My springs are rated at 125 lbs @1.78 in, so i should step up to a 165~170lbs spring.

So when i tell my friends that my spring will have a pressure of 80 lbs or 36 kg at max lift instead of 77 kg , which is 50 percent off what it should be.


I have up graded TFS springs 135 seat / 365 over the nose..
Don't have any issues up to 18 Lb boost...
Also have track heat heads and springs..

 

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Last edited by EBXR8380 on Tue Aug 31, 2010 11:46 pm, edited 1 time in total.

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 Post subject: Re: help with valve springs for boost
Posted: Wed May 05, 2010 9:07 pm 
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Is that a missprint Graeme, 118lb of Boost, or 18lb
Brett

 

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 Post subject: Re: help with valve springs for boost
Posted: Thu May 06, 2010 10:00 am 
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eboost wrote:
thanks^. I worked out my valve area is 3.14 in. And with a max of 15 lbs boost will have approx 45 lbs holding the valve open. Which is 20 kg each valve. My springs are rated at 125 lbs @1.78 in, so i should step up to a 165~170lbs spring.

So when i tell my friends that my spring will have a pressure of 80 lbs or 36 kg at max lift instead of 77 kg , which is 50 percent off what it should be.


It's your seat pressure (valve closed, no lift) that is reduced. The open pressure (valve open, max lift) will be virtually unchaged as pressure (force) on the backside of the valve will be almost identical to pressure on the frontside of valve as cylinder pressure approaches intake pressure.

I would agree with your calcs if you have a hydraulic roller cam that you rev to around 6500. If the cam is smaller or is a flat tappet design you could set the seat pressure to around 150lbs with safety.

If in doubt it is almost always better to run slightly more spring pressure, than slightly less.

 

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