4x4dually
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- Joined
- Jan 22, 2007
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I am disappointed you didn't measure short and curlies...
Keeping it clean.
I am disappointed you didn't measure short and curlies...
The sad part is, Zach can rattle all this off talking face to face, without blinking, pausing or otherwise.. lousy smart people!
Exhaust valves are usually smaller on competition builds so that will offset the higher pressures in the exhaust. With 1.87" intake valves that are popular these days, there is 2.74 sq. in. 75 psi is not uncommon these days which would mean 206lbs pushing the valves open. This is why 60# springs don't cut the mustard. At full lift 206lbs of the nose pressure the rocker sees is being offset. Spring pressure is a big deal to power production on competition engines. On street trucks, too much is not cool.
Jory, I need a measurement on raccoon hair for posterities sake.
Exhaust valves are usually smaller on competition builds so that will offset the higher pressures in the exhaust. With 1.87" intake valves that are popular these days, there is 2.74 sq. in. 75 psi is not uncommon these days which would mean 206lbs pushing the valves open. This is why 60# springs don't cut the mustard. At full lift 206lbs of the nose pressure the rocker sees is being offset. Spring pressure is a big deal to power production on competition engines. On street trucks, too much is not cool.
Jory, I need a measurement on raccoon hair for posterities sake.
See, you say there is 206lbs of pressure pushing the valve open....but you seem to be forgetting that there is pressure on the opposite side of that valve.
Intake stroke the valve is open, so pressure pushing it open only helps the valvetrain.
Compression stroke the pressure under the valve will go from near nothing to what, 5-600lbs?
Power stroke there is ALOT of pressure under the valve
And exhaust stroke there is still pressure in the cyl.
So with a 60psi spring, at what boost level would the valve actually push open, during actual engine operation, and not on a test bench with 0psi under the valve??
The way the logic in my head works, since drive pressure (aka backpressure) is usually higher than boost, the exhaust valve would be the valve more in danger of being pushed open, and this would only happen on the intake stroke.
But the valve spring wouldn't be the component holding the valve open against that force. Wouldn't the rocker, pushrod, and cam be "holding" the valve open at full lift. Right ? And springs hold it closed against full boost/drive pressure.
See, you say there is 206lbs of pressure pushing the valve open....but you seem to be forgetting that there is pressure on the opposite side of that valve.
Intake stroke the valve is open, so pressure pushing it open only helps the valvetrain.
Compression stroke the pressure under the valve will go from near nothing to what, 5-600lbs?
Power stroke there is ALOT of pressure under the valve
And exhaust stroke there is still pressure in the cyl.
So with a 60psi spring, at what boost level would the valve actually push open, during actual engine operation, and not on a test bench with 0psi under the valve??
The way the logic in my head works, since drive pressure (aka backpressure) is usually higher than boost, the exhaust valve would be the valve more in danger of being pushed open, and this would only happen on the intake stroke.
I will answer you at a later time, but I need you first to do some math for me. Please tell me how much pressure is on the other side of the valve and calculate the diminishing effects of this offsetting pressure.
I think two things will come of this
-your arguments have merit to a lesser degree
- you will struggle to fully find data as a function of rpm, factoring for boost/drive pressure, factoring for valve surface area, factoring for inertia and mass etc.
All that aside, practical application of simple concepts will take you a long way. That is my favorite tool, thought experiments using practical concepts. Sit on top of the piston and watch what is going on 42 times per second and then sit on the piston and watch the same thing at 15 times per second.