Floating Valves / Redline
Thread Starter
Joined: Jan 2006
Posts: 20
From: Abbotsford, B.C., Canada
Floating Valves / Redline
I'm a noob and proud of it! 
I'm wondering what it means to float a valve? I'm assuming it means things are moving so fast the valve can't open/close properly (they're on springs, right?) Does floating a valve ruin the engine?
Also, why are people tuning their engines to 6700/6800 RPM redline when stock is ~6250? Doesn't torque drop a whole bunch after it peaks anyway? (i.e. wouldn't you want to shift instead of redlining?)
Thanks!
I'm wondering what it means to float a valve? I'm assuming it means things are moving so fast the valve can't open/close properly (they're on springs, right?) Does floating a valve ruin the engine?
Also, why are people tuning their engines to 6700/6800 RPM redline when stock is ~6250? Doesn't torque drop a whole bunch after it peaks anyway? (i.e. wouldn't you want to shift instead of redlining?)
Thanks!
1st Gear Member
Joined: Nov 2005
Posts: 57
From:
RE: Floating Valves / Redline
There are two commonly accepted theories of what is valve float. I say "theory" because no one has ever recorded footage of actual valve float.
1st example: The Item following the cam profile (depending on valve train configuration this could be a lifter, rocker, valve, cam follower etc.) no longer contacts the cam as it goes over the nose (peak of the cam lobe) of the cam. This is the most commonly thought of occurance and probebly the least frequent.
2nd example: The valve bounces off of it's seat after the lobe finishes it's lift cycle. This occurs when the valve spring has the least amount of tension on the valve stem and the valve is most vunerable to floating. This is when I believe most power robbing valve float takes place. If this happens on the intake side, then cylinder pressure is lost as the piston is coming up the bore on it's copression stroke. It's less common to have damage to the engine at this point because the piston is still far enough away from the valve as the piston is "chasing the intake valve up the bore.
If the exhaust valve floats, chances are good that you will have problems because the exhaust valve is open when the piston is at top dead center on the exhaust stroke. clearances are usually around .120 to .160 so any float will likely cause contact and a bent or broken valve.
Rest easy though. Exhaust valves are far less likely to float than intake valves because exhaust valves are usually smaller, weigh less and have cylinder pressure working in their favor. Blower and torbo motors need lots of intake spring pressure when running big boost because boost pressure works against the intake valve spring.
1st example: The Item following the cam profile (depending on valve train configuration this could be a lifter, rocker, valve, cam follower etc.) no longer contacts the cam as it goes over the nose (peak of the cam lobe) of the cam. This is the most commonly thought of occurance and probebly the least frequent.
2nd example: The valve bounces off of it's seat after the lobe finishes it's lift cycle. This occurs when the valve spring has the least amount of tension on the valve stem and the valve is most vunerable to floating. This is when I believe most power robbing valve float takes place. If this happens on the intake side, then cylinder pressure is lost as the piston is coming up the bore on it's copression stroke. It's less common to have damage to the engine at this point because the piston is still far enough away from the valve as the piston is "chasing the intake valve up the bore.
If the exhaust valve floats, chances are good that you will have problems because the exhaust valve is open when the piston is at top dead center on the exhaust stroke. clearances are usually around .120 to .160 so any float will likely cause contact and a bent or broken valve.
Rest easy though. Exhaust valves are far less likely to float than intake valves because exhaust valves are usually smaller, weigh less and have cylinder pressure working in their favor. Blower and torbo motors need lots of intake spring pressure when running big boost because boost pressure works against the intake valve spring.
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