What exactly is valve float?

[1BAD351 21]

ok guys I have a 24 valve DOHC engine that im worried might have valve float (its not fully assembled yet just preventative here) and im wondering what exactly it is and what can happen if I have it. From my research I see the spring can wobble and the valve actually flex and lose contact with the cam lob (hence the term valve float). This can be bad for me as I have lash caps installed under my shim buckets and if the valve floats high enough it can dislodge one of them, (iv seen it happen) and that could jam the valve open and possibly destroy some stuff in there. I also came across the valve bouncing on the seat problem which can cause premature wear on the valve and seat. Now I have seen some engines that were damaged from valve float such as the piston hitting the valve. My main question is, is if a valve floats, does it actually fly out further then the actual lift of the cam, or just not retract fast enough. The reason I ask is when I had the pistons made, I had them made to clear my higher lift cams, making this a non interference engine. God it sounds so weird saying cams and dealing with 24 valves lol, BTW we are talking about a Taurus SHO engine if anyone even knows what that car that is lol.

[ForceFed70 14]

Valve float is when the valve doesn't "close" as fast as the cam lobe will allow. Generally this happens at higher RPM. When this happens, you loose cylinder pressure and power. It's also more of a problem for forced induction vehicles where the positive pressure "pushes" the valve open. Stronger springs will help with this, but at the expense of more friction and wear on the valvetrain components.

[1BAD351 21]

well my engine will (I would like it to be able to) see 8k rpm possibly more but ill say 8k is the cap. It revved to 8k ok in stock trim but now with the higher lift cams im not so sure the springs will pull the valve back far enough. Other guys that have the higher lift cams say they have taken it there w/o trouble and they had stock pistons which means their engien is no longer non interference. However mine also has over sized valves in it too which I assume weigh a little more then stock so will be even harder to pull back. I had the pistons made for the OS valves and the higher lift; so what im looking at is, if i get it together and rev it, I wont damage anything so long as I don't hold it up there long and damage the seats? Ill just feel the loss of power and hear something funky going on and then I know when I hit valve float and ill be able to shift accordingly?

[ForceFed70 14]

You are honestly planning on revving that thing to 8000rpm? If so, this should be a strip only engine. Street/strip engines generally don't go over 6500-7000RPM. Anything higher and you are really getting into race only territory (even with a cleveland)

 

Cam profile has a lot to do with this. If the cam has "steep lobes" valve float will occur at a lower RPM than it would with a less agressive cam.

 

You might want to look into roller rockers/lifters if you are planing on revving this engine to the moon. With a rollerized valvetrain, you can get away with running stiffer springs, which in turn allows higher RPM before valve float.

[1BAD351 21]

You are honestly planning on revving that thing to 8000rpm? If so, this should be a strip only engine. Street/strip engines generally don't go over 6500-7000RPM. Anything higher and you are really getting into race only territory (even with a cleveland)

 

Cam profile has a lot to do with this. If the cam has "steep lobes" valve float will occur at a lower RPM than it would with a less agressive cam.

 

You might want to look into roller rockers/lifters if you are planing on revving this engine to the moon. With a rollerized valvetrain, you can get away with running stiffer springs, which in turn allows higher RPM before valve float.

 

this is a DOHC not an old school v8, stock rev limiter is 7300 rpm, iv already had it at 8krpm in stock trim. The saying for this engine from the ford fuel injection & electronic control book tuning book is, "the SHO engine is rev-limited by the control module to 7300 rpm to prevent over revving the accessories. If the pulley diameters are changed to underdrive accessories, engineers say the engine is safe to 8500rpm.":biggrin: Also I know a guy who accidentally had it at 10,500 rpm and she didn't blow.

[ForceFed70 14]

Ahhh.. Ok, that makes more sense. It's probably already got a rollerized valvetrain then. Sorry, most of my valvetrain knowledge ends with the pushrod...

[1BAD351 21]

yeah I wish I could put an old school engine with a carb in this thing, new stuff is to complicated, but the govt wont allow it...:censored: