I've seen this mentioned a few times and am curious about what it means. I have a general idea, but would like a clearer explanation.

Thanks,
Victor

 

Is when the Valves don't open for some reason... this is in general terms of course.

 

No, thats not correct.

The opposite actually.

1st understand that your valves closes by a spring which is attched to them.

Your cam shaft opens the valves with a lobe on the cam shaft. (which depending on design may or may not use a lever (rocker arm) to connect the cam shaft to the valve)

When a valve floats, it means that the spring wasn't able to close the valve in time before the cam shaft lobe was starting to open it again. Hence the valve never seats itself, as such it never closes.

This can happen at high RPMS.

Usually most people upgrade the valve springs to counter this.

EDIT:

(obviously this isn't your engine) But it's the same principal.


(ill update the pic in a minute, I'm adding text on it)

That big spring you see around the valve closes it. At high rpms it won't have the strength needed to spring the valve back shut fast enough. Hence the valve floats, since it never closes.

 

It's not going to happen to us. It was common on pushrood engines with their clumsey valve train. (Actually I still have a push rod engine in my Harley). However, when you overrev an engine (No rev limiters in the old days), if the spring is too weak to snap the valve (intake or exhaust) shut, you have what is referred to as valve float. Springs have to walk a fine line, to strong and they wear the cams, to weak and they float. You can also relieve float and wear by using two springs (one wound inside the other). It's a science.

Lou

 

Siggy and Lowrider are right. Race engines have valve springs with extremely high spring rates to eliminate high rpm "valve float". Happens when the speed of the cam outraces the ability of the spring to recoil and close the valve..... the cam's trying to open the valve again before the spring fully closes it.

I think Formula 1 engines that turn 18,000+ rpms have pneumatic or hydraulic valves that are opened and closed using gas or oil pressure because there aren't too many springs that can react in 0.0016 of a second ....that's opening and closing 600 times per second if my math is right!!!!

 

Sorry for not be more specific but I know this things in my language (Spanish) but what "SiGGy" said is what I know about it... when the valves don't do they work!

 

This is untrue. Any engine that uses a mechanical valvetrain is susceptible to valve float during overrevving. This is because at RPMs that are over safe specs, the lobes on the cam will start "launching" the part of the cam that contacts the lobe off of the cam due to the extreme speed and lack of spring stiffness. The physics of this will always be the same for any engine, no matter the valvetrain.


Of course this is horrible for your engine because your intake and exhaust gasses will mix, there will be no no firing in the cylinder, any ignited gasses will blow through the intake and exhaust systems, and in extreme cases pistons will smash up valves in interference type engines.

 

r34 racer

You misunderstood me. I wasn't saying it couldn't happen to us because of the type of valvetrain. I just used push rods as an example. (Valve float was a lot more common in the "old" days). It wont happen to us, because of the rev limiter. What I said was and is true.

Lou

 

What about those folks that raised their limiter to 7100rpm?

 

The harmonics of the valve spring can also cause the valve not to close

koryo

 

Aside from stiffer springs or dual spings, lighter valve retainers and keepers will help reduce the amount of weight that the spring has to move. Under cut valve stems are also a good "trick" to help alleviate this plus it increases air flow into and out of the cylinder. The ultimate are titanium undercat valves which are serious cash....

 

350ed

7100 RPM is safe with our engine. They always put in some tolerance. However, with the higher limit more stress is placed on all reciprocating parts. I wouldn't exceed the stock 6700 redline often.

Lou

 

It may be "safe" mechanically but why? It is clear from dynos that the stock engine is wheezy after 5500 RPM or so. I see no point in spinning an engine higher that does not continue to develop HP. Now, if you have, at a minimum, the crawford plenum then, you may get better acceleration.

 

Then there's always Ducati's answer to valve float, Desmodromic Timing.

Here's a couple tech articles on it.

The history: http://www.ducati.com/bikes/techcafe...hnical&artID=5

Desmo for dummies: http://www.ducati.com/bikes/techcafe...hnical&artID=2

Its good stuff. Other than the fact you have to get a major valve job done at 6000 miles, i think its an amazing design. I can't wait till i get my Duc next spring.

 

Which Duc are you getting? 999S? Monster?

Victor

 

Hah, nah, i dont really like the 999's all that much. The look just rubs me the wrong way. Well, that and the price tag. I'm probably going to try and find a 748 maybe a 996. But most likely a 748.

It was really a struggle for me to decide on a Duc over a Honda or Yamaha. Now, the Jap. bikes will have much better reliablity, and as of late, more power, and better handling (sorry all you duc lovers, but its true. Ducati needs to catch up). But there's just something about the bikes i love. A couple friends of mine have Duc's and they love it. Expensive parts, repairs, and maintenance included. They say its just part of owning a Duc. Damn, thinking about it is getting me excited, and i still have to wait till the spring time!

 

i am glad to see someone knows about Desmo valves.
These are the only way to beat float- BUT you pay dearly for this- everything is critical- clearances on the guide and valve finger, grind on the valve and valve seat, wear on the various contacted parts.
BUT, there is essentially no limit other than the mechanical resonance frequencies, and the reciprocating mass limits, to the valve reaction time since it is mechanically coupled to the actuating finger. As Dr. Cold noted, the maintenance is high, complexity is a toss-up (sort of in the eye of the beholder)- but you can really get to the high rpm without floating the valves.
Feeding the air and getting the exhaust out becomes the biggest problem....