Armitage

iTrader: (15)

I remember reading about this in another thread but no explanation was given as to what it was. It was in a thread about using a lightweight pulley and lightened flywheel being used together.

davidv

iTrader: (2)

Its what you get at red line. Thats why horsepower falls. Get some stronger springs.

What does this have to do with pulley and flywheel? I haven't the faintest.

Black_Sunshine

I could explain myself, but I'll just let Wikipedia do it for me.

http://en.wikipedia.org/wiki/Valve_float

Oh, Wikipedia, what don't you know?

It should be noted that valve float does not magically happen at redline. Lighter weight valvetrain components (titanium retainers and locks, hollow stem valves) and higher rate springs are usually employed to counter this phenomenon. Also, davidv is correct, this has absolutely nothing to do with pulleys or flywheels.

zz350zz

David you're gettin close to 10,000 posts buddy?

You goin to celebrate with a good picture?

Armitage

iTrader: (15)

Thanks for the responses guys.

daveZ insanity

iTrader: (4)

The definition of valve float in the link above is "a condition which occurs when the valves on an internal combustion engine do not stay in contact with the camshaft lobe". But there is no contact between the valve and the camshaft lobe - in an OHC engine, there's a part between the two called the "lifter" or "tappet". In an OHV engine, there's a pushrod seated in the lifter instead of the valve stem - and a rocker arm driven by the other end of the pushrod transfers movement to the valve stem tip.

If the valve stem tip rode directly on the cam lobe, there would be no way to adjust clearance for wear - and both would wear much more rapidly than they do in reality. The lifter may be a simple inverted cup sitting over the valve stem tip and separated from it by a stack of shims, or a piece of round metal stock with one end threaded into the other to permit changing its length for changing valve clearance, etc. The lifter also withstands the angled forces of the cam lobe and turns them into pure axial forces on the valve stem (i.e. pushes the valve stem up and down but not side to side - one of many reasons OHC designs can reach higher revs than OHV).

Valve float is a physical phenomenon. The valve springs have to exert enough force on the valves throughout the cycle to make sure the valve follows the cam lobe contour. When the revs rise high enough, the valve springs aren't fast enough to follow this motion and the valves begin to "float" - they no longer move up and down exactly as the cam lobe tells them to do because the springs can't keep enough force on them to hold them against the lifter. But the valve is never in contact with the cam at any time.

Reducing valve train mass raises this point, as does increasing valve spring rates. But higher spring rates cost power. That's the reason for desmodromic valve trains - cams both open and close the valves, and there's no high-rate springing involved. But desmos have their own drawbacks, which is why they're used so rarely on street engines.

Wikipedia is a perfect example of how the web can mislead you. What they're trying to say is correct (valve motion no longer follows cam lobe contour) - but what they actually say is not.

davidf

iTrader: (18)

Yes we need pics. of the smartest man on the board.

roast

iTrader: (2)

I noticed that too. The good thing about wikipedia is that anyone knowledgable enough and willing can help improve the information available.

davidv

iTrader: (2)

No. 10,000 posts means that I have no life. Celebrating would be too depressing.

daveZ insanity

iTrader: (4)

True enough - but the bad thing is that you have no idea whether what you're reading is right. This is true for anything you read anywhere, of course - but reputable publishers (even on the web) use authoritative sources. Wikipedia uses readers with little or no qualification as experts in the field. I've been racing for decades and have built a few dozen engines and about 15 cars for myself. But I continue to run into things I just plain didn't know.

We grow old too soon and smart too late!

Black_Sunshine

I noticed that too, however, I thought the explanation would suffice for a neophyte. It could be said that it loses it's connection with the cam lobe, which is at least theoretically correct.

godmans

iTrader: (10)

my question would be
how does the high rev engines like the ones in S2000 and the new RS4 do the trick of high rev and high horsepower?

titamium valve train components?

ouch1011

The S2000, along with many other Hondas, use a different style of valvetrain. The S2000 uses solid lifters versus hydraulic ones. The solid lifters are lighter and are better for all out performance, but the valve clearance has to be adjusted occasionally, whereas with hydraulic lifters, you don't have to adjust them. The S2000 probably also uses stiffer valve springs and hollow, sodium filled valve stems, which are lighter than solid steel valve stems. Its really just alot of little things that add up.

Cannysage

iTrader: (29)

sounds like something that involves ice cream...

daveZ insanity

iTrader: (4)

"Less weight, more power". And, just like beer, the head's pretty important.....

ouch1011's got it right (and I think Honda uses solid lifters on all their engines for this reason). The mass of the valves and associated parts is one important factor in how high an engine can rev and produce power. The valves with their springs and other reciprocating parts form an oscillating system, and every system like this has a natural resonant frequency (the frequency at which it will oscillate when driven by external energy and unrestrained). A guitar string oscillates when you pluck it, producing a tone at the resonant frequency determined by the string's mass, length and tension. An engine's redline is related to these factors as they apply to the valve train.

You can move the theoretical point of valve float up the rev range by using stiffer valve springs. But it takes more power to compress them, so you won't achieve your goal unless you somehow offset this (e.g. lighter valves, more flow, etc).

The collective mass of the valve, lifter, keepers etc and the rate of the valve springs determine the resonant frequency. The valves will "float" in resonance at this point and above, which is why they no longer follow the cam's instructions to open and close fully. Lower the mass of the valves and they can do their thing higher in the rev range - the same valve springs can keep them "on the cam" at greater rotational speeds. This is one advantage of "multivalve" heads over conventional designs with one intake and one exhaust valve. Two smaller valves each weigh less than one big one.

Another problem with pushing up your redline is that there's less margin for error in an interference engine (i.e. an engine in which the valves can hit the piston tops if valve timing isn't perfect). So the cost of engineering and manufacturing goes up because tolerances have to be closer to prevent your valves and pistons from getting intimate. Honda's a master at engineering and manufacturing to tight tolerances. But a base S2000 lists for about $35k as I recall, and a lot of the price premium is for the engine - there's little difference in the cost of raw steel and other structural components between a base Z and a base S2k. The 2.2L 4 is beautifully engineered and made - but you have to row it down the road with the shifter to keep it in its power band.

SergEK

People also forgot to mention that cam design has alot to do with valve float

Honda's VTEC system incorporates a third lobe for high rpm use

Its the cam's job to orchestrate the valve motion and an agreessive/high rpm cam will put more stress on the valve train ---- BUT a properly designed cam will have a slower ramp rate and keep the valvetrain from being too stressed

Without VTEC cam designers are forced to design one lobe with a compromised ramp for both high and low speed (350Z cams) while Variable timing helps its still a compromise

Many cams that have the same lift and duration specs but give different power bands --- its the ramp rate that will make or break or valve train --- even if the upper limits of lift arent touched if it ramps too fast the valvesprings cant handle it and will float

An aftermarket cam may have slightly more agressive lift and duration specs but a fast ramp up rate --- people start getting float and blame the springs ---- but another cam comes out that works with the stock springs and is just as agressive with the lift and durations specs

So lighter components help but properly designed cams are another part of the "valve float" battle as well

That is how I understand the valve/spring/cam balancing act anyway FWIW

LegionnaireZ

iTrader: (1)

for those interested in purchasing cams... this is definately a good read...

daveZ insanity

iTrader: (4)

A radical cam profile is useless unless the valve train is optimized for it. Lighter valves just raise the resonant frequency - they don't guarantee anything else.

I'm always amazed at how people make one or two changes in isolation and expect them to produce more power (like a huge header and exhaust without increasing breathing).

Variable valve timing is pretty cool too - it's amazing how well engineered even a garden variety econobox is these days.

Armitage

iTrader: (15)

Lots of good info here. Thanks!

daveZ insanity

iTrader: (4)

I lOVE it! One of my biggest fears (and I share this with most of the over-50 car people I know) is that there won't be anybody left who understands and plays with the innards when we're gone. As kids, we used to buy an engine or transmission for $5 at a junkyard and take it apart to see how it worked - then we'd fix it, stuff it into some oddball junker like a Renault or MG, drive he11 out of it for a few weeks and sell it for $100. There are no more $5 engines, and most kids today would rather pay to have an aerokit installed than learn to change a cam themselves. Of course, it was a lot easier to work on an early small-block Chevy or Austin 4-banger than it is on many of today's cars - but there are tons of dirt-cheap little Chevys, Civics and Sentras out there waiting to be somebody's automotive love toy.

It's true that many well-designed cams hit the valve more lightly on initial opening. But duration has to be increased if the average rate of rise is reduced throughout and lift is the same. That means increasing overlap, which can require radical changes to avoid poor running. You can grind a cam to smooth the initial lift, but there are only so many crank degrees available for the whole process - so, unless you increase duration (the arc of crank rotation during which the vale is open) it has to open faster somewhere at some point to reach max lift at the right time and hold it long enough to pump the mixture in and the exhaust out.