Valve Float! What Is It, And How To Avoid It | Engine Masters | MotorTrend

We all know valve float as the evil that lurks beyond the redline, the dreaded robber of horsepower and destroyer of valves. But what is it really? What does it look like when it happens? And how can you put it as high in the RPM band as possible?

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[cornelius]

Downshifting to 1st at 100mph might cause some float.. among other things.

[BlownCommodores]

Valve float, valve bounce or valve lofting are all different things and can have different causes, including spring pressure, harmonics and cam lobe design. There is a great video on youtube taken with a high speed camera under the rocker cover and shows valve bounce which is what I believe we are talking about in this video.

[carlob]

quite some years ago i had an Alfasud which has a flat 4 overhead cam per bank motor boxer motor just like a Subaru, it could rev to well over 8, 000 rpm as it only had a 54mm stroke, one day i experienced valve float at around 6150rpm no misfiring just a strange sensation while reving , i didn't know it was valve float at the time, a friend of mine who worked at an Alfa dealer told me what the problem was he said that probably some retainers had split causing a loss of spring tension on the valves he said because of the flat 4 design the pistons were closing the valves affected !, luckily there was no damage to the pistons or valves i could see once i had the heads removed Phew close call, he overhauled the heads and found that 2 valve retainers had cracked/split in the cone part of the retainer where the valve stem collets are seated causing a loss of valve spring tension, i fully understand why spring tension on a Valve is important as your video has pointed out cheers

[lessmith]

The person i bought my 67 el camino from had a auto shut off on the ZL1 engine. It had trick heads, had roller rocker arms, triple valve springs and steel retainers. Victor jr manifold, 750 holley carb. Solid lifter cam 500" lift + 260° duration. 4.56 posi rar 1 ton axles and 4 speed. Lots of fun.

[JETZcorp]

Ahh, valve float. One of my favorite things about 2-strokes and rotaries is the sheer "who cares" factor with rpm. It's a neat experience to feel the power nose over, and then hold it wide open for two minutes, knowing that it's just resonance timing and the engine has 4, 000 more rpm of mechanical overhead. Just... make sure you have good premix that day, lol.

[Trucker_Dashcam]

No titanium valve? I guess I missed the valve choice discussion. Mushrooming stems or what?

[DeliveryMcGee]

Preventing valve float is also why Ducati motorcycle engines sound like, as one person put it, "a Cuisinart full of clarinets" -- actually looking at the system as implemented breaks my brain with the linkages involved, but short version, the top end works kinda like the bottom-end rotating mass -- the valve stems are pushed /and/ pulled by the cam, no springs, so no float, but it's very noisy and probably a bit harsh on parts without the tolerance buffer that a spring provides. But when you need a twin to make twice the revs of the tested V8, and thus fire each cylinder ... not gonna lie, but I must've slept through the session of physics class covering the square-cube law. A 1.3L twin is working a lot harder per cylinder to bet to 12kRPM than a 5L V8 at 6k, that's just basic math, right?

[maxreed]

My dad had an 83’ CJ7 with the inline 6 and it used to float the valves going up hills, had over 300k miles on it too

[davebaker]

I have a panhead harley that I've owned for over 40 years, am running a ridiculous big cam, and use the stock FL valve springs (way to light seat pressure for the cam) and I rely on the valves floating as a rev limiter, but having it for as long as I have, I don't abuse it, also the light pressure on the valve train has the benefit of not wearing stuff out fast, I do weird things with my vehicles but would never do on a customer's bike, car whatever, but for myself I go way out in left field, and it works for me

[cuckleberryfinn]

Floating a valve is my biggest fear in my old Chevy. I’ve got a 1978 Big 10 with the 454 and 400 turbo trans. It’s all original parts, and at 128k lol

[tedjones]

Valve float is also a function of the spring harmonics, should have talked about BeeHive springs. Beehive springs have less of an issue with harmonics and you are able to use less spring pressure to control the valvetrain

[shammon]

What a remarkable clip. Thanks for explaining this issue great!

[FSHSKainon]

Engine Masters PRO Sessions 🙂thanks Guys!!

[edengates]

Thank you techno hippies I learned a lot

[brianf]

Why did he state don’t use Titanium values?
I missed that part.
Always great information in the show.

[SlimeyFukR]

Boy you were made for the camera!Always interesting listening to you David,
Thank you for the awesome content

[redtailhawk]

Eng Masters is the best car show, Tell use about the intakes on the shelves behind you. Thanks

[claycopopo]

Why no Ti valves? No explanation for why this is bad?

[johnbujalski]

Why no Ti valves? They are fantastic at high rpm and allow for more aggressive cam profiles. The other I would like to see are Sodium filled valves for higher cylinder pressures.

[rogermason]

Ok, you added spring stiffness to resolve this particular engine's valve float problem: so what is the limiting factor to adding even more spring stiffness and running to higher RPM's?