valve float

[tomsfe64]

At precisely 5800 rpm my fe hits a wall and loses power. Feels like a rev limiter but worse. I have a hydraulic roller cam from comp cams. I thought it was my old dual point distributor so I bought msd distributor coil and box. Same thing at at 5800 rpm. The engine builder told me that the flat tapped valve springs that were on the heads originally were fine so he did not install new springs. He says it's not related to the springs but still has no answer for me. The old springs were about 10 years old on a non running engine so several valves were open for that long. Could these springs create this 5800 rpm fall off? What does valve float feel like?

[blykins]

What are the spring pressures?

The off-the-shelf cams from Comp have a more aggressive lobe.   If the springs are marginal, the cam will find the weak spot. 

[fastback 427]

Fuel pressure issue?  Mine did the same thing, caught it on the chassis dyno when the fuel pressure dropped.  It was a very dirty filter.

[cjshaker]

Does it do it in every gear, and does it recover as soon as you shift? Like instantly?

In my experience, valve float typically just feels like a power drop, not like you're hitting a rev limiter (if you know what that feels like, which is a very pronounced stuttering and obvious missing). But it recovers as soon as you shift and get the RPM's back down. A fuel issue can feel the same, or it can be like a sporadic miss, but typically takes a few moments to recover after shifting, if it does recover.

I'd be skeptical of springs that have sat compressed for 10 years, especially if then used on an aggressive lobe like Comp usually has.

[Barry_R]

Hydraulic roller needs more spring than a flat tappet.
I say its floating.
Looks like heck on the dyno - you will see a couple dozen pounds of torque just "disappear" in one hundred RPM
A sharp edged drop in a curve is often a sign of float - you might also see a drop in oil pressure at the same RPM as the parts all come uncoupled from each other - not visible in the car but obvious on a dyno sheet..

[Falcon67]

One other might be fuel feed.  First big 351C I put in the Falcon went flat about 5800.  Tried several tweaks, pretty consistent.  Finally traced the problem to the fuel feed out of the stock tank.  It was " 3/8" pipe out but the crimp inside the tank where it turned down to point at the tank sump was crimped to around 1/4 if that.  Mushed the pipe when it was bent during manufacturing.  Drilled the tank for a #8 bulkhead in the sump and set up a return fuel system, problem solved.  A proper fuel cell also removes that potential issue.

[chris401]

At precisely 5800 rpm my fe hits a wall and loses power. Feels like a rev limiter but worse. I have a hydraulic roller cam from comp cams. I thought it was my old dual point distributor so I bought msd distributor coil and box. Same thing at at 5800 rpm. The engine builder told me that the flat tapped valve springs that were on the heads originally were fine so he did not install new springs. He says it's not related to the springs but still has no answer for me. The old springs were about 10 years old on a non running engine so several valves were open for that long. Could these springs create this 5800 rpm fall off? What does valve float feel like?

I must have missed you build on another thread. Sounds like you could be running out of cam. When I put a stock replacement cam in my 352 and advanced it 4° it hits a wall at 4450. Unlike my previous valve floats where it is rough running and usually leaves witness marks on the pistons.

[plovett]

You can also hear valve float.  It sounds almost like an ignition misfire.  The power doesn't fall off on a mild or even a steep slope, it falls off a jagged cliff, bouncing up and down, mostly down. 

I would recommend just adding more spring pressure as a first step.  I've done that it cured the problem like magic.  Sometimes just a little more open pressure will do it.  You can also reduce valvetrain weight with lighter retainers.  Sometimes just changing the specific springs will change the situation as the harmonics are different, even if the spring pressures are similar.

sounds like valve float to me,

JMO,

paulie

[WConley]

A good trick to control valve spring harmonics is to shim the spring.  Set it up so it's within 0.050" of solid when you're at full lift.

I've seen what a difference that makes with my own eyes.  If that's not enough, you'll need more pressure as mentioned above.

Best of luck tracking down the problem!

- Bill

[Barry_R]

A good trick to control valve spring harmonics is to shim the spring.  Set it up so it's within 0.050" of solid when you're at full lift.

I've seen what a difference that makes with my own eyes.  If that's not enough, you'll need more pressure as mentioned above.

Best of luck tracking down the problem!

- Bill

I really enjoyed all the valve spring and rocker development stuff you did on the SOHC.  We all learned a lot.

For those that do not know about that - Bill had some really impressive video online (is it still available - have not looked)

[wayne]

Change them i was going to be smart and save a few bucks on springs that set. They checked ok one broke and a the keepers came out the valve broke the cyl wall it was a 300 six so easy to find a block. Now if i rebuild a lawn mower it gets new ones.

[WConley]

So many projects...  I've been wanting to build a Gen-II spin machine that bolts up to a long block (no pistons) on a stand.  That would let you test pushrod engines.

I'm trying to get my butt over to PRI this year.  May chat with some folks about that...

Anybody interested in some spin test videos from the old machine can look here:

https://www.youtube.com/user/Ottodyn/videos

[Heo]

Lot of movment and flexing going on there......
Wounder how much power loss the whole valve assembly
stands for...If one could eleminate springs rockers puschrods
camshaft and open the " Valves" or what ever to use instantly.
So you had full lift the whole duration......like a the opening of
a fuelinjector.....

[Barry_R]

.If one could eleminate springs rockers puschrods
camshaft and open the " Valves" or what ever to use instantly.
So you had full lift the whole duration......like a the opening of
a fuelinjector.....

Been done a couple ways already.  Practicality and cost get in the way a bit.

Formula 1 pneumatic and Ducatti desmodromic are a couple examples....

[Heo]

Yes   i was tinking practical and economical on a street engine
The desmodromic is a old invention on motorcycles and Mercedes grand prix racers
from the 30s but its more like a crakshaft that both close and open the valves mecanical
with a small hairpin valve spring if i remember it right and you have the "ramp"opening
but no valvespring and no float
I think Catepillar have hydraulic valveopening on big engines  but there you have space
and they are constant rpm engines 2000-2400 rpm somewhere
Köningsegg is experimenting With a camshaft less engine
https://www.youtube.com/watch?v=H2mTriy9oQM
have you guys seen a knight slidevalve engine there is two sleves in each cylinder with ports like in
a two stroke engine thats driven by a crankshaft
https://www.youtube.com/watch?v=NSMYjRNQ8Rs
I have one of those fron a willys knight some day whe i have some spare time im going to tear it apart
just to see how its enginered.must bee a lot of extra friction with those extra sleves per cylinder

[Falcon67]

>What does valve float feel like?

From my experience -

Fuel - motor just stops pulling, doesn't necessarily break up but runs out of power and feels like you started dragging a trailer.  Usually, not always, recovers on the shift if you're going 1 to 2.

Float - things go to hell quick, car noses over, maybe some sound indicators if the intakes are open during the firing sequence.  Valve bounce nearly as bad.

One more like "We're out of juice" and the other more like "Mayday! We're hit, goin' in!"   

[tomsfe64]

Thanks for all the info. I'm changing the valve springs one at a time and i found a completely broken pushrod snapped off 1/2" from the top. Somehow it stayed together until i removed the rocker shaft. So i suppose that the valve hit the piston. Hopefully the new springs solves the problem.

[WConley]

If you think a valve hit a piston, you may want to check that it's not bent.  Do a compression test on all cylinders once you've got the new springs installed.

(I'm assuming you're not pulling the heads.  If you did you could just check them on the bench...)

[fryedaddy]

valve float or not,what is the most rpms anyone ever seen out of a flat tappet hydraulic cam on an fe.i have hit 7000 on my limiter during a burnout.im sure it was losing power but the rpms kept rising

[tomsfe64]

new valve springs solved it. now runs as it should. pulls hard at hi rpm.

[cammerfe]

Actually, desmodromic valve actuation goes back to the late 1800s and was used by Mercedes, in the '30s I believe, for its race cars before being used in the Ducati motorcycles.

KS