Boost induced valve float?
Boost induced valve float? Well boys....I sent her a little too hard this weekend i guess. I've been running 20ish psi here lately. lemme back up...... its 5.3, billet 7875, summit ghost cam....222/233, BTR dual springs (I'll find out part number). I've been shifting at 6500-6800 for the longest. I decided to raise the rev limiter about 3-4 weeks ago to 7100-7200 and i've been shifting at 7000-7080, sometimes hitting the limiter. Its revs quite fast so sometimes when the tires are spinning i dont shift soon enough.
Took a buddy with a low 8 second car for a ride, I didnt want to disappoint so I might have ran her a little harder than i normally might and I dont remember if I hit the limiter, but it did make more boost than it ever has, 24.5psi is what i saw. Yikes. I wasnt prepared to go that high, but it did. When I let off it was not running right. Missing real bad. stuck a camera in the spark plug hole, saw a bent exhaust valve. To confirm, pulled the valve cover, loosed rockers, straight edge across the retainers and a two exhaust valves were further down...one about 1/4" and the other about 1/8....
So I was thinking it was boost induced...and it came to me like an epiphany......but then i realized its the exhaust side.....more plausible would be the intake side?
Took a buddy with a low 8 second car for a ride, I didnt want to disappoint so I might have ran her a little harder than i normally might and I dont remember if I hit the limiter, but it did make more boost than it ever has, 24.5psi is what i saw. Yikes. I wasnt prepared to go that high, but it did. When I let off it was not running right. Missing real bad. stuck a camera in the spark plug hole, saw a bent exhaust valve. To confirm, pulled the valve cover, loosed rockers, straight edge across the retainers and a two exhaust valves were further down...one about 1/4" and the other about 1/8....
So I was thinking it was boost induced...and it came to me like an epiphany......but then i realized its the exhaust side.....more plausible would be the intake side?
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Someone will chime in that knows more than me but I'm leaning towards valve float plus "lifter pump". Your hydraulic lifters basically don't get enough preload pressure while the springs are allowing the float and this lower preload pressure allows them to fill all the way up with oil. Then, when you let off the throttle and the valve float stops and the hydraulic lifters are filled too full and they force the valves down into the cylinder further than normal until that extra oil bleeds off.
Could be a lot of hot side pressure at the throttle let off too I suppose.
Boost induced in the sense that HP tends to find these weak points.
How old are the valve springs? How many miles on them?
Could be a lot of hot side pressure at the throttle let off too I suppose.
Boost induced in the sense that HP tends to find these weak points.
How old are the valve springs? How many miles on them?
Last edited by LetsTurboSomething; Nov 18, 2020 at 03:41 PM.
Someone will chime in that knows more than me but I'm leaning towards valve float plus "lifter pump". Your hydraulic lifters basically don't get enough preload pressure while the springs are allowing the float and this lower preload pressure allows them to fill all the way up with oil. Then, when you let off the throttle and the valve float stops and the hydraulic lifters are filled too full and they force the valves down into the cylinder further than normal until that extra oil bleeds off.
Could be a lot of hot side pressure at the throttle let off too I suppose.
Boost induced in the sense that HP tends to find these weak points.
How old are the valve springs? How many miles on them?
Could be a lot of hot side pressure at the throttle let off too I suppose.
Boost induced in the sense that HP tends to find these weak points.
How old are the valve springs? How many miles on them?
well at the end of my original post I had a realization that the valve float probably wasn’t from boost. The reason I don’t think it is now from boost is being on the exhaust side. Make more sense on the intake side. But hey, this is my first turbo project so I’m not sure.
the Springs are maybe 20k miles old.
i like your theories too btw.
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well at the end of my original post I had a realization that the valve float probably wasn’t from boost. The reason I don’t think it is now from boost is being on the exhaust side. Make more sense on the intake side. But hey, this is my first turbo project so I’m not sure.
the Springs are maybe 20k miles old.
i like your theories too btw.
the Springs are maybe 20k miles old.
i like your theories too btw.
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Verdad Gallardo I would say if anything it makes more sense on the exhaust side. Because on the intake side that same 25psi is also shoving 25 extra psi of air into the cylinders which translates to a lot of extra pressure as the piston begins to compress the intake charge. There is way more than enough pressure there to shove that intake valve closed against the 25psi of resistance. Now the exhaust on the other hand will be absolutely filled with pressurized exhaust gasses in the hot side from the full throttle pull you just did and the moment you let the throttle close and that cylinder only has vacuum to draw from there is just what ever the cranking psi of the cylinder is left in there and 4-8(divided hotside or not) full cylinders worth of spent gasses in the exhaust. I doubt you made over 150psi in the hotside though unless you had raw fuel igniting in there. Given the spring is still doing some work to close the valve, you would need an amount of pressure in the hotside higher than the spring pressure+ cranking compression to force it back open like that. Not impossible but improbable. That kind of pressure would blow my hotside apart like it was a pipe bomb I think.
I’m going to shim the springs and add more seat pressure. I’ll figure out what it is now and do some consulting and figure out what would be best to set it at. I may change the cam timing and lower the limiter.
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I think it has more to do with the event durations than the max lobe height. Does the cam crack the valves open slowly over a longer duration or do they open the full amount in only a few degrees of rotation? See what I'm saying? It's like a dirt bike jump: whether the springs and suspension can soak up the jump depends entirely on the angle of the ramps and the dispersion of the momentum over a greater distance. Same principle with lobe duration and spring rates.
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From: Norn Iron
A small cam will be easier on the valvetrain that a big cam.
And the only thing that is going to bend the exhaust valve, is contact with the piston.
Now idea how tight PTV is with your cam etc ?
And the only thing that is going to bend the exhaust valve, is contact with the piston.
Now idea how tight PTV is with your cam etc ?
well I didnt check that because popular consensus was it wasnt big enough to worry about checking. lol
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From: Norn Iron
If a valve was indeed bent, at rest, the valve spring top would be noticeably lower than the rest.
Bit of an unusual occurrence really, especially to bend an exhaust valve as they're usually fairly robust. Although PTV contact more robusterer lol
Bit of an unusual occurrence really, especially to bend an exhaust valve as they're usually fairly robust. Although PTV contact more robusterer lol
At the end I mentioned the valves in question were a good bit further down. I did type alot tho and you may have skimmed over it. I do the same thing. A lot.
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From: Norn Iron
Time to get the heads off anyway, and hope it hasn't caused too much other damage.
I'd doubt a lifter could ever pump up enough to cause such a thing, but it would do no harm to look at them when it's apart too
I'd doubt a lifter could ever pump up enough to cause such a thing, but it would do no harm to look at them when it's apart too
Yea I might take the heads off this weekend. I’m still thinking about putting a bigger motor in it so I can lower the boost. I wish head studs weren’t so damn expensive, I’d put some of those in.
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From: Norn Iron
If you are prepared for some drilling and tapping, ideally with some proper guides...1/2" studs can be cheap
Or China studs, but that isn't a route I'd do.
LS head studs do seem unusually expensive.
Or China studs, but that isn't a route I'd do.
LS head studs do seem unusually expensive.
