Tech / General EngineWhy is my car making that sound? My car won't start! Combination questions? Don't see a board for your problem or have other technical or engine specific questions? Post them here!
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Ive been having this strange issue with my valvetrain/cam.
There are certain RPMs where the lifter appears to partially collapse and allow the lash to open up. Basically at idle the valvetrain is quiet. But, as I raise the RPMs up, there are regular intervals where the valvetrain will become noisey, then quiet back down, then noisey again as the RPMs are increased up to redline. Although it does this at all engine loads, the engines performance also is noticably effected by this at WOT. The power from one cylinder to the next will pulseate and the engine vibrates. If you look at the first second or two of the video here, it does it right before the speedo reaches 20 MPH. You can hear it in the mic as it distorts real bad from the noise. http://www.youtube.com/watch?v=2uH31z3qKhQ. Does it again right before 60 MPH, but not quite as bad.
Ive run into this type of thing before with my old setup, but never this bad. Whats the cause? Is it torsional vibrations in the cam? Possibly a spring issue? Whatever it is, it seems to be overloading the lifters and causing them to collapse. I'll put a timing light on it tommorow and see if the spark is scattering as well. That would at least give a clue as to where the problem is.
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I would tend to say no as it occurs at all RPM ranges. It first starts doing it around 1200 RPM, and then has harmonics at intervals up to 5500 RPM (and beyond, I assume). The valvesprings are dual coil 986's.
I checked the timing, and its rock solid through the RPM band. No apparent scatter. So, I dont think its the cam itself causing it.
What could be the cause, then? Resonance issue between the valvetrain mass and lifters? Possible resonance in the springs? I know that dual coils tend to oscillate a bit even with dampers. Could they be the issue?
The heads where originally set up (by me) for a high lift roller, so they where good to at least .550" if I recall from my measurements. The cam itself has .471" of lift at the intake and exhaust with the current 1.52 rocker arms. The springs should be pretty far from coil bind given the cams a flat tappet and not the orignal planned roller.
Everything looked good the last time I had the rocker covers off to re-adjust the lash. I set it to around 1 full turn, which made the valvetrain much more quiet. But, it still clattered at certain RPMs. When the valvetrain is quiet, the engine runs very smooth and has good power. But, when it gets into those areas where it begins to make noise, the power drops and it becomes uneven under load. Its strange to say the least.
I have a set of CC beehive springs from my old vortecs. They are smaller than the spring seats on my trick flows, but Ive been tempted to try them as they are much less prone to oscillations than the standard coil springs from what Ive seen. I cant really think of what else could be causing the symptoms other than some sort of interaction between the springs and the rest of the valvetrain.
The beehives would have roughly the same closed/open spring pressure as teh 986's. They are not installed on the heads now, the dual coils are. IIRC, the spring pressures for the 986's are around 115 closed and 260 open as installed, give or take.
I think Im going to try out the beehives. What the hell, dont have anything to loose, other than the $50 dollars or so to buy the correct retainers for my app. Probably wont fix it, but at least its one less variable to think about.
If nothing else, it should lighten the load a bit at high RPMs.
