Valve Spring Pressure - BBC stocker

[Chipper Chapman]

My new motor 396/325hp stocker broke a valve first weekend out (had dyno time too), upon disassembly it was found that all 8 intake valves had been making light contact, of course it was one of them that broke.

My tightest ptv clearance upon assembly ( and checking again during autopsy) was .070" which should be plenty, which leads me to believe it must have been lofting the valves open due to lack of adequate spring pressure. Springs were setup to be 250# on the seat and 500# over the nose which all recommendations said should be enough. Obviously not however. Just wondering if any of you have suggestions of what we should set it up to this time. Cam is steel core, dlc coated trend lifters, pac 1325 springs and steel retainers. Fire away with any suggestions, and thank you.

[HP HUNTER]

Originally Posted by Chipper Chapman

My new motor 396/325hp stocker broke a valve first weekend out (had dyno time too), upon disassembly it was found that all 8 intake valves had been making light contact, of course it was one of them that broke.

My tightest ptv clearance upon assembly ( and checking again during autopsy) was .070" which should be plenty, which leads me to believe it must have been lofting the valves open due to lack of adequate spring pressure. Springs were setup to be 250# on the seat and 500# over the nose which all recommendations said should be enough. Obviously not however. Just wondering if any of you have suggestions of what we should set it up to this time. Cam is steel core, dlc coated trend lifters, pac 1325 springs and steel retainers. Fire away with any suggestions, and thank you.

Can I ask what it made on the dyno and the peak RPM HP?

[Chipper Chapman]

445@5700. My dyno is known to be stingy, but it really should have made peak near 62-6300. Should have been the first clue but unfortunately it wasn't.

[Alan Roehrich]

If you indeed have 250# on the seat, and over 500# open, you have over 0.050" piston to valve clearance on the intake, and you have a valve control problem, then there is something terribly wrong. I can spin mine to over 8,000 RPM with those pressures, and a true 0.050" piston to valve clearance.

[nhramnl]

Originally Posted by Alan Roehrich

If you indeed have 250# on the seat, and over 500# open, you have over 0.050" piston to valve clearance on the intake, and you have a valve control problem, then there is something terribly wrong. I can spin mine to over 8,000 RPM with those pressures, and a true 0.050" piston to valve clearance.

I completely agree with Alan, but have a question. Is the camshaft you're using one you've used in the past without problems, or is it something new, perhaps with lobe profiles that are somewhat "exotic"? If the intake lobe ramp profile is too extreme, even with the kind of valve spring pressures and piston-to-valve clearance you're talking about, the lifter may not be able to stay in contact with the lobe, and you end up with an uncontrolled valvetrain. Just a thought.

[Chipper Chapman]

Originally Posted by nhramnl

I completely agree with Alan, but have a question. Is the camshaft you're using one you've used in the past without problems, or is it something new, perhaps with lobe profiles that are somewhat "exotic"? If the intake lobe ramp profile is too extreme, even with the kind of valve spring pressures and piston-to-valve clearance you're talking about, the lifter may not be able to stay in contact with the lobe, and you end up with an uncontrolled valvetrain. Just a thought.

This is what my fear is, When I called to have cam ground I had called Chris P when he was at crane, he left while my cam was to be done and it got shuffled over to comp and ground totally different than we talked, in a more aggressive fashion. I usually check the ptv the simple way with clay, but also checked it by opening valve with checking springs and degree wheel, same result both ways.

For reference camshaft is COMP .396/.396 lift, 288/294 .006, 248/254 .050 and on the big core

[Alan Nyhus]

Originally Posted by Chipper Chapman

My tightest ptv clearance upon assembly ( and checking again during autopsy) was .070" which should be plenty......

Chipper, how was v-p clearance checked? Where was the contact on the piston? -Al

[Jim Hanig]

[QUOTE=Chipper Chapman;566240]My new motor 396/325hp stocker broke a valve first weekend out (had dyno time too), upon disassembly it was found that all 8 intake valves had been making light contact, of course it was one of them that broke.

My tightest ptv clearance upon assembly ( and checking again during autopsy) was .070" which should be plenty, which leads me to believe it must have been lofting the valves open due to lack of adequate spring pressure. Springs were setup to be 250# on the seat and 500# over the nose which all recommendations said should be enough. Obviously not however. Just wondering if any of you have suggestions of what we should set it up to this time. Cam is steel core, dlc coated trend lifters, pac 1325 springs and steel retainers. Fire away with any suggestions, and thank you
who told you needed 250 on the seat? I have ran several 396/454 oval port engines with no way near that much seat or open pressure. I wou call pac and get the correct spring.

[Darrin Christen]

[/QUOTE]I do not use clay to check. I check as run, complete the springs. Manley makes a tool to open the valve using the rocker arm. Check every 2 degrees between 25 degrees BTDC and 25 degrees ATDC. Your piston to valve clearance, as run, should be equal to or greater than your TOTAL piston to deck clearance. On the exhaust, you need 0.075", checked the same way.
Alan,
I know many people use this method for checking ptvc, but when you open the valve using the rocker arm to find the clearance you basically take all of the load off of the pushrod and lifter which doesn't account for any pushrod flex. Seems to defeat the purpose of checking the clearance in an as run situation. I do realize that the larger diameter and thicker wall pushrods would have minimal flex. Just a thought.

[Ed Wright]

There ya go!! ^^^^^^^
I did not know anybody used modeling clay these days.