Valve Spring Pressure - BBC stocker

[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

[pmrphil]

What "group" is that intake lobe from? They have some violent stuff that really shouldn't be in almost any engine. Your opinion may vary.

[X-TECH MAN]

Originally Posted by pmrphil

What "group" is that intake lobe from? They have some violent stuff that really shouldn't be in almost any engine. Your opinion may vary.

Not surprised........Stock is no longer stock since 1985. I would prefer to run S/S than try to deal with these crazy cams and spring pressures of today. The pressures you are running (250/500) with .632 lift with 1.6 roller rockers is what I ran in my SS/IA 350 Camaro back in 1978 with roller lifters. Never broke a motor and ran right with Bobby Warren and Phil Hardee. Have fun !

[gsa612]

The spring you have is to long.To have 250 on the seat your probably in at 2 in.,now subtract .400 lift your at 1.600.Coil bind is at 1.150,I can see the spring being unstable.The problem with the 325/396 is you only have.398 lift,so you have to find a short spring that would put you .50-.100 from CB.That would make it more stable.CC should have a spring that would work in those parameters.If you find one I would put it on the dyno and see if the rpm and HP go up.On your initial pulls your down quite a bit both ways.If it's still down after the change, give Bullet a call and see what they recommend for a cam.The good thing here is you didn't have the Shubeck's in it.. gsa612