Lofting, light load overrev, pushrod mechanics
Posted: Wed Sep 04, 2019 8:02 pm
Ran into an interesting issue last weekend at Norwalk that I'd like some feedback on.
My brother heard a light pop during a burnout but nothing that hasn't happened before.
Saw low oil pressure so the run was aborted and pushed back to the trailer.
Took the valve cover off and found both #4 pushrods broken.
They are Trend Performance pushrods so they are not junk.
The intake roller rocker bearing bore is a bit out of kilter and both lifters were pushed high enough in their bores to cause the low oil pressure (BBM, mains fed from cam galleys.)
Looked at the data log and bumped 7500RPM on the burnout which is about 250RPM higher than RPM through the traps.
A leakdown shows good and the cylinder actually held residual pressure as confimed by feedback when tapping the valves with a hammer.
He threw two pushrods in and oil pressure returned to normal so the brand new crank seems to be unhurt.
Knowing all this, and that the intake valve is a steel 2.35" piece sitting on a PAC spring with about 40 laps on it, what would you speculate for the cause of the pushrod breaking? I've experienced poor head setup for spring height and the subsequent bent pushrod, broken rocker runaround until a spring dies and the piston creates a a two piece valve. But theses heads were just gone through to correct any coil bind issues and a light valve job along with new pushrods, new springs and lifters maintained by Isky.
Can it be possible to loft a valve enough to make the spring go solid or cause the pushrod to leave the rocker?
Even if the spring goes solid, the rocker wouldn't be applying pressure to the valve if it loses contact, right?
The "sticky" intake rocker tells me that it saw a force bigger than the valvespring and that is typically the piston punching the valve back into formation.
A quick borescope showed no intake valve shadow on the piston.
There are some markings on the intake pushrod where it has been contacting the head gasket. This may have contributed to an eccentric load, albeit pretty small (think of tapping a pop can as you're stepping on it) during the overrev but I don't know at this time.
Nose pressure (force) is a little over 800lbf at a bit over .750" lift.
Once the first pushrod broke, it seems to be like bowling as the partner pushrod gets bumped and then that one breaks also.
Anyway, I'm just looking for a plausible what might have happened from the experienced engine folks here.
http://ktosolutions.com/images/BBM%20pushrods.jpeg
My brother heard a light pop during a burnout but nothing that hasn't happened before.
Saw low oil pressure so the run was aborted and pushed back to the trailer.
Took the valve cover off and found both #4 pushrods broken.
They are Trend Performance pushrods so they are not junk.
The intake roller rocker bearing bore is a bit out of kilter and both lifters were pushed high enough in their bores to cause the low oil pressure (BBM, mains fed from cam galleys.)
Looked at the data log and bumped 7500RPM on the burnout which is about 250RPM higher than RPM through the traps.
A leakdown shows good and the cylinder actually held residual pressure as confimed by feedback when tapping the valves with a hammer.
He threw two pushrods in and oil pressure returned to normal so the brand new crank seems to be unhurt.
Knowing all this, and that the intake valve is a steel 2.35" piece sitting on a PAC spring with about 40 laps on it, what would you speculate for the cause of the pushrod breaking? I've experienced poor head setup for spring height and the subsequent bent pushrod, broken rocker runaround until a spring dies and the piston creates a a two piece valve. But theses heads were just gone through to correct any coil bind issues and a light valve job along with new pushrods, new springs and lifters maintained by Isky.
Can it be possible to loft a valve enough to make the spring go solid or cause the pushrod to leave the rocker?
Even if the spring goes solid, the rocker wouldn't be applying pressure to the valve if it loses contact, right?
The "sticky" intake rocker tells me that it saw a force bigger than the valvespring and that is typically the piston punching the valve back into formation.
A quick borescope showed no intake valve shadow on the piston.
There are some markings on the intake pushrod where it has been contacting the head gasket. This may have contributed to an eccentric load, albeit pretty small (think of tapping a pop can as you're stepping on it) during the overrev but I don't know at this time.
Nose pressure (force) is a little over 800lbf at a bit over .750" lift.
Once the first pushrod broke, it seems to be like bowling as the partner pushrod gets bumped and then that one breaks also.
Anyway, I'm just looking for a plausible what might have happened from the experienced engine folks here.
http://ktosolutions.com/images/BBM%20pushrods.jpeg