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Lofting, light load overrev, pushrod mechanics

General engine tech -- Drag Racing to Circle Track

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Rick!
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Lofting, light load overrev, pushrod mechanics

Post by Rick! » 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
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Warp Speed
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Re: Lofting, light load overrev, pushrod mechanics

Post by Warp Speed » Wed Sep 04, 2019 8:17 pm

Man, it's been a long time since I've seen a pushrod break, and that was oem sbc stuff back in the late 70s, doing real stupid shit! Lol
I would look for a serious coil bind, retainer to guide bind ect. Thats wild stuff for everything else not to be destroyed!

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Re: Lofting, light load overrev, pushrod mechanics

Post by Racing68 » Wed Sep 04, 2019 8:38 pm

What rockers? We always had problems with the solid aluminum rockers "no roller bearing" style, that would constantly be eating pushrods, adjusters and rocker bodies deforming turning 8000. Eventually went to a set of jesel rockers to eliminate the problem.

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MadBill
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Re: Lofting, light load overrev, pushrod mechanics

Post by MadBill » Thu Sep 05, 2019 12:34 am

When both p rods in a cylinder let go, the exhaust is usually the culprit. Once it fails, the intake has the daunting task of opening against up to 800 psi or more of cylinder pressure and it often doesn't. This would of course also explain the intake rocker distress..
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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Re: Lofting, light load overrev, pushrod mechanics

Post by Sparksalot » Thu Sep 05, 2019 4:22 am

Parts aren't always what you expect them to be. Hidden flaws are well, hidden.

I come from an aerospace career where extreme efforts were taken to make materials and processes as near perfection as possible. The essence is perfection is not possible. Flaws that are hidden to all efforts to examine, inspect and prevent by all means possible happen.

Example: a helium leak test of a rocket part on the launch pad discovered a leak through 4 inches of solid very first class 17-4 stainless steel. An oh S..t moment. That put a major hitch in our nation's launch schedule.

The point is you don't know what the first failure was nor why it happened. Could have been... a pushrod, or not.

Failure analysis is our most powerful learning tool.

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Re: Lofting, light load overrev, pushrod mechanics

Post by CamKing » Thu Sep 05, 2019 10:23 am

Was it up against the limiter, during the burnout ?
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Rick!
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Re: Lofting, light load overrev, pushrod mechanics

Post by Rick! » Thu Sep 05, 2019 12:40 pm

Racing68 wrote:
Wed Sep 04, 2019 8:38 pm
What rockers? We always had problems with the solid aluminum rockers "no roller bearing" style, that would constantly be eating pushrods, adjusters and rocker bodies deforming turning 8000. Eventually went to a set of jesel rockers to eliminate the problem.
T&D aluminum rollers, shaft mounted, individual head oil feeds (not spraybar). It's interesting to note that the head mfr now pushes Jesel rockers over T&D.
CamKing wrote:
Thu Sep 05, 2019 10:23 am
Was it up against the limiter, during the burnout ?
It's got a limiter at 7499RPM in the ECU but I don't know if it's ignition signal (MSD points signal) or fuel cut or some combination of both. Datalog shows an RPM dip near there but then recovering just before the end of the burnout.

MadBill's explanation makes sense for the order of breakage of exhaust first and giving the intake side a hernia. I'm just scratching my head as to how the exhaust might have failed first. BBC guys just throw in big diameter pushrods and never have the fun that BBM guys have livin' on the edge.

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Re: Lofting, light load overrev, pushrod mechanics

Post by tenxal » Thu Sep 05, 2019 1:18 pm

I'd say the intake was opening against exhaust pressure. As to why it killed the exhaust push rod..hard to say. Could be as simple as a bad push rod.

If it was mine, I'd put two new valves in it, as well.

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Re: Lofting, light load overrev, pushrod mechanics

Post by CamKing » Thu Sep 05, 2019 1:22 pm

Rick! wrote:
Thu Sep 05, 2019 12:40 pm

It's got a limiter at 7499RPM in the ECU but I don't know if it's ignition signal (MSD points signal) or fuel cut or some combination of both. Datalog shows an RPM dip near there but then recovering just before the end of the burnout.
This would be something to look into.
When the Indy Racing League went to their normally aspirated engine package in the 90's, they had what we called a "Hard" limiter(shut down cylinders). If the engine spent much time, up against the limiter, the chaos it would cause, would float the valves. To fix this, the teams went to a "Soft" limiter(altered fuel and spark), that came in before the hard limiter.
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