What causes a valve spring to break?

KobaltBlau · 12:28 PM

I have done a little research and it seems the SCs breaking valve springs is not entirely uncommon, as mine has.

A (mostly race) mechanic I know said that normally not just one spring breaks. In my case only one has broken, but I hope the rest don't go. He indicated that an overrev could cause several to break in succession. I don't understand how an overrev would break a valve spring.

My engine has not been overreved for at least 30k miles (no missed shifts), I think never. I have run it up to 7000 and perhaps even 7200 at DE events (just a few times, not all the time), and the car/engine has probably had a pretty quiet life prior to my ownership, where I imagine it was run up to 6 rarely. The spring broke during my ownership, but I noticed it well after DE season had ended, during street driving. I could have cracked or weakened it during a DE, though.

My hope is that I found a spring that had an original flaw or just fatigued faster in some way, by really exercising the engine.

But I'd love to know why people think valve springs break, and whether an overrev is a possible cause.

Thanks in Advance,

KobaltBlau · 09-30-2005, 12:56 PM

Now that I think about it, the mech said that overrevs cause the valve springs to overheat and then they start breaking.

I'm not clear how an overrev would overheat the valve springs, but I can see where sustained high RPM operation at the track and highish oil temps could keep the valve springs much hotter than street driving. I'm not sure this would cause a failure, though.

DW SD · 09-30-2005, 01:01 PM

I'm guessing that the increased mechanical motion can turn into heat energy. (conservation of energy) Any coil spring (or torsion spring for that matter) gets hot when it is cycled fast enough.

I suspect I'm not adding much for you, but that is my guess. The material properties of all of the springs would be similar, as are the tolerances, so they'd all see similar stresses and have similar breaking points. A specialist like camgrinder would probably tell you if all of the other springs will begin to break (or not).

Hope that is the smallest bit helpful.

Doug

speedkillz · 09-30-2005, 01:11 PM

When yours broke ...what kink of sound did your engine make?.. My SC has a ticking/klanking that comes and goes..we pulled the engine and looked as well as possible at the valves and springs. checked the tentioners and though everything was fine...installed engine only to find the same noise. any help ???

wholberg · wholberg's Garage

My own opinion is that valve springs are subject to conflicting accelerations at high revs. A compressed valve spring that is suddenly relaxed upon valve closure has some harmonic motion, or wobbling/vibration that takes a finite period of time to dissipate. If the spring is violently compressed before this residual vibration has stopped, the acceleration of the spring from compression may collide with the vibration going the opposite direction, creating shear forces that are localized in a very small area. This would accelerate metal fatigue in that small area and cause breakage.

Read the link below about causes of spring failure. Several other possible causes are listed, many of which can be directly extrapolated to valve springs at high rpm.

http://www.lapham-hickey.com/Resources/CausesofSpringFailure10.cfm

KobaltBlau · 09-30-2005, 02:24 PM

Quote:

Originally posted by speedkillz
When yours broke ...what kink of sound did your engine make?... any help ???

I did not hear anything loud when it occurred, but afterward it had a ticking like a valve adjusted wrong to 150-200% of its designed clearance. It definitely came and went, even with the car sitting there idling. I was able to localize which valve area it was using careful ears, along with an automotive stethoscope; mine was #1 intake. I still feel lucky that it was exactly the one I thought I was hearing. with the rocker arm and shaft out the spring was clearly broken (seen with a mirror), the outer had broken all the way through and was in two pieces. I think you should try to listen carefully with a stethoscope to see where it is coming from, then pull that rocker and shaft.

Quote:

Originally posted by wholberg
My own opinion is that valve springs are subject to conflicting accelerations at high revs.

I buy this whole argument, but I would think a stock SC cam at even 7300rpm would be relatively gentle, compared to people I know using RSR sprint cams with stock valve springs at 8000 or 8500. However, I can certainly see why an over-rev might cause this condition in a nasty way.

hopefully I just found a weaker spring than the rest of the lot (knock on wood). I'm sure oil temps of 250 or so don't help a lot.

Quote:

Originally posted by wholberg
http://www.lapham-hickey.com/Resources/CausesofSpringFailure10.cfm

Thank you, that is helpful, I should have searched for something like that.

Cheers,

Grady Clay · 09-30-2005, 03:49 PM

Andy,

I think the primary culprit is rust. One little site on a carbon steel spring wire will spell its failure. Yes, this is accelerated by high cycle rates (very high RPM). Yes, this is accelerated by high temperatures (the springs need oil cooling also). Yes, resonant harmonics have an influence also (note the springs are wound opposite directions and more).

What is the solution? Replace all the springs at every overhaul or xx miles? Sure, at 500,000 miles.

I think the best preventive measure is often oil changes, drive it to minimize water in the oil and drive the pi$$ out of it all the time. I prefer like new street springs with 10K miles for a race engine.

Best,
Grady

Hoffman912 · Hoffman912's Garage

yup rust.

kstylianos · 09-30-2005, 04:01 PM

For the early 80's SC, this is not uncommon, like you mentioned. I had an outer intake spring break. Walt at CE mentioned it was the spring material for this particular vintage that caused problems. To be honest, I didn't pay particular attention to the springs until Walt gave me the heads up, upon further tare down, there it was.

randywebb · 09-30-2005, 04:41 PM

Either Jerry Woods or Bruce Anderson has said there seemed to be a run of bad steel for springs...

At the level of the grain in the metal, the cracks from corrosion are the same as stress cracks...

Steve@Rennsport · 09-30-2005, 05:28 PM

Walt @ CE, Jerry, and Bruce are all correct; Porsche installed a batch of defective outer valve springs in the 79-81 model years and this has been well known problem for many many years now.

I started seeing those in 1980,...

I tell anyone with an SC of that vintage to press on each valve with your thumb at valve adjustment time. If you can push the assembly down manually, the outer spring is broken and requires replacement. This can be done with the engine in the car,.....

You'll hear a ticking (much like a loose valve) when a spring has failed and its VERY important to fix this right away.

Vipergrün · 09-30-2005, 06:02 PM

Don't want to hijack this thread.....but I'm gonna

If one were to replace the stock springs and retainers with race springs and Ti retainers, could the bottom end of a stock 3.0 handle the higher revs? I know there is no reason power-wise to rev higher, but in the event of a missed shift, would the valve train have a better chance of escaping damage? Just wonder if these things would be cheap insurance if doing a rebuild.

speedkillz · 09-30-2005, 06:12 PM

Thanks Andy...I couldn't see anything even with the engine out. I turned the engine over by hand 40 times looking for ??? pieces ect. nothing.I reset the valves and put it back in the car. It did and does sound like a super loud ticking,but not all the time. when you increase the rpm it gets quite loud.

Steve@Rennsport · 09-30-2005, 06:38 PM

Quote:

Originally posted by bb80sc
Don't want to hijack this thread.....but I'm gonna If one were to replace the stock springs and retainers with race springs and Ti retainers, could the bottom end of a stock 3.0 handle the higher revs? I know there is no reason power-wise to rev higher, but in the event of a missed shift, would the valve train have a better chance of escaping damage? Just wonder if these things would be cheap insurance if doing a rebuild.

Brad:

The only benefit of installing better springs & retainers in an otherwise stock SC motor would be a greater resistance to valve float if you missed a shift.

Fact: Porsche OEM springs lose some tension over time and the RPM threshold of valve float goes lower as that occurs. Really good springs like the Aase ones and Ti retainers maintain some insurance against piston-to-valve contact when you make a mistake,....

I make the decision about what to use based on the engine's configuration and application. Being a proactive-type person, I like to install the components that will help PREVENT the missed shifts in the first place.

john walker's workshop · 10:54 AM

all the springs i've replaced for breakage over the years were on the intakes. any ideas why?

KobaltBlau · 10-01-2005, 11:12 AM

Quote:

Originally posted by Steve@Rennsport
Walt @ CE, Jerry, and Bruce are all correct; Porsche installed a batch of defective outer valve springs in the 79-81 model years and this has been well known problem for many many years now.

I started seeing those in 1980,...

I tell anyone with an SC of that vintage to press on each valve with your thumb at valve adjustment time. If you can push the assembly down manually, the outer spring is broken and requires replacement. This can be done with the engine in the car,.....

You'll hear a ticking (much like a loose valve) when a spring has failed and its VERY important to fix this right away.

Steve, this is AWESOME info and 100% consistent with my experience. if a single spring breaks, what do you recommend for a street/DE? replace all the intake (or all) outers, or replace the broken one and keep an eye on things? I plan to do the latter, but if there could be very expensive consequences I could be convinced to change them out.

Thanks!

Sameer · Sameer's Garage

Over reving

Grady Clay · 10-01-2005, 12:24 PM

Andy,

Fix the immediate problem in the car.

For the longer range, do a complete top end with all new valve springs and associated parts. This gives you the opportunity to look at P&Cs, rebuild the heads to better than new and attend to all the other issues. If done correctly, another 200K isn’t unreasonable.

BTW, I’m working on the Rubbermaid Solution for you today.

Best,
Grady

KobaltBlau · 02-23-2011, 12:27 PM

Quote:

Originally Posted by Steve@Rennsport

Walt @ CE, Jerry, and Bruce are all correct; Porsche installed a batch of defective outer valve springs in the 79-81 model years and this has been well known problem for many many years now.

I started seeing those in 1980,...

I tell anyone with an SC of that vintage to press on each valve with your thumb at valve adjustment time. If you can push the assembly down manually, the outer spring is broken and requires replacement. This can be done with the engine in the car,.....

You'll hear a ticking (much like a loose valve) when a spring has failed and its VERY important to fix this right away.

Bumping a very old thread here, but I wanted to update the records: I believe the bad run of valve springs also includes the '78 models, not only 79-81 as Steve previously mentioned.

I just replaced a broken #4 intake on a 47k original mile 78 SC, which is my second SC to have this problem - at the time of this thread I had just done one on my 81 SC, and the problem is almost exactly the same. That car had a broken intake as well, I believe it was also #4, though I think the cyl. number is just a coincidence. In both cases, the outer spring was broken but the inner spring was OK.

On the 78 I didn't notice a ticking as I had with the 81, but I noticed that I could push a rocker down by hand without trying particularly hard during my first valve adjustment on this car. The outer spring was broken into 4 pieces, and looked like it had been for a good long time.

Still curious why these break on the intake side, as John Walker mentions.

I'm really surprised these cars can be run to redline with a broken outer spring without valve to piston contact, but hey, I'm not complaining!

Grady Clay · 02-23-2011, 12:51 PM

Andy,

At least you had the tool at hand.
See … I told you.

The piston must have the exact correct shape to help close the valve without bending it.
Just to be sure, do a cylinder leak test.

A spec of rust is the usual staring point for a fracture.
The intake (top) part of the cam box gets more moisture (cooler) and gets less oil.
I suspect these are contributing factors for intakes breaking more often.

It was good to see you last month.

Best,
Grady