930 Turbo Snapping Rockers

[porsche930turbo]

Hi,

I've just had my 930 turbo fully rebuilt and upgraded with the following:

AASCO springs and retainers
3.2 cams
3.4 Mahle motorsport pistons and barrels
Flame ring machined into heads and barrels
Ported and polished 930 heads
930 sodium filled valves
930 heavy duty rockers
Rocker shafts with seals
Micropolished crank
ARP headstuds
ARP rod bolts
All new bottom end bearings
K27 Turbo
1 bar wastegate spring
Full bay intercooler

It's still on CIS and distributor spark

It conducted 1,500 miles of run in with multiple oil changes and then went for dynamometer tuning where it made 445bhp.

The problem started a few hundred miles after dynamometer session, whilst going up the gears there was power loss. It turned out to be 2 snapped rockers on cylinder 3, the following was checked:

Borescope - no witness marks on piston
Compression and leak down - all cylinders 120 PSI with 8%
No bent valves
Springs all in tact

2 new rockers where put into head no.3, the engine ran perfect again but 10 miles into the test drive on a 2nd to 3rd gearchange power was lost again, yet again both rockers on cylinder no.3 snapped with no further damage, yet again the following was checked:

Borescope - no witness marks on piston
Compression and leak down - all cylinders 120 PSI with 8%
No bent valves
Springs all in tact


I'm really struggling to think what it can be, there is a theory the engine is over revving but the distributor arm limiter is 6,500RPM and works as expected, the engine builder thinks it might be going over this as the engine accelerates so fast so has suggested removing the distributor and going with electronic ignition setup with toothed pully.

Has anyone ever experienced the same? Any recommendations on what to check? After spending so much on the engine rebuild it's a huge disappointment.

[quattrorunner]

Are you sure about those springs? Like removed them?

[908/930]

Valve guides too tight?

[356911930]

Your valve spring height might not be set correctly? Possibly causing coil bind and snapping off the rocker arms? I'm not sure if this is possible but worth a look.

Rahl

[Rawknees'Turbo]

Originally Posted by 908/930

Valve guides too tight?

Usually when valve guides are not reamed/honed properly to size, the valve stems wear the hell out of them in short order (become egg shaped quickly) and you see a lot of bronze colored paste in the rocker box (not saying it isn't possible that is what is causing the rocker snappage, however).

As has been mentioned above, it is likely a problem with either the spring preload, or the springs themselves.

[jwasbury]

You seem to have ruled out valve piston interference, but that is (of course) the most obvious cause of broken rockers.

I'm curious as to how one can be sure the valves are not bent, and valves have not kissed the pistons without tearing the thing down.

[Midlife930]

I would check for coil bind first thing.
Had a bind issue at my last rebuild that was caught before full assembly.

[David]

I regularly hit 7,000 without issue especially in first since one second at full throttle is 7,000. Sounds like something's binding.

[dannobee]

Check the clearance between the retainer and the valve guide/seal. And check for spring bind. Should have at least 0.020" between the coils.

[porsche930turbo]

Hi Guys,

Thanks for all the help and suggestions - the engine is now out of the car and being disassembled to assess what could be causing it.

I'll ensure they check what you guys have suggested:

Valve guides too tight
Valve spring height
Spring bind

And check for spring bind. Should have at least 0.020" between the coils - is this at maximum valve open i.e maximum spring compression?

Thanks again

[porsche930turbo]

Thanks for all your help guys it's greatly appreciated. The engine is out now and various things being checked. The minimum clearance between coils is that at maximum valve open? No sign of bronze coloured paste.

[Midlife930]

Yes, coil clearance when valve maximum open.

[Alan L]

There should be room for the springs to move further when the valves are fully open - max cam lift. You don't want them coming to a sudden stop because they can't depress any further. There are two spings - inner and outer. BOTH have to be able to move further. No binding on either. Hard to understand but same cylinder/rockers twice. :-( Suggests something not right in valve train on #3. About the only thing you can influence in that respect is the shimming of the springs - which affects the spring travel.
Alan

[Ollies930]

Absolute minimum valve travel to coil bind for most valve springs is supposed to be .060" past maximum lift. Most valve springs will perform best at or near there, to help eliminate spring surge and other associated problems. But you might want to ask AASCO what their design installed compressed height is.

[porsche930turbo]

Hi Guys,

Engine has come apart a little further - no sign of coil bound, the springs have a few more mm of travel after max valve lift and there is a sufficient gap between them. The cubic capacity of each combustion chamber was measured to ensure they were the same (which they were). Valve lift the same on each cylinder - nothing found to suggest why it is snapping 2 rockers on cylinder 3.

I think I might get the injectors tested and cleaned to make sure that all the injectors are flowing the same, if cylinder 3 is running much leaner maybe the heat is causing the issue.

No sign of the valve seals being too tight however some of the rockers felt a little tight on the shafts and don't rotate easily in the hand - is this normal? The shaft has some visible bronze colour on them but they are on other cylinders too.

Thanks

[boosted79]

remove the springs on #3 and have them checked on a spring tester vs. Aasco specs. Check the #3 stem to guide clearance, not with the stem rock method but with mic and ball gauge

[Alan L]

The fuel situation and the compression of the cylinder will have nothing to do with snapping rockers.
Yes - worth checking valve spring tensions. You can do this simply if you have a w/shop press and some bathroom scales. Compare to another cylinder. But again, the spring specs would have to be well off to snap a rocker.
The rockers should be free to move - no binding. Not sure where you went with those - if re- bushed, they should have been checked for proper clearance. The problem has to be in the valve/rocker area.
Check the installed heights of the valves against book values - but it sounds like they should be correct - still got spare mm of travel.
Alan

[1979-930]

It's not the injectors, but I would have replaced them with the work you did. They are not rebuildable and are a maintenance item.

Are your camshafts new or regrind?

[nocarrier]

Pull the valve springs for cyl 3.

install checking springs with the keepers and retainers back in.

Set valve lash to 0.

Put a dial indicator on the top of the adjusting screw for the valve you want to check and set to zero with cylinder 3 At TDC.

Rotate engine and notice when the cam starts moving the rocker opening the valve.

Manually push against the rocker and checking springs while watching the dial indicator.

Record your valve clearance up to tdc and to maybe about 15 degrees or so after.

What is the smallest number you come up with?

[Alan L]

"Rotate engine and notice when the cam starts moving the rocker pressing down towards the valve.

Record your valve clearance up to tdc and to maybe about 15 degrees or so after.

What is the smallest number you come up with?"

In case it helps him out.... I don't understand this bit.
Alan