924Board.org

[Lizard]

as most of you know I had 2 valves blow out on the head that I had on the car origonally, this was VERY shortly after a headgasket failure, it failed between cyl #3, and #4, then the valves blew on the exhaust valves of those same cyle,
then I swapped a second head I had onto the car and after awhile I had a valve fail on it as well valve #3,

here is a pic http://www.geocities.com/cjensan928s/Valves/img_1968.jpg
the 2 with small damage are from the first head, the last is from the last head, which I ended up stealing a valve out of the origonal head and putting the second head back on.

but any ideas as to cause of failure, I did hear abit of detonation on the second head and knew the guides and seals were not all that great and that it was probably due to oil being mixed in as when I went back up to prem gas it went away.
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3 928s,

[Mikri184]

Well detention or you dropped a washer or something broke inside there. Better find out the cause though. Heads can add up.
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Life is to short to be looking for something?

[Lizard]

well on the first head, I thought that it was possibly that a piece of the head gasket when it blew got caught in the valve and make this happen, I have no idea what caused the next one, I can say for sure that NOTHING was dropped in there though.
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3 928s,

[augidog]

Are you running any modifications?
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1978 924 95 mile daily driver.
Audi TB/POR174M/High Flow Cat/2.25" exhaust
I knew that positive thinking thing wouldn't work.

[jpab924]

WHY VALVES FAIL

Any valve will eventually wear out if driven enough miles. But many valves call it quits long before they should because of burning or breakage.

Let’s talk about burning first. Exhaust valves are the ones most likely to burn because they run hotter than the intakes. The intake valves are cooled by the incoming air and fuel, and consequently operate at about 800 degrees F. Exhaust valves, on the other hand, receive little such cooling and are blasted by the hot combustion gases as they exit through the exhaust port. Exhaust valves run at 1200 to 1350 degrees F. on average, which makes them much more vulnerable to erosion and burning than intakes. The higher operating temperature requires a tougher alloy, so exhaust valves are usually made of stainless steel or have stainless steel heads (typically 21-2N or 21-4N alloy with a high chromium and nickel content). For heavy-duty gasoline and diesel applications where heat is even more of a problem, a tough Stellite facing (cobalt alloy) may be needed on the exhaust valve face to control wear.

The intake and exhaust valves rely on physical contact with the valve seat and guide for cooling. About 75% of the combustion heat that’s conducted away from the valve passes through the seat, so good seat contact is essential to prevent burning. The remaining 25% of the heat is dissipated up through the valve stem and out through the guides. Sodium filled hollow valve stems in heavy-duty applications are sometimes used to draw even more heat up through the stems to aid cooling.

Anything that interferes with valve cooling or creates extra heat in the valve or head can lead to premature valve failure. A buildup of deposits on the valve face and seat can have an insulating effect that slows cooling and makes the valve run hot. So too can poor contact between the valve and seat if the seat is too narrow, nonconcentric or off-square. If deposits build up in one spot or flake off in another, it can allow leaks that create hot spots on the valve and result in "channeling" (grooves eroded or burned into the valve).

Weak springs or insufficient valve lash can also prevent good valve-to-seat contact and allow excessive heat to build up in the valves. A loose seat or poorly fitting guide can also hinder heat transfer to the head and contribute to burning.

Not paying attention to the installed valve height when doing a valve job can lead to burning. When valves and seats are ground or cut, the valves sit deeper in the head than before. This causes the stems to stick up higher which changes the rocker arm geometry and may lead to a loss of valvelash when the engine gets hot. If the proper geometry can’t be restored by grinding the tips of the valve stems (no more than about .010 maximum or you run the risk of grinding through the case hardened layer), the seats should be replaced to correct installed height (an expensive fix but cheaper than a comeback). Another option is to install valves with slightly oversized heads (.030 in.) that ride higher on the seat to compensate for seat machining.

Valve recession can cause the same kind of problem. As the seats wear away and the valves recede into the head, valvelash is lost. Eventually there’s little or no lash left and the valve makes poor contact with the seat, overheats and burns. Valve recession tends to be more of a problem on older engines that lack hard valve seats and are used in heavy-duty truck, marine, agricultural or industrial applications. The cure here is to install hard seats. Stellite or hard faced valves may also be necessary if the valves show evidence of erosion.

Cooling problems in the engine itself can lead to valve sticking and burning if the operating temperature gets too high. Low coolant, a defective thermostat, a weak water pump, a radiator obstruction, a defective cooling fan or fan switch, etc. can all make an engine run hot. This, in turn, makes the valve stems swell which may cause them to gall or stick in their guides if there isn’t enough clearance. If the valve sticks open, it can burn or be destroyed if it smacks the piston.

Blockages caused by casting flash inside the head or a head gasket that doesn’t have the correct coolant holes can allow hot spots to form that can cause valve and guide problems. So too can a buildup of scale inside the head the interferes with good heat transfer.

Valves can also run hot because of elevated combustion temperatures. Factors such as retarded ignition timing, lean fuel mixtures (often due to vacuum leaks), detonation (from too much compression or low octane fuel) or preignition (from hot spots caused by deposits in the combustion chamber or too hot a spark plug) can all play a role here. Likewise, exhaust restrictions such as a clogged catalytic converter or crushed pipe can make the valves run hot.

Just a little fodder for ya Lizard...JP

[Joes924Racer]

Nice pics Lizard hope I never have that happen to me
havent had a prob with detonation that I can recall
maybe its the way you drive it
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

[augidog]

[Paul]

Retarded timing causes the fuel to be still burning as it leaves the exhaust valve, thus causing it to run hotter.

[augidog]

[Joes924Racer]

and sometime time is everything. gotta have time ya know oops.
_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

[Lizard]

interesting read jpab924 thanx.

to answer some of the questions, I am running an MSD ign system, however I now realise that my gap might still be set for the old ign system,
my exhaust well both times the valves failed I was running a facotry header and downpipe followed by straight pipe to a 40 series flow master, no cat no resonator, so I dont think that would have a restriction in the exhaust, now however I am running the headers I got from you jpab924 going into 2.5" pipe which goes into a cherrybomb right under the middle of the car, and then comes out the drivers side just infront of the drivers side wheel. so I dont think I will have to worry about restriction in that for the future.
prior to the first set of valves failing the distributor did become stick inside and was not advancing properly so that might have been a factor, and prior to the last valve failing, I know I had abit of a vacuum leak as the idle would sometimes be at 2000 RPM and other times 850, I now know that I have absolutely NO leaks as I have nothing to leak except for the vacuum booster, the line running to it, and the timing advance, however I have verified neither leak, I have NO other vacuum operated pieces on my car, no AAV or anything else, and the crank case breather does NOT run into my air box either.
in regards to carbon build up blowing off, that is not going to be the case, I have had my engine open a few times and there is NO carbon buildup, or virtually none, pistons are clean as are the ports,
I have also inspected the valve seats on the heads and they were not damaged at all by these incedents.
now I have had detonation in the past as mentioned which was cured by raising the octane used, however that was not the only cure, this detonation ONLY happened at LOW RPMs going up a hill or putting a great load on the engine at low RPMs, so now when going up a steep hil I downshift into 4th, now I am also running a G31 (yes G31 not 016/Z) so i also have a REALLY tall 5th, sitting at 100kmph(62mph) I am just at 2k RPM,

I also believe that the engine was rebuilt by a previous person a very few miles before I purchased it, the reason being is that I purchased it from someone who had bought it for his wife as a christmas present and then turned out she couldnt drive stick and didnt want to learn, but they purchased it from a shop in chilliwack who did deal with a few porsches and had parts all over the place, I have inspected the head and the origonal head had new aftermarket valve seals, and the guides looked relitively new, also when tearing the engine apart, the honing marks were still VERY apperant where in the other 2 block (1 924, 1 931) that I have sitting here those marks are not visable.

so it looks like it is possibly due to timing and too lean.
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3 928s,

[Joes924Racer]

whats the correct percentage of total that the co is to be set I cant remember. whated you do think if a little to the lean is good a little more is better its easy to do cause if your goin by the punch of the throttle.
Its so easy to think its still good. and your in the mix with the CO.
_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.