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I can help. This is a typical result of poor Valve seating or a lash problem holding the valve off the seat. The heat has to be transferred into the coolant through the Valve seat into the head and then into the coolant. If the valve is held off the seat it will over heat and always fails at the area where the valve is the hottest, thinnest and where it is under the most contact stress. This is a typical "burnt" Valve.

Cause. Poor seating.
Fix. Go through the heads and rebuild properly. The head with the burnt seat should be checked for hardness as it may a have become soft and a new seat could fall out if the hardness has dropped. I would also recommend the whole "build" be checked to see if other areas are OK. Tune included. Do not spend any $$ on anything until the mechanicals are repaired and correct.

I will come back on later and go through what needs to be checked etc in more detail.

You suffered a burnt Valve(s).

 

Ok..I googled it and it looks like you are right. Please let me know how I should go about fixing this problem. Much thanks....I think you've cleared a lot of misinformation I've gotten and made me feel like there may be a solution and long term plan for my baby....

 

I guarantee you that if you knew who was helping you it would blow you away...the fact that industry leaders have agreed to provide their expertise to our members blows me away...stay tuned

 

Ok, I have some more time to add to what I have already written.

Looking at photos is always a little difficult to exactly know the causes as shadows etc., all can play tricks on you. The Piston does appear to show signs of some detonation erosion at the 1 o’clock to 3 o’clock position, but it may be just the photo. The only reason I bring this up is that you look at it clearly and make sure. If it is detonation erosion, then the mapping may need to be revised.
Also the same photo of the Piston shows what could be some Valve contact in the pocket. Check this as well. The Valve timing could also need to be adjusted. This is electronic on the Intake so this is also controlled by the mapping.

These engines suffer from weak and soft Valve Guides. They are made of a copper alloy that is almost 100% copper. They wear out with use and heat. In lay terms, they become “out of round” and the Valve stem rocks back and forth and it not held concentric to the Valve seat. This give the Valve very poor seat memory. The Valve does not seat back onto its seat properly and the seal between the two is poor. This creates a leak, a portion of the valve that does not sit tightly onto the seat.

Simply said, Valves hang down into the chamber during the engine cycle. The Intake Valve hangs down on the Induction stroke when the Piston is going south towards BDC. Cool fresh charge enters the cylinder which has a cooling effect on the Intake valve. The Exhaust Valve hangs down into the chamber during the Exhaust stroke. Hot gases are expelled past the Exhaust Valve by the Piston moving from BDC to TDC. This hot gas super heats the Exhaust Valve. It needs to be held back onto the Valve seat to transfer that heat into the seat, then into the Head and finally that heat is transferred into the coolant fluid. If the Valve does not seat properly, the Valve will over heat and break apart. This is why burnt Valves seem to always appear on the Exhaust side.

I suspect the Guides had a lot to do with this failure. Another variable on these engines is the camshaft and Cam followers. The Intake Cam Follower does not spin in its housing. The Cam strikes the follower and all of the loading goes all in one direction. This adds to the wear factor on the Intake Guides. The Exhaust has the same issue but slightly different. The Exhaust Follower is Flat and round and typically these spin in their housings. This is done intentionally so the wear on the surfaces is not concentrated in one place. If aftermarket cams are installed, some do not have any taper cut into the nose that along with an offset lobe causes the Follower to turn. Some cams come with flat noses. Again, the loading is all directed in one direction and with soft guides the wear rate goes up.

The assembly of the head cannot be overlooked either. Making sure there is no excessive play between the Guide and the Valve stem. Making sure the Valve sits onto the seat properly and seals. Making sure there is enough seat pressure when the Valve is held onto the seat to seal the valve under high Exhaust back pressure and to transfer the heat into the seat. Seat margin widths is another variable that needs to be considered. If you are running very high manifold pressures, you should expect you will also see high exhaust back pressures and high exhaust temps. Turbo engines can see upwards of 1800°F and higher. If the seat margin is too thin, then the contact surface between the Valve and the seat is small and the transfer of heat will be less. It becomes a “trade off” between boost pressure and air flow across the seat. The material the Valve is made from is another. If stock Valves are used, caution should be taken with high Exhaust temps. They are not made for extreme temps often seen in performance engines. There are special steels made just for these purposes.

If the camshafts are an aftermarket set, are the base circles all the same? If one is slightly bigger than the rest, this could hold the valve off the seat too. It would only take one and the engine will start but would run rough I expect. So check the base circle diameters on all Camshafts.

Make sure you have Lash even with Hydraulic Followers. If there is no lash and the followers pump up with running oil pressure, you could open all valves and the engine will stop. The engine will show no faults and will start only when the engine has sat for some time and Followers drain down and close the Valves. Most of the time to gain lift the base circles are cut down, so this may not be the case but is something that needs to be checked.

This is a typical result of poor Valve seating. Unfortunately the engine has to come all the way down, cleaned and rebuilt. I see you have bought all new parts. Make sure the Oil tank is thoroughly cleaned along with the heat exchanger and the oil galley plugs are removed from the engine case and the oil galleys flushed. Trash has a habit of hiding from view during the rebuild process only to re appear once the engine is running.
Hope this gives some simple insight into the causes of this type of failure.

 

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While the bronze guides can be (usually are) more durable if the new guide causes the valve stem/head to not be concentric with the valve seat, the valve head will not be centered in the seat. A misaligned valve can burn as it does not make full contact with its seat and thus can't transfer sufficient heat to the valve seat and thus keep the valve below some critical upper temperature threshold. In some cases even before the valve can burn the head can snap off from the stress.

If the valve seat need to be machined to bring the seat concentric to the valve stem/head this can sink the valve in the valve seat. It also thins the seat. If the seat is thinned too much, the seat can run hotter than it would otherwise, and this can result in it deteriorating to the point it can develop heat damage and lose its sealing, become loose and can fracture or even just fall out.

If the valve sinks into the seat because the seat was re-cut the valve stem protrudes higher and this can result in insufficient valve lash (clearance).

As poster 411 touched upon insufficient valve lash can hold the valve slightly off the seat. In fact insufficient valve doesn't have to hold the valve off the seat the reduced lash means the valve spends less time on the seat and thus runs hotter all the time in all circumstances.

Some material can be removed from the tip of the valve stem to keep the valve stem's height the same so the lifter does not cause the valve to run with too little even zero lash. But the tip metal must be removed in such a way the tip doesn't get too hot. If it does this can soften the metal of the tip and it will mushroom and then one doesn't get the benefit of the valve fully opening. And one doesn't want to remove too much metal for even if it is removed using the appropriate procedure the hard portion of the valve tip can be thinned to the point again the tip mushrooms or it wears away and then the valve has too much lash and engine output suffers.

Even with a stock 996 Turbo the tune is critical. With a modified engine super critical doesn't begin to cover it.

If the engine detonates due to any number of reasons this can have small particles of aluminum -- "blasted" from the from the piston crown by the shock wave from detonation -- in the exhaust gas. It a particle sticks the exhaust valve or valve seat and although the valve will flatten this out to less than foil thickness -- and eventually the aluminum will be gone -- in the meantime it can hold the valve off the seat a tiny amount and enough to compromise the cooling of the exhaust valve and a burned valve is the result.

The aluminum doesn't have to hit the valve or seat right away either. It can stick to a bit of combustion chamber deposit and when this comes loose and carries the alum. with it this can get caught between the valve and the seat. The softer deposit material gets pulverized and expelled but the alum. can remain awhile and wreck havoc.

 

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We were very fortunate to get several industry leaders to agree to answer questions here anonymously as 411...I created the master account and sent out the invitations with log in info to the gurus that agreed to participate...they can all log in as 411...in this particular instance I happen to know which guru has been helping you....I don't always, but they are all industry leaders and Porsche experts...we are very lucky that Chad's idea for this forum has worked out....we are also fortunate that private experts like Macster are willing to contribute here

 

While these are good points, they are the basics required to rebuild Cylinder Heads correctly.

No guides should ever be replaced without the seats being re cut. It is almost impossible and unheard of that the replacement guide is installed without the seats being re cut.

When re cutting the seats so they become concentric to the new guides, if it lowers the seat height beyond an acceptable level, the seat needs to be replaced. Anytime the seat is lowered into the head the valve essentially get longer and the required lash between the Valve tip and the follower is lost. Cams are designed with lash included. Without it or with too much the whole valve motion and geometry is off. If you take the time to "time" the cams correctly but the motion is off, you have wasted your time. Airflow is compromised as well as the Valve will not lift off the chamber floor the same distance. Seat angles will be all wrong for max flow. Often the lower the valve sits in the seat the more the top angle changes which affects the flow. If you were to lose 5-10 CFM per Cylinder, that's upwards of 60 CFM for the engine. That can be around a 23 HP loss in power.

Tipping or removing any material off the stem is important to make sure the ends are flat. You can do some lash adjustment there but make sure the valve has enough room above the keeper groove for this to be done safely. 996T engines have approx 11.00mm above the groove. Seat height dictates the position of the valve, so taking a lot of the valve tip to gain lash after sinking the seat is not the correct and proper way to rebuild heads.

Unfortunately all to often this is the case. I have seen heads that have been repaired this way and the closed spring pressures never adjusted. Now we get back to the issue of a burnt valve from not enough pressure holding the valve against the seat when closed. The heads look nice and clean but with hidden faults that come evident only when the engine is built and run.

As for any mushrooming, don't really see that with hydraulic followers as they adsorb the shock, unlike a rocker arm set up.

"""As poster 411 touched upon insufficient valve lash can hold the valve slightly off the seat. In fact insufficient valve doesn't have to hold the valve off the seat the reduced lash means the valve spends less time on the seat and thus runs hotter all the time in all circumstances."""

Not sure if this is the case, but regardless good warning to make sure you have lash. Anytime the Exhaust valve is held off the seat the Exhaust valve is being heated and only when its on its seat does it transfer the heat out of the Valve.

Make sure the place that is rebuilding the heads for you knows what they are doing and CARE!! In my humble opinion, these 4V heads need to be repaired by specialists and not the "corner" machine shop. Too many areas that need to be considered.