Open Header = Burnt Exhaust Valves? Myth? 1

[spectreeng]

Gentlemen, I've been trying to find a reputable source for the cause of this but I'm having little luck. There seems to be a well known idea that running an engine with just open headers or just a collector with no long exhaust will eventually "burn the valves." The idea is that they warp and then lose enough contact with the head for heat dissipation. I've also seen people saying that exhaust leaks can do this also. The theory is that upon shutdown, cold air can reach the valves and warp them especially if they are open (unseated from the head). One counterpoint to this theory is that losing an exhaust causes the engine to run lean, and this is what actually causes damage to the exhaust valves. Does anyone know the truth behind this?

Also, during the valve overlap portion of the combustion cycle, isn't a fresh dose of cold air hitting the exhaust valve anyway?

 

[Zapster]

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[SomptingGuy]

Nobody should penilise typos!

- Steve

 

[patprimmer]

It sees I even managed to break a non existent part before I burnt an exhaust valve.

I never ran an engine lean and always retuned the carbies after major changes to cam or exhaust.

Regards

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[xlch58]

Pat, conscientious builders/mechanics are less likely to burn a valve on a VW. Typically the valve gets overheated and the stem stretches if you run them at high output levels for extended periods(like the flying VWs). You compensate the next time you adjust the valves. Eventually you replace the valve when you cant get it in spec. If you don't stay on top of it, then you toast a valve when the seat pressure drops to the point that either the heat transfer is compromised or the valve fails to clear a bit of carbon on the seat and once again heat takes its toll. In autos where you spend very little time at high output levels the cooling does ok for big bore VW engines. In aircraft conversions you can't escape the fact that there are plenty of big bore kits out there, but all of the aftermarket heads are made to fit the existing cooling tins -- so are limited in fin area. The fins are also less than optimal for cooling the critical area between the valves(no pompadours). There are some heads that are marketed to cool better -- citing more material in the head, but the additional material comes at the expense of fin area. Any engineer should be able to see through that. The technically naive seem to buy the explanation and for short bursts I expect it can act as a sink, but no joy for continuous operation.

Charles

 

[Lou Scannon]

good tech, Charles!

 

[patprimmer]

My real point is irrespective of brand names or exhaust system design, if the engine is tuned and maintained correctly and you use quality parts and specify and install them correctly, you don't burn valves.

WW11 aircraft ran high output supercharged and even compound supercharged engines at high boost levels and still did not burn valves unless something else was wrong. This applies equally to VW or Porche or small block Chevy or Crysler hemi or Nissan or Toyota or Honda or Rolls Royce Merlin or Lycoming or whatever



Regards

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[thruthefence]

Charles, When a valve "stretches", the clearance between the stem & guide increases, reducing the heat transfer out of the valve. This exacerbates the problem, hotter it gets, the more it stretches. See it on the little aircraft engines all the time. I think it's caused by overly aggressive leaning, trying to save a buck on fuel.And it's amazing how many operators decline a simple valve clearance check during routine inspections to "save money". After all, it's "running fine". I used to carry a chunk of exhaust valve, ( recovered from the opposite bank intake pipe ) to show folks the consequences of not keeping up with the equipment, but few were impressed.

 

[xlch58]

I am pretty sure most WWII aircraft ran sodium valve stems. The exhaust valve has always been the TBO limiting factor for aircooled aircraft engines. For a flying VW you will probably find yourself doing a top end overhaul at least every 200 hours. The WWII era radials also had regular top end overhauls, even though the major overhaul interval for postwar transports was approaching 3000 hours. Overspeeding was the bane of the bottom end. I have an Airforce chart somewhere that lists the min rpm overrun that demands an overhaul. In any event, when people say burned valve, they are generally referring to fairly sudden catastrophic failure of the valve, but if you go through a couple of buckets of old exhaust valves, you may come to the opinion that it is just a matter of degree and interval - but they all burn. As an aside, I do have buckets of old valves to god knows what engines. I find sitting in front of an old Sioux 680 and refacing them exceptionally relaxing. The ones that are too far gone, I forge into flowers (with the exception of the old Lycoming exhausts, as they are sodium filled stems as well).

Charles

 

[evelrod]

Charles, I agree that under the right conditions a valve stem can 'stretch'...I've had a couple of overly lightened Tilton valves do so. However, I have never seen an overheated ex valve stretch in the guide area...it was always under the valve head! By the time a valve gets hot enough to stretch the stem, it probably is not seating at all or has a large chunk melted out of it! At least that has been my experience over the years.

Also---In 1967/68 my I was helping my cousin build a Benson Gyrocopter and the original plans called for a VW. After careful study of the plans (and me lobbying against it...I don't particularly like Vdubs) we scrapped the idea and went with a McCullough out of a surplused R-CAT.

Sorry Pat...I still don't like those aircooled VW's. I know that is irrational...Whoever said I was 'rational'? ;-)

Rod

 

[thruthefence]

You are correct regarding exhaust valve; they are one of the few 100% mandated replacement items for an aircraft engine overhaul, ( as defined by 14 CFR ) no matter what it's condition appears to be, visually, dimensionally,or time in service. If it's on the engine at overhaul, it's scrapped.

 

[patprimmer]

Rod

Air cooled VWs are something people either love or hate rationally.

I like the light weight, low CG and extreme versatility for modification. Also the chassis and drive train are tank tough.

The things to hate are the noise, compression leaks under the head, flexible valve train components, oil leaks, extreme number of individual components for a 2 valve 4 cylinder, and need to dramatically improve oil cooling if hot rodded.

I think in their heyday, they were second only to SBC in term of numbers hot rodded.

I agree with you that if valves overheat, they stretch near the head.

Almost by definition, valves do not overheat in a correctly tuned engine, even with exhaust stubs.

An aircraft engine that has been excursively leaned to save fuel is not correctly tuned and a failure in that case is pilot error not mechanical failure a such.


Regards

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[xlch58]

A solid exhaust valve has a considerable temperature gradient. It is coolest at the tip of the stem and hottest at the center of the face. For a non sodium valve, I think you will find that 75% or more of the heat flows through the valve face and less than 25% percent flows through the stem into the guide- at least in aircooled engines. So yes, since the higher the heat, the lower the elastic limit -- it is going to stretch and or deform at a point near the head. I would think this would be obvious. Where exactly it deforms is going to be a product of the temperature gradient and the cross section. Heat distributions show the hottest part of the stem in an area just slightly above the head and just below the guide where some valves in an effort to improve exhaust flow, are actually necked down just before flaring out for the tulip head -- exactly where stem temps are highest, exacerbating the issue from both a heat transfer and tensile strength stand point.
As far as it goes, I have never mic'ed one after the fact, it could just as easily be caused by recession. At the end of the day, the clearance is taken up and the valve fails to seal, and disaster follows.

As far as burned valves go, I don't necessarily buy the short headers burning valves independent of mixture issues. If this were true, then air injection systems would be trashing heads.