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Open Header = Burnt Exhaust Valves? Myth?

Open Header = Burnt Exhaust Valves? Myth?

Open Header = Burnt Exhaust Valves? Myth?

(OP)
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?

RE: Open Header = Burnt Exhaust Valves? Myth?

spectreeng;  It has been my experience that those engines being run with open headers, are being run without an eye to ultimate longevity. i.e., they generally are race engines.  That being observed, do race engines burn exhaust valves more often than non-race engines?  Certainly.  However, if engine longevity is a key consideration for a particular race engine, then many factors come into play, which may effect total exhaust valve life life:  Valve seat width.  A/F ratios.  Camshaft timing. (Including intake to exhaust overlap, and centrelines.) Header tuned length. Header primary diameter, Valve materials. Ect.  This a loaded topic, not easily condensed into one response.  I'm sure others can give you the advice you are looking for, in much more detail.  I for one however remain skeptical that convection of cold air up the header will cause the valves to warp.

j79 guy

RE: Open Header = Burnt Exhaust Valves? Myth?

I've never bought into the theory myself.  Even with short, open headers, there is plenty of hot cylinder head that would heat any air on shutdown.

RE: Open Header = Burnt Exhaust Valves? Myth?


An engine with short headers will run leaner than if it had longer exhaust tubes due to sucking back fresh air diluting the mixture. If the mixture is weak enough or the engine load great enough, it will go into detonation and/or pre-ignition. That will wreck valves for sure. My 2c.

RE: Open Header = Burnt Exhaust Valves? Myth?

Dan, do you really believe fresh air can travel all the way back up the header, past the (closing) exhaust valve, into the combustion chamber, and significantly dilute the mixture compared to what is coming in thru the intake valve/carb?  It sounds like you're postulating a negative pressure condition at the end of the exhaust stroke, yet all the hot exhaust has cleared the header?  I'm not seeing it.

RE: Open Header = Burnt Exhaust Valves? Myth?

If the open exhaust port goes lean with out primary tubes why does it backfire from unburnt fuel? If the engine is properly tuned and not abused the engine will last without burnt or wrapped valve's but low on power. The exhaust system either common collector, y or crossover will help rid the exhaust gases via a scavenge effort.Ever run a lawn mower without a muffler? Just a lot of noise but will run foor ever.

RE: Open Header = Burnt Exhaust Valves? Myth?

As a youngster I put an expansion chamber on my 125 Yamaha dirt bike without enrichening the jet ( something I'd never heard of ) and burnt a piston pretty quick. The additional scavenging of the new exaust leaned her right out. Learn sumthin' new ever day.

RE: Open Header = Burnt Exhaust Valves? Myth?

that would be pulling more air in thru the intake, not bringing fresh air back thru the exhaust...
 

RE: Open Header = Burnt Exhaust Valves? Myth?

look at boats, they have open pipes with water coolers.  They still have long life on big block V8's.

RE: Open Header = Burnt Exhaust Valves? Myth?

Ok...From the old guy, one more time.
Exhaust reversion is real.  However, it is not the problem that it is made out to be in this context, ie, burned valves.
I'm guessing this idea (I won't dignify it by calling it a theory) came about pre war as I heard it early on, in the 1940's.  From my racing experience, I find burned valves were generally due to metallurgical considerations. Crappy valve material.  Last set of really crappy exhaust valves I bought were from Ford Racing, UK.  Replaced with Datsun valves and never another problem using the same type OPEN HEADERS in 28 years.
I used short, open headers on my '61 Vette for two years on the same engine (Jack Moss Automotive, Amarillo, TX.) and maintained the same compression and leak down numbers. Two years! That was then. Today I am finishing up my Mini and the cylinder head and valves are on their seventh season, still without significant wear...always with open headers.

Their may be some significant possibilities of 'burned valves' from a broken header, or no header.  I don't recall anyone ever testing this as under these conditions, power is greatly reduced.  I had a cracked exhaust manifold on my 83 Chevy 3/4 ton work truck.  It ran that way for several years.  All I ever checked on it was fuel mileage and that remained about 12 mpg the entire time.

Rod
 

RE: Open Header = Burnt Exhaust Valves? Myth?

...consider the length of the exhaust headers used on the fighters of WWII...short, and certainly not made that way to 'shorten' value live...especially in the heat of battle.

RE: Open Header = Burnt Exhaust Valves? Myth?

The problem as I've heard it described is that momentary flow reversals expose the valves to cold ambient air on the backside, or at least to high temperature differential. If that's the real failure mechanism, a very short exhaust - probably only a few inches - would be sufficient to prevent this. I've never actually seen a failure definitively attributable to this despite seeing engines operating in many conditions that could theoretically trigger the problem.

RE: Open Header = Burnt Exhaust Valves? Myth?

Well, I guess that settles it then.
Is it possible---Yes.
Is it probable---NO.
Has it happened---probably.
Has it happened often---probably not.

Rod

RE: Open Header = Burnt Exhaust Valves? Myth?

Hi All,

This is a very generalised question with a huge amount of variables.

To fix issues we have to find the CAUSE of the problems, after some thought i see it may be quite possible to OVERHEAT the exhaust valves with open headers without a collector being used, depending on the state of tune of the engine itself.
Performance n/a engines without the collector are not going to Scavange worth a Damn.
So in fact maybe the Cause is the LACK of cooling doing the damage & not being cooled to quickly as mentioned.
With increasing Temperature rise at the exhaust valve from the Lack of cooling with NO scavanging to aid the issue.

With a collector i can't see this happening unless the collector is of poor design & not suited to the application.

As already talked about is the metalurgy of the valves themselves, not to mention the amount of modifications one can do to the backside of the valves to increase flow or hinder reversion which in some cases can & will reduce the life of valves.
I myself in an attempt to make components as light as possible or to configure flow or to take anti reversion measures to the next level have fallen victim to my share of Burnt, Warped & Bent valves for various reasons.
This has never happened when tuning or playing with exhausts though..

Running high spring pressures with highly modified or CHEAP valves is a good source of Warped valves & a starting point for burning valves.

Another source of "ultra lightweight" valve warping has been NOT using Torque plates for the valve seat cutting operation. Heavier unmodified valves seem to retain there shape better than valves with less material to support them.

Ok we can run a lawn mower without an efficient exhaust but a race engine is a different animal & the Exhaust can make or break the engine.

RE: Open Header = Burnt Exhaust Valves? Myth?

I was referring to real short headers, just a couple of inches.
 

RE: Open Header = Burnt Exhaust Valves? Myth?

I researched this myself a short time ago after coming across a manufacturer that recommended a minimum exhaust backpressure.  After speaking to one of their applications engineers I learned this was to ensure exhaust components were sized properly to prevent condensing of the exhaust gases inside the exhaust piping and muffler.  I didn't come across any burnt-valve proof either, just a lot of hearsay...

RE: Open Header = Burnt Exhaust Valves? Myth?

I experienced a set of burnt exhaust valves about 15 years ago in a Honda CB900 motorcycle that had been fitted with an aftermarket exhaust system without adjusting the jetting. The explanation that I was given by the bike shop was that the decreased back-pressure of the more free-flowing exhaust led to a leaning-out of the engine which in turn led to the damaged exhaust valves. The shop replaced all 8 burnt valves (ouch!) and re-jetted the carbs. Problem solved.

RE: Open Header = Burnt Exhaust Valves? Myth?

It it my understanding that the short header=burnt exhaust valves theory is pointing toward that fact the exhaust system has a vacuum of sorts when you let off the throttle, hence the reason that engines with cracked or broken headers backfire under decel.  When you decel, there is still fuel being drawn into the engine, and you can draw cold fresh O2 rich are in from the exhaust system, if these two meet at the hot exhaust valve, it can act as a glow plug of sorts, creating a secondary combustion event right at the valve.  The contact area between the seat and the valve is usually only wide enough to dissipate the heat from one combustion event per 4 revolutions, if you start introducing 'extra' combustion events into the mix, you could overheat the valve and burn it.  This is very common with the VW aircooled engines, as their exhaust systems are very short, and prone to rust holes.   

RE: Open Header = Burnt Exhaust Valves? Myth?

I built a lot of high performance Air cooled VWs. I never ever burnt an valve. I broke a few cranks, dropped a few valves, blew out a combustion chamber, broke a dudgeon pin and cracked a few crankcases, but never burnt a valve no matter what exhaust or what damage to the exhaust. Some were in Speedway Midgets, some in off road buggies and some in everyday street cars.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Open Header = Burnt Exhaust Valves? Myth?

Pat, I couldn't fine "dudgeon pin" in my automotive dictionary... is this some kind of component unique to the VW?  :D

RE: Open Header = Burnt Exhaust Valves? Myth?

gudgeon pin = wrist pin
dudgeon pin = a badge to let everyone know you are offended/angered bigsmile  

RE: Open Header = Burnt Exhaust Valves? Myth?

Nobody should penilise typos!

- Steve

RE: Open Header = Burnt Exhaust Valves? Myth?

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

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Open Header = Burnt Exhaust Valves? Myth?

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

RE: Open Header = Burnt Exhaust Valves? Myth?

good tech, Charles! thumbsup

RE: Open Header = Burnt Exhaust Valves? Myth?

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

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Open Header = Burnt Exhaust Valves? Myth?

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.   

RE: Open Header = Burnt Exhaust Valves? Myth?

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

RE: Open Header = Burnt Exhaust Valves? Myth?

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'? winky smile

Rod

RE: Open Header = Burnt Exhaust Valves? Myth?

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.  

RE: Open Header = Burnt Exhaust Valves? Myth?

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

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Open Header = Burnt Exhaust Valves? Myth?

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.   
 

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