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?
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?
j79 guy
RE: Open Header = Burnt Exhaust Valves? Myth?
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?
RE: Open Header = Burnt Exhaust Valves? Myth?
RE: Open Header = Burnt Exhaust Valves? Myth?
RE: Open Header = Burnt Exhaust Valves? Myth?
RE: Open Header = Burnt Exhaust Valves? Myth?
RE: Open Header = Burnt Exhaust Valves? Myth?
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?
RE: Open Header = Burnt Exhaust Valves? Myth?
RE: Open Header = Burnt Exhaust Valves? Myth?
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?
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?
RE: Open Header = Burnt Exhaust Valves? Myth?
RE: Open Header = Burnt Exhaust Valves? Myth?
RE: Open Header = Burnt Exhaust Valves? Myth?
RE: Open Header = Burnt Exhaust Valves? Myth?
Regards
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RE: Open Header = Burnt Exhaust Valves? Myth?
RE: Open Header = Burnt Exhaust Valves? Myth?
dudgeon pin = a badge to let everyone know you are offended/angered
RE: Open Header = Burnt Exhaust Valves? Myth?
- Steve
RE: Open Header = Burnt Exhaust Valves? Myth?
I never ran an engine lean and always retuned the carbies after major changes to cam or exhaust.
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RE: Open Header = Burnt Exhaust Valves? Myth?
Charles
RE: Open Header = Burnt Exhaust Valves? Myth?
RE: Open Header = Burnt Exhaust Valves? Myth?
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
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RE: Open Header = Burnt Exhaust Valves? Myth?
RE: Open Header = Burnt Exhaust Valves? Myth?
Charles
RE: Open Header = Burnt Exhaust Valves? Myth?
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
RE: Open Header = Burnt Exhaust Valves? Myth?
RE: Open Header = Burnt Exhaust Valves? Myth?
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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RE: Open Header = Burnt Exhaust Valves? Myth?
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.