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?

 

[j79guy]

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

 

[RossABQ]

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.

 

[Dan320]

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.

 

[RossABQ]

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.

 

[hirollcenter]

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.

 

[thruthefence]

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.

 

[ivymike]

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

 

[dcasto]

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

 

[evelrod]

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

 

[70AARCUDA]

...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.

 

[JSteve2]

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.

 

[evelrod]

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

 

[willeng]

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.

 

[Dan320]

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

 

[Bribyk]

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...

 

[Paulusgnome]

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.

 

[vwenginebuilder]

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.

 

[patprimmer]

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

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[Lou Scannon]

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