I'M new here, and need this question answered. ITS SO ANNOYING!
Hey guys. I'm not completly new. :cheers:
I've been reading some posts here and there (damn you non-LT1 C4's respond to mods well :eek: ). This is my first post. I was brought in by Kevin, hes a new guy..Stolemydads67 or something along those lines (yeah he stole my Z on several occasions and took it for a spin...sometimes before he got his lisence!)
I got 1300+ posts on CamaroZ28.com, so i'm not really new to this system either...
I've noticed you guys tend to be a dat wee bit more knowlageable than the Cz28.com guys (and they are perdy dang smart) so heres my question
Fact or fiction:
Running strait headers/open exhuast can burn out your valves
:confused:
A frew guys at lunch tried to convince me it would (kevin!).
I think otherwise
they tried to tell me a lack of backpressure will contribute to this :confused:
eventually the piston head will hit the valve :confused:
I wanted to call BS on this and Hoist the moron flag, but Kevin is usually pretty good with this kinda schniz
I'm planning on going longtubes, !cat and cut out and i just rolled the 100k miles marker adn don't want anything to die...
I got mixed answeres on the Cz28.com board
one said too much backpressure will burn out valves
another said his valves burnt out(r a friends) after running strait exhuast
Another said "BS" and "no"
Lost and confused
trey
Re: I'M new here, and need this question answered. ITS SO ANNOYING! (TreyZ28)
No headers will burn your valves up...open header just need to have the program updated for the setup. You should not tune your car to a full exhaust and then simply open it up and expect the computer to compensate. Cutout are loud and over-rated. Just get yourself a nice free flowing exhaust setup and skip the cutouts.
Re: I'M new here, and need this question answered. ITS SO ANNOYING! (mountainmotor)
I'm not exactly an expert... but I do some reading :) and I would tend to agree with mountainmotor.
If simply removing the headers causes your valves to burn what could possibly be the reason for that?
One of the first things that I did when I got my '58 was to rebuild the carb. I had never adjusted a carb before and got it where I thought it should be and took it for a spin around the block. When I got back the engine was SOOOO HOT that it turned the paper air filter brown near where the choke is. Then, luckily, my neighbor (a mechanic) came over and helped me do it right.
Since then, I have often wondered exactly why it gets so hot during a lean condition. Here is what I have come up with... (I have NO IDEA if these are the real reasons or not)
Guess #1) The fuel is partially used to cool the cylinder/valves - not enough fuel, not enough cooling.
Guess #2) It has something to do with compression/expansion of gasses. Compressing a gas generates heat. Expansion of a gas absorbs heat. The compression cycle will generate the same amount of heat regargless of the amount of fuel. (You still have the same amount of air). If you do not have enough fuel, then during the combustion process there will only be a reduced time of expansion because there is not enough fuel to burn. The net result a steady build-up of heat eventually causing damage to the valves.
Can anyone out there offer an explanation of what, really, is going on during an a lean condition? Did I get it right or am I way off base?
If you had a fuel injected engine, usually there is an O2 sensor somewhere in the exhaust to tell the computer if it is putting the correct amount of fuel into the cylinders. If you simply remove this sensor (headers) it COULD (I don't know what would really happen) cause the computer to generate a lean condition. This would not be a problem with carbureted engines because they don't have any feedback mechanism.
Very interesting question. I'm glad you posted it. I look foreward to hearing what others have to say.
Re: I'M new here, and need this question answered. ITS SO ANNOYING! (aobrien)
Quote:
If simply removing the headers causes your valves to burn what could possibly be the reason for
that?
The fact that without headers, there is no scavaging of exhaust gases which kills flow and the heat will simply increase to the point of burning the valves. Now, if you are running just open headers, this is not the case because the exhaust gases are still scavanged. The only thing with running open headers on our cars is that you will boil your transmission fluid not to mention the passenger compartment may not much care for the heat either.
Re: I'M new here, and need this question answered. ITS SO ANNOYING! (TreyZ28)
If open headers caused burnt valves, the the auto parts stores near a drag strip wouldn't be able to keep them in stock. I see a lot of the faster car guys ADJUSTING valves between rounds, but none of them CHANGING valves. I think the misunderstanding stems from NO headers or exhaust manifolds. With open ports in the head, cold air can WARP a hot exhaust valve.
Re: I'M new here, and need this question answered. ITS SO ANNOYING! (Corvette55)
Just figured I'd chime in with my 2cent.
When the fuel mixture is leaned out too much the Exhaust Gas Temp's will soar. This excess heat has been responsible for many damaged engines. Cylinder Head Temp's are directly related to EGT. All combustion engines have a optimal Air Fuel Ratio wich will corespond to a optimal EGT wich will equate to a optimal CHT. That's how your filter turned brown!
Some old timers used to beleive that with open headers under the rigth conditions the engine would try and breath cold air back through the headers when you dump the throttle. It's my understanding that it's the cold air hitting the valves that causes the damage. I think that most modern cams along with the correct length/diameter primaires will prevent this condition.
I tested my 406 at the track with the full exhaust vs open headers. I found that my car offered no MPH or ET increase. But it did sound way better. I used header turn downs that were cut to length based off their bluing.
Re: I'M new here, and need this question answered. ITS SO ANNOYING! (aobrien)
Aobrien
If you're looking for finer details, then someone else will have to help out. But in a nutshell The temps go up because the amount of fuel in the mixture is less than optimum.
If I recall correctly, All 4stroke combustion engines share an ideal fuel/air ratio. It doesn't matter how big or how small the motor. ALL Engines make best power at this predetermined ratio. Ofcourse there is also an optimal temp that goes along with the ideal ratio.
Fuel does tend to cool things off in the chamber! If you were to observe an engine with an EGT gauge while the mixture is varied you would see an imediate fluctuation in the EGT. If you increase the fuel, the temps will go down and so will power, or if you decrease the fuel the temps will go up and eventually power will fall off once again.
It has something to do with the fuel giving off the most energy under certain conditions. Too rich, and the fuel will burn ineficient and wont give off anywhere near the energy it should. And if things get too lean, then there would not be enough fuel in the mixture give off the energy contained in it.
I hope I haven't confused you even more. But hopefuly you're starting to understand why temperature is directly related to perfromance.
Re: I'M new here, and need this question answered. ITS SO ANNOYING! (TreyZ28)
Thanks everyone for your responses, I have learned much. Well, here is the real answer to my question... quoted from SWCDukes responce to my post on the C1/C2 section http://forums.corvetteforum.com/zerothread?id=228631
Quote:
The issue is "ignitablility". First I will define the term "equivalence ratio". This is the ratio of the actual fuel-air ratio to the stoichiometric ratio, so stoicihometric is 1.0; greater than 1.0 is "rich" and less than 1.0 is "lean". Equivalence ratio is usually represented by the small car Greek letter phi.
Rich mixtures are very ignitable, and will usually fire even with a weak spark. As mixtures become leaner misfires or incomplete burning can occur. These misfires may not be noticeable to the SOTP until phi is below 0.9 depending on the specifics of the engine and ignition system design.
If a cylinder misfires the mixture will usually begin to burn as it passes by the hot exhaust valve, or combustion may not be complete, but begins again as the unburned mixture passes by the hot exhaust valve. Thus EGT goes up rapidly, and the exhaust valve can burn. Lean mixtures also have more tendency to detonate so piston crown damage can occur.
Many experiments in lean burn technology (phi usually between 0.6 and 0.8) have been conducted over the years, but few have proved satisfactory because of the ignitability issue - it's just to unreliable at such lean mixtures.
One that ended up in the public hands was Honda's CVCC. The engine had it's combustion chamber divided into two sections. One was feed by a small carb venturi that provided a rich mixture; the remaining chamber volume had a very lean mixture fed by a separate venturi. The theory is that you ignite the rich mixture with a spark and it will burn and expand into the lean mixture zone and ignite and maintain combustion of the lean mixture. It worked in theory, but the carb was complicated and prone to problems. Honda finally ditched it and went with modern digital EFI with O2 sensor technology. This system maintains phi very close to 1.0 under most operating conditions, and the exhaust gas constituency is correct for proper functioning of three-way catalysts that promote the oxidation of CO and HC and the reduction (disassociation) of NOx. Lean burn technology experiments continue, but it's tough to beat the modern engine control architecture for emissions, fuel economy, power, driveability, and long term reliabililty.
There are many myths out there about combustion, so be careful what you believe. I got my combustion education while doing a MS with the internal combustion engine research group at the University of Wisconsin, so my explanation is based upon the best knowledge taught in universities, not oversimplified explanations (which lead to myths) that you read in consumer car magazines staffed by journalism majors.
BTW, best power usually occurs at a phi of about 1.2. This is the point where essentially all of the oxygen is consumed. Best fuel economy is usually at a phi of 0.9 (assuming ignition is reliable) as this consumes all the fuel and releases the most energy per pound of mixture. Because of this, a phi of 0.9 will reach a higher peak combustion temperature than any other phi. Lean mixtures also burn slower, so more advance is needed. The reason why modern engines don't run this lean is the high peak combustion temperature creates too much NOx. The old Rochester FI units could be run at phi near 0.9, even with the crummy point ignition system, and that's why a well tuned unit with a good ignition system will deliver 22 MPG on the highway with 3.70s.
If you can run lean with reliable igntion and have the proper spark advance, the EGT of lean mixtures is only marginally higher than with phi equal to 1.
Re: I'M new here, and need this question answered. ITS SO ANNOYING! (aobrien)
In regards to the original question (I think I answered you on Cz28.com also) - they type of "burning valves" you have to be worried about when running a short exhaust is cold air rushing back up through the exhaust and hitting a hot exhaust valve.
If you have no headers/manifolds obviously this is very easy - hence burned valves.
If you are running a full exhaust - near impossible (exluding massive header gasket leaks).
Now running open longtubes headers? I would say no chance still, they should be more than long enough.
You get into a gray area when you start talking about open mid-length or shorty headers - personally I wouldn't run anything open that was shorter than 23-25inches. Open manifolds can most certainly burn a valve.
In regards to the other question, the reply posted was accurate, but to answer the original suppositions
Quote:
Since then, I have often wondered exactly why it gets so hot during a lean condition. Here is what I have come up with... (I have NO IDEA if these are the real reasons or not)
Guess #1) The fuel is partially used to cool the cylinder/valves - not enough fuel, not enough cooling.
fuel can be used to cool the intake charge, but once ignition has occured it of course has no cooling effect. As was pointed out if you monitor temps with an EGT then a lean mixture will display hotter and hotter temps up to a point of around 16:1 or so - and then after that you will start reading cooler temps simply because you aren't oxidizing enough fuel to provide the energy to heat it up anymore. As you go richer you will read cooler at the EGT up until around 11:1 or so - and then you will start reading hotter as you will have a good bit of fuel burning in the header.
Of course spark and valve timing, etc. will also have effects on this. You can burn a valve simply from running it to hot (this can be a big problem in turbo motors especially - that's why if you notice quite a few of them are switching to incolnel valves). You can also "burn" a valve (more like crack) from heat cycling - which is typically what happens with to short an exhaust track - heating the valve up to temp, then shocking it with cold atmospheric air.
Quote:
Guess #2) It has something to do with compression/expansion of gasses. Compressing a gas generates heat. Expansion of a gas absorbs heat. The compression cycle will generate the same amount of heat regargless of the amount of fuel. (You still have the same amount of air). If you do not have enough fuel, then during the combustion process there will only be a reduced time of expansion because there is not enough fuel to burn. The net result a steady build-up of heat eventually causing damage to the valves.
What you say above is true in an adiabatic system (no heat loss) - but don't forget in an IC engine the expansion/power stroke is driven by combustion so you have a large input of heat/energy on this stroke. You also have a cooling system which actually takes a majority of the energy produced and dissipates it - what you say is true, it just isn't an apt analogy.
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I'M new here, and need this question answered. ITS SO ANNOYING!
