What would you get?

[blackhat620]

Jurgen,

First off if you are going to quote someone elsess words, you need to give them credit for their write-up and post the link to the full text. Second there is some truth in what you cut and pasted but there is also a lot of mis-information.

A burned valve is the result of valve overheating. This is more common in exhaust valves than intake valves. There are multiple things that can cause a valve to overheat and burn; valve face & seat wear, deposits on the valve, to small valve lash (ie. improper adjustment, cam or lifter wear, worn follower, worn rocker, bent pushrod), anything that elevates combustion temperatures ( ie. Cooling system problems, lean mixture, pre-ignition, detonation, incorrect ignition timing, failure of EGR system). A burnt valve will cause a loss of power, rough running, low compression in effected cylinder.

#1. The valve damage theory as per exhaust gas, or atmosphere is a myth. Exhaust valves cool when they are seated on the exhaust seat via the cooling system (water jackets), oiling system
(valve guides). Otherwise, the exhaust valve is subjected to about 1500 degrees F on average throughout the cycle (typical gasoline performance engine).

If any air is reverted back to the exhaust valve, it will come in the form of a reflected wave, but we're talking ambient temperatures of 60'F-90'F within a period of milliseconds. This is not enough time to cool the valve to a point
of damage. If anything, the valve temperature may reduce 40 'F from the reflected pulse.

What normally happens is the air fuel ratio will go lean when you run an open headers because of the increased air flow. You must then adjust timing, and jetting (fuel pressure/ injector timing) to restore the fuel ratio.

If anything, you can overheat the chamber and heads to cause damage to the valves when running open headers, but that is only under extreme lean conditions.

I would not get to worked up about ambient air from open headers damaging exhaust valves.
The problems with open headers, leaking exhaust manifolds and reduced backpressure is that it changes the flow characteristics of the engine. The flow characteristics start at the begining of the intake and stop at the end of the exhaust, and are effected by head design, valve size, cam profile etc., and changing the exhaust backpressure will change the scavenging and flow characteristics of the engine. If the exhaust backpressure is incorrect for the engine flow & chosen cam design (valve overlap/timing) you can get any one or a combination of the following; reversion of exhaust gas back into the intake manifold (reversion can be caused both by inertia wave pulse & vacuum from the down stroke of the piston), bleed over of exhaust from one cylinder to another, intake charge going straight out the exhaust instead of into the cylinder (over-scavenging). If you have a cam with earlier & wider exhaust opening and wider LSA you will need an exhaust with more backpressure (restriction).
You can overheat the cylinders & exhaust when running open headers in all the same ways you can with any chosen exhaust system. Elevated combustion temperatures occur from cooling system problems, lean mixture, pre-ignition, detonation, incorrect ignition timing, failure of EGR systems and it can happen no matter what exhaust system is used.

#2. The exhaust valve is never open when the
intake valve is closed AND the piston is moving downward. Therefore the piston is not responsible for sucking exhaust gas back into the chamber.
Any bit of exhaust reversion is from the low pressure in the cylinder at TDC during the valve overlap period when the intake pulse is at a lower pressure value than the reverted exhaust pulse.

This is just plain incorrect, most cam designs open the exhaust valve long before BDC on the power stroke to aid in proper breathing characteristics, and this requirement becomes more pronounced as the RPM increases.

#4. The only performance loss from running open
headers comes from a shifted torque and HP peak UP the RPM band. You will notice a decrease in torque at lower RPM due to the shorter secondary
exhaust length.

The car should go faster down the track, or at least trap a higher MPH due to the increase in power.

To restore the torque without restrictions of cat(s), mufflers, etc., you can calculate the proper tuned length and fabricate collector
extensions such as these:
Using software and valve timing, you can tune the exhaust to gain TQ and HP anywhere you want in the desired operating range of your motor.

It will not go faster down the track if the higher rpm range of the power band is not correct for the redline of the engine and the transmission & gearing of the vehicle.

As i have stated before, you change any one thing in the engine or drivetrain, it will effect the performance and may not be for the better.

 

[F 150Cobra]

Jurgen,

First off if you are going to quote someone elsess words, you need to give them credit for their write-up and post the link to the full text. Second there is some truth in what you cut and pasted but there is also a lot of mis-information.

A burned valve is the result of valve overheating. This is more common in exhaust valves than intake valves. There are multiple things that can cause a valve to overheat and burn; valve face & seat wear, deposits on the valve, to small valve lash (ie. improper adjustment, cam or lifter wear, worn follower, worn rocker, bent pushrod), anything that elevates combustion temperatures ( ie. Cooling system problems, lean mixture, pre-ignition, detonation, incorrect ignition timing, failure of EGR system). A burnt valve will cause a loss of power, rough running, low compression in effected cylinder.



I would not get to worked up about ambient air from open headers damaging exhaust valves.
The problems with open headers, leaking exhaust manifolds and reduced backpressure is that it changes the flow characteristics of the engine. The flow characteristics start at the begining of the intake and stop at the end of the exhaust, and are effected by head design, valve size, cam profile etc., and changing the exhaust backpressure will change the scavenging and flow characteristics of the engine. If the exhaust backpressure is incorrect for the engine flow & chosen cam design (valve overlap/timing) you can get any one or a combination of the following; reversion of exhaust gas back into the intake manifold (reversion can be caused both by inertia wave pulse & vacuum from the down stroke of the piston), bleed over of exhaust from one cylinder to another, intake charge going straight out the exhaust instead of into the cylinder (over-scavenging). If you have a cam with earlier & wider exhaust opening and wider LSA you will need an exhaust with more backpressure (restriction).
You can overheat the cylinders & exhaust when running open headers in all the same ways you can with any chosen exhaust system. Elevated combustion temperatures occur from cooling system problems, lean mixture, pre-ignition, detonation, incorrect ignition timing, failure of EGR systems and it can happen no matter what exhaust system is used.




This is just plain incorrect, most cam designs open the exhaust valve long before BDC on the power stroke to aid in proper breathing characteristics, and this requirement becomes more pronounced as the RPM increases.



It will not go faster down the track if the higher rpm range of the power band is not correct for the redline of the engine and the transmission & gearing of the vehicle.

As i have stated before, you change any one thing in the engine or drivetrain, it will effect the performance and may not be for the better.

i understand you completly Blackhat! thanks for the info read!

the write up was from a google search from a Z28 forum from Tim i believe..


here is a friend of mine the guy who helped me build my engine from GEO from anderson performance.

Do you think all those uncapped race cars have burned exhaust valves? If your engine is running lean “with mufflers” then you will lean out the engine without them. This will cause pistons to burn. If you don’t have good valve springs or the guides are worn then it is possible to burn a valve but it’s under extreme conditions for long periods of time at that. You are running rich so you have no worries… unless you have fuel starvation from an inadequate fuel pump… still, burned pistons before valves.



A burned valve has a misfire at idle and you can feel it bumping. Take a plug wire off of a good running engine just to get the feel and you will know what I mean. Next, the compression test would have found the valve burned by low psi in that cylinder. I’ve actually seen burned intake valves but they are rare, most of the time it’s an exhaust valve. A flat cam will cause the same misfire at idle, (again; 90% flatten an exhaust lobe) but when you rev the engine it will have a popping noise back through the carb. This happens because the exhaust has nowhere else to go because the exhaust valve doesn’t open. They flatten on the exhaust lobe on the cam because the cam is trying to open the valve when the cylinder is under the most pressure; during the power stroke. The more power, the more pressure.



EFI: The o2 “must” be at least 14” back in the collector (or open end of the exhaust) or you will get an erratic run problem from “pulsing” fresh air back into the pipe.



Geo