Burnt valves from open headers

[Johnm]

does it happen? i want everyones

[heep70]

I only have one cent. I here it happin cause of the cool air that somehow gets up the header.

[Joshpow]

It happens when the engine drops rpm's. If you idle or go at one steady rpm it won't. Due to the drop in exhaust pressure, cold air gets back through the header to the valve and causes it to burn. Anyways that is what I've always been told. My hearsay .

[Dragon]

I never saw warped valves on open header race engines. None of mine or any engine i ever built did that.

When they get trashed it is a cast iron manifold or no manifold at all and the engine is run for a length of time. the exhaust valves get it when the manifold engine is shut off and they get it anytime the engine fires with no manifolds.

[Joshpow]

Huh You got a point about race engines. Maybe just a myth!

[willowbilly3]

That's one of those things I hear but haven't seen. We raced plenty of hard miles on roundy-round cars and never burned any valves. My take is this; if it did happen it is because the drop in back pressure caused the engine to lean out, which could burn a valve. If the engine was properly tuned for open exhaust, then I would say myth busted.

[Fordlorn]

Very short or no headers can (as I've been told) cause warped valves but I'v never heard of them causing burnt valves. If you look at some old drag car photos you will notice tennis balls in the open ends of some of the cars shorty headers. This was done to keep cool air from cooling the valves too soon. Going from "hot as the hubs of hell" to "frigid" could warp open valves, valves not against the seat to stabilize them. Don't have any personal experience with this but it is what I was told long ago.

[Dragon]

Very short or no headers can (as I've been told) cause warped valves but I'v never heard of them causing burnt valves. If you look at some old drag car photos you will notice tennis balls in the open ends of some of the cars shorty headers. This was done to keep cool air from cooling the valves too soon. Going from "hot as the hubs of hell" to "frigid" could warp open valves, valves not against the seat to stabilize them. Don't have any personal experience with this but it is what I was told long ago.

You still can see zoomy (upward pointing Rail dragster pipes) tubes get covered at the end of a race because all they are for is to direct the exhaust away from the car and are really short. Funny cars with their weed sweeper pipes don't have to because each tube is 32 inches or more long and that is enough to keep cold air from coming back up to the valve. My funny had 36 inch pipes to clear the 73 Mustang FB style body.

Funniest thing I saw was a newby at the track walk right up and look down into Zoomy tubes as the crew chief turned on the mag and squirted gas into the blower. It took all his hair off.

[Fordlorn]

Man, I'd pay money to see that! Reminds me of the (several) times I've lost my eyebrows to a lean misfire while messing with carbs..

[papabug71]

I guess it depends. On what I dont know. I have been told that if you run open headers on a motor that was originally to run on leaded fuel you are more likely to burn a valve or a seat.

i.g. my 1959 Ford tractor lasted about a month after I took the muffler off & ran straight pipe. Now it has a stuck valve.

But, I ran straight pipes on my 71 for 3 years in highschool & it still runs like a scalded dog.

Who knows...........

[Dragon]

I guess it depends. On what I dont know. I have been told that if you run open headers on a motor that was originally to run on leaded fuel you are more likely to burn a valve or a seat.

Mythbusters time

i.g. my 1959 Ford tractor lasted about a month after I took the muffler off & ran straight pipe. Now it has a stuck valve.

Most likely from something else. Warped valves don't get warped stems but warped edges that can and will, if left not repaired, cause burnt exhaust valves.

[Thunderfoot]

Very short or no headers can (as I've been told) cause warped valves but I'v never heard of them causing burnt valves. If you look at some old drag car photos you will notice tennis balls in the open ends of some of the cars shorty headers. This was done to keep cool air from cooling the valves too soon. Going from "hot as the hubs of hell" to "frigid" could warp open valves, valves not against the seat to stabilize them. Don't have any personal experience with this but it is what I was told long ago.

The guys I race with use the tennis balls (or cover) to keep any rocks / sand etc from the tires flinging into the header tubes and into the engine if the exhaust valve is open... They never used them for worry of the valves warping...
By the time they shut the engine off (for the alcohol/nitro cars) at the lights and decelerate to the end of the track any serious heat on the valves is gone already. same thing for those that just decelerate to the end of the track and then turn the engine off and tow back to the pits, any heat that could damage the valves is already dissipated from the engine running without any load (decelerating)

I think this was a problem back in the day (before early 70's) when the material valves were made out of is not what it is after that (made for unleaded fuels) not to mention the stainless steel racing valves. Unleaded fuel needs to run hotter to burn and also without the lead it doesn't transfer the heat to the valve seat as well so this is why the new material was needed for the exhaust valves (satellite). The old material would warp easier then the new stuff.

Valves burn when they can't cool off from the heat of combustion, the only time they get to cool is when they are on the valve seat. If the valve is not seating completely on the valve seat, then that part that is not touching will not cool and will eventually burn, the more it burns the worse it gets...
Week valve springs will cause valves to burn also, because the valve doesn't stay seated on the valve seat long enough, they will bounce on the seat instead of staying closed. This is very small but remember that at higher RPM the valve is only on the seat a mili-second and that is its only time to get a chance to transfer its heat to the seat and cool. If it bounces even once it takes away from it's efficiency to cool...

There are lots of guys at the drag strip that run street cars with open headers with no problems... In the 30 years I have been out at the drag strip racing I have never heard of anyone having burnt valves from open headers, including myself.

I also agree that the engine tune (rich / lean) will be affected by open headers compared to full exhaust...

I agree this would be a good one for Myth Busters to do as it seems to be a Myth (to me anyways...)

[papabug71]

i.g. my 1959 Ford tractor lasted about a month after I took the muffler off & ran straight pipe. Now it has a stuck valve.

Most likely from something else. Warped valves don't get warped stems but warped edges that can and will, if left not repaired, cause burnt exhaust valves.[/quote]

Dragon. Most likely. Could be that its the original motor, never been rebuilt & has been worked like a gutted mule all of its life ..... Poor old thing is just wore slick out. I guess I should get around to fixing it. Its still a stout tractor, even with a dead hole!!

[Dragon]

LOL the valve head can warp and get caught in the seat area and get stuck.