Running motor with nothing for exaust on 1 side

[vandelay industries]

I thought the flap was to keep rhythm at idle.

Someone told me that the flap was there "because when the engine shuts down it turns a little counterclockwise just before it stops turning. And the flap is there so the engine doesn't suck in cold air and damage the valve."

i immediately very probably thought this was hogwash, but i wanted to know for sure.

However, on the rare occasions i watch bracket drag racing on friday nights at a local track, i'll occasionally see a dragster with up-swept headers and there is a flap on at the end of the collector. What is this for?

 

[82355]

They call that flap a "rain cap" on a tractor.....

Martin

 

[ktmoutfront]

Someone told me that the flap was there "because when the engine shuts down it turns a little counterclockwise just before it stops turning. And the flap is there so the engine doesn't suck in cold air and damage the valve."

i immediately very probably thought this was hogwash, but i wanted to know for sure.

However, on the rare occasions i watch bracket drag racing on friday nights at a local track, i'll occasionally see a dragster with up-swept headers and there is a flap on at the end of the collector. What is this for?

I was told it was for keeping cold air out.

I figured they were there so you could tell the engine was running.

 

[max 02]

I was told it was for keeping cold air out.

I figured they were there so you could tell the engine was running.

Its a rain cap! Thats it... but It does work great to see if a tractor is running if your not in that one driving another one. If that makes sense?

I look at ours all the time on our old mixer wagon tractor when I`m loading it with the Loader.

BTW I`d drive it on a trailer with no manifold and not even think about it.

 

[ktmoutfront]

I was told it was for keeping cold air out.

I figured they were there so you could tell the engine was running.

I was talking about the comp drag cars.

 

[76zimmer]

a FEW dragsters have flappers on their upswept headers still, but most do not, and I can't remember the last time I saw tennis balls put in a set of headers...old school myth stuff

...the nitro engines get torn down right after a pass and are hot enough to melt pistons...yet the valves seem to hold up even when the heads are taken off the engine and exposed to ambient temp air.

See my first response to the OP's question...it was all in jest!

 

[campfire]

First, thank you for taking the time to school me on this, as this is something that I have not understood thus far. I've not even thought about it very much to this point. I do follow what you've said, but I still have some questions.

Glowing exhaust manifolds we have all seen also reach ~1200*, while manifolds EVENTUALLY crack and fail, they are exposed to "open air" and don't fail spectacularly because of the heat cycles. If heat cycles were an issue, headers alone would cause valve failure, as they shed heat far faster than a cast manifold.

Valves are in contact with cast iron or aluminum, which is the sink. Somewhere around 75% of the shed heat from valves, is through the head.

First of all, I've been thinking that the iron/aluminum head is what's causing the problem here. That the head, when up to temperature, cannot cool as quickly as the valve can. That when outside air, cold relative to the temperature of the head, is introduced, that this disparity in cooling (and shrinking) rates encourages warping in the valve as different parts cool at different rates, and the valve shrinks faster than its seat. That the convection of air is the problem, not any heat shed slowly via the manifold or anywhere else. That is what I have been thinking, and that is what directed my comment about summer temps vs. winter temps.


Second, I've never thought that valves (which can easily see >150,000 motion cycles per hour of runtime in addition to thermal stresses) were in the same category as manifolds, which typically see no motion cycles at all. I've thought that the valve is much more delicate than the plumbing downstream.

Removing an exhaust manifold (or installing headers) on the traditional small block, without correcting the AFR, WILL cause a lean condition because combustion efficiency has increased...more air in, but its up to a human to correct the fuel. Bad enough, that lean condition will lead to burned valves, pistons, etc.

Thirdly, when this (^^^^) happens, why does AFR change? Why is this increase any different from the increase that happens when the throttle is opened? More airflow should mean more fuel is mixed in, no?

See my first response to the OP's question...it was all in jest!

I actually missed the jesting, as I have heard several of those concerns from other sources. Clearly, I have much to learn about this...

 

[max 02]

I was talking about the comp drag cars.

Fair enough.

 

[dyeager535]

If valves were delicate, they wouldn't hold up under normal use, let alone the extremes they see during racing. Think about the heat cycle...that valve is at ~1200* if not more, yet it gets hit with much colder air and fuel every cycle. Additionally, the valve has a good seat to keep it straight. Filled valves are used to further help disssipate heat.

AFR changes because you've changed how well the cylinder can ingest air, and you've decreased any sort of "self EGR" that you can see with poor flowing exhaust...I've *heard* that some of the negative backpressure EGR valves don't operate properly when you install headers, because they rely on the restriction the stock exhaust provides.

AFR can't and won't change unless the engine has a way to adjust it's AFR. In a carb, it can't, since the fuel flow under any given operating condition is governed via the rods and jets. Take a "perfectly" tuned engine that provides optimum fuel under heavy throttle and at cruise, then change the exhaust (but true of cams and heads as well), as long as the induction system will flow enough, the cylinders will pull in more air. Since the fuel metering is "fixed", it is now lean. I've experienced this personally by removing EGR and not adjusting the carb, the lean surge was very obvious.

There is also a myth that fuel injection eliminates this problem, but that's not true either...unless there is a wideband O2 sensor, the only accurate adjustment FI can make to AFR is during cruise, and lean cruise isn't nearly as bad as lean WOT, which is where it will still run lean. FWIW, took about 20% more fuel to keep from running lean at cruise in a 454 with a mild cam and headers. I'm hearing some of the new new engines are running wideband O2 sensors that could be used to maintain a proper AFR even under a heavy load, that's not most systems though.