cracking exhaust valves

[MrTube]

I've always been told running an engine without an exhaust system of some sort can crack a valve.

Here is a picture of a 1920s Wright J-5C Whirlwind aka R-790.
Notice the short little exhaust pipes, or whatever you want to call them. Why weren't engines like this at risk for cracking valves?

http://upload.wikimedia.org/wikipedi...ght_R790_2.jpg

[PC/SR]

I think the theory is that the "cold" ambient air would cool the valves too quickly when the engine was shut down. I dont know about the J-5 but many later radial engines had a collector that directed exhaust to the side. However, the in-line water cooled engines did not. (Mustang Merlin, etc) I have never seen, or heard from a first hand source, a valve cracked or warped from cooling too fast. Maybe its one of those old myths that won't go away.

[jrelliott]

They ran a number of Pietenpol aircraft with Model T and Model A engines. Gar Williams on this site has one of these planes with a Model A engine which he flew earlier this year and reported on this site. They had short exhaust tubes. Google Pietenpol quite intresting. Contact Gar Williams he is a very knowledgable expert ont he Model A

[Purdy Swoft]

I've ran model A engines, straight out of the exhaust manifold in the old days with no valve problems. The big problem was fire belching out the manifold . A gas leak at the sediment bowl would be a real danger . I had one to burn in the driveway in 1962. Luckily, I was able to bring it back to life and still have it .

[Aerocraft]

If the Pietenpol builder follows the original plans, the exhaust stacks are cut 8 inches long. I have heard comments in the past that the exhaust valves will crack or warp but have not had that experience.

I have had a number of aircraft engines, both radial and opposed, that had short stacks and have not experienced any problems with exhaust valves. One small racing airplane that I built with a 190 cubic inch Continental had quite short stacks, less than 3 ", just long enough to clear the cowling. The valves on that engine did not have any problems even though I was running the engine some 500 RPM faster than the factory redline.

Gar Williams

[Patrick L.]

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I too think that is just an old wives tale.. Some folks think that the valve face can shock cool, but, it doesn't appear to happen in real life.. They may be the same folks that will say an exhaust system is needed for back pressure or valves will burn,, thats not true either.. These engines go through extreme temperature changes during every stroke..

[Flathead]

A myth.

[BILL WILLIAMSON]

Originally Posted by Patrick L.

I too think that is just an old wives tale.. Some folks think that the valve face can shock cool, but, it doesn't appear to happen in real life.. They may be the same folks that will say an exhaust system is needed for back pressure or valves will burn,, thats not true either.. These engines go through extreme temperature changes during every stroke..

Pat,
Why do wives get blamed? They think an exhaust valve is that flapper in the bathroom fan! Bill W.

[just plain bill]

Originally Posted by BILL WILLIAMSON

Pat,
Why do wives get blamed? They think an exhaust valve is that flapper in the bathroom fan! Bill W.

I can remember guy's running out to the race cars when they shut them off in the pits and stuffing rags into the exhaust pipes so the valves, or seats, or something wouldn't crack. I was impressed!

[MrTube]

Originally Posted by just plain bill

I can remember guy's running out to the race cars when they shut them off in the pits and stuffing rags into the exhaust pipes so the valves, or seats, or something wouldn't crack. I was impressed!

Or maybe you should have been laughing at him for wasting his time.

The Wright J-5C Whirlwind was known for being extremely reliable (ergo why it was used in the Spirit of St Louis) in its day and apparently didn't crack valves.

[Milton]

I ran my first model A right out of the block ports some times, sounded like a pistol with a 4" barrel and good for about 9-12 inches of blue flame out of each hole.

[Tom Wesenberg]

Not quite the same, but years ago my neighbor had an early 60's 2 stroke single cylinder 250 cc Zundap. It wouldn't start or run without an exhaust pipe to help scavenge the cylinder.

[Patrick L.]

Tom, how many valves did your friend's Zundap burn ?? Hehehehe....

[George Miller]

Your lawn mower has a short exhaust, tractors have a short exhaust.

[MrTube]

George, true but they almost always have a muffler which would likely provide some heating.

[George Miller]

Originally Posted by MrTube

George, true but they almost always have a muffler which would likely provide some heating.

You should always let a engine that has been running hard, like plowing with a tractor, climbing a long hill with a truck, idle for a couple of minute before you shut it off.

[BILL WILLIAMSON]

Originally Posted by Milton

I ran my first model A right out of the block ports some times, sounded like a pistol with a 4" barrel and good for about 9-12 inches of blue flame out of each hole.

Milton,
Thanks for giving me goosebumps, just thinkin' of all the fun we've had! I can hear Jerry Lee Lewis singin' "GREAT BALLS OF FIRE!" bILL w.

[BILL WILLIAMSON]

Originally Posted by Tom Wesenberg

Not quite the same, but years ago my neighbor had an early 60's 2 stroke single cylinder 250 cc Zundap. It wouldn't start or run without an exhaust pipe to help scavenge the cylinder.

Geez! I thought I owned the only one left in the world, with it's chrome muffler, salvaged from a Jag XKE and it's stoplight switch made from a spring loaded antenna toggle switch. Gregg asks why I don't remember really IMPORTANT facts???
(I just looked at him with a thousand words)
Bill W.

[BILL WILLIAMSON]

Originally Posted by just plain bill

I can remember guy's running out to the race cars when they shut them off in the pits and stuffing rags into the exhaust pipes so the valves, or seats, or something wouldn't crack. I was impressed!

Just some of the silly things we did to appear like SERIOUS racers, GREAT FUN!! Drags in the '50's were a blast, everyone was some kind of MAD inventor of one sort or another. If I could type well, I could write volumes of fun stuff. When Gregg goes back to work, I should hire a cute red head to clean house, cook & type stories as I narrate while pettin' the dog. Bill W.

[just plain bill]

Originally Posted by BILL WILLIAMSON

Just some of the silly things we did to appear like SERIOUS racers, GREAT FUN!! Drags in the '50's were a blast, everyone was some kind of MAD inventor of one sort or another. If I could type well, I could write volumes of fun stuff. When Gregg goes back to work, I should hire a cute red head to clean house, cook & type stories as I narrate while pettin' the dog. Bill W.

Well, this was taken pretty seriously by the circle racers in the day. As to open exhaust, most farm tractors years ago had no muffler. Our Case "D" had an exhaust stack about 6" long .

An old time race engine builder told me that back in the "teens" and early "20"'s he would tune the carburetors until he had 1 1/2" to 2" blue flame out of the ports or short stacks.