Short open pipes "burning valves"...fact or fiction?

You always hear the old addage that running short, open pipes (like zoomies) on an engine will quickly lead to burned valves in the exhaust port(s)...but is this true or not?

I've done it on mini-bikes and stuff like that and never saw a valve magically fry itself (what constitutes a "burned valve" anyway?!)...but admittedly...how far do you drive a mini-bike (okay, I used to take mine on 20-some mile trips, but that's still short work compared to what a car sees!)?!

Back eons ago, I had a crappy VW Bug with NO exhaust on it...open PORTS only! I dove that pile a few months like that and it always ran good...just super LOUD with random flame shows at night! I want to make some short, open tubes for my current Bug-Based Bomb, so I thought I'd toss the question out there to see what the collective HAMB Bank of Wisdom thought of this notion!

(Of course...Bugs can never be driven all that far either, so how much difference can it actually make, right?!)

Anyone got any hard evidence to support this silly old wive's tale about burned valves...or is it just another urban myth??

 

F.H. I used to hear it as very short or no pipes would cause the exhaust valves to bend or warp. I dont know the truth to this one but,,,I know on a motorcycle you gotta richen up the carb/s if you pull off the mufflers. Is that a clue to what were talkin' about? N.N.

 

i always heard it would bend them due to temperature change after turning off the motor...... brandon

 

I can't see burnt valves happening unless it's a rich mixture with "late" timing and gettin some extra heat at the valve from atmosphere/air. It'd have to be sustained throttle at that particular point. I never did buy into the backpressure thing either. It's not about that. It's about scavenging spent gases and keeping the exhaust path hot. The biggest restriction to spent gasses is the extreme temp change on exit. While I know this whole response seems over-simplified I think we get the idea.

 

I have been told that also. But look how short the stacks are on a P-51..

 

I've seen the switching of exhaust to straight pipes without rejetting cause real problems with the exhaust valves. It's not instant and it's not magic. But creates a lean condition and increased heat to the exhaust side. To me it's more about the fact that they don't make power, they lose it....

 

Good point, I hadn't even thought about that as an example!

 

So jetting up the carb would sorta offset the "problem"?

I'm not worried about making power...just noise!!

(It's a BUG after all...it'll never make POWER, so it might as well be loud!)

 

It should, though you need to watch out for pulling raw fuel through and into the exhaust, it can cause issues... like flames... One of the downsides (or ups depending) of no back pressure...

 

oooooh! Flames would be cool!!!

 

Oh man, don't EVEN go there.....

 

We have some 1940s farmall tractors that have been in use for 40+ years no muffler just straight from the manifold never had a valve problem. I know of a guy that put a 5 ft long piece of driveshaft on the exhaust of a 3T D7 cat dozer it was ouiter and kept the exhaust out of his face. But it burnt some exhaust valves in about 20 hours of running. Randall

 

The old 21stud flathead in my old 34 coupe threw an exhaust valve seat just after I fitted shorty headers.
Coincidence? I don't think so.
Mart.

 

generally a burnt exhaust valve comes from valve lash being to tight or the guide being worn out badly, generally.

people always say that about zommies and stuff, but whos run zommies long enough to burn a valve??

JEFF

 

How short is short to burn valves? Millions of Harleys with short,36 inch, drag type pipes running around.
Dropped a valve seat? Improper fit,some kinda flaw?

 

In my old drag race days, guys would stuff a rag up headers or cover zoomies to cool them off gradually. This kept the exhaust valves from warping.

 

Theoretically it could have been caused by increasing the exhaust velocity hence pulling harder and causing it to run lean and run hotter with unspent fuel burning in the exhaust...

 

I greatest problem with short stacks is the lack of scavenging effect that actually helps in the torque factor which makes Horsepower work!

I've heard stories about P-51's having one or several stacks shot off and adversly affect the performance of the engine!

On the cylinders that were missing the stacks... they usually found it didn't burn the valves but it did affect the performance.

P-51's were not designed to last more than 75 hours in combat conditions and are maintainence hounds requiring 17 hours of maintenance for every hour of flying!

 

Ha Ha...I reworked your quote a bit...cuase that's just like a Vee-Wee!!

 

How I understand it the only thing to be worried about is when you shut it off & with a short pipe it allows cold air to immediately hit an hot open exhaust valve, thats where your damage to the valve would happen from hot to cold.

 

VW, Bug, Beetle...etc!

 

Yes evidence not repeated stories.

The theory according to the wives tale (my opinion) is that the cold air will warp the valve stem which makes the valve tulip leak (same result as a burnt valve but not actually burning the valve)

I believed it for many years until I started doing exhaust every day. I've never seen any concrete evidence just the often repeated stories.

My evidence has been to the contrary. I've had customers drive every day for months with broken manifolds and snapped off header pipes with no affect on the engines. Subaru's don't have manifolds. The cat. converter bolts right to the head like a motorcycle and were famous for snapping off at the head. They were expensive and often run for months with out repair. I never saw a warped or burnt valve.

 

There are thousands of Lincoln pipeline welders out there running nothing but a 12 to 16" straight pipe on the flathead continental engines. Mine runs like a champ with no valve problems.

 

I heard the same thing I run 16" pipes on my 50 Merc with no Problem <br> of course the local Police dont like them but you can't please everybody <br> just my 3 cents

 

Here is a photo of a Rolls Royce "Merlin" for WWII. It looks like RR did not worry about short exhast stacks.

 

It's NOT burnt valves. It's warped valves that the myth is based on. My opinion? Myth, NOT fact. Let's face it. The exhaust valve's a bad mutha to take temps north of 1000 degrees for hours on end. And to think the cooler air is force fed to that hot valve when the engine is shut off is rediculous. That valve will stabilize itself even with no exhaust. This is my mechanical opinion, and I'm sticking to it

 

One thing with the aviation engines, like the Merlin, is that they would have a substantial amount of time to cool down while taxiing in and shutting down. The average would be something like 10 minutes or so. This would allow for the temperatures to drop considerably before the engine was shutdown. Hence there would be very little stress on the exhaust valves. While in the air, in the traffic pattern, they would be in a rich mixture setting, with plenty of fuel available for cooling.
I heard the same story as a kid in the 50's. I ran on the highway for long distances with open lakes pipes, and my old nailhead never burned a valve.
My vote goes with the posters who think it is the cold air entering the pipes after the shutdown and warping the valves.
Bob

 

my idea is its BS ,
only because the cyl. head is hot anyway , so how cold must the air be to warp the valve in a hot head ?

the heat of the head will keep real cold air from ever getting to the valve until the head loses enough heat to equal the outside temp....

time for mythbusters to have a new show !

Beaulieu

 

Even if its true ( I don't think so ) a VW ought to be up to the challenge. They're well built from quality materials.

And even if it does go pear shaped, an engine R&R is only half an hour.

 

There were many reports of warped valves caused by short/no pipes. But this was way back in the early days of the internal combustion engine and most likely caused by the inferior alloys used for the early valves.
There was a lot of "black magic" used by the early (Millers/Offy's etc) race mechanics to keep valves in these engines. It is amazing how much better valve materials have gotten since the 40's