Ok....I have heard for several decades that if you run too short of exhaust you will burn a valve. But then you see tons of rodders running these short slash cut headers with no baffles or anything in them. (I will be one of these guys this next spring ) So is it true? Are all you guys burning vavles and just no talking about it? Or is this just an old wive's tale? DuckusCrapus
Good post. I remember hearing the same thing back in the early 60's. I never had any first-hand experience, so it would be interesting to me to hear about other people's experinces.
You will burn up the valves if you don't set your mixture and timing to suit the shorter exhaust. I'm not going to say it will happen everytime. How critical they are depends on the engine. Shortening the exhaust will drop the backpressure and lean the mixture. When you richen the mixture you get a bigger longer burn. Retarding the timing slightly reduces the temperature across the heads and piston crowns considerably.
Good to know but I hadnt ever heard of burning one cause of the exhaust being too short. But I was also going to run just come cut off pipes, so now I'll watch out for it.
I dont know about burning an exhuast valve with too short of an exhaust system,but think about this... On a typical OHV V8 engine,at least one of the 8 exhaust valves will be open(off its seat) when the engine is shut down.This leaves the head of the valve off its seat,allowing it to cool off WAY quicker than the rest of the valve itself,and the surrounding head,piston,block etc on that particular cyl. This could cause the head of said valve to warp/bend while it's cooling too quick,especially with short pipes. Run those straight pipes,but stuff rags in 'em for awhile after shutdown to keep the heat in there.
The story as I always heard it was that when you shut off the engine, cool air would hit any exhaust valves that were open and bend them. With longer pipes the air would warm up before it got to the valves. In all my years of racing, on gas or fuel cars, with zoomies or open headers, I've never seen it happen. This was on engines that were leaked down and taken apart every week to check them so if the valves bent even a little bit it would have been found. I'd have to call this a myth but I'd be interested in hearing from anybody else with first hand knowledge of it being true.
Im leaning towards it being a myth as well,but I think it could happen on certain cold weather days. Where's the Mythbusters when ya need 'em !
In the early '60s' we ran 8 inch straight exhaust pipes in a nitro injected dragster (SBC) no water in the block, and a lot of heat- never- ever seen a burned valve from that.
I heard this years ago. But I have run short straight pipes for years on my Harleys and never had it happen. Luck? You be the judge.
Interesting question. I remember asking my dad when I was very young (10 maybe?) why couldn't you run an engine without any exhaust on it at all, no header, manifold, zoomie, or anything. Ofcorse I don't remember his reason, but he said you couldn't or shouldn't.
I went to San Felipe about 15 yrs ago for vacation. I took a walk into town & there was a guy that had an early 60s Impala with NO manifolds on it.He had cut the top completely off the car(trunk lid too) & collected bottles for a living.I saw the dude driving around in it every day I was there. I finally found someone who could translate Spainish,& he said alot of the local cars were like that because the salt air rusted the manifolds away,so they just take em off. There was also a guy with a baby Bengal Tiger running around on the beach unleashed,but that's another story.
I've got no technical back-up for this, but think of WWII aircraft V-12 engines; with the exception of the the P-38 with turbochargers, most of them (P-40, P-51, Spitfire, Mosquito - to name a few) all ran short exhaust stacks, about 6"-8" long, and I've never heard of exhaust valve problems in those engines.
I've explained what happens when you shorten exhausts. It leans the mixture by reducing backpressure. The fuel washes out and burns in the port and manifold. End result if carbs are not adjusted to suit the exhaust is excess heat in exhaust port, and across pistons and heads. Heads warp, pistons melt, valves crack. Don't you figure that aircraft engine builders, drag racers or anybody else that takes the time to build an engine would bother to calibrate the fuel to the exhaust? This started as a topic that might actually help people and deteriorated into a waste of space.
Wow....thanks for the replies. I have a set of slash cut headers to run on my T coupe and everytime I look at them I think about that "burning valve" syndrom. I am going to run 3x2's so I am sure I will be plenty fat on the mixture....but I never would have thought about retarding the timing. I have also heard of covering zoomies and have seen several people at the track doing so right after every run. But then again you see many more that just don't ever seem to care about it. I will run my short pipes and I guess I will be the first to let the world know if it does burn a valve. Thanks for the input and I am open to any other opinions if you think different. DuckusCrapus
Since you are the only one that knows what he's talking about I will ask you this question.. If I am running lake pipes with down tubes to a rear exaust and want to take off my block plates and run the lake pipes open Do I have to adjust my carbs? Or run the risk of burnin my valves...
Tuned headers with collectors will scavenge exhaust better than short straight pipes or zoomies. This would cause a leaner condition than short (or no) pipes. I assumed the question was about the effect of running short pipes on a properly tuned engine.
Every time you modify or alter the inlet or outlet sides of a motor the fuel mixture changes. Best investment I know of is a colortune plug. They tell you how the mixture is while the motor is running. They cost about $50 each. You only need one. A balancer is useful too if you have multiple carbs. Forgetting the extra heat in the heads a minute, A colortune and balancer are a must for any performance minded owner. You can get balancers for as little as $13, and I actually prefer the el cheapo to the 40-50 dollar mercury versions. The Morgan carbtune is about the best,( that I've seen ) but that is $100
Man I wish they would have taught this stuff when I was in school. HAMB should be required reading for every highschooler!
My father in law had a allis-chamer lawn tractor with a koelher emgine that was completely missing the exhaust pipe, you could watch the valve opening & closing,He ran it like that for ever & never had a problem!! NOW for the next myth: Did anyone on the HAMB ever get there eye shot out from a BB gun?? or has anyone ever shot anyones eye out!!! I don't mean " a friend of mines brothers, cousin , sister in law, lives down the street form a vet who treated a beagle thats owner knows a guy whos son did", i'm talking first hand, finger on the trigger shit!!!lol jimV
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