Straight pipe question

chris4x4 · Jan 8, 2012

http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm

Destroying a myth.

Some say that "an engine needs backpressure to work correctly." Is this true?
No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.
The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.
The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.
The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.
Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.
Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.
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Bobber Bill · Jan 8, 2012

chris4x4 said: ↑

This is an old mechanics tale based on miss information. First, the valves cool from the seat. Valves are heat tempered, and dont expand/contract to the same degree other metal parts do. Being the exhaust system runs between 1200 and 1500* when driving normally, and can get as high (safely) as 1800/1900* when under load, a 40* change in ambient temps (shuting off vehicle on a cold night after a day of driving) isnt going to cause any issues. Consider that back in teh 'ol days of flight, radial engines ran the exhaust right out of the head. These engines mostly suffered oil starvation issues, rather than burned valves. Valves that were found burned, were mostly found burned due to poor fueling, or timing. As for the

You do realize that valves stay where they are supposed to be in the timing cycle when the engine is stopped, right?
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The information I presented is my personal belief and opinion, based on my experience with rebuilding motors. I have seen corroded internals and had the pleasure of rebuilding the motors myself.

Valves that aren't seated or adjusted properly won't ever close, regardless of valve train position.

Now, how likely it is that this will happen to a daily-driven vehicle with properly lubricated valve train is a whole 'nother matter. Again, my belief and opinion - based on my experience.

I'm sure I can google a number of websites/articles that argue this point in accordance to my beliefs.

*misinformation

Bobber Bill · Jan 8, 2012

chris4x4 said: ↑

http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm
Click to expand...

On the other hand, this is one "copy & paste" that I can agree with.

chris4x4 · Jan 8, 2012

BFF BILL said: ↑

The information I presented is my personal belief and opinion, based on my experience with rebuilding motors. I have seen corroded internals and had the pleasure of rebuilding the motors myself.

Valves that aren't seated or adjusted properly won't ever close, regardless of valve train position.

Now, how likely it is that this will happen to a daily-driven vehicle with properly lubricated valve train is a whole 'nother matter. Again, my belief and opinion - based on my experience.

I'm sure I can google a number of websites/articles that argue this point in accordance to my beliefs.

*misinformation
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Ok. well your very welcome to your belief. Most of my knowledge comes from first hand experiance as well, as do many others on the site. Dont let me spelling lead you to think I havent taken a few engines apart. While I agree that valves that are seated incorrectly, or not adjusted, wont seat, that has no bearing on weather or not the exhaust, or lack there of, will lead to bunred valves. Burned valves are caused by a poor A/F ratio, and unmless you are running a carbed engine, or one with a bad ECU, the engine is going to maintain the correct ratio, regardless of the length of the exhaust.

Bobber Bill · Jan 8, 2012

chris4x4 said: ↑

Ok. well your very welcome to your belief. Most of my knowledge comes from first hand experiance as well, as do many others on the site. Dont let me spelling lead you to think I havent taken a few engines apart. While I agree that valves that are seated incorrectly, or not adjusted, wont seat, that has no bearing on weather or not the exhaust, or lack there of, will lead to bunred valves. Burned valves are caused by a poor A/F ratio, and unmless you are running a carbed engine, or one with a bad ECU, the engine is going to maintain the correct ratio, regardless of the length of the exhaust.
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Improper spelling/grammar: meh, I'm over it

Taking engines apart: my neighbor's 6 yr old has taken an engine apart, it's still scattered across the garage floor

So you're saying that the EFI cars/trucks/bikes out there that pop on deceleration, bog down on acceleration and spew out 7 different kinds of smoke all have bad ECU's?

Also, the only person who made any reference to any variation in exhaust length is you, so...

fireturk41 · Jan 8, 2012

Bobber Bill · Jan 8, 2012

By the way... I don't take anything I've said up to this point back, but.... it's all in the spirit of intelligent debate and fun. There's no need to get all bent outta shape over any of this, like so many people seem to on the internet... I look forward to your rebuttal

chris4x4 · Jan 9, 2012

BFF BILL said: ↑

By the way... I don't take anything I've said up to this point back, but.... it's all in the spirit of intelligent debate and fun. There's no need to get all bent outta shape over any of this, like so many people seem to on the internet... I look forward to your rebuttal
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Completely agree!

BFF BILL said: ↑

Improper spelling/grammar: meh, I'm over it

Taking engines apart: my neighbor's 6 yr old has taken an engine apart, it's still scattered across the garage floor

So you're saying that the EFI cars/trucks/bikes out there that pop on deceleration, bog down on acceleration and spew out 7 different kinds of smoke all have bad ECU's?

Also, the only person who made any reference to any variation in exhaust length is you, so...
Click to expand...

No, Off throttle, they can run lean, as you know being a bike builder, but not lean enough to cause damage. Im thinkin we're getting off the original subject, and things I questioned you about, which was exhaust being a cause of burned valves.

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chris4x4 With sufficient thrust, pigs fly just fine. Moderator

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chris4x4 With sufficient thrust, pigs fly just fine. Moderator

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