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Discussion Starter · #1 ·
In reading discussion on exhaust and mufflers in other posts, I find I have more questions. (I am beginning to think I should just unsubscribe from this forum before I drive myself and may be the rest of you crazy!)

When I see "backfire" in other posts, should I read this as "afterfire" i.e. unburned fuel igniting some where in the hot exhaust?

Afterfire is due primarily to momentarily "running rich", especially on deacclerating / downshift? (backfire usually due to running lean with ignition occuring in intake manifold, not exhaust manifold or back from there?)

The "pops" that are described with Tubi (and even the stock exhaust) are afterfire?

I would have thought if the above is correct, i.e. unburnt fuel in exhaust, that this would cause problems for emission testing.

When is afterfire bad?
 

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to avoid the "pops" instead of downshift before coming to a stop just put it in neutral.

http://www.thrashercharged.com/tech_htm/exhaust.shtm
http://autospeed.com.au/cms/A_108185/article.html

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 (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.

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.
 

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Coasting

to avoid the "pops" instead of downshift before coming to a stop just put it in neutral.
In the manual it says that you can coast a bit in neutral coming up to lights etc. but then goes on to say that it's dangerous to coast in neutral for a long period as you would be out of control of the car. :rolleyes:

I've always been told never to put an auto car in neutral and coast as it screws the gearbox, but is this true of the CC gearbox or can it indeed be put in neutral for going down hills in traffic etc ?
 

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Discussion Starter · #4 ·
Stopping in neutral

The big problem I have found with putting the car in neutral while in motion, is that to go back into gear you have to have your foot on the brake.
 

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The big problem I have found with putting the car in neutral while in motion, is that to go back into gear you have to have your foot on the brake.
that's not entirely true. it will try and figure out the best possible gear, though not perfect, when it should be in the 2nd it will engage in the 4th... the pops only occur when decelerate drastically in lower gears. namely, from 4th to 2nd then stop or 3rd to 1st then stop. I can usually make it pop from 2nd to 1st if I heel and toe it. Anyhow, you won't be coasting in neutral per se...so...
 

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Discussion Starter · #6 ·
that's not entirely true. it will try and figure out the best possible gear, though not perfect, when it should be in the 2nd it will engage in the 4th... the pops only occur when decelerate drastically in lower gears. namely, from 4th to 2nd then stop or 3rd to 1st then stop. I can usually make it pop from 2nd to 1st if I heel and toe it. Anyhow, you won't be coasting in neutral per se...so...
I stand corrected! You are correct, it is only when at a stop that you must have foot on brake to shift back into gear.
 
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