Exhaust and Back Pressure Myth
11-19-2007, 06:06 AM
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Below is an excerpt from a member on Corvette Forum regarding back pressure:
Larry,
I'm well aware of valve overlap and exhaust tuning. I've been a mechanic, and a shop manager for over 16 years. I work on everything from 170 hp snowmobiles (now that's where you find TUNED exhaust!), to cars that I've built, to 500 hp snowcats.
Your concept of the exhaust gas "holding" the incoming a/f charge in the cylinder is way off -this isn't a 2 stroke.
During the valve overlap period, some fresh air and fuel DOES escape out the exhaust, and that is desireable**. The desired result is to "wash" ALL of the old exhaust from the cylinder, and the valve overlap period is when this is accomplished. Any remaining exhaust gas in the cylinder at this time will dilute the fresh charge, making it weaker.
**From a perfromance perspecitve, but this is why OEM's have to keep the lobe separation wide and duration as short as possible, keeping "leakage" to a minimum. This is necessary to reduce hydrocarbon emissions and optimize fuel economy.
The "holding" of fresh charge into the cylinder is MANAGED, by VALVE timing; the closing of the exhaust valve. Not by a bouncing-back-wave in the exhaust system. If you want to stop the fresh charge from exiting the cylinder through the exhaust valve...you close the exhaust valve. Simple. Again, this isn't a 2 stroke. 2 Strokes DO use the theory you described....because they don't HAVE any exhaust valve to close! They must use resonant wave tuning because that is they only means that 2 strokes have to contain a fresh charge. But we are discussing 4 strokes here an they have the luxury of controlled valve movement.
The "tuning of the exhaust" in a 4 stroke is desired, but not for the reason that you stated nor does it behave the way you explained. The desired behavior is for the exhaust to use the pulses to create a draw, or "negative" pressure on the back side of the exhaust valve as a result of VELOCITY and the momentum of the exhaust gasses leaving the cylinder. This "negative pressure" helps to draw the fresh charge into the cylinder during the valve overlap period achieving what I said above; a clean cylinder w/a fresh, undiluted charge. This is why exhaust (headers) that are "too big" hurt low end power; because at low RPM, the exhaust gas doesn't have enough speed (and momentum) to create that draw on the chamber.
"Tuning" is not the same as "back pressure". "Back pressure" is the result of an out right restriction and is NEVER a desirable feature.
WHO EVER IS RIGHT HERE, There is zero "Tuning" happening by the time you get all the way out to the mufflers and the tail pipes which is what this thread is about; mufflers and tail pipes. Any increase in back pressure at the mufflers or tail pipes will ONLY HURT POWER, and any decrease in back pressure at that point in the system will only help power.
I think the mod described in this thread w/the perforated inserts is a darn clever one, and I dig it. When I finally get my car, I WILL be doing my own exhaust system for it (my own, meaning I will fab it from scratch) to achieve the performance and proper SOUND that I want. And this thread is a cool "notation" in my head of things to play with.
-Tom
Larry,
I'm well aware of valve overlap and exhaust tuning. I've been a mechanic, and a shop manager for over 16 years. I work on everything from 170 hp snowmobiles (now that's where you find TUNED exhaust!), to cars that I've built, to 500 hp snowcats.
Your concept of the exhaust gas "holding" the incoming a/f charge in the cylinder is way off -this isn't a 2 stroke.
During the valve overlap period, some fresh air and fuel DOES escape out the exhaust, and that is desireable**. The desired result is to "wash" ALL of the old exhaust from the cylinder, and the valve overlap period is when this is accomplished. Any remaining exhaust gas in the cylinder at this time will dilute the fresh charge, making it weaker.
**From a perfromance perspecitve, but this is why OEM's have to keep the lobe separation wide and duration as short as possible, keeping "leakage" to a minimum. This is necessary to reduce hydrocarbon emissions and optimize fuel economy.
The "holding" of fresh charge into the cylinder is MANAGED, by VALVE timing; the closing of the exhaust valve. Not by a bouncing-back-wave in the exhaust system. If you want to stop the fresh charge from exiting the cylinder through the exhaust valve...you close the exhaust valve. Simple. Again, this isn't a 2 stroke. 2 Strokes DO use the theory you described....because they don't HAVE any exhaust valve to close! They must use resonant wave tuning because that is they only means that 2 strokes have to contain a fresh charge. But we are discussing 4 strokes here an they have the luxury of controlled valve movement.
The "tuning of the exhaust" in a 4 stroke is desired, but not for the reason that you stated nor does it behave the way you explained. The desired behavior is for the exhaust to use the pulses to create a draw, or "negative" pressure on the back side of the exhaust valve as a result of VELOCITY and the momentum of the exhaust gasses leaving the cylinder. This "negative pressure" helps to draw the fresh charge into the cylinder during the valve overlap period achieving what I said above; a clean cylinder w/a fresh, undiluted charge. This is why exhaust (headers) that are "too big" hurt low end power; because at low RPM, the exhaust gas doesn't have enough speed (and momentum) to create that draw on the chamber.
"Tuning" is not the same as "back pressure". "Back pressure" is the result of an out right restriction and is NEVER a desirable feature.
WHO EVER IS RIGHT HERE, There is zero "Tuning" happening by the time you get all the way out to the mufflers and the tail pipes which is what this thread is about; mufflers and tail pipes. Any increase in back pressure at the mufflers or tail pipes will ONLY HURT POWER, and any decrease in back pressure at that point in the system will only help power.
I think the mod described in this thread w/the perforated inserts is a darn clever one, and I dig it. When I finally get my car, I WILL be doing my own exhaust system for it (my own, meaning I will fab it from scratch) to achieve the performance and proper SOUND that I want. And this thread is a cool "notation" in my head of things to play with.
-Tom
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