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why is back pressure good?
- BarMan
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- Duck
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- e vica na i sau na ga
If we just opened the valve to the air, it would work and be very loud.
If we add pipe, we can use the momentum of the prior exhaust pulse to help the next one along. Done properly things will be quieter and we reduce the pressure in the cylinder for the charge stroke. This momentum generating pipe is where folks get the idea that 'back pressure' is needed.
Once your head pipes hit the collector, the rest of the system is there to quiet, to couple the mass flow out the end of the pipe, and to give a pleasing tone. If the tone is not pleasing to you, you may remove the baffle with no ill effect on performance.
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- Corith
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Engines work by air flowing from the intake, through the piston chambers, and out the exhaust. In a non-turboed (read non-supercharger, NOS, etc...) engine, the air flows into the piston chamber via a vaccumm.
Here's what happens.
1. A door opens in the piston which leads to the air intake. As the piston moves there is a vaccumm in the chamber which must be filled. The only thing to fill it is the outside air.
2. The door to the air intake closes, and a door to the exhaust opens. At the same time a HUGE explosion happens in that chamber. Which things explode the expand. The only place for the expanded gas to go is out the exhaust door, so it does.
So technically, back pressure ISN'T a good thing. In the perfect solution you would have a constant flow of gases from your intake, through your pistons, and out your exhaust.
So I'm with you. I want to know why having a bit is good. Cause from my understanding of how the combustion engine works back pressure doesn't make any sense.
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- vach
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Post edited by: vach, at: 2007/05/21 22:15
1979 kz650 B. Chicago, Illinois
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- arobsum
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well said...I doubt you'll notice any difference in performance. Exhaust is a whole mess of fluid dynamics.
If we just opened the valve to the air, it would work and be very loud.
If we add pipe, we can use the momentum of the prior exhaust pulse to help the next one along. Done properly things will be quieter and we reduce the pressure in the cylinder for the charge stroke. This momentum generating pipe is where folks get the idea that 'back pressure' is needed.
Once your head pipes hit the collector, the rest of the system is there to quiet, to couple the mass flow out the end of the pipe, and to give a pleasing tone. If the tone is not pleasing to you, you may remove the baffle with no ill effect on performance.
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- pumps
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Check out our site. kcvjmc.org
1977 Yamaha XS650
2000 Kaw W650
2 KZ440 LTDs , a 79 KZ400H and an 83 Belt Drive
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- vach
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1979 kz650 B. Chicago, Illinois
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- steell
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So, increase Tq = increased HP.
Open headers shift the torque higher in the rpm band, thus the increase in HP.
More restrictive exhaust shifts the torque lower in the rpm band , thus less HP.
Torque is a measurement.
HP is a calculation.
Typical big block hot rod magazine build, 600 hp, 650 tq.
Typical high hp diesel truck motor, 600 hp 2100 tq.
HP is all in the rpm
KD9JUR
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- vach
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I have bent valves running open headers using the motor to stop the car. The backfires are ignition before the valve is completly closed. You may or may not need heavy duty valves and springs. I learned the hard way. I wouldn't recomend letting the bike backfire when stopping with open headers as it is very stressful on valves and springs.. Sorry fellas, I had a very bad day..
1979 kz650 B. Chicago, Illinois
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- AR15Ron
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Ron
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- wireman
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- pumps
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I remember we used to call them "Lead Sleds" because of the weight but we still loved them. Got any pics of your little 66? My dad had a sweet little 62.My buddy "Slink" had a cool 66 Caprice SS/327 with bucket seats etc.The little motor is in a 66 Impala, a pretty heavy car.
Post edited by: pumps, at: 2007/05/22 00:42
Check out our site. kcvjmc.org
1977 Yamaha XS650
2000 Kaw W650
2 KZ440 LTDs , a 79 KZ400H and an 83 Belt Drive
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