Scavenging vs non scavenging
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BEATAV8
The Engine Whisperer
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Scavenging is a thing for naturally aspirated engines.
The manifolds on a turbo car are highly pressurized so there’s no considerations for scavenging
The manifolds on a turbo car are highly pressurized so there’s no considerations for scavenging
BEATAV8 hit the nail on the head, but this could be a fun topic for discussion. On small turbos like our cars run the exhaust pressure vs intake pressure is high enough that you are not going to get any scavenging in the sense of a naturally aspirated engine. If we had longer tubes on the exhaust, could we get a marginally lower cylinder pressure via the exhaust pulse, maybe, but the intake charge is still going to be diluted because you are not going to drop the cylinder pressure lower than the intake pressure like what happens on an NA application.
Now, if (a big if) the newer turbo technology is allowing for much better exhaust pressure vs intake pressure, a long tube header could be a benefit. A balance between spool and efficiency has to be managed. It is possible to have the exhaust pressure equal to or lower than intake pressure, but historically that has resulted in a spool up time that is not useful except for a purpose built machine.
Depending on that pressure ratio, the header could be optimized along with cam timing. If you could actually have a turbo designed to run at a lower exhaust pressure than boost pressure (and still have good spool response), the wasted exhaust energy could be recovered as a pumping gain because opening the intake valve would pressurize the cylinder above exhaust pressure and recover some of the pumping losses.
Consider as rough figure as I don't have the heating values for any fuels at my finger tips along with the efficiency figures for typical turbo engine, but roughly 2/3rds of the HP available in the fuel goes out the tail pipe and the radiator. Think about that. A turbo running an exhaust pressure less that the intake pressure can improve that figure.
Now, if (a big if) the newer turbo technology is allowing for much better exhaust pressure vs intake pressure, a long tube header could be a benefit. A balance between spool and efficiency has to be managed. It is possible to have the exhaust pressure equal to or lower than intake pressure, but historically that has resulted in a spool up time that is not useful except for a purpose built machine.
Depending on that pressure ratio, the header could be optimized along with cam timing. If you could actually have a turbo designed to run at a lower exhaust pressure than boost pressure (and still have good spool response), the wasted exhaust energy could be recovered as a pumping gain because opening the intake valve would pressurize the cylinder above exhaust pressure and recover some of the pumping losses.
Consider as rough figure as I don't have the heating values for any fuels at my finger tips along with the efficiency figures for typical turbo engine, but roughly 2/3rds of the HP available in the fuel goes out the tail pipe and the radiator. Think about that. A turbo running an exhaust pressure less that the intake pressure can improve that figure.
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Not cheap....
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I posted this video in another thread and at the 8 minute mark exhaust scavenging is discussed .
www.youtube.com/watch?v=2tZYJHtXG9s
www.youtube.com/watch?v=2tZYJHtXG9s
TIMINATOR
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Just because there is a lot of backpressure in the exhaust from the turbo doesn't mean that HP gains are not possible from a well designed higher flow header. A log has a lot of backpressure/restriction. A collector type system (properly designed) has a much lower back pressure and thus greater efficiency. The reduced turns and 90° intersections from a well designed (T/A Performance race headers for example) header drops the backpressure at the exhaust port and facilitates more flow with less backpressure at the turbo. The backpressure is lessened at the exhaust port, allowing a higher more efficient flow to the turbo where it can be turned into boost.
You can't get something for nothing, so the exhaust side of the turbo will always have a higher pressure than boost produced.
Properly designed headers will show a better ratio of boost to back pressure AT THE PORT. All things equal, dropping the backpressure at the port increases developed HP. The more work the piston must provide to attempt to evacuate the cylinder, means less HP to the crank. Also, the more efficient the turbo header is, the less residual exhaust is left in the cylinder taking up space that should be used for fresh intake charge.
For those of you that have never run anything but turbos, if you run a belt driven blower like a Roots (I.E. 8-71) if you improve the exhaust port or header flow (larger, freer flow) the boost will drop! But the H.P. will increase. There is less residual exhaust to take up space in the cylinder which acts like a smog EGR slowing the burn and lessening the amount of fresh, burnable charge. Blower guys get in trouble this way because you then need to reduce timing and sometimes richen the mixture to avoid detonation, even at this reduced boost.
Bottom line, the collector may not allow scavenging, but it allows lower backpressure increasing efficiency. And yes, I did port and blend my T/A race headers before installing them.
Lesson over.
Questions?
TIMINATOR
You can't get something for nothing, so the exhaust side of the turbo will always have a higher pressure than boost produced.
Properly designed headers will show a better ratio of boost to back pressure AT THE PORT. All things equal, dropping the backpressure at the port increases developed HP. The more work the piston must provide to attempt to evacuate the cylinder, means less HP to the crank. Also, the more efficient the turbo header is, the less residual exhaust is left in the cylinder taking up space that should be used for fresh intake charge.
For those of you that have never run anything but turbos, if you run a belt driven blower like a Roots (I.E. 8-71) if you improve the exhaust port or header flow (larger, freer flow) the boost will drop! But the H.P. will increase. There is less residual exhaust to take up space in the cylinder which acts like a smog EGR slowing the burn and lessening the amount of fresh, burnable charge. Blower guys get in trouble this way because you then need to reduce timing and sometimes richen the mixture to avoid detonation, even at this reduced boost.
Bottom line, the collector may not allow scavenging, but it allows lower backpressure increasing efficiency. And yes, I did port and blend my T/A race headers before installing them.
Lesson over.
Questions?
TIMINATOR
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