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Scavenging vs non scavenging

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xlr8ngn

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Just curious. Looking at our driver's side exhaust tubes that create a collector (albeit very short) versus passenger side exhaust log design, would 1,3,5 not run leaner because of scavenging?
 
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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.
 
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
 
That is a good summary Timinator. You can, though, have less exhaust (drive) pressure than intake manifold pressure. It is not the pressure that drives the turbine, velocity and mass flow drive it. If you can make the velocity and pressure, and you have a turbine that can use it, you can make the shaft power. This is more difficult to do with low pressures and avoiding a larger/heavier turbine wheel. The trade off is the response of the turbo. I think as the turbos have gotten better along with converters and everything else, in addition to the understanding of what makes it work together, this will become more common. With our low rpm V6, we are more challenged, so it would be interesting to see the experience of those monitoring back pressure, what they are seeing, and what is their combo. I don't think a turbo, that has lower back pressure than boost is practical for the street yet, but again who knows what others have done. If I had more time and money, this would be fun to figure out what is possible.

We had a guy on this site, a number of years ago, build a really cool setup. He was running lower drive pressure than intake pressure. Somebody else may remember, I think it was Don Wang. I can try to find it later.
 
With our normal street/strip Buicks, I have seen around 2 to 1 and more, backpressure to boost ratios. Flow in the exhaust port and headers and intake to exhaust duration/ lift are also factors in this ratio.
Thanks Nigel.
P.S. I talked to Larry in Auburn a month ago or so. Remember the P/U truck roof deal from the 70s?
TIMINATOR
 
The 70's is a couple years before my time, but please elaborate.
 
Duh!
Yup.
Talked to Larry a while ago. Reminisced about the trucks and you. Lots of big laughs, Until u mentioned that they were supposed to be scrapped, we didn't know. Poor customers....
Funny thing, the guy that replaced me at Larry's (1977 when I moved here to Phoenix) has been a customer of mine for 5 years, and we just figured that out about 6-8 months ago! He moved here about 20 yrs. ago.
TIMINATIOR
 
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Ok good, glad I was not missing something, other than my normal issues I'm still Ok. :)
 
Us old guys are getting slower, but I don't trip/fall on stairs (or airplane stairs) and I don't think I am president.
I'm ahead there...
TIMINATOR
 
I like this twisted collector. Not sure if it does any good but it is different.

 
Ya know Chuck, I have wondered about those trucks since u said they were supposed to be destroyed.
 
Super scavengers were found to be less effective than equal length, rotational firing collector, or better yet, with a rotational and venturi collector. A hand made set for my 1979 2300 4 cyl pinto (ran 12s back in 1982) picked up around 3 tenths and almost 4 mph. I modeled mine after formula One headers of the time. I don't think they have been much improved since then.
Before anyone asks, they only work on 4, 8 or 10 cyl (or more) engines. Or inline 6s, but the collector is big.
TIMINATOR
 
Super Scavengers were designed to be streetable..........As in the avg guy could bolt them on.
They were equal length but obviously didn't have the other features Tim mentioned.
I ran a set until they rusted out.
 
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