Negative pressure on exhaust?

[Ecky]

Reading "How do you get your car to warm up quickly?" I was struck with an idea. I haven't fully thought it through yet, and would like some feedback. It might be completely idiotic for reasons I haven't considered yet.

My premise:
Having too large an exhaust pipe can rob an engine of low-end torque because it disrupts scavenging which is carefully tuned by the manufacture to assist the engine in removing exhaust gases, right?

If that's correct, I'd surmise that it increases engine efficiency. What happens then, if there is decreased air pressure on the exhaust side? Would it function something like a reverse-turbo, giving "free" power, so long as you don't actually suck any unburnt fuel out of the cylinder during the period of valve overlap that many engines have?

Of course, something would have to create the vacuum, but you don't necessarily need a belt-driven turbine, which would almost certainly defeat the purpose. What about using something like the venturi effect? Use restriction someplace like behind the radiator (where you can't help but have turbulent airflow anyway) to create vacuum, and run a small hose to somewhere on the tailpipe.

If the added vacuum-assistance is non-negligible, would this do anything for engine efficiency?

Thoughts please! I may go out tomorrow with a shop-vac and some duct tape and experiment with different positive and pressures on my exhaust pipe. If I pull the IACV, I should be able to tell whether I'm generating power by the idle RPMs.

[drmiller100]

a bigger exhaust gives more horsepower, but less efficiency at part throttle and lower RPM.

Sucking on the exhaust, if you can maintain it, will build more top end horsepower, but have less efficiency.

partly Plugging the exhaust can very easily give better low end efficiency and mileage. This is basically what variable cam timing does - opens the exhaust sooner as RPM increases for more power on the top end, but good economy when the exhaust valves are staying closed longer.

[jakobnev]

drmiller100: Did you just deduct all of that from a priori assumption that there is always a trade off between power and efficiency, and you can never do anything that improves both?

[Ecky]

This is a really interesting article:

http://www.veryuseful.com/mustang/te...Scavenging.pdf

If you think that leaving some exhaust gas behind in the cylinder before the next intake stroke is not important (in a bad way), look again at Figure 5 below. This is, once again, Jim McFarland's classic graph comparing the volumetric efficiency curve versus the torque curve.

The tests came about because Kevin has developed a patented variable-flow exhaust that uses a butterfly within the exhaust pipe. He initially expected to use the system to cause some backpressure at low loads 'to help torque.

However, he soon changed his mind when any increase in back pressure proved to decrease torque on a properly tuned engine. What increasing the backpressure does do is dramatically quiet the exhaust.

and

If, in fact, power does increase with increased exhaust back pressure, it is most likely the air/fuel ratio and/or ignition timing that are no longer optimal for the altered state of engine tune.

Larry Widmer comments on the above textbook quote:

At less than WOT and peak power rpm, the diameter of the tubing should change in ID. Just as with intake ports (unless we're just running off port volume), cross sectional area should be only sufficient to supply the flow rate necessary to feed the engine.

and

The exhaust system is much the same. Just changing backpressure is a bogus way of trying to create the "ideal" pressure in the system. The exhaust system should work like a correctly conceived header. It should extract the exhaust from the header, to minimize pumping pressures.

The only way to create a system that will serve as an extractor is to properly size the tubing to allow the flow velocity to create a sort of "vacuum" behind it.

Just as with headers, creating a system that will provide the best of all worlds at all throttle positions and rpm ranges is impossible. It's all going to be a trade-off. You can tune for the throttle positions and rpm ranges where you desire the greatest performance, but you'll sacrifice performance at the other
end of the rpm range.

I'm running an LS engine in my Del Sol, which does not have VTEC.

[drmiller100]

Originally Posted by jakobnev

drmiller100: Did you just deduct all of that from a priori assumption that there is always a trade off between power and efficiency, and you can never do anything that improves both?

No, There absolutely are things you can do which improve both low RPM part throttle efficiency and WOT.

More open intake. Decent fuel injection. Higher compression combined with more cam overlap. Long, tuned intake tracts. 4 valve heads.

Go look at Honda Civic engines. Amazingly powerful, amazingly efficient.

[jakobnev]

Yes, but from what did you conclude that sucking on the tail pipe would decrease efficiency?

[drmiller100]

camshafts, valve trains, etc. are designed to have at least 14.7 psi absolute backpressure. The goal is to open the exhaust valve late enough to get maximum thrust from the power stroke, but early enough to get the exhaust flowing so the intake can get fresh charge.

if you suck out the exhaust, you might lose a bit of thrust.

Further, if you leave a bit of exhaust gas in the chamber, there will be less pumping loss on the intake stroke at part throttle.

is it a big deal? Nope, but if you are looking for the next 5 to 10 percent efficiency at part throttle, it can add up.

also, what is going to provide the energy to create the vacuum for the exhaust?

[Ryland]

If you've ever tried running an engine without an exhaust at all then you have seen what it's like to run it without a negative pressure on the exhaust side, that is why tuned exhaust is so important, the momentum of the exhaust gasses create a vacuum that pull the next pulse out behind it, when you remove the exhaust system then the exhaust is entering in to the open air where their is no vacuum and the engine hardly runs.

[jakobnev]

if you suck out the exhaust, you might lose a bit of thrust.

Having less pressure some of the way down, but all of the way up would be a net gain.

[gone-ot]

...remember, it's about WHEN the exhaust and intake valves are both open, so it's ALL ABOUT TIMING, ie: when the low-pressure exhaust rarefaction get's reflected back up the exhaust pipe so as to arrive back at the simulataneously open exh and intake valves.

...no sudden change from pressure to rarefaction means no reflection back up thru the exhaust gas to the valve(s), and thus no "tuned" exhaust scavenging action.