NSSANE02

iTrader: (14)

If an engine didn't "suck" air, it would have to have a positive intake air pressure just to run. Better yet, how would you explain the vacuum in the intake?

Old SStroker

"Delta P" or pressure differential.

It's not really a vacuum in the intake manifold, is it? It's just at a pressure lower than local atmospheric pressure.

It's that same concept that makes wind in a weather system with all those big "H" and "L" on the weather map.

Sometimes people even get confused between "suck" and "blow".

NSSANE02

iTrader: (14)

^^^ Fair enough. It's just so commonly reffered to as "vacuum" it's easier to explain it to most people that way. I guess I should have said how else could you explain the pressure drop inside the intake manifold if the engine wasn't sucking in air.

Old SStroker

The descending piston creates a slightly lower pressure in the cylinder than atmospheric pressure (~14.7 psi @ sea level), so the atmosphere pushes air molecules in to try to equalize the pressure. It's the atmospheric pressure doing the air moving.

FWIW, at higher rpm on a good engine making BMEP in the 200 psi range, there is only about 0.2-0.5 psi "vacuum" (below atmospheric pressure) in the intake manifold. So is it time to get into "intake ramming" resulting from intake runner length tuning? That can add ~5 psi to the charge in a NA engine, and it's not caused by the piston. Probably we shouldn't go there yet.

With the throttle plate(s) closed, the atmosphere can't find a place to force the air in so we get high intake manifold vacuum readings. Think snapping the throttle closed at max rpm like at the end of a drag race.

It's how you look at it, but it's all "Delta P". Try to think like Ma Nature, who prefers to blow rather than suck.




Jon

NSSANE02

iTrader: (14)

Ooooohhh, I see. I had it all backwards. Thanks for the explanation

Troux

iTrader: (4)

The way I learned the multiple "ramming peaks" is this:
(Primer first: )
The primary peak occurs when the valve slams shut, and shoves the air backwards up the runner. This charge then "bounces" off the more stagnant air in the plenum, and goes back down the runner.When the timing is right, the charge will be rushing back towards the valve just as it's opening, and this perfectly-timed rush of air is what causes a small "boosted" effect, and getting the timing right is about tuning the length of the runner. At higher rpms (shorter duration between valve openings), a short runner will get the charge back quicker to meet the valve as it's opening, and you can see how this works for longer runners at lower RPM. A nice bonus is the valves are also moving faster as RPMs rise, and hence throwing the charge up and down its path faster, so you get a "range" of gradually increasing and decreasing valve resonance benefit.

(Now for multiple peaks: )
Basically, the principal is that at a certain RPM, the charge is moving fast enough that it has time to go up the runner and back twice, so that it's still being force fed into the valve at just the right time, it's just lost a bit of steam since its first trip back. It takes a bit of math to find just where the second resonances are, but tuning for the first one is what's most important. In general, though, you can see how the shorter the runner length, the more resonances it's going to have, as going back and forth twice takes as much time as doing it once in a runner double the length.

Also, this is why ITBs can benefit so much from not having a plenum. By being exposed to atmosphere, there is a greater "bounce" back effect off the more quiescent air at the end of the runner when compared to a plenum, where the air inside is still moving quite quickly.

joecar

Does header scavenging/pullthru interplay with intake ramming, and how...?

Troux

iTrader: (4)

Good point to mention!

It can, if you have enough valve overlap. Without repeating the details that most of us mostly know, scavenging basically "pulls" on the upstream air. If your intake valve is opening as the exhaust valve is still closing, there is still some scavenging effect from the escaping exhaust gasses that will help to fill the chamber with the intake air.

I got excited when I first learned of this, but I couldn't help but think the effect would have to be so minor. When you think of how we hunt down small gains in an exhaust system, it's a constant flow through a system of perfectly-shaped pipes with flowing bends and controlled, 'tuned' radii. Comparing this to a less-than-perfect path taken along the head of two barely open valves, during a very short period of time, where there exists a huge turbulent chamber between the smallest pinches of each air pulse's path, I would have to see some pretty solid evidence to get over how theoretical it all sounds.

rsz288

In simple terms yes. If you cant get it out (exh) you wont get it in (int). .

The optimal trick is that the last of the exh gas is leaving as the exh valve seats with the intake charge right behind it for maximum cylinder filling with minimal to no dilution with non burning exh gas.

The intake ram effect helps to optmise this.

Ram effect is largely created by intake charge velocity. Once the valve closes the momentum of the rapidly moving intake charge compresses behind the valve. Optimised timing would have the valve opening as this pressure peaked. Variable runner lengths are used to optmise this ram effect as a longer column of intake charge is needed at lower speeds to achieve a similar amount of kinetic energy in the charge vs higher rpms where a shorter column works as the gas speed is higher.

Exhaust being hot high pressure gas by comparison with the intake charge relies on preserving the energy of the main "slug" of exhaust gas so that it tends toward creating a depression behind it to help evacuate the cylinder as the pressure drops off late in the exh cycle.

Reflected shock waves need to be optimised in the exhaust as well so they dont shove exh gas back into the cylinder at the wrong time or kill the speed of the main "slug" of exh gas.

For any one who is listenging engines dont really suck. They create a depression which is then filled/replenished by atmospheric pressure. But then that can be a circular discussion about what is sucking or blowing and when.

IFRYRCE

iTrader: (3)

No, it isn't. An engine that makes 100hp/l at 6k RPM is probably over 100% VE. Something like a Honda that makes 100 hp/l at 9000 RPM is probably NOT over 100% VE... Your "easy reference" is totally wrong, because HP is RPM dependant.

Torque per liter would be a better (though not great) reference of VE.... HP is torque x rpm and therefore dependent on RPM and VE... Torque is basically determined by VE, engine size, and compression ratio, and therefore a better measure of VE.

Peak VE RPM is usually close to/is peak torque RPM for what that is worth.

Old SStroker

Don't get pressure waves confused with charge motion. You may already have.

The pressure wave (compression and rarefraction) is moving in the tract at the local speed of sound (about 1100 ft/sec). The charge is moving at an average speed (velocity) of about 1/3 the speed of the pressure waves, and the piston (at it's maximum point, about 75° +/- after TDC), is moving about 1/3 as fast as the average charge velocity. At that's at power peak rpm.

Think organ pipe. There is a primary and multiple harmonics so there are a number of pressure waves of different frequencies dancing from one end of the intake runner to the other all the time. The intake runner length (plenum to valve) determines which harmonic(s) the intake system is using. In an OEM LS engine, the primary is the second harmonic, I believe. The intake tract is too short to use the first. Each harmonic is weaker than the previous one. Does that give a clue as to why the LS runner lengths work better than the LT1 (for example)?

It would be useful to have a ramming peak (higher pressure point) arrive at the intake valve just as the valve is closing to cram the last bit of charge in there even as the piston is moving up the bore. It might be nice to have one just as the intake is opening also. Remember that it is the difference in pressure across the valve that moves air in or out of the cylinder.

So how would you accomplish this with a fixed length intake tract? I'd choose an intake length that had ramming peaks at/near the rpm where I needed the power to peak, and then open and close the valves to take advantage of when the peaks were arriving at the valve. Easier said than done, of course, but still doable.

There are also other rpm points below peak power rpm where there are peaks. That allows a fairly flat torque curve if you do it right. It isn't all a free lunch, however...Keep thinking about it.


Jon

Arfdog

SSStroker pretty much explained the difference between pressure wave and charge motion.

I like to think of it like this. The intake valve slaps both the valve seat and the column of air. It makes a sound, which travels at the speed of sound, back towards the plenum. That is one source of pressure, though probably slight (not sure). That's the sound pressure wave.

The other source is the momentum of the air column. As it travels and slams against the closed intake valve, air will pile up in the intake port. More air piles up and air pressurizes. This gives rise to potential energy (change in pressure * volume). Now the energy can be released either by 1) waiting for the oncoming air to subside and pushing the air column back up the intake port or 2) pushing the air through the intake valve. Now would be a good time to open the intake valve.

I do not believe air molecules are actually traveling very far during this process, but the sound wave is.

chevynation

iTrader: (8)

How does the relationship between cam duration and runner length affect the way the cylinder sees the ramming peaks? In other words, would a huge cam 'catch' a different or more than one harmonic because of the long duration?

I'm curious because I was thinking maybe certain cams would never work well with a given runner length because the 'timing' of the pressure peaks or charge motion with the valve opening would not be ideal... If thats the case, does anyone know what cams work really well with LS plastic intakes?

This thread all of a sudden has me paranoid my 236* is giving up power (and drivability) to both a 240 and a 232, lol.