Thats a large modifier. I am trying to listen to all fronts on this, as like I say to Todd "You know this stuff, I just sell insurance".

I had no idea that the wave would be affected by pressure, but it makes sense now that you explain it.

A new development is that the intake may need to be designed with turbo pressure in mind instead of just CS. The reason for this is because, well, it may be a multiple step process, and the intake should be forward compatible for me IF I move from CS to turbo some day. Or by the time the project is done. I would have to drive a powerful turbo car to see if the non-linearity is something I could get used to.

Even at 30psi with a CS, your notion holds alot of weight though. 1.73x15 inches of runner and port length makes... Well, maybe if I did the math this way. I know i can fit 10. 4 in the port and 6 out of it. Plenty of room for that. 10 x 1.73 makes a 17 inch runner calc at 30psi.

I wonder if this calculation would affect the base calculation of the induction wave, or do we just use the pressure as a modifier of the original number? I could use an earlier wave (2 maybe) instead of 3rd or 4th.

Is the PR linear? So 15psi (half the way to 30) is 2?

Turbo would make this a very different game. You could have 30psi at 2500rpm and hold it at 30 untill 7krpm. Wow.

Anyway, thanks. This is much more flexible, and thats good.

 

+1 to what Louie said. The more pressure you have in the intake, the shorter you want your runners. They should definitely be shorter in your application than for the stock NA manifold.

Dan
'91 928GT S/C 475hp/460lb.ft

 

Well, the point is on a CS car the engine has to do alot of off boost work. So it needs to work as an NA alot.

Secondly, I have read that the "characteristics" of the runners and how the air "flows" are in no need of change - RE, Diameter and shape. Length is the only thing Louie was talking about.

Nobody wants a dog of a motor that switches on like a light when boost comes on. Everyone wants a tractable, "sweet reving" compromise.

 

Use it as a modifier. Most of the simplified sound velocity formulas assume air pressure of 1 bar. The "modifier" is a sq rt function of the pressure ratio between 1 bar and the new air pressure. That is at 1 bar (atmospheric) absolute pressure, as it would be if N/A, your modifier would be sq rt of 1/1 = 1. At 15 psi boost which is 2 bar absolute, the modifier would be sq rt of 2/1 = 1.414. At 30 psi boost (3 bar absolute) the modifier would be sq rt of 3/1 = 1.732. So it isn't linear. One downside of using a lower order wave, like 2nd, is that you have some RPMs below the 2nd order where it will be the 3rd order. That's ok, but what about the RPMs beween the 2nd and 3rd order. There is a dip there. Maybe not a big deal, but realize that the bigger peak for the 2nd order comes at the price of a bigger drop off below the peak.

Air temperature also affects the sound velocity. Hotter air makes sound travel faster. That means you'd need longer inlets for a SC charge heated intake temp. That's also a sq rt function, but the base temp for the ratio is 0 Kelvin. A temp ratio of 30C rise from standard isn't going to make a whole lot of velocity change, but it's there and somewhat balances the higher pressure change.

 

Thanks Louie. I see now that its not linear. I remembered when I was reading what you wrote that each pipe length will have its proper place at each individual rpm, and thats why different lengths are used to soften the "on power" surge. But that also twists the engine in an uneven manner, so I was going to stick with equal length intake, and possibly try unequal length headers at the exhaust side (but if its turbo, that all changes)

Back to the topic of length. You are saying that a "dip" in the regular process of power delivery through the rpm range will be almost directly due to moving through the pipe length required for taking advantage of 1, 2, 3, or 4th order resonance wave? And porsche used the center flap to CHANGE the wave used during the rpm rise.

I think this connects why when people talk about the true intake length at least on an NA, they go all the way out to the filter. Because the stock S4 intake runners stop at the side pod. But the center flap is between the two pods - so obviously the real length of the runner is way past the bell mouth, and is actually past the throttle valve.

INteresting read: http://www.profblairandassociates.co.../Bellmouth.zip

You will need an unziper, but I think its worth it. It specifically talks about bell mouth shapes. Basically, the outcome of the discussion with days of CFD profiling is that:

1) A straight pipe is worst.
2) A simple radius at the entry to the pipe gets you the most increase in flow and decrease in entry losses(he is calling it exit (since its an exit from atmosphere - get it? )
3) Changing the bell mouth from a simple entry radius to an elliptical entry (fatter entry at an angle) increases the good side only by about 4%.
4) Oval or "square with edge radii" bell mouths are not good at all. Avoid them.

RE temptature: San Diego is 75 today. It may get to 100 in the summer if I have to go east. The coldest it gets where I drive is about 45. Maybe. So I am hoping that I can just factor in a 20c envelope and call it a day. Warm, hotter, hottest. No Cold ratios. I pay good money for sunshine here in SoCal.

 

Density will decrease the speed of sound, but pressure will increase it. In air, the two cancel each other out, making temperature the greatest factor affecting the speed of sound.

Just a couple of references:

http://en.wikipedia.org/wiki/Speed_of_sound
"In a given ideal gas the sound speed depends only on its temperature. At a constant temperature, the ideal gas pressure has no effect on the speed of sound, because pressure and density (also proportional to pressure) have equal but opposite effects on the speed of sound, and the two contributions cancel out exactly."

"In fact, assuming an ideal gas, the speed of sound c depends on temperature only, not on the pressure or density (since these change in lockstep for a given temperature and cancel out). Air is almost an ideal gas."

http://www.sengpielaudio.com/calculator-speedsound.htm
"The speed of sound c depends on the temperature of air and not on the air pressure!"

http://www.sengpielaudio.com/SpeedOfSoundPressure.pdf
"Note: The speed of sound c in air is only dependent on the temperature . It is completely independent of the air pressure p.
Reason: The air pressure and the air density are proportional to each other at the same temperature."

If anything, as far as the runner length goes, you should maybe be thinking about how efficient the intercooler will be, and not about whether boost is present or not.

 

Hi Brendan,
I'm not an advocate of unequal pipe lengths. Like you said, the torque from each cyl won't be the same. Our springy driveshaft doesn't like that. Also, you can't get a good mixture tune unless you have an ECU capable of an individual map for each cyl and want to take the time and effort to do that level of tuning.

I have seen that paper on bellmouths by prof Blair. Very interesting.

The temperature change I was thinking about that may affect tuning length was that from the supercharger raising the charge temp. That would be more than ambient temp change.

The S4 intake design and the flap is really complex. With the flap closed, it uses a phenomenon called Helmholtz resonance. That resonance depends on an inlet source which for the S4 manifold is the chamber downstream of the throttle. That chamber has no resonance properties. It's a simple air source. Next requirement is an inlet tube of certain cross sectional area and length. That is the short riser up to the side chambers. Then the side chambers themselves, and finally the individual cylinder runners. The side chamber and runner volume control the RPM of the Helmholtz resonance. Not runner length. The side chambers feed four cylinders that have even spaced intake pulses. Two cylinders from each side of the engine. That is vitally important. For Helmholtz resonance to occur, the intake pulses can't overlap. Therefore, you could have up to a six cylinder motor fed from a single Helmholtz resonance chamber. The large single chamber and 6 runners would have a volume so large that the resonance RPM would be really low. That's why a six is fed from two chambers (like a 911) and that's ok too. However, you can't have Helmholtz resonance in a single chamber being fed from all 8 cylinders. The intake pulses overlap and won't cause enough pressure variation in the single plenum for the Helmholtz resonance (reverberation) to occur.

To recap, with the intake flap closed we have true Helmholtz resonance. The frequency (RPM) is dependent on the S4 intake side risers, the side plenum volume, and the volume (not length) of the 4 cylinder runners on each side. That resonance is around 3000 RPM. Helmholtz resonance is quite strong and also fairly sharp. That's the reason for the sharp dip above 3000. When the flap opens, we lose all Helmholtz characteristics. The intake then functions like a large single plenum intake with the plenum volume being both side plenums, the cross connecting passage, and all the volume after the throttle. There is intake tuning, but it is then the individual runner length. They are all different and any resonance effect is spread out across a fairly wide band.

 

Ah. It was more complex then I thought.

 

Well then. Is the resonance wave truly sound, or is it pressure? That may not make sense.

The intercooler in this instance will be as efficient as Todd's. But the Fuel cools the air in my case, theoretically, much more than the intercooler can.

 

Both. Sound is a series of alternating pressures, which is why there is no sound in a vacuum. The pressure pulse travels in the runner at the speed of sound because it is sound.

There was some interesting information mentioned and discussed on a web site and forum about the use of electrically generated pulse waves being directed into intake runners. Besides providing pressure pulses in towards the open valve, it's supposed to have a beneficial effect on fuel vaporization. Apparently it was outlawed when used in some professional racing. Think along the lines of one or more speakers mounted in the side of the plenum, pointing at the bellmouths of the runners.

 

Nice. Probably F1. They do get crazy.

 

Hmmm. Ok sorta. Apparently, air density and air pressure are on both sides of the fraction that is subject to the sq rt operation therefore cancel out. As pressure increases, apparently so does density increase the same amount. Ok. Got it. The speed of sound (mach 1) for an airplane at sea level is 761 mph. The same airplane at 40,000 feet reaches the speed of sound at 660 mph, 99 mph slower. That's all due to air temperature? Nothing to do with the air pressure, and density, change?

 

I had a discussion about the principal of air density and air pressure affecting the speed of sound within an intake runner of an internal combustion engine with my father-in-law last night after seeing the discussions here. He's an engineer, and while he was the first to admit that he doesn't know the subtleties of internal combustion dynamics, he said the exact same thing that Z has posted. That the temperature would be the primary factor as density and pressure would cancel out. At that point in the conversation, I think we had reached a point where I believed he didn't understand the nature of my question, and he probably believed that I didn't understand the nature of his answer.

I'll post Louie's query/response to him and see what he says.

 

At first, it seems counter intuitive that a change in air pressure would not change the speed of sound. However, density also affects the speed of sound through a medium. Air pressure change also changes the density. Formula wise, they do cancel. I should have realized that, but didn't. Then you have the "Z" factor. When the "Z" speaks, better listen. Air temperature changes air density, but not the pressure. That's why temperature is the only variable that affects the speed of sound in air. However, (on thin ice here) changing air temperature changes only air density in free air. If the air volume in question is constrained in a vessel, then changing air temperature (density) also changes the pressure. When a tire heats up, the pressure goes up. In that case, the speed of sound may remain constant with changes in air temperature. Inside an operating engine intake manifold, or even within an intake runner, the air is more or less constrained within that volume. I'm not so sure air temperature change would have the same effect on speed of sound as in free air. More study and experimentation required.

 

Another thing to consider is that air is never completely dry. At sea level, air humidity is pretty high most of the time. The more water there is in air, the further it is from an ideal gas where it's no longer as simple as PV=nRT.

Dan
'91 928GT S/C 475hp/460lb.ft