TheLS1Lover

I was reading an older Car Craft Mag last night and when I came to the letter/question section I fould something somewhat interesting. The question was very straight forward, how does Forced Induction affect Head Flow Charateristics? Now I know the basic stuff, but I'm looking for info from the more experienced i.e. Liljohn. Please discuss.

chiaj144

iTrader: (4)

I asked the same question a while back on the turbo forums. Here is the link.
http://www.theturboforums.com/thread...ead+flow+boost

TheLS1Lover

The dense thing makes more sense actually, I need to think about this some more.

98Z28CobraKiller

iTrader: (17)

It's the same principles. Same goes for intakes. If it sucks NA, it'll still suck under boost.

TheLS1Lover

I figured that much, just curious as to different port work or CNC programs when boost is involved. Something like the "nitrous" exhaust port.

pwrtrip75

iTrader: (8)

I skimmed over that post, no mention of velocity vs. turbulence.
I agree the volume of air will remain the same, just the mass will vary depending on boost. What i didnt see taken into consideration is the speed in which it travels to fill the cylinder will vary depending on pressure. An N/A engine draws the air in, 1psi of boost pushes the air in, 20psi of boost pushes it faster and harder. So if there are any turbulence in the runner I would think they would be greatly amplified as boost increases. What exactly that will affect.... could be a number of things.

TheLS1Lover

Thank you! Thats what I was thinking of, just couldn't put it in words.

chuntington101

From my understanding is any boosted app the air is NOT pushed into the cylinders. It is sucked in like an NA engine only the air density is higher.

chuntington101

N2o is different as you are basically injecting the O2 into the cylinders as a liquid. So the intake ports only need to be big enough to flow the air and the liquid n2o. The exhaust powers need to be larger (in relation to the intake ports) as you are flowing much more exhaust gas compared to the intake air.

Nitroused383

iTrader: (10)

At sea level atmospheric pressure is 14.7 psi. Turbos and blowers compress air to increase density.

pwrtrip75

iTrader: (8)

That is why they call it "forced" induction. Air is "forced" in. Im sure at a small PSI in a big cylinder there might be more air demanded than than be pushed in leading it to still suck it in. But when you start turning up the boost you have more pressure (force) pushing air in and it is being pushed in.

If you are measuring positive and negative pressures (vacuum) you start out with zero. What that zero is in terms of atmospheric pressure is irrelevant.

chuntington101

What's the cylinder presure as the valve opens? I am happy to be proven wrong but very knowledgeable people have told me that the above is not the case. Happy to proven wrong though.

If you think of the event of the vavles opening then what is the engine seeing when WOT? All it's seeing is the air in the plenum is it not? Or dose it see the whole intake track? I thought it was just the plenum. So by increasing the air presure in the plenum the engine will just suck in dencer air would it not?

Really wish I understood this stuff better.

Wnts2Go10O

iTrader: (12)

think of when the intake pulse wave compresses the air in the port against the back of the valve: when under boost, all of the air is forced against the closed valves. the higher the pressure, the more air molecules are forced into a given spot. because boost = resistance to flow, at the same settings a better flowing head will make more power at the same boost level. i would however like to know what effect the compressible mass flow has in boosted apps.


there is also, i think, an effect on the speed of the air moving through. in calculations ive made, air moves at under 1/4 the speed of sound under n/a conditions. how much speed would positive pressure add to the equation?

6.0mustang

iTrader: (2)

nothing on earth "sucks", there is always pressure trying to balance out.

Wnts2Go10O

iTrader: (12)

there is a positive pressure on the back of the valve but, that is when the valve is closed. its more like a motorway backup that finally gets let loose.

Sales@Tick

iTrader: (3)

Some are of the thought that the entire intake tract I.E. cold side piping and all is part of the "plenum" in a boosted application.

pwrtrip75

iTrader: (8)

Example. Take a bottle of coke and put a straw in it. How does the coke get in your mouth? You suck it in. Now seal the straw in somehow, tape, cork... pinch the straw off And shake the living hell out of the bottle. Now suck with the same force, or all you can for that matter. You now have pressure in the bottle. Is it still you sucking that is putting the coke in your mouth? Unless you really suck that coke will be entering your mouth faster than you can suck it in..... Thus the pressure driving it is greater than you can draw it out by sucking.

Same concept for entering the cylinder.

Also, turbulence wise; say the straw had a kink at the top. You sucking probably did not disrupt the flow pattern at all... but when you have a bunch of pressure driving it that coke might be shooting out of the straw and that little kink could be causing it to shoot out of the straw crooked.

Make sense? If the air did enter the cylinder by being sucked in you would never see any pressure inside the intake....it would be a vacuum.

chuntington101

I See what you are saying but what difference is there if the engine is -100 ft below see level or +4500 feet? Same goes for there being boost or not. the engine only see the air in the intake manifold (plenum) and has no idea if this is presureised or not. It's just air at a set density. For the air to be pushed in there has to be a presure difference. If you increase airflow then the residual presure in the cylinders will also be higher. Plus with turbos it will probably be a lot higher than NA thanks to the back presure needed to drove the turbine.

In your system you are using the atmospheare as your base measurement. In a engine it different as the engine dose not see the atmospheric presure, only the air presure in the Plenum. I know that probably makes no sense. Sorry.