Kento

Then you do know that the hood (where you were originally referring to installing the ram-air duct) is not a positive pressure zone?

Sorry, I don't want this all to sound like your idea is completely without merit. If you feel this is a good idea, you should press ahead with it. I just think you need to do a bit more research before going through all the trouble of fabricating something.

Kento

Aggh...double post. Man, this server sometimes....

FD Seeker

Not trying to be defensive, or offensive, just objective. Don't be se sensitive.

Anyway, without trying to get into semantics, *I think* you can "ram" air into (or just to) a turbo. That is, all else being equal, create more pressure on the inlet side (probably less negative pressure). I also *think* that the result would be more pressure at the turbo outlet.

But, it's already been proven I don't know what the hell I'm talking about.

FD Seeker

That's the first I've heard that the hood area I mentioned was actually a negative pressure zone - although I think that was inferred at some point. Makes sense, I guess. Bummer.

bigmack000

damn i wihs i could put what in my mind in words. but yeah you can up the boost because your sendign more air to the turbos. the restiction the more air goin to them. but you saw when he made stuff bigger how it hurt the flow more. if you have positive pressure that means there will be a steady flow of cool air that the turbo doens't really have tp pull in because its allready tryign to get in there so less work on the trubo. that the best i can put it into word. not good whit exsplaning what i under in words.
joel

areXseven

iTrader: (1)

The Turbo (as it's designed in our Fds) works by and with the exhaust flow. The spool and spool RPM is created by the Exhaust as it hits and passes by the Turbine (exhaust side) Wheel and out through the manifiold into the Pre-Cat, or DP...so forth.

The Turbo Wheel blades will have a limit as to the volume it can create, regardless of the amount of air flow "wanting" to go into the Compressor (intake side) Wheel.

Kinda think of it as a Large fan blowing into the back of a smaller fan set to a specific speed. The smaller fan will not have an increase in speed(because it's set) and and the flow coming out of the smaller fan will not have a significant gain in air-flow?? YES - NO??

Also, The FD Hitachi Twins are made to operate at 120-125K RPM safe maximum. Any significant increase in spool RPM, .. you'll be seeing hot little Turbo parts exiting the Muffler.

FD Seeker

Good info and examples. Thanks!

I understand that the turbo rpm is dictated by exhaust flow, but can't other factors alter the rpms? I'm thinking that friction, or pressure differentials on either side of the compressor can and will alter the actual speed of the turbo.

I think the same factors apply to your fan example, so I may try that and see if I notice an increase in speed of the smaller fan. I think I might!

Thoughts?

FD Seeker

Here's an interesting question:

If enough air were forced through the (intake side of the) turbo while the engine is not running, would the turbo begin to spin? (assuming an unclosed system between the turbo and engine)

If so, does this mean anything?

areXseven

iTrader: (1)

The number one factor that will decrease/increase spool rpm is the Exhaust flow created by the Engine. A lower engine rpm will spool the Turbo slower than at high engine rpm. Turbo mechanical condition ie; bad bearings, broken/bent blades, clogged Compressor/Bearing housing will play a major roll in Turbo performance as well. Even a little exhaust leak will deprive the Turbo from it's total performing capacity.

areXseven

iTrader: (1)

Of course. Spinnig will occur. But that example does not apply when the Engine is fired. When the engine is running, exhaust is flowing, then the Turbo is spooling at a set speed, according to the amount of exhaust flow created by the rpm the engine is making.

FD Seeker

So, I'm saying that if you can optimize the other factors (again, more pressure at intake side), then you will get more boost pressure. I think the question would be: How much?

areXseven

iTrader: (1)

Boost will increase if back-pressure on the Turbine (Exhaust) side is relieved. That's why when you install a DP or MP or a straight flow muffler, you'll usually end up with excess boost issues. Opening up the exhaust flow, means more flow throught the Turbine, which equates to more spool RPM, which should create more Boost.

Bio-Weapon

I'll try to reword what was said a bit: Think of it like this... You can only swallow at a certain speed, no matter how hard you try. There will always be a limit to how fast you can swallow. Having someone shove food into your mouth (but not down your throat) isnt going to increase your swallowing speed, it will just let food build up in and infront of your mouth. Now think of the entry to your throat as the front of the turbocharger. It can only take in so much air, so stuffing its face with more air isn't going to make it take in more air. It has a limit based on the internals of the turbo. The only benefit, as jim said before, would be taking air thats cool, as opposed to hot engine bay air.

Fred Sickert

Not an FD, but
http://www.autospeed.com/cms/A_0629/article.html
pretty much covers what you are talking about.

Kento

A aerodynamic "positive pressure" zone is an area where airflow "builds up". In order to really realize the benefits of ram-air, you need to position your intake at the point of highest positive pressure, which in the FD's case would be at the leading edge of the nose. If you just put a "scoop" in the hood, you would not be creating a positive pressure zone (unless you did some sort of wacky intake that totally protrudes above the hood, and then the aerodynamic drag would cancel out most of the benefits). Just because an area is not a "positive pressure" zone does not mean that it's a negative pressure zone.

Kevin T. Wyum

I've been dreading responding to this thread anymore for fear of being sucked into hours of typing. The thing that seems to be missed in all of this is that the amount of air that will be moved by each compressor blade is not fixed. We're talking about a compressible medium. The lower the pressure in front of the compressor wheel the lower the pressure will be at each blade of the compressor, meaning less air. The fan example is actually a very good one. If you place a large fan blowing into the back of a smaller fan with a set amount of current being delivered to the small fan (such as having a set amount of exhaust flow to the turbine) it will speed up, delivering more air. Alternatively if it runs at a fixed speed it will require less current to turn it and actually deliver a slightly larger amount of air.

Of course we run at a fixed boost because of a wastegate, but if more air is delivered because of availability per compressor rotation more exhaust can be diverted through the wastegate causing less backpressure and better combustion chamber gas expulsion and less resistance to rotation of the motor, essentially raising efficiency. I of course can't say anything for certain because as I'm going to point out this is a ridiculously complicated issue. Due to time constraints I'll just try to highlight some issues, not providing any answers.

Increasing pressure in front of the compressor may increase the volume and hence mass of the air being carried by each blade. To change the direction of mass requires energy proportional to mass. The energy in this case comes from the exhaust driving the turbine. It's possible that the increase in mass and slowing of shaft speeds require the same energy of the turbine as the faster turning blade, turning a heavier wheel as opposed to turning a lighter wheel faster.

Then we have the frictional losses from higher shaft speeds at lower compressor supply pressures.

Combustion chamber contamination from higher backpressure reducing the A/F charge entering.

Turbulent airflow in front of the compressor causing unpredictable variations in density to each blade.

Some have mentioned the compressor speeds being so high that it somehow negates any higher pressure "air supply." Are we really reaching the absolute vacuum of space before the compressor wheel? Clearly that isn't the case or you couldn't raise the boost.

14 psi of boost is not 14 psi of boost, a less restricted system will actually deliver more volume at the same psi. The easiest thing to do is probably cut and paste a post Paul Winter made on the big list last week which was probably the best written explanation I've seen in a long time. I'll stop here, my turn to watch the kids : ).

Portion of Paul's post.

"It is a good thing to remember that BOOST is only a backpressure measurement
(how much the setup downstream of the turbo is resisting airflow from the
turbo outlet) and a worthless number EXCEPT in the context of a single,
completely defined mechanical setup. A complete definition includes the
parts list for everything upstream of the motor that isn't stock (eg piping,
IC, intake config, any turbo change, etc), same for downstream of the motor
(downpipe, midpipe, cat and catback, pipe diameters, etc) and the motor
(porting, or using a higher than stock redline or shift point). Or better
yet, an actual airflow/rpm curve for the setup (NOT just calculated engine
airflow) that is, of course, generally not available.

AIRFLOW is the real deal, not boost. Comparing boost numbers from different
setups is the first major piece of foolishness most folks do on the way to
blowing their engine. Its oxygen (not a boost number), fixed at 20.9% in
air, that is mixed with fuel!! And its too little fuel compared to oxygen
that you're worried about first and foremost (and maybe ONLY given the
rarity at which other things except overheating kill motors). Boost related
detonation is very uncommon compared to motors killed by lean mixes and
that's part of the reason that most of us no longer carry things like
enhanced knock sensors in our cars- they help only in a very few instances
and are no substitute for getting AFR correct. We've finally got the ECUs
and the tools to ensure good AFR commonly available so lets use them!! (Or
maybe you really like being a trog who turns carb screws, changes jets and
hopes for the best? Heck, even a lot of those guys are starting to use the
tools)"

areXseven

iTrader: (1)

Okay. Well, that settles that argument! Finito

FD Seeker

I assumed that because you stated the area was not a positive pressure zone rather than stating it was a neutral prezzure zone implied that it was a negative pressure zone. Apparently, I zoned out.

Fred Sickert

"wacky intake that totally protrudes above the hood"

You mean like, a hood scoop ?

The WRX seems to run ok with one.

FDNewbie

iTrader: (10)

Man...I just wanted to tell everyone who's contributed to this thread how much I appreciate it...cuz I've learned a GREAT deal from it.

I personally really enjoy such threads because when ideas get kicked around and scrutinized as they were here, you really get an in-depth understanding of just how things work, as well as how they DON'T work. And for newbies like me, that's simply priceless. So my hat's off to all of you

Kento

Negative, I meant like the shroud of a blower like a funny car/fueler.

Now install a manometer in the WRX's IC duct and see how much positive pressure you're actually getting, compared to having a duct directly on the front bumper. To get the most benefits of ram-air, you need to have the duct as close as possible to the highest aerodynamic positive pressure point on the car.

DamonB

That's why stock cars for instance plumb their intake to the base of the windsheild...

Fact is you can get high pressure air anywhere you wish on a moving car. The vehicle is moving through the air at some velocity and so the air in relation to the vehicle is moving at the same velocity IF you bring your ram air from outside the boundary layer of the vehicle. The boundary layer is the portion of air closest to the surface where friction literally slows the air down and causes turbulence among other things. The thickness of the boundary layer depends upon the aero design of the surface and every surface in front of it. The nose generally has the cleanest air; that's a given.

All you have to do to get "good" ram air is raise the duct outside the boundary layer of the surface, that's why you frequently see things like engine intakes or brake cooling ducts mounted to snorkles on the vehicle rather than slapping the duct right onto the body work.

If you have a good laminar flow (the air flows smoothly along the area) a NACA duct offers the lowest drag and yet the same flow characteristics as a large duct sitting out in the airstream.

NACA duct


Duct on snorkel to receive ram air


A truly well designed and elegant duct can actually provide a postive thrust to the car. Think of an oil cooler in a racecar for instance. The mouth brings the air into a slowly expanding duct before the oil cooler (this slows the air down and reduces the pressure, giving the air more time to absorb heat from the oil cooler), the air passes through the oil cooler and has energy added to it from the heat exchange that takes place and then another duct behind the oil cooler slowly constricts again to speed the air back up before releasing it outside the bodywork. Since energy was added to the air (heat from the oil cooler) there is no reason at all that we can't actually make power from this duct. The problem is that this takes quite a lot of room inside the car and nothing short of a very high speed racecar would see much benefit from it anyhow. The beauty of it is that the faster the car goes, the more thrust the duct makes.

ZeroBanger

it looks like my old thread from about a year and a half ago was deleted, but I did extensive testing at the drag strip and the short story is I found that by ramming air under the car into both the M2/rx7fashion cold air box and the stock air box in both instances I gained a consistent 5-6 mph on the trap. This was done using averages from about 60 runs at the strip.

pugg57

basic Fluid dynamics here guys... the turbos are creating a pressure gain of 10 psi. running a normal intake, we'll assume atmospheric pressure exists at the intake. this means the engine will see 10psi above atmosphere. if you increase the pressure infront of the turbos to say, 1 psi... the engine will now see that 1psi plus the 10 from the turbos... = 11psi above atmosphere.

the big fan infront of the little fan was wrong... if they were ducted together, you would be increasing the pressure between the fasater flowing front fan and the lower flow set rear fan. this means that the velocity of airflow from the rear fan will stay the same as without the the front fan, HOWEVER, do to that pressure increase (this applies to the turbos also) you will be putting through a MORE DENSE slug of air, i.e. increaseing the MASS FLOW of air, which is what we are trying to get with forced induction... its the MASS that counts, not the pressure, pressure is just an easier way to measure the difference.

the ideal gas law states that pv=mt where p is pressure, v is volume, m is mass, and t is temperature. if you keep v and t constant, and raise p this means you get more M.

secondly, the hood scoop on the WRX Sti, i believe, is actually only used to force air through the intercooler thus adding more hp.

bigmack000

^ just put everyhing i was thinking , but couldn't put itnto words down. and yeah the sti scoop only for ic.
joel