gt37vgt

and compress it and compress it and compress it and add fuel and discharge it

V2Rocket

i think hes trying to reference ramjets and scramjets that use the sheer volume of air hitting the fans at supersonic speed to compress it a little bit and make a little more power.

ehall

Those examples don't apply to this application...unless someone has a 951 doing hundreds of miles per hour. No matter how fast a 951 is going, it is still sucking air in rather than having rammed in.

carlege

im gonna have to agree that looks really nice and simple. would be easy with a MAF

951_RS

Yes, but the pressure difference is still there. A really good ram air system might give 1 psi of positive pressure at speed, that's just that much more air going to the turbo, giving it an easier time to suck in that air.
Otherwise you might as well say it doesn't matter what the ambient temperature is because the turbo is going to heat it up anyways and then the intercooler will cool it back off. Just doesn't work like that.
More pressure before the compressor wheel should cause the turbo to spool quicker at least.

gt37vgt

OK looking at the driving light space it would only yield 177cfm @100mph so it's fair to say it's near impossible to make a ram air system to give a noticeable performance gain . But saying increasing vucum in front of the turbo is simply wrong the net affect of this is shifting the demand plot UP on the compressor map with out increasing inlet manifold pressure .

ehall

Turbos pull air on one side and push it on the other. No matter how much air you push towards the pull side, it still cannot pull beyond it's maximum efficiency. So, if your goal is to maximize the efficiency at which the turbo can pull air, just open up it's inlet.

951_RS

I agree with this, but I think the ram air would really just give it more available air as compared to without the positive pressure. So if ever there was a time when the turbo was having to suck harder because the intake tract was restrictive, then positive pressure would help provide more air. Though technically speaking the intake tract before the turbo is never under positive pressure once the turbo is spooled.

V2Rocket

if there is positive pressure in the fenderwell area where the factory pickup is, and the turbo creates a vacuum, the positive pressure would want to flow towards that vacuum, right?

TurboTommy

That is totally not the case at all.
If you drive a car at highway speeds, the tail end of the vehicle has noticable negative pressure around it. So for example, if a turbo were to draw air from this area it's already behind in its' ability to make positive pressure because right in front of the compressor blades is less than ambient pressure.
Conversly, the front of a car at speeds, has slightly higher than ambient pressure. When the turbo is sucking, as you put it, it will never encounter less than ambient pressure.
Look at the all out turbo drag cars. They've got the compressor inlet totally exposed, somewhere in the front grill.

TurboTommy

951RS;
all what you're saying is correct

And yes, it's not like there's huge gains leaps and bounds, especially, if some of us are just placing the air filter somewhere at the front of the car (in the fender, hood sealed area, naca duct, etc)
Real ram air is if you drive the air into a sealed box, which the turbo can draw from, and that the flow area into the box is significantly greater than the area of the turbo inlet.

Also, none of this has anything to do with increasing or decreasing compressor efficiency.
The issue is what pressure ratio is your turbo working at to make the desired boost (the less pressure ratio, the better).

ehall

The outlet is exposed not to utilize ram air pressure, but create as little restriction as possible, to the inlet air, so that the turbo gets max intake vaccuum. In fact, if you look at jets, such as the SR-71, cones are used to reduce the effect of ram air, so as not to create a compressor stall. As the cones go forward, they drasctically reduce the amount of air available to the engines, and infact cause even greater compression and both more power, as well as more efficiency. In an SR-71, you actually go faster to increase range. Too much air and the compressor stalls.
Obviously this is the extreme, but drag cars don't move anywhere near fast enough to gain enough ram air to create stalls, so they just create the largest available opening to allow the compressor to get all that it wants. At some point, that is all that is needed, and more becomes less efficient.

CPR

Good to see ya E

951_RS

Right, but there would still be vacuum before the compressor wheel (in reference to ambient pressure, obviously there won't be a true vacuum). Without the vacuum air would never be drawn in, but pushed through if you will.

I'm pretty sure we're all saying the same thing, but starting from different ends of the same spectrum.
The ones saying how positive pressure would be beneficial are referencing situations when the added psi would increase available air while the ones saying it would provide no improvement are talking about when there is already enough air available without positive pressure.

User 41221

Surely there is a fluids engineer in here that can back me up. The turbo inlet has a set restriction, caused by its size. Short of an external way to compress the air ahead of it, the air will be at roughly 1ATM (depending on elevation). Adding a high pressure source (aka "Ram Air" by most car guys) won't compress the inlet air, it just moves it around and causes turbulence. The air at the inlet is still going to be at 1 ATM.

If the air is at basically 1 ATM, then you are probably thinking that it doesn't matter, right? Wrong. The air flowing in is subject to laminar flow properties, which is weird science all by itself and does all sorts of things that the layperson wouldn't predict. The best way of thinking about this would be to think of air flowing around an airplane wing. Ever have a plane ride with a lot of turbulence? Same thing applies to a turbo. By adding a high pressure source, it causes disruptions to the flow (called "eddies") that increase the internal viscosity of the air and slows the flow down. Instead of flowing through the turbo in a laminar fashion so that the turbo can compress it, the kinetic energy is expending bouncing around against other air.

Theres a LOT of science that I throw at you, but ram air doesn't work with turbos and fluid dynamics will back me up on this. Pick a low pressure source with the coolest air you can find and your turbo will work as efficiently as possible, as long as the inlet source exceeds the capacity of the turbo inlet, which it will pretty much anywhere in the front end of the car.

Its interesting to note (if you are thermodynamics geek anyway) that the opposite applies to radiators and intercoolers. You want a high pressure source, as the turbulence helps with the heat transfer and mixing.

For the purposes of a 951, I think this is mostly an academic argument anyway.

Regards,