Bell-mouthed inlets, eh? Didn't they try that in the 50's on Bonneville? My (admittedly limited) understanding was that, as they increased the hood ram scoops size to increase the 'ram' effect, they were also increasing vehicle drag, and, at the higher speeds they were driving, you were costing most power than being gained.

Cliff notes version- nothing is free, or as easy as it looks-

 

I did note the effects of relative pressure in the cabin by opening and closing the windows at various speeds and found no significant differences on the manometer. (a C5 is vented out the rear left quarter).

The duct openning is 43 si. which alows simple flow of 1500 cfm at 60MPH and 2600 cfm at 100MPH.

I then ran a test logging the MAF, with and without the ducting (ram air). At 6000 rpm, 100mph shifting from 2nd to 3rd there was no significant difference on consecutive runs. Reading was around 430 gm/sec. Im guessing 600cfm.

So my conclusion was that the ram air affect at 100mph that creates no more than 6" of water had no performance gain.

But it still like the looks of it.

 

Hey phantom, I did get carried away with the sarcasm, sorry. And I still have the same question, if ram air does not work, why do so many drag racers use it and why did it came as an option on a car (this could have been the cool factor).
It just seems funny to me that so many people would use something that doesn't work. I can not base any of my observations on facts, they are just observations. But there still is a whole lot of folks putting a lot of time and effort into something that doesn't work.

 

Just because it doesn't knock half a second off your 1/4 mile times doesn't mean ram air is bunk. Any number of textbooks will explain exactly how and why ram air does work. Air speed indicators and boat speedometers use the exact same principal.

The effect for a drag race car is miniscule below 100 MPH and even at 200 MPH can add less than 5% when perfectly executed, so even a Pro Stocker is only going to gain ~20 HP at mid track and maybe 50 HP in the lights.

As has been mentioned, cold air is much more effective unless you're running 300 MPH at Bonneville, so it makes sense to concentrate on it. However there is really no reason not to go for both.

 

Chris 1313 has the correct idea, his theory is that the ram air in addition to bringing in cooler air to the air box is acting in a positive manner to overcome the vacuum of negative atmosphereic pressure created in the air box. His quote is below.

It is not so much the ram air effect (which many have proven is non existant) as it can only get to about what ~.3 psi. It is the idea of unchoking the engine and not causing it to have to suck and not causing a vacuum inside of the lid.

"Ram air" intakes work not by compressing air but by lessing the vacuum inside the intake (lid) along with supplying cold air.


A few years ago at a dyno session I forgot to take out my paper air filter and install my K&N the K&N is obviously more porous and lets in more air and dirt, that's why I only use it for racing and dynoing. Any I didn't have the K&N so we ran without any filter and gained 7-8 rwhp, so my point is that any positive pressure no matter how small it is will help at least to a minimal extent any vacuum created by the engine gulping air.

 

There is something called "total pressure", which is the pressure a fluid (air) will be at if it is slowed to a rest with no losses. For example, air traveling at Mach 0.9 at a pressure of 14.7 psi will have a total pressure of 24.9 psi. The difference between total and static pressure will be lower at low Mach numbers, for example at Mach 0.1 it would be 14.7 and 14.8 psi.

Let's say your car is stopped and your engine is drawing air from 14.7 psi atmospheric pressure. Now let's say the mach number at some point in your intake is Mach 0.2. Since the atmospheric air is at rest, that means atmospheric pressure is the intake's total pressure (with no friction). The pressure at that point in the intake would be 14.3 psi based on that mach number. Or from another perspective, the intake pressure must be that much lower than atmospheric to accelerate the air from a rest to Mach 0.2.

Now let's say your car is moving at Mach 0.2. The atmospheric pressure is still 14.7 psi, but the total pressure of the moving air (relative to the car) is now 15.1 psi. If the air in the intake is still at Mach 0.2 at some point, the static pressure at that point will be the same 14.7 psi.

I find it's easier to think of things in terms of static and total pressure. If you know the mach number, then you know the ratio of the pressures, and vice versa.

Air that's not moving won't start moving unless it sees a pressure difference, then it will start moving toward the lower pressure. For air to get moving from outside the car to inside the engine, it must see a lower pressure inside. If the air is already at speed, there will be no pressure drop to get moving into the engine (minus friction). If the air is moving faster outside the car than it is inside the engine, its pressure will increase as it slows down.

 

Hey I appreciate all your replies, I thought I was crazy putting all this time into ram air for my T/A. And after reading everything about this (more then I wanted or probably needed) Ram Air is probably the wrong term to use, "I am opening the intake of my engine so it can breathe better". Anyway when I am down with the project, I will post pictures.

 

You mean my dad's old tuba bell that I was going to mount on my hood is not such a good idea after all? Dam, all that polishing for nothing.

 

Ram air does work to some extent if done right. A boost of .1 psi is achievable at 70 mph.

this is a good read on the subject http://naca.larc.nasa.gov/reports/1938/naca-tn-631/

to summarize ....

You want your inlet to be no bigger than needed as it just creates more drag if it is bigger. You then want a smooth gradual expansion to the size of the air filter.

A rule of thumb for finding this inlet size at a particular speed is

(engine CFM at rpm for this speed )/ 60 = vel (in feet per second) * area of inlet (in square inches).

Actually you want the area to be slightly bigger than this as the air will compress ever so slightly.

EDIT: Note I made a mistake in the formula it should be

(engine CFM at rpm for this speed )/ 60 = vel (in feet per second) * area of inlet (in square feet).

 

Ram Air = Cooler Air.

 

Ram Air = Cooler Air, with a Marketing spin to it.

Max psi increase from any inlet size that will fit on a modern car at 100mph = 0.18psi!

Confucious say: Focus on Ram Air only make bullshit flow faster.

Fast car builders focus on truly beneficial changes, rather than marketing slogans. Feed your car the coolest, densest air possible, and move on to something else.

Jim

 

When compaired to the cost and difficulty of getting more power another way utilizing ram air to some extent is worth the effort. Its not something I would loose sleep over knowing my system is 35% short of the theoretical maximum. An edge is an edge.

If you have a turbo the gains are more appearent. You get up on boost quicker. When you are up on the waste gate a .1 psi boost at the air filter means about a .2 psi drop in backpressure. Your intake manifold temps drop a degree or two due to the fact that your compressor wheel doesn't compress air as efficiently as a duct.


EDIT: Note I made a mistake in the formula it should be

(engine CFM at rpm for this speed )/ 60 = vel (in feet per second) * area of inlet (in square feet).

 

So, to sum this all up: You will probably never achieve positive pressure in your intake, but you *MAY* reduce some of the negative pressure (vacuum) by having a functioning ram-air. The real benefit mainly comes from the fact that you are providing the densest, coolest air possible for your engine....

Good discussion, lots of numbers and formulae- I like both the 'hard science' and the seat-of-the-pants approach

 

Back in the muscle car era when "ram air" was popular on factory muscle cars, one of the car magazines such as Car Craft or Hot Rod ran tests on the various ram air designs to see what worked best. Their conclusion basically was that the pressure from such devises was negligible but the cool air was a big plus, 1% increase in HP per 10 deg F cooler air. Because all of these cars were carberated, many of the air cleaners actually sat in a vacuum area of the engine so any system that corrected that was a plus, however, some of the systems themselves actually created a vacuum such as the hood scoop on the '69-70' Mustang Mach Is, not high enough, but still a plus because of the cooler air. The best systems were the ones used by the Olds 442s that brought air in thurgh holes in the front grill or the cowl induction used by the Chevelles. Note that Nascar uses this method. Does this date me?

 

On a friends pickup, We achived a little more than 2" of water on a manometer at 70mph . Thats about .07 to .08 psi. We took the pressure just before the air filter. Because we wanted to see the change we made we had the static vent located next to the air cleaner. We could have played with the inlet sizing some more but decided to leave it.

The only reason I'm here is there are people who have some idea of whats really going on and are aware of the science behind it. I don't even own a GM product (I know I'm going to take heat for saying that).

 

It sounds like you are arguing there would be significant gain to be seen from "ram-air" on a street car ... but then you post at 70mph you found a .07 psi increase ... which sounds pretty insignificant to me. Which is it?

So what kind of HP increase do you calculate that a .07 psi increase would equate to?

 

well, you take some muffler bearings and a flux capacitor and some blinker fluid, heat it up to 55 degrees celcius and not one degree more or less!! this is important because if you do so, you have failed this stupid idea. sorry to waste your time. haha

 

He said he achieved .07 above atmospheric, not a .07 increase if i'm reading it right.

 

This makes sense to me.

Even in you don't have ANY positive pressure, wouldn't just hitting zero be better than SUCKING air?

 

Hitting zero would be positive in relation to the 'current' manifold pressure in
your thought process.

As long as your ram air system is creating a positive influence within the manifold,
then you're doing something right.

I have never seen a turbo, ram-air, or supercharger that lowers the intake
manifold pressure...but it would be funny to see one installed upside down/
backwards (ha)!