Ram Air - Fact or Fiction? Need a science guy here!

[Achilles99]

Read this post:

http://www.vetteguru.com/ramair/

His argument for why Ram Air is a myth seems to have some validity (at least to my limited knowledge). However, it CAN'T be true, right?

 

[Chuck 98 RT/10]

Sheesh, I can’t read all that. He lost me after he mentioned Newton. Seems like a case of semantics to me though. From my days of Pontiacs 25 years ago we all believed the benefits of ram air was more the free flow of cooler air than an actual ram affect. So his conclusions are really old news. I do remember however, that there were carburetors that weren’t meant for ram air and carbs that were. Still it was the free flow of cool air that everyone cut out their scoops for. Hey it’s GM, whadya expect? Marketing is everything, performance is last.

 

[BruceW]

On the whole I agree with what is said in this. The confusion is what you call ram air. The systems out there that work are providing cool outside air at a pressure equal to or higher than the manifold pressure. That helps performance but how much it helps depends on the overall efficiency of the engine.
On the Viper I think ram air is not providing much of a pressure boost because at 6000 rpm you are sucking air through stock throttle bodies at close to 90 mph, my gut says that you probably aren't getting any pressure boost until you're doing 150+ and even then its a matter of how effective the duct is. . People can argue Bernoulli's equation and the limits of incompressibility assumptions all day long. I only believe test results. One of these days I am going to put pitot tubes on the V and get some real data.

 

[Matt M PA]

I didn't read the link above, but I have brought this whole ram-air thing up before. A newer Viper has a NACA duct, which is not a scoop...but an inlet. An inlet that was designed to create little or no drag. No drag means no ram-air effect. The 69 1/2 Road Runners for example, had a great big hood scoop that was good for ram-air at some speeds. Anyone who forgot to put the hood pins on found out how well that scoop worked when the hood blew over the roof.

 

[Vreracing]

Its an interesting report. Primarily having started my life as an Engineer, most engineers don't write or speak much. I've never seen an engineering report so lengthy.

The bottom line I got from it was that to get a Ram Air effect, you have to be going Mach .5 which would be 300 plus miles per hour.

 

[Mike Brunton]

There is no "ram air" effect at any speed a Viper will be seeing because air is essentially incompressible at car speeds. In order to increase pressure, you need to compress the air and that ain't happening in a Viper.

However....

There are 2 benefits you can get from a "ram air" system.

First is you generally get cooler air. Second is if you tune the intake to resonate correctly you can get a much higher fill in the cylinders than with a standard air intake. "Ram Air" is not a term used to describe either of these effects, so when people say "does ram air work?" the answer is "no it doesn't". Does a scoop on the front of the car have benefit? Yes, at least some (cool air) and possibly a fair amount (tuned intake size) if it's setup right and tuned correctly.

 

[99 R/T 10]

"The bottom line I got from it was that to get a Ram Air effect, you have to be going Mach .5 which would be 300 plus miles per hour."

I disagree with Vreracing on the speed in which it takes effect. I fly the UH-60 helicopter, and our charts that we use to calculate the performance of the helicopter shows at approximately 80 KIAS we start to see a boost in performance due to Ram Air Effect. At 160, (our top speed straight and level) shows an approximate increase of about 4% from 0 A/S. These test are done by an actual flight test, not a calculation. Just my $.02.

Mike

 

[Jack]

My fluid mechanics class was quite a few years ago, but... Not sure I entirely agree with his claim that air is incompressible below .5 mach. Maybe from a practical standpoint, but from a technical standpoint, water in incompressible, air is compressible. There were really two benefits of hood scoops on 70s cars, one was the ram-air effect, it can be argued how on effective it was. The second was cooler air fed directly into the carburator(s) (remember them?) since it didn't mix with hot air under the hood. Cool air is denser, so it helps in burning more fuel. Chrysler really had the best ram air systems. Most articles of the time claim the most effective set-up was the Dodge Challenger T/A and AAR Cuda. The scoop was about one inch off the hood. Scoops flush with the hood didn't function as well, since there is a "boundary layer" of air that slows down as it comes in contact with the hood (friction). The cool air probably contributed more to small hp gains than the ram air effect, but it did look cool.
My roomate in college borrowed a piezometer from the mech lab to try it out on his '74 Trans Am. It did show different pressures, and that Pontiac really did put the hood scoop in the right place.

Jack
'97 RT-10

 

[Vreracing]

Its okay to disagree with me.

I just quoted the Mach .5 out of the article.

When I was in College (I was in Electrical Engineering) I quoted Bernoulli's equation and almost got thrown out of the building.

If you know UT, (Austin for you Tennessee people) at least when I was there, we had our own building and were very territorial. If it isn't Electrical or related to computers who cares.

Interesting discussion. If at 160 there is a 4% increase in power that could be significant on a track.

 

[1TONY1]

My airplane (Mooney) has ram air. Its a 2 1/2" hole in the front of the cowling (lower hood) that has a flap kind of like a gate valve, it is cable actuated. It bypasses the air filter and creates a shorter path to the throttle body. It is worth 1/2" vacuum....= more power. Would it be worth more without bypassing the filter and longer path. Maybe not. It is only used at altitude to keep the air filtered on the ground. I think a lot of the cars do better (like the Viper, ram air T/A) because of the short/straight path to the throttle bodies. If you look at the stock breather filter/tubing on some vehicles there is a lot of restriction....turns, elbows, small inlets etc. It certainly does not hurt to have 100 mph air blowing over your intake point.

 

[joe117]

I've seen this "ram air debunked" science before. It sound's good but, like the man said, it just isn't intuitive.
One question would be, if you have air coming into a scoop of some kind, why not take advantage of that moving air. The air must move to enter the engine intake, and here we are presented with a stream of air that is already in motion. That must be worth something? Right?

 

[jrkermode]

This ground has been covered before, and was started by the same nonsense from a Vette board poster.

Perhaps the problem comes from thinking of ram air as some magical supercharger, which it is not.

Instead, think of ram air as access to air at the highest available pressure. It doesn't matter what the pressure is, just that it is highest available. If you don't believe higher is better, stop reading now. If you don't believe pressure varies over the surface of the car, stop reading now.

Typically, the highest pressures are found right on the nose of the car and at the base of the windshield (it's no coincidence this is where the radiator and vent inlets are located).

A scoop can access this "high" pressure if it's routed to one of these areas. Think cowl induction, or a snorkel scoop.

The NACA duct on the Viper is a little too small and a little bit too far back on the nose to be a reaaly effective scoop.

The inlet temperature associated with ram air is also a big benefit. When you think of a place likely to have high pressure air, it is also likely to be air which has not yet been heated by the cars engine or radiators.