Ram air

[janz]

Just wondering why I have never seen a post on this. Would a functional ram air unit work on a 3.2 motronics setup? Seems to me the engine intake air is sitting in a negative atmosphere, engine fan sucking air to cool engine already creates a negative atmosphere for the fuel air intake. Can a ram air setup be introduced or would it mess up the fuel injection, you know mass air flow sensor and give false reading to the DME so the sensors wouldn't work right? Seems a ram air system would contribute to a bit more cooling and better MPG. They seem to do this all the time on muscle cars for a slight benefit why not Porsches.

[Formerly Steve Wilkinson]

Well, the ram air on my Mooney started working at about 150 mph airspeed. Added about an inch and a half of MP, as I remember...

[rusnak]

I thought that the plastic velocity stacks on the MFI units, and the 3.2 intake plenum were designed to compress air, especially above 4K rpm.

[Formerly Steve Wilkinson]

That's not a "ram air" effect, it's a result of the tuning of the air-intake length for the proper augmentation of intake air through ideal frequency--okay, ju-ju to me--of the reaction between the incoming charge and what's happening during the engine's intake part of the cycle. Ram air is putting a funnel out your Pontiac's window and piping the air to the carburetor.

[Flieger]

Yes, the tapered MFI stacks and carburetors' vertical or side-draught venturis increase air velocity according to Bernouli's equation/principle. They are relying on the engine sucking a particular volume of air through a smaller opening, increasing fluid velocity as the density remains constant. This is not a perfectly accurate equation at high speeds, near Mach 1, where the compressiblity of air becomes noticable. The density starts to increse as the air molecules are blasted at the wing like particles in a sand blaster rather than a smooth flow of fluid. Therefore, the pressure changes that accompany changes in velocity are different and do not adhere to the equation. This reduces lift and sets up pressure waves on transonic aircraft/airfoils.

Ram air only works at high speeds and uses dynamic pressure (in the same direction as vehicle velocity) to create more intake pressure and increase the mass of air molecules into the engine.

Get a turbo unless you are going supersonic.

[Formerly Steve Wilkinson]

Well, you hardly have to go supersonic--as I said above, 150 mph will do very nicely--but you're right that there are more productive ways of packing air into an engine.

[Flieger]

Love those Mooneys- the Porsche of the private aircraft world in a way.

Hey they even had Porsche motors in the top models!

[davidbir]

The simple answer is; you have to be going over 80 mph for it to have a measurable effect. You would also need to contour the air intakes to maintain airflow to the engine intake-might make the 911 look like a U-Boat!

[Flieger]

or this:



Note the air ducts on the side of the engine lids/spoilers.

[RWebb]

"according to Bernouli's equation/principle"

No. However, it does involve fluid flow.

You can do the math (wave superposition, usually taught in Soph. physics class) or you can analogize to it as sort like tuning a trumpet or sax or something.

I am going to vote for Bernouli's equation as the most abused physics consept on this bbs...

[Flieger]

The wave superpositon principle would apply to the tuned intake chambers that harmonize the intake reversion pulses (or the exhaust extraction principle in tuned headers).

The Bernouli equation would still apply for the increase in a fluid's dynamic pressure with the decrease in static pressure needed to flow through a smaller cross-secional area of tubing.

The "ram air effect" applies in the high-overlap S engines that develop their highest horsepower at high RPMs. This is due to the momentum of the incoming air charge overcoming the intake reversion pressure from the piston coming up. ?This allows the air to compress a little and cram more air mass into the combustion chamber. This is the increase in efffective compression ratio with speed.

So, "ram air" is still only at high speeds-engine or vehicle.

[rusnak]

I doubt those GT2 scoops are a "ram air" scoop either. Intake too small, too much length, etc. More likely the intake is being moved to a high pressure area to encourage airflow, vs leaving them in a low pressure area, such as janz pointed out regarding the original 911 setup. Think ducted airflow, and that is what you have with the GT2. Otherwise, every scooped form is a "ram air" setup, including brake cooling scoops, and the hood scoop on my Uncle Jim's '67 Impala.

[911pcars]

The key for any type of effective ram air effect is high terminal velocity (of the vehicle). At real-world road speed, the exposed air-box to ingest intake air would create unnecessary aero drag.

And to kill two birds with one stone, the term "cold" air intake is also a misnomer with our engines as well.

Sherwood

[Flieger]

You could put a "cold" air intake outside of the engine bay- where the GT2 scoops are and get cooler air than in the engine bay.

There is a small amount of "ram air" in any forward facing scoop- but the compression is not significant until high speeds.

NACA ducts are the best at reducing the compression of the incoming air in order to reduce drag on high speed airplanes.

[RWebb]

Originally Posted by 911pcars

The key for any type of effective ram air effect is high terminal velocity (of the vehicle). At real-world road speed, the exposed air-box to ingest intake air would create unnecessary aero drag.

And to kill two birds with one stone, the term "cold" air intake is also a misnomer with our engines as well.

Sherwood

That is good, because "cold ram" just does NOT sound that appealing to me....

[Phat Euro Cab]

I'm new to the air cooled world but one of the first things I wondered about on my car was the air intake. I've been under the understanding that cooler air allows the engine to make more power. Since hot air rises why would the intake be pointed right over the engine in the first place? Why wouldn't cooler air be used from the bottom of the car or from a scoop?

One of my other cars has a Ram Air scoop and it is functional in a high pressure area near the base of the windshield. I made a "plug" so I could block off the outside air and drive it in the rain while on a vacation trip. At highway speeds the plug was pushed into the air cleaner assembly even though both snorkels of the air cleaner were open allowing underhood air to feed the engine. Granted the "Ram Air" effect probably isn't really worth much but what about cooler air. A "cold Air Intake" can be purchased for most modern vehicles.

[cdrik915]

This system permit a maximum air intake , just to be careful if it rains........

[RWebb]

just do a search - this has been discussed before and data on actual air temperatures has been recorded & presented - the effects on hp were calculated

nil

[Flieger]

At the velocities the air reaches and the distance/time to get from the grill to the cylinders, there isn't time for the air to heat up appreciably.

If you sucked in air from below, you would get lots of road debris and the hot air exhausted during engine cooling.

[Wayne 962]

The practical application of something like this would be difficult, particularly since the engine is in the back. I don't have too much faith in ram air systems on "typical" cars with the engine in the front. In order to do this right, you'd have to probably have some type of scoop a la the 1998 GT1 cars...

-Wayne