Ram Air Post

The following is an extract of a thread about ram air system

Does RAM induction help my car go faster??

This post was made on the board as a reply to a members query about putting in a rampod on his car.

Don't kid yourself that "ram air" gives any real induction pressure boost.

At 100km/h in 1.2kg/m^3 air, stagnation gauge pressure is about 500Pa. That's about 0.07psi.

Stagnation pressure is the maximum pressure at the front of the car - at points where the air flow comes to a complete stop (ie at the point right in the middle, between the areas where air flows under the car and over the car, between where air flows around either side of the car).

If you make an air intake at a stagnation point, it's no longer a stagnation point.

Remember that the engine is sucking in, say, 600cc of air every rev at WOT. Revving at 5000RPM, that's 50 litres per second.

Next part of our thought experiment - make a 10cm square intake (or 11.3cm diameter, take your pick, they're the same area). 50 L/s through this area will be moving at 5 m/s, which is 18km/h.

So, if you put this big-arse intake right at the highest of high-pressure points at the front of your car, the air would only see a "ram" velocity of 82km/h when the car is doing 100km/h at 5000RPM. That drops the ram pressure down to 0.045PSI.

For less than .05 of a PSI, you may as well just bolt on one of those $10 "electric superchargers" you can get off Ebay

Move the intake away from the stagnation point, you loose most of the ram effect.

Make the intake smaller, you get higher intake velocities, you loose most of the ram effect.

Start to include real-world intake and duct losses, and it's all downhill.

Quote:
in the bonnet have a Naca duct

NACA ducts are good, but they can't magically create a pressure difference where none exists. Stagnation pressure is as good as it gets. And you won't get that good on the bonnet.

And remember what we were saying about intake area. You'll need one monster NACA duct if you're planning to draw all of your intake air through it without losing whatever tenuous benefit it gave you in the first place.

Quote:
As well as literally no induction track to create drag, and any performance benifits that a rampod sucking cool forced air may have....

The NACA duct itself will become the restriction, but you're mostly right.

Effectively, you'll have an intake duct as big as the box you have put around the pod, which shoudn't be very lossy at all.

And cold air is very noice in an engine, I must say. I am enjoying the drive to work in <5dgC, 99% humid air...

Quote:
Does anyone think that'd have a problem getting engineered, or am i just completed out of it at nearly 1 o'clock in the morning?

I think you'd have trouble getting through a thorough cost:benefit analysis. Lots of cost, SFA benefit.

But that has never stopped anybody from modding a car before...

tim