Since there is so much talk of ram air lately I thought I would inject a little sanity.

I did some calculations based off forumlas I found for airplane speedometers, which use air pressure induced by the forward velocity of the plane to calculate their speed.

Taking this forumla, you can figure out the pressure increase you get at various speeds. With each % increase in pressure you get approximately that % increase in air, which gets you about that % increase in power.

Now, this isn't entirely accurate, bends in the intake duct, imperfect seals, and other properties of engine airflow that I don't pretend to understand will almost certainly make the actual power increases much lower than these ideal values.

I do believe though, that this represent a nice approximation of the theoretical max power gain you could get from ram air:

Assumptions from aviation sources:
psi = 1/2 mph^2 * 1.07832e-4
based on P = 1/2 v^2
101mph = .55psi impact pressure
atmospheric pressure = 14.7psi

Speed <> pressure(psi) <> hp increase on a 400hp motor:

0 <> 0 <> 0
60 <> .19 <> 5.2
100 <> .5 <> 14.6
170 <> 1.5 <> 42
200 <> 2.1 <> 58.6


So these varam claims of +40hp, did they actually go 170mph and do a power measurement somehow?
did they do some calculation like this to come up with the theorertical max power gain from their product? if so thats kinda questionable.

of course im sure they will make some claim about COLD air being a benefit too. as if the stock intake wasn't already getting cold air.

hmmm

anyone more trained in this sort of stuff, please feel free to correct me here or add to the discussion.

 

Well, I think that your calculations are a bit flawed. I've seen a reputable publication quoting the RAM air effect as only 1-2% HP increase at 120 mph.

I think it is cold air that drives the HP up. it is not unusual to have a 20-30 degree difference between outside and hood air. That would significantly improve density - thus more HP.

 

here is some more interestin info
http://www.installuniversity.com/ins..._12.262000.htm

regarding temperature, if they achieve a 30 degree temperature drop, that should make for a 5% incresae in air density, and about 21hp

but the stock air intake already sits in front of the radiator shroud, with vents that funnel cool air up there.

has anyone used a temp sensor to actually see what the air temps are like at diff. points in there?

 

An aircraft pitot tube for measuring airspeed works on the principal of "isentropic stagnation pressure". In a moving air stream the TOTAL PRESSURE is the sum of static pressure and dynamic pressure, which is the pressure energy associated with velocity. If we slow the air stream "isentropically" (no friction or heat transfer) to "stagnation", which means zero velocity, pressure will increase, and, this is important, so will temperature.

Since a real pitot tube is not isentropic the deviation from this ideal behavior is handled by calibration.

Now, in an automotive inlet system what really counts is air density, so any increase in pressure is somewhat offset by temperature increase as the pressure increases, so density increase is less than pressure increase. Further, the air in an automotive inlet system is never slowed to stagation - zero velocity - so maximum recovery of dynamic pressure/density can never be achieved, even if the inlet is isentropic, which no real system can ever be.

Any fluid dynamics textbook will have what are called isentropic flow tables where the stagnation pressure, temperature, density, and area ratios are listed in tabular form as a function of Mach number.

At 150 MPH, which is approximately Mach 0.2, the isentropic density ratio is 0.9802, and it would take a good diffuser of substantial area increase to essentially slow the velocity to zero.

This isentropic density ratio means that the maximum density increase is about two percent at 150 MPH with a "perfect" diffuser and the air slowed to zero velocity, but the realities of packaging, and the fact that the airstream never can be slowed to zero in an operating engine means that even this modest two percent increase in density cannot be a approached in a real world design. Thus, there is no such thing as "ram air" in a car.

By contrast, density/pressure recovery is very important in jet aircraft. At Mach 0.8 the density ratio is .74, so density can be increased 35 percent, and the inlets of jet engines are very carefully designed as diverging diffusers to recover as much density/pressure as possible because this reduces the amount of compressor work required to achieve the design pressure ratio.

The differences between airbox/filter systems available for Corvettes is due to variation in overall system restriction (such as size of inlet and detailed duct design), filter restriction, and their ability to pick up air as cool as possible.

Duke


[Modified by SWCDuke, 9:54 AM 5/9/2003]

 

duke - thanks for your post, very informative.

I have people I know with ram air setups and boost gauges seeing as much as 1psi increase in manifold pressure at speed. But I didn't consider the fact that temperature would also increase and offset the power gain.

What is the critical difference between pressure derived from a ram air effect and from say a supercharger? clearly these add loads of power from pressure increases, even without any intercooling.

 

Not possible at automotive speeds!!! This would require an isentropic duct slowing the air to zero velocity from Mach 0.32 or about 243 MPH. Normal pressure gages work on the principal of gage pressure, which references the surrounding static pressure as zero, and static pressures vary in and around a moving car so recorded pressure is going to depend on where the gage is located. In order to measure any increase in inlet static pressure you would have to use a very sensitive absolute pressure gage, and have very accurate ambient pressure data from a local barometer.

Dynamic pressure recovery is "free" assuming you have enough velocity to make designing for it worthwhile. A supercharger is mechanically driven by the engine and takes a lot of power, so for the increase in density you pay a price. If you double manifold pressure from ambient, you won't see anywhere near double the power due to isentropic heating (Boyle's law) and heating due to the less than 100 percent efficiency of any pump, both of which conspire to reduce density. Turbosuperchargers are more efficient than mechanically driven superchargers because turbos recover some of the energy in the exhaust gas to compress the inlet air - energy that is just throw away on a naturally aspirated engine.

Duke

 

Extremely informative. Makes me wish I finished grade school. :hat SO, what would you two recommend for a upgrade to the stock air intake system? Thanks :flag

 

I don't know.

I'd like to do some testing with an air temp gauge to see if theres any gains to be had from temperature drops from setups like vararam or halltech

 

I've already posted info numerous times about IATs with an aftermarket cold-air box. Do a search.

 

thanks, I will

 

If your C5 is anything other than a '02-'03 Z06, install the '02-'03 Z06 airbox cover. It has about four times the opening area. If you have a '02-'03 Z06, I'd leave it be. The restrictive inlet on the all C5s through '01 is why the aftermarket inlets would outperform them. When the '02 Z06 showed up with its much larger inlet the data between it and the aftermarket boxes got real muddy.

Duke

 

Good stuff SW. I'm surprised the air box resonance effect hasn't been used as a marketing ploy yet. I'll be sticking with my stock air box. :)

 

Actually, air box resonance tuning, typically in the form of expansion chambers, is used by OEMs to reduce or enhance inlet sound quality, particularly on four cylinder engines whose inlet frequencies tend to have an unpleasant industrial quality. Not so with a V8 or most six-cylinders!

The inlet runners on the LS engines do take good advantage of inertia tuning between the manifold plenum and cylinder to enhance the mid to high end volumetric efficiency.

The basic rule of thumb for power as a function of temperature is that power increases/decreases about one percent for every 10 degree F decrease/increase in inlet temperature, so the most important design criterion for an air box is drawing air as close to ambient temperature as possible.

Duct sizing should be sufficiently generous to keep velocities well below Mach 0.3 (about 330 ft/sec) as this is the point where compressiblity effects begin to create meaningful losses, and if velocities are kept below 150 ft/sec and the duct does not have any sharp bends or rapidly diverging flow area that create eddies, losses are at the noise level.

Duke




[Modified by SWCDuke, 6:48 PM 5/9/2003]