ram air?

[sofadriver]

At what speeds does ram air start to be effective?
(I don't know enough about it to ask any more questions  )

[Milwaukee Midget]

Hopefully Woody will chime in on this one.

I can say it's largely a function of shape and volume, but details are often application specific.

[TheBaron]

Ram effects start to appear North of  80 mph….

Standard Day conditions and 90 mph equals about a  1% pressure gain if the inlet is properly located….

Robert

[Koncretekid]

A. Graham Bell says 1.9% increase in pressure at 200 kph (124 mph) which may result in 1.9% increase in power or more if your intake system was suffering from restrictions before adding ram air.  According to rule of cube roots, this could result in speed increase of .6%, or more. I've looked at it and figure that I might be able to add 1.5 mph to my current 149.9 mph speed, enough to get me over 150.  But, hey, 150 mph is a lot faster than 149 mph!
Tom

[fwillyj]

-here's a chart I made up some years ago with several different methods of measuring the pressure.

-we were primarily running between 130 and 150 mph at that time and we used both an inches of water mechanical gauge and a millimeter of water mechanical gauge---both showed us extremely close to the numbers on this chart in a fully enclosed production car.

-there is a huge variation between trying to get decent believable gauge results between an open car and an enclosed car when using a mechanical gauge----because the internal gauge components are affected by whatever the atmospheric pressures happen to be.

-trying to get decent numbers in an open roadster was real challenge but we finally did get some decent numbers by enclosing the gauge inside an enclosure with a vent tube to a quiet place on the car.


----------------------------------------------------------------------------Milli-  
-------------------PSI------------inches--------------inches-------------meters
MPH/FPS---------RAM------------Mercury------------Water-------------water
-70/103---------.086#-----------.18"hg-------------2.40"h20-----------4.47
-80/117---------.113-------------.23-----------------3.13----------------5.84
-90/132---------.143-------------.29-----------------3.97----------------7.40
100/147---------.176-------------.36-----------------4.90----------------9.13
110/161---------.213-------------.44-----------------5.93---------------11.05
120/176---------.254-------------.52-----------------7.05---------------13.15
130/191---------.298-------------.61-----------------8.30---------------15.43
140/205---------.346-------------.70-----------------9.13---------------17.90
150/220---------.397-------------.81----------------11.02---------------20.55
160/235---------.451-------------.92----------------12.54---------------23.38
170/249---------.510------------1.04----------------14.15---------------29.59
190/279---------.636------------1.30----------------17.70---------------32.96
200/293---------.705------------1.44----------------19.59---------------36.53
210/308---------.784------------1.59----------------21.63---------------40.33
220/323---------.852------------1.74----------------23.64---------------47.70
230/337---------.940------------1.97----------------54.98---------------48.86
240/352--------1.024------------2.14----------------59.72---------------53.22
250/367--------1.111------------2.33----------------65.02---------------57.75

[WOODY@DDLLC]

Ram air pressure is density [lb/ft³] X V² [mph] / 4287

Chris, consider yourself chimed! 

[stay`tee]

fwillyj, those figuers pretty much align with calculated figering for my Kawasaki zx12, 

[Fiatdude]

Put a Turbo on it and you can Ram Air on demand




( Boost dude here and loving it, lol )

[wobblywalrus]

It is difficult to jet a bike carb for a ram air setup.  The big main jets applicable for running into a head wind on a cold day are a lot different than those needed for a hot day and a tailwind.  About six weeks of monkeying around with various ram air arrangements showed me that carbs drawing still and cool air from an airbox were easier to jet correctly.  Some folks with more talent than me might make these systems work, but I could not.

[sofadriver]

Thanks for the knowledge, everyone. I asked this question when I noticed I had two areas on the bike that would be perfect for ram air intakes.
But, as Wobbly said, tuning will be a problem on my build schedule. There are gains to be made with ram air but I think I'll hang on to this info for future use. Like when I'm refining a bike (not building a bike). 

[55chevr]

Having tried ram air a few years back, I can say that is very difficult to size the plenum required effectively.


Joe

[TheBaron]

Intake plenums are sorta like two-stroke expansion chambers,,,, if they are too large too fit on the bike then they are most likely close to correct .

Robert

[Rex Schimmer]

Robert is correct, you want the plenum to have lots of volume. The incoming air at vehicle velocity needs to be slowed down so that the air velocity can be turned into pressure (Bernoulli still works here) also you need to probably have some sort of dampening device (big air filter works well) to control some of the internal turbulence that may be present. If you run a forward facing air inlet that is approx the area of your engine inlet you are asking for tuning problems as the air will be very turbulent right at the engine inlet because it is suddenly slowed down by the inlet restriction and if you are running a carb it responds to this turbulence and probably not at the tune you want.                   Rex

[fordboy628]

Ram air pressure is density [lb/ft³] X V² [mph] / 4287

Chris, consider yourself chimed! 

As a "carb guy" and "tuner", coupled with being a dyno engineer and data analyst, here are some practical thoughts to consider, based on my experience.

Not in any order:

Carb/engine "sensitivity" to "ram air pressure" generally increases with:

1)  smaller engine displacement
2)  smaller main jet diameter
3)  smaller plenum/airbox volume
4)  larger delta V

Some combinations create real tuning nightmares.    And you always need to tune for how the conditions might change.    If you gamble and guess wrong, well, bad things can happen . . . . . .     although this is less likely nowadays, a sudden cold front could melt your engine.

Also:

a)  Make da** sure that any part of the carb that "needs" to "see" the pressure increase has access to it.   This is especially important with Webers/Su's and M'cycle carbs.
     Blocked vents or air bleeds can be a real nightmare . . . . .
b)  You may also need to pressurize the atmospheric vent of a mechanical fuel pump . . . . . .
c)  Bigger plenum is usually better.
d)  Reduced turbulence is always better.
e)  Large differences in airflow or turbulence to cylinders is always a pain.
f)  Come prepared with all the jets/parts you will need, ahead of time.    Figure out what conditions might occur, including pressure increase.    Plan ahead.    Soldering/drilling
    jets can be done, but it wouldn't be my first choice . . . . . .

And I'm sure there is more.    Other guys can chime in with their thoughts.

Guys usually run carbs because they think they are "simpler".      Properly set up EFI is much easier.
Just my 2 cents.
 
Fordboy