Discussion in '4-Stroke Engines' started by ZnsaneRyder, Nov 22, 2008.

  1. ZnsaneRyder

    ZnsaneRyder Member

    I notice that when you are going fast on your bike, that around 25-35mph or more the push from the wind against you feels really strong.

    It would be nice to use some of that resistance to help you go faster. I've been thinking of using a cone of some sort with a tube to the intake for the Ram-Air.

    I figure it may make more HP, and probably will need the carb tuned a bit richer, but that's no problem. I'm also wondering how much it may help MPG.

    Also I wonder if it would be as beneficial to a small engine as it is to a large engine.

    Has anyone else here considered a forced Ram-Air setup before?

  2. arceeguy

    arceeguy Active Member

    "Ram-Air" is pretty much a marketing gimmick, I wouldn't waste my time.

    For Ram-Air to be the least bit effective, you'd need to be traveling a lot faster than 25mph. (100+ comes to mind)
  3. mabman

    mabman Member

  4. Simonator

    Simonator Guest


    Ram air isn't effective until well over 100mph. Maybe over 200mph. The engine is sucking in air way faster than it can be pushed in. it is effective on a car because it draws in cooler air as opposed to warm air in the engine compartment.
  5. RdKryton

    RdKryton Active Member

    Let me tell you what happens on a Whizzer when you ride into a strong head wind. The NE5 air filter cover will actually force enough air into the carb to lean it out to the extent that it stalls the engine remove the cover and the problem goes away. These carbs are not able to compensate for the extra air and if you jet them rich you will be way out of tune unless you are at top speed with wide open throttle. It's just not worth the hassle in my opinion. There is a thread about this. http://www.motoredbikes.com/showthread.php?t=9049
  6. arceeguy

    arceeguy Active Member

    If a Whizzer crapps out in that manner, I suspect that it just has a very sensitive main fuel circuit. Even a slight increase in pressure from the "Ram Air" effect will have the undesirable effect of eliminating the "Venturi Effect" which is how the fuel gets drawn up from the fuel bowl and into the carb throat. (If there is positive pressure in the venturi relative to the fuel bowl, air will get pushed into the fuel bowl) So it isn't so much that the carb cannot compensate for the extra air, it is the extra air pressure essentially shuts off the fuel supply. This is why a carbureted engine with a turbo or supercharger draw air through the carb before boosting it.
  7. RdKryton

    RdKryton Active Member

    I stand corrected in my theory but the result is the same I believe. I just don't believe it will work.

  8. arceeguy

    arceeguy Active Member

    Yup - At least you'll know right away if you are actually ramming air down the intake - your engine will cut out!

    If the engine were fuel injected, with an air flow sensor, you might just see a small increase in power but nothing really significant until that 100 mph mark.
  9. ZnsaneRyder

    ZnsaneRyder Member

    When I removed the airfilter cover off my engine as stock, it gained much more torque and revved quicker, and deeper note with a load on it, similar to how any car engine has been after an intake mod.

    With the cover on, it breathes with just two small holes, one near the hot air by the cylinder fins and another small one opposite of that before getting to the air filter. Now with the cover off, the filter is in the open and the engine runs best. Wind blows right by the filter, but not into it. Airflow does make a lot of difference, however I don't want to overdo it.

    I see what you mean about the suction needed from the carb, so you can't shove a lot of air down the carb for it to still get gas. I'm just wondering with a slight bit of forced air, but not enough to kill it, if it will increase the velocity of what's already being sucked in to aid with total flow. My engine runs out of peak powerband RPM around 47-49mph, so I have to hold it at full throttle @ 46mph, and it slowly creeps up to 49mph because the revs are high, and it needs to breathe more to go faster, and I'm going to get a better filter to help, but want to reduce what bottlenecks I can to gain a bit more. I'm not going 100mph, and I'm not wanting a ton of air, just enough to make a difference and raise the RPM some.
    Last edited: Nov 26, 2008
  10. Honda50

    Honda50 Member

    The way the air stream enters a carb is important. If the air enters the carb as a straight stream, it enters the carb's venturi in a more orderly fashion. It also builds inertia before entering the carb venturi. This will intern provides more concise airflow into the venturi at wide open throttle and more even velocities means even fuel mixing in the main venturi as velocity and pressure remain stable. At wide open throttle, ambient air pressure is pushing the air into an engine. It is not sucked in. Since an engine is an air pump, improving the intake side should be met improving the exhaust side.

    Ram air would only benefit engine performance at wide open throttle and high speeds (as mentioned). In four stroke engines, there is a point at which both intake and exhaust valves are open at the same time. This is known as valve overlap and is measured in crankshaft degrees. Since regularly aspirated engines are not 100% volumetrically efficient, meaning that some of the exhaust gases remain in the cylinder, diluting the incoming air-fuel mixture. The idea is to increase the engine's volumetric efficiency. This can be done by finding ways to evacuate as mush inert exhaust gases as possible. The engines camshaft profile and valve timing in relation to piston travel comes into play here. Stock cam profiles are designed to find a middle ground in terms of performance and reliability. Attempting to increase volumetric efficiency without addressing other known 'choke points' will result in possibly lesser performance and upset the balance of the other components working in concert with one another. In some applications, a small width spacer placed between the carb and head will at a certain rpm, will aid in airflow into the valve port. No 'trick' works throughout the entire rpm range.
    Now, if you increase volumetric efficiency, the airflow through the engine is more efficient, resulting in more air or denser air. Theoretically we have more air entering and exiting the engine. Suddenly the stock carb does not match the new airflow or remains a major restriction to the improvements just made. See where I'm going?

    When squeezing more performance from an engine, it's a trade off game. Every advantage has a disadvantage. It's a fun game to play. It involves a lot of basic physics and math, trail and error. It's a lot of fun just the same.
  11. ZnsaneRyder

    ZnsaneRyder Member

    I'd have to differ about the first statement about WOT. The throttle adjusts the allowed rate of airflow, and WOT is just max flow. I can clearly run an engine at WOT and it will suck air in clearly shown when I put my hand by the intake of an engine. Atmospheric pressure always pushes air into an engine, because of the suction.

    Also ram air only making a difference at or near WOT is EXACTLY WHAT I WANT. I'm looking to gain RPM range, as I have more than plenty of low-end torque. I even lowered my clutch to 1800RPM to take more advantage of that torque, and also allow lower speeds without slipping the clutch. However, with ram air, I want this baby to rev as well. More low and high speed range = driveability.

    Now the stock carb and exhaust, I'm aware they can be improved. I clearly understand how that works. They even sell adaptors for a GX390 carb (13HP) for my engine to increase HP. But increasing airflow helps no matter what size carb, and I obviously would upgrade my carb when I have the money since I want performance anyway. I'll upgrade parts as I go along. I'm just using what I have now, and enjoy the learning of hot-rodding an engine. :biggrin:

    Happy Riding
  12. arceeguy

    arceeguy Active Member

    So is it sucked in, or pushed in, or pushed in by sucking?

    Air moves from one place to another because of a pressure differential. The piston creates a low pressure area and air is pushed in by atmospheric pressure. The "Ram Air" effect you seek is to raise the atmospheric pressure at the intake of the carburetor to push more air in every stroke. Superchargers and Turbo driven superchargers do this mechanically.

    I still maintain that any significant pressure increase will cause the mixture to lean out and you will lose power. There must be a negative pressure differential for fuel to be pushed into carb throat through the main jet. If you pressurize the intake, you will create a positive pressure in relation to the fuel bowl and fuel will cease to flow into the engine.

    Removing the factory air cleaner and replacing it with one with a lower restriction will make more power because air will now be pushed (sucked or whatever) with less restriction, but the pressure drop in the venturi will still be there and fuel will be drawn into the venturi.
  13. Honda50

    Honda50 Member

    Say, some of you guys might be old enough to remember the old muscle cars of the 60's-early 70's. Remember the ram air set ups? It was a marketing gimmick. The engines were NOT benefiting from more air and fuel getting 'packed-in', it was the benefit of cooler, more dense air. Another trick was to cool down the fuel temperature to in turn make it dense as well. Cool Cans were used. They consisted of the fuel line coiling up inside a coffee can filled with ice, to chill it before heading out to the carb.

    One misconception was to add a larger cfm carb, thinking more power could be gained. It works up to a point. The name of the game is to keep the venturi air velocity high throughout the rpm range if possible. The engine may like it at WOT but will probably fall on it's face at sudden lower rpm throttle opening unless some sort of power enrichment is incorporated to compensate until velocities pick up again.
  14. Honda50

    Honda50 Member

    As the piston travels downward on intake, a negative pressure area is formed on the head surface of the piston. Pressure travels from high to low and wants to equalize. At sea level the pressure is about 14.7 psi., depending on the weather. The weight of earths atmosphere rushes in to fill the void. A simplified term for this is 'suction'. To measure that 'suction', a vacuum gage is used and is read in inches of mercury. That measurement is taken below the throttle plate with the engine running or in some cases, while cranking the engine for test purposes. A healthy engine will have between 17 and 21 inches of mercury at idle (17-21 Hg.) 'Snap' the throttle to the WOT position and it will momentarally drop to zero. It drops to zero since the restriction was removed (throttle plate opening rapidly). What really happens is that air flow actually stops and then catches up. If left at WOT, the carb venturi becomes the new 'restriction'. On car engines we have an enrichment phase (accelerator pump) to compensate to prevent a stumble until air velocity resumes and fuel is again 'pushed' into the main jet and into the engine.

    Super charging or turbo charging effect would assist in reducing air velocity decline at sudden throttle opening, thus continuing to take fuel with it. Once we 'force' air into the engine, it is no longer 'normally aspirated' and a new set of rules comes into play. It really can get interesting with all the science involved! There are certain physical rules that cannot be ignored.
    Last edited: Dec 1, 2008
  15. DesmoDog

    DesmoDog New Member

    As for the engine leaning out because of the raised pressure from the "ram air" the solution is simple and used in blow-through turbocharging. You vent the float bowl so it sees the same boosted pressure as the intake. Pressure will still drop as it goes through the venturi so you still get flow. Blow through turbos on carb'd engines might put the entire carb in a box, then use the turbo to pressurize the whole box.

    The venting of the carb can be very sensitive. I've heard of a case where vent tubes on motorcycle carbs were routed wrong or maybe the airbox was installed incorrectly - in any case the pressure the vent "saw" was lowered and the carb stopped working at higher speeds.

    All that said, I have seen results on a dyno from ram air below 100mph, more like 70 IIRC. But this is with a well designed intake system. It takes more than the intake being aimed towards the direction of travel to get a decent ram air effect. I don't have the math handy but at the speeds these bikes go you're seeing something other than a true "ram air"** effect if simple mods make the bike run better.

    **By "ram air" I mean you are pressurizing the intake air to above atmospheric pressure, not just lowering the restriction of the stock intake.

    BTW - The engine isn't sucking anything in. It's being pushed in by atmospheric pressure as already mentioned. If we say the piston is sucking in the air, then do we also think that putting an ice cube into water sucks the heat out of it? Entropy increases. Nature abhors a vacuum. Heat transfers from high to low temperatures. Air travels from high to low pressure. The only reason we see vacuum or "suction" in a manifold is because we're measuring gauge pressure, not absolute. If we measure absolute pressure, a vacuum would be zero. Nothing's there. If the container leaks, this "nothing" doesn't pull anything in - the pressure normalizes by the high pressure pushing into the no/lower pressure zone.

    And so endeth my rambling for the evening! :biggrin:
  16. Honda50

    Honda50 Member

    Nicely put. I find the air box thing-a-ma-jig theory very interesting.
  17. arceeguy

    arceeguy Active Member

    Yes, this is true but in a gravity feed fuel system, you might end up blowing bubbles in the gas tank if you got enough "Ram Air" effect. Point is that it is not worth the effort, even if you went through the trouble to build a box around the carb to "pressurize" the fuel bowl and intake.
  18. DesmoDog

    DesmoDog New Member

    I'd agree it's not worth the effort, but only because you won't generate any true "ram air" to begin with. If I thought you could generate enough pressure in the system to blow bubbles in the gas tank, I'd say it WAS worth the effort! (The fix to gas tank bubbles would be to vent the tank to the the same place you're venting the carb)
  19. ZnsaneRyder

    ZnsaneRyder Member

    So it seems, just have a good flowing intake and filter, and you are good to go.
  20. Jax Rhapsody

    Jax Rhapsody Member

    Wouldn't want vapr lock.

    Ram Air is and isnt a gimmick depending on design. In car terms a simple hood scoop or cowel scoop isnt ramming air it's making it easier for air to reach the engine. either through a direct opening or from back pressure[cowel]. Pontiacs loved the RAM AIR thing, sometimes it was real and other times it wasn't. One way is to have an extra opening like what it called true ram air is when you havee a scoop that pops up via a vacuum solenoid for extra air. either way to actually ram air is a t/c or s/c.