Ram-Air

At wide open throttle, ambient air pressure is pushing the air into an engine. It is not sucked in...........

Ram air would only benefit engine performance at wide open throttle and high speeds (as mentioned)................

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?..............

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. :D

Happy Riding
ZnsaneRyder
 
..... Atmospheric pressure always pushes air into an engine, because of the suction......


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.
 
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.
 
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:
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.


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! :D
 
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..........

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.
 
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.

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)
 
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.

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.
 
Back
Top