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Almost every motorcycle I have seen has its air intake mounted such that it is sucking in air from the direction opposing travel.

According to fluid dynamics, if the intake is made to face forwards, the engine would benefit from ram-air effect, which can result in higher engine power output.

Now the Wikipedia article does say:

While ram-air may increase the volumetric efficiency of an engine, they can be difficult to combine with carburetors, which rely on a venturi-engineered pressure drop to draw fuel through the main jet. As the pressurised ram-air may kill this venturi effect, the carburetor will need to be designed to take this into account; or the engine may need fuel-injection.

Surely a carburetor can be reengineered to suit the change in pressure profile across the intake? Plus, even motorcycles with fuel injection have their intakes point backwards.

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    I would take issue with the wiki article in that if this were exactly true, there wouldn't be supercharger applications where it just uses a hat nor would there be ram air applications in the first place, like the '69 Pontiac GTO. Neither of these applications have redesigned carbs to compensate.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Aug 25, 2015 at 12:06
  • Why would you re-engineer the carburetor under these circumstances. Venturi effect performance is based upon differences in pressure. These motorcycles aren't competing in motogp they are driving down the road with other vehicles.
    – DucatiKiller
    Aug 27, 2015 at 16:37
  • @DucatiKiller : I do agree with you. But my point was that even if the changes were that extreme, reengineering the carb doesn't seem like a big deal to a motorcycle manufacturer.
    – Zaid
    Aug 27, 2015 at 17:22

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First of all by "most production motorcycles" I am assuming commuter level motorcycles used in South and South East Asia

its not true that all production motorcycles have intakes facing backwards, its more or less a ergonomics based design decision , some motorcycles have intakes facing 90 degrees to the side. Some have them facing towards the back as you say.. For example the intake duct location on the TVS Apache Series.

enter image description here

A common point in all of these mass production motorcycles is that they use carburettors

Your point on Ram air effect on carburettor is correct it will cancel out the venturi effect but reingnieering the carb to make use of the Ram air effect only works if there is a specific way air can be channeled to the intake via a duct or port which is not the case In case of a commuter bike..

  • secondly Ram air effect will work only when you achieve a specific speed , most production motorcycles are not designed to go at high speeds to use the advantages provided the Ram air effect .

Update: There is literally no point in making a intake face in the forward direction when the engine is at the back of the intake , you will need additional space to bend the intake tube which will create ergonomic issues and frankly there is no need to do the exercise , iterating my previous point,(please dont mind the bad drawing)

enter image description here

  • You neeed a specilized channel or duct to force air to the intake which is not there in these commuter segment bikes which are mostly naked.
  • These bikes do not have the necessary top speed to achieve the advantage of the RAM air intake effect. Most of these bikes have a top speed of 60MPH.
  • Agreeing to the points mentioned by ALLman, having the intake facing in the backwards direction also prevents weather and road debris from entering the engine through the intake.
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  • Yes, I meant commuter-level motorcycles. Can you give some examples of mass-produced motorbikes that have intakes at 90°? I agree that ram-air has an impact at higher speeds, but I guess my question is why it is not possible to have forward-facing ducts on commuter motorcycles. What's stopping manufacturers from making them that way?
    – Zaid
    Aug 25, 2015 at 7:56
  • The TVS Apache RTR 160 has side facing intake duct..
    – Shobin P
    Aug 25, 2015 at 8:14
  • I'm thinking about this. A side-facing duct is not much better than a reverse-facing one from a pressure perspective. Why not angle it and make it forward-facing?
    – Zaid
    Aug 25, 2015 at 8:18
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    Rainwater, dirt and bugs also comes to mind
    – Allman
    Aug 25, 2015 at 10:56
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    @Allman Already done bro , thanks, through a minor thought, makes a huge difference.
    – Shobin P
    Aug 25, 2015 at 13:31
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You guys are looking at it from the wrong angle...

It’s not about the carb or the ram sur effect, it is all about the exhaust.

The exhaust side of the cylinder is the hottest side so it needs to be facing forward so it’s gets a bit of cooling.

Plus if you’d put the exhaust facing backward it could get too warm where the cables and electronic are, under the seat.

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  • I agree that this is a packaging issue. I have a v-twin (not HD) that has one intake nominally aimed forward and the other to the back. The exhaust is the same, one forward, one back. The intake air actually comes from above into both intakes. On an in-line multi-cylinder motorcycle I think having the exhaust in the back would cause heat-related issues, plus the carburetors/fuel injection/intake would be exposed to rain, debris, solid objects, etc, hanging off the front of the engine.
    – Tim Nevins
    Nov 21, 2018 at 20:43
  • This is not much of an issue in low displacement liquid cooled engines, they dont get as hot as lets say a Harley. Like for example the BMW G310R , all comes down to the packaging costs.
    – Shobin P
    Jun 13, 2022 at 6:46
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Perhaps a forward facing intake would be good for the engine but the disturbance it would create in the air infront of the bike would be quite bad for handling, straight line stability and completely negate the effect of the fairing.

On a basic commuter bike, the problems with a forward facing inlet would be compounded by bikes that are ridden in all weathers. In heavy rain they would be susceptible to ingesting moisture and in low temperatures they could suffer from icing

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    While this is a good thought, just like the bike not traveling fast enough to really enjoy the benefits of the Ram Air effect, the small cross section the inlet would provide will not damage aerodynamics, either. The bike just isn't going fast enough to develop any issues. Besides, the rider is by far the worst impediment to aerodynamics on a bike. Their cross section is huge ... :-)
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Aug 25, 2015 at 12:13
  • True, I guess the real reason is to prevent carburettor icing.
    – Steve Matthews
    Aug 25, 2015 at 12:15
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In my opinion while designing the vehicle, designers consider the density of air coming into the air box. Because of the vihicle design air boxes are fixed behind the engine. So the air in front of the air box will be coming through the engine cooling fins. Which make it hot and reduces the density. Inordar to get cold and fresh air designers will never provide the air inlet mouth directed towards the engine.

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    "In my opinion". Can you please give some evidence to support your opinion.
    – Chenmunka
    May 19, 2021 at 14:28
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Kawasaki introduced “ram air” in the 1990s with the ZX-11 (US, elsewhere the ZZR-1100), and from there it appeared in other models.

The Zx-11 was carbureted at the time, which required the ram air intake (which led to increasing pressure in the air box) to have an additional air path from a central tube to pressurize the fuel bowl. So if you see photos or diagrams, that’s what the central tubes are in the front air intakes.

It was marketed as a performance feature.

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