Question about the air to fuel mixture in carburation/injection that’s been bugging me - why do some sportsbikes have obvious air intakes/scoops e.g. my old ZX7R with two big holes each side of the fairing feeding ducts which go directly into the airbox - whereas other sportsbikes like the Yamaha Thunderace the air box sits under the tank with no big scoops/ducts feeding into it and just a modest air scoop showing?

(I know these bikes are very old school and not cutting edge - but I’m interested in the different approach to induction)

Looking at the two examples you would think that the Ace would be positively asthmatic compared to the Kawasaki - but this is obviously not the case - so why the difference? :slight_smile:

ZX7R AIRBOX (underside - two holes at the front for the intake ducting)

Ace airbox (just a modest scoop on the top that sits under the tank and is not connected to anything)

It could be that large intakes aren’t needed with the EXUP.

Interesting question. My Thundercat has virtually the same airbox but the air intake is via two pipes, from a scoop under the headlight. The 'cat doesn’t have the EXUP valve.

Even the bking doesn’t have ram air and neither did the gsr.
The gsr was a totally enclosed airbox whilst the King has 2 small pipes to breath through. They admittedly end directly over the hot engine about 3 inches away from the airbox.
I’ve done the airbox mod so now it has a cold air feed but not true ram air. It sounds like a Citroen saxo with a cone air filter :smiley:
The bike has lost a little mid range now but I need to address that with a retune once the catalytic converter has been removed :slight_smile:

The ram air increases the air going into the intakes, and hence the pressure of the air up to the head and into the cylinder. The carbs rely on a (predictable) pressure drop in the venturi to get fuel into the air, and I gather that it’s hard to get that right when you’re forcing induction.

I’d imagine it’s easier to get right with injectors plugged into a computer, but that doesn’t explain why they used to make bikes like that but don’t any more…

Yeah - interesting stuff - I get can get my head round ram air for big power at high revs and exup for torque and mid-range - but stil wondering how two sportsbikes have such a different approach to induction - or more accuratley same technical approach - but one just has these two big scoops and the other doesn’t!

It’s all about seeking high volumetric efficiency whilst maintaining the optimum stoichiometric values to produce the best output in terms of bhp & torque in as close to a linear curve as possible… maybe.

:ermm: WTF. Where did THAT come from at 8am on a Saturday? … That’s about all I recall from my college days… well… the only useful thing I recall anyway!

Interesting subject, very interesting…

I imagine a lot of complex calculations go into some of the ‘ram air’ systems… Like you say my 2003 ninja has a complex system with air ducts, little pipes that go everywhere and all sorts. As you carry speed I guess more air gets progressively forced into the system so a balance has to be calculated. I tried running my ninja without the air ducts, and it didn’t run very well at all! All choppy and lost power at about 50mph upwards.

But my Fireblade just has a simple air box under the tank…

Was it Suziki that first brought ram air to bikes with the ‘SRAD’ system?

Suzuki SRAD was the first bike I was aware of with the big air intakes and they made a big thing of ‘ram air’ at the time - but I’m still at a loss as to why some sports bikes have them and others like your Blade - seem to be able to do without them.

The concept of getting as much air into the system to help make big power at high revs makes a lot of sense - but if it so crucial then why have powerful bikes like the Blade/Ace only got modest intakes that are hidden under the tank - not only are they not connected to any external intake - they are also shielded from wind/air blast by the tank sitting on top of them - compared to a ram air bike it looks like they would have trouble getting enough air into the box (although obviously they don’t).

My ZXR750L only has one of those holes at the front

At a guess, because there’s cheaper and more reliable ways of doing it. Predicting the pressure difference you’ll get from the ram intakes is never going to be wholly reliable - there’d be a huge change when it’s pissing it down with rain, for example - and tuning the engine and carbs to make the most of that pressure will require some work; you won’t gain much from just bunging a ram intake on a normal airbox & carbs. If you can instead spend that work on a more predictable gain - perhaps fancy stuff with the timing or exhaust - that would normally be a better bet than the ram air. Modern engines certainly seem less needful of that sort of thing, perhaps because they’ve so much more control over what goes into the cylinders and knowledge of what happens in them that any benefit can be better gained in a more dynamic way - I can’t see the vagaries of outside atmosphere particularly helping your average ECU do its job.

Also, I suspect some of the ram air is a salesy gimmick. It’s very obvious when it’s there, it intuitively makes sense that it would add to the power and it’s got scope for cool names and initialisms.

A larger intake into an airbox doesnt mean a performance gain. Take the Ducati airbox for instance. That relies on a very precise shape of tube to allow the air to swirl and then fill the airbox, same with the snorkel on the SV650 and SV1000 - which alot of people remove thinking its restrictive. A large airbox opening can create a void which prevents air entering the airbox at the correct rate and keeping it filled with air.

A worthy read if you have time:

Good read up Sarah.
A question though. If the airbox was modded and larger tubes fitted for colder air, naturally a dip in the midrange would happen. This has happened on my BKing. If the king was adjusted by a decat pipe and Ecu remap, would this then regain that midrange and add more?

Yes Big Red S and Sarah Jordan - It seems that marketing and styling may account for the appearance of large intakes/scoops on the front of various sporty bikes over the last twenty years - rather than any physical need for extravagant intakes and ducting.

I`m guessing ram air et al are not ideal for high speed fordage?

Go fast enough and you’ll plane over the top.

Suzuki’s patented Ram Air system was
designed to cool the central cylinder
head on three cyclinder two stroke engines,
e.g. the GT550.