INCOMING, Happy 50th to me.... Yeah, it's a Yamaha... it's the easy option.

[Supermofo]

I wouldn't do it, modern bikes have properly designed airboxes (modern being almost anything made since 1985

Pfft;
K&N by MrLongbeard, on Flickr

He said 1985 not 1895

[singlesman]

I wouldn't do it, modern bikes have properly designed airboxes (modern being almost anything made since 1985), why fuck the fuelling up for a hypothetical 7 bhp, if you want more power buy a more powerful bike.

Pah! My cunningly modified 1050 speed triple air box, an afternoon on the dyno saw it making just about the same horsepower as a bog standard one
Kept me amused for an afternoon though, and it did sound nice

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[Mr. Dazzle]

Takes it from 70bhp to 77-80bhp after a flash tune.

Whilst leaving a big hole in the bottom end

[weeksy]

Takes it from 70bhp to 77-80bhp after a flash tune.

Whilst leaving a big hole in the bottom end

It's a good Qn mate and one i don't know the answer to.

[Mr. Dazzle]

Put it this way.

Even F1 cars and MotoGP bikes have air boxes.

[Taipan]

Open air box mods and pod filters etc are for full throttle power gains, but normally at the cost of the bottom end and midrange.

[Supermofo]

I wouldn't do it, modern bikes have properly designed airboxes (modern being almost anything made since 1985), why fuck the fuelling up for a hypothetical 7 bhp, if you want more power buy a more powerful bike.

Pah! My cunningly modified 1050 speed triple air box, an afternoon on the dyno saw it making just about the same horsepower as a bog standard one
Kept me amused for an afternoon though, and it did sound nice6C61E80E-CF7E-4F5B-9F4A-B8C6DBCA2545.jpeg6EC423F9-4746-4ED0-8775-D72BF47AD831.jpeg

Looks like one of the spaceships in The Night Garden. Is it the new Ninky Nonk.

[Le_Fromage_Grande]

Open air box mods and pod filters etc are for full throttle power gains, but normally at the cost of the bottom end and midrange.

I think the only reason anyone fitted them in the 80s was because getting the carbs onto an old inline 4 with the airbox on is so bloody difficult

[Count Steer]

Put it this way.

Even F1 cars and MotoGP bikes have air boxes.

Q? How do they work? I understand a bit about getting the gases out the other end and how 2T zorsts can improve performance. I've experienced the complete lack of bottom end on something where I faffed with the air filter side of things. How does an airbox size/shape/flow characteristics fit in with tuning across the range of air intake/throttle opening?

Ta.

[lostboy]

Put it this way.

Even F1 cars and MotoGP bikes have air boxes.

Q? How do they work? I understand a bit about getting the gases out the other end and how 2T zorsts can improve performance. I've experienced the complete lack of bottom end on something where I faffed with the air filter side of things. How does an airbox size/shape/flow characteristics fit in with tuning across the range of air intake/throttle opening?

Ta.

I was wondering the same thing and came to the conclusion that it's all to do with tuning the flow of air to get the right volume delivered at the right time to get the best bang for your buck across the whole of the rev range - rather than just saying "there you go, have as much as you like" and trying to deal with it, which must be inefficient.

[Taipan]

Intakes and airboxes are pretty complex. You'll often have a baffle plate opposite the intakes which I believe is to stop resonance corrupting flow. Also, inside an airbox you'll quite often see they still have intake trumpets (Bellmouths) as you can tune airflow speed based on their length (velocity stacks). Some racers you used to glue an epoxy ridge in the airtake to disrupt smooth flow and cause turbulence for better mixture. Loads going on in an airbox, which is why its best left as it is.

[Le_Fromage_Grande]

Some F1 engines had (and may still have) intakes that alter length with engine rpm.
With flatslide carbs you can get different length bellmouths, I'm trying to remember which ones do what.
The intake on a 4 stroke is more complicated than the exhaust, and easier to fuck up

[Count Steer]

Put it this way.

Even F1 cars and MotoGP bikes have air boxes.

Q? How do they work? I understand a bit about getting the gases out the other end and how 2T zorsts can improve performance. I've experienced the complete lack of bottom end on something where I faffed with the air filter side of things. How does an airbox size/shape/flow characteristics fit in with tuning across the range of air intake/throttle opening?

Ta.

I was wondering the same thing and came to the conclusion that it's all to do with tuning the flow of air to get the right volume delivered at the right time to get the best bang for your buck across the whole of the rev range - rather than just saying "there you go, have as much as you like" and trying to deal with it, which must be inefficient.

Yebbut, it's a static device ie no moving parts apart from the air so I wondered how, for example, you could look at Singlesman's airbox and work out why it was worse than standard and how it could be improved? How do you get, what is effectively a box and a few tubes, to behave linearly or otherwise across the range of flows, guessing that there's more to it than trial and error. Just intrigued 'cos at first sight it seems like voodoo.

Edit: From what Fromage says above they may not all be static.

[Mr. Dazzle]

At the base level it's actually pretty simple. The engine doesn't suck constantly right? It "puffs" - each intake stroke sucks in air, but 3/4 of the time the cylinder is not sucking. Even on the intake stroke the airflow isn't consistent because the piston speeds up and then slows down during the stroke. Therefore the air flow down the inlet track isn't a constant breeze, it's a series of gusts. Throw in more than one cylinder and you've got a series of interacting gusts. Then throw in the interaction of the exhaust and it gets more complicated. It's very very similar logic to 2 strokes, not that different at all - 2 strokes are just even more 'susceptible' because they don't really have valves to help control ("really" because you can obviously get a 2 stroke with various sorts of valves too!).

What you want to do is set it up so "gusts" arrive at the cylinder just when you need them too. I.e. so when the intake valve opens the wind is already blowing 'into' the engine and you're shoving in air and fuel straight away. Obviously the term "gust" is something of a layman's simplification. You can talk about standing waves and pressure fronts and all the rest of it if you prefer, but at the base level that's what's happening.

Think of it in terms of musical instruments if you prefer. On a recorder you cover differnet holes on the side to change the standing waves in the tube and hence change the frequency of the note played. It's exactly the same in inlet systems, only rather than trying to play a particular note you're trying to match the harmonics in your inlet system to the engine's needs.

FWIW F1 used to use variable geometry inlets but AFAIK they're banned now. But even so you can tune the shape of the airbox to 'shove in' more air at certain rev/throttle combos. You can also get clever and make it so different standing wave patterns are set up at different speeds to change the interactions. F1 gets even cleverer than that, 'cause the airbox is fed from a ram intake on the top of the car, so the pressure coming into the box changes as the car speeds up and slows down. You're never going to be at high speed AND low revs for example, so you'd consider how you tune the box design to account for / take advantage of that.

[Count Steer]

Cheers. I've experienced when the standing wave type thing is out of phase with the engine demand. It felt like the engine was tying itself in knots - but that was a simple 2T experiment when I was a yoof.

[Mr. Dazzle]

If anything 2Ts are easier to get your head around in this context because the effect is so pronounced. You've probably seen this animation before?



See how the gases flow down the exhaust and the sort of 'bounce' off the end? Exactly the same thing happens on the inlet side too, the time it takes for the 'bounce to come back' defines the RPM at which it works best. This happens in pretty much any tube you blow into, you can try it yourself by humming into a poster tube

[Yorick]

When we were racing we all raced with the air box top removed. Heaps more power.

But on monster power bikes like my GSXR the airbox is to keep the air pressure constant. Together with Ramair, this avoids massive highs and lows.
Then the engine can get what it wants at any given time.
It's quite simple and common sense.

[lostboy]

If anything 2Ts are easier to get your head around in this context because the effect is so pronounced. You've probably seen this animation before?



See how the gases flow down the exhaust and the sort of 'bounce' off the end? Exactly the same thing happens on the inlet side too, the time it takes for the 'bounce to come back' defines the RPM at which it works best. This happens in pretty much any tube you blow into, you can try it yourself by humming into a poster tube

The singing one is interesting and where I'd formed my ideas from. In my youth (and beyond it) I designed a lot of speaker enclosures and ports and the smallest difference in shape and size of both the enclosure itself and the port makes an enormous difference in what the speaker can deliver. Made sense to me that it's the same trying to get a volume of gas into an engine!

[ajlog]

Quick explanation here from Kevin, the tech guy at Cycle World in the US:



He has some good videos on YouTube.

[Le_Fromage_Grande]

LCs like Yorick raced are easy to get running right with no airbox or a less restrictive air box, from memory you go from a 240 main jets to a 280 and drop the needle one clip, the same era 4 strokes can be gits to get running with out airboxes, CB900Fs and FZ750s were particularly difficult, nearly everything else needs an air corrector to reduce the size of the air bleed into the emulsion tube.

Exhausts are pretty easy, Graham Ball's engine tuning books have the equations to work out exhaust dimensions for both 2 and 4 stroke engines, the 4 stroke ones definitely work as I've used them to get the correct tuned length for silencers, what's also pretty obvious is that Micron, Motad, Kerker and Vance and Hines worked to the same equations.