1216 bandit with modded airbox

[Foz]

I'm just exploring a few options , my bike is a 1216 with a lot of headwork and gsxr cams , akrapovic exhaust . Currently I'm running rs38 carbs but occasionally have a hankering for the refinement of cv carbs , previously when it was a 1157 I've tried it as a stage 3 with pod filters and tbh I just never got on with them and they drank fuel for England when giving it the beans .  I then swapped to a modified airbox and had the same performance but with better carburation and better fuel economy. 
  How would a 1216 engine respond to cv's and a modified airbox ??

[MeanBean49]

If your getting the same performance and varying fuel economy then the setup of one is way better than the other.

The most powerful and economical setup for tuned motors is pretty much big carbs on open bellmouths.

The less work the engine has to do to suck the air in, the more efficient it is.

Bad fuel economy on a less restrictive system is just bad setup. At constant speed a free flowimg system will use less fuel as the engine can make the power required to maintain that speed easier, which takes less fuel.

You will use more fuel though when setup right and using full throttle because making more power means more fuel and air.

You can get any setup riding spot on when setup well on a dyno

[Gixer1460]

A M8 with a 1216 bandit used DJ'd 36 CV's and modded airbox and would do a tank after about 90 miles - now uses RS36's still breathing through the modded airbox and has more mid range, more top end power, smooth throttle response and is getting 120 - 130 miles to a tank, more if he's 'driving Miss Daisy'

[Foz]

Last time I used cv carbs on pod filters it was a 1157 all correctly set up on the dyno using 150 main jets and it was doing 121 bhp but was slightly hesitant between 3-4K and drank fuel if giving it the beans , on a modified airbox with perfect fuelling, good economy and 127.5 main jets it was doing 120.2bhp  

[MeanBean49]

4 minutes ago, Foz said:

Last time I used cv carbs on pod filters it was a 1157 all correctly set up on the dyno using 150 main jets and it was doing 121 bhp but was slightly hesitant between 3-4K and drank fuel if giving it the beans , on a modified airbox with perfect fuelling, good economy and 127.5 main jets it was doing 120.2bhp  

If thats what it was doing it wasnt setup perfectly

[Foz]

It was just as you picked it up on the throttle coming out of islands, bloody annoying and couldn’t get rid of it. Still doesn’t alter its thirst or the marginal power difference between the two set up though  

[Oilyspanner]

An airbox does a number of jobs. Here's just a portion of them - The intake tract resonates due to the movement of the pistons and valves opening and closing (you are aware of this from the noise of open carbs) - the resonance can be used to aid flow, particularly at low/mid-range revs. The intake length has an impact on the resonance, so this can be used to boost various bits of the rev range - so helping overall power curve. There's a load of boring/interesting physics in play along the inlet tract, modern bikes make better of the airbox and intake system than our bikes, which were designed in the early days of airbox dynamics.  A carb with a velocity stack will flow very well at high flow rates, but an engine with overlap will be boosted by the use of a tuned inlet length and airbox, not necessarily at peak, but it can be made to be a lot more effective throughout  the rev range - as most bikes are ridden on the road and at a variety of throttle openings this is very helpful An airbox allows cooler, denser air to enter the carb, although it's only slightly helpful on most of our bikes - slingshots do try to vent some cool air to the rear of the airbox.

An engine doesn't really 'draw'  (as in suck) in air, the piston's downward movement causes a lowering of pressure within the cylinder and it's the higher atmospheric pressure that 'pushes' air through the carb and inlet , to cylinder- the velocity of air causes a lowering of pressure, which enables the carb to force fuel/fuel vapour through the jets, because the float bowl is at atmospheric pressure .

I don't pretend to know everything, but I've tried to learn as much as I could over the years on how carbs work. I promise not to talk any more on this subject...….but some info into why airboxes can and do help seem to be needed.

[Gixer1460]

53 minutes ago, Oilyspanner said:

An engine does 'draw'  (as in suck) in air, the piston's downward movement causes a lowering of pressure (vacuum) within the cylinder and it's simple physics that atmospheric air which is at 'normal pressure tries to equalise - this 'flows' air through the carb and inlet to the cylinder. The increasing velocity of air passing the venturi (the narrowing in the carb) causes a lowering of pressure, - a depression - which literally sucks fuel up the jets that then gets atomised into the air stream. The float bowl is at atmospheric pressure so the fuel is drawn up the tubes to replace that which gets atomised. Nature hates vacuum's!

8/10 - made a few corrections for you LOL!

[Foz]

But, science aside. Is utilising the airbox going to to help or hinder my bike if I revert back to cv carbs ? 

Edited June 22, 2018 by Foz

[vizman]

horror vacui*

 

 

 

*'arris tottle

[Rene EFE]

5 hours ago, Foz said:

rs38 carbs

Dibs 

[bluedog59]

If you want to look at "squares under the dyno curve" rather than a peak BHP figure I'd go for the airbox.  As mentioned, the airbox is more than just a place to stick the air filter. If you look at the carb to airbox rubber they are basically  a  velocity stack, nice rolled entry, shallow taper, they project into the airbox (meaning they can draw air from all around them) and crucially, the right length for them to help make the inlet pulses work together. Now look at a pod filter straight on the back  of a carb . You've lost all the good stuff and got a solid end to the pod that screws up the intake pulses. A good compromise could be a set of correct length stacks with a big foam filter over and around them but not touching them at the end.

But then you've started building an airbox.

The airbox also provides a still air chamber for the engine to draw from helps in the harmonics between cylinders (could be why twin pods work better than separates). 

[Rossco(Iceman Josros)]

Had my 1052 ( flowed head v&h zorst bellmouths with ramair socks) dyno’d a couple of years ago as it was struggling to pull cleanly from a closed / part throttle first run 118 bhp on std size DJ jets but showing 26% rich dropped down to 127.5 mains ( 2 sizes below std) and gained 5bhp and still 20% rich.

since then I’ve dropped down to 120’s and it is a lot crisper just not had it back on the dyno 

[zedhead]

As an aside, there's a reason why Prostock bikes are fitted with an airbox... they're making over and above 300bhp, normally aspirated, with Lectron carbs...

(although there's no requirement for good low rpm manners or closed throttle transition, both things which many 'tuners' will tell you come from using an airbox)

[Duckndive]

2 hours ago, zedhead said:

As an aside, there's a reason why Prostock bikes are fitted with an airbox... they're making over and above 300bhp, normally aspirated, with Lectron carbs...

(although there's no requirement for good low rpm manners or closed throttle transition, both things which many 'tuners' will tell you come from using an airbox)

The NHRA Pro-Stock bike guys in the US are very secretive about airbox's you rarely see more than the clutch when they service between rounds every thing else is covered up away from prying eyes or the bike goes in the trailer with a shield behind it for service.....

Just like the Pro Stock car boys used to do before the NHRA banned the practice ....they just run a huge fake air box on the car,s now to hide the trickery ......

F1 teams spend millions on air box design and wind tunnel time....

[dupersunc]

Air boxes are pretty simple devices, the biggest problem car guys have is how they interact with the rest of the aero package.

The primary issue as has already been said is the resonant length of the inlet tract.  Bell mouths do 2 things, 1 they provide a clean efficient transition from atmosphere to inlet, and 2, provide a very clear transition in cross section and pressure which gives a very strong signal for the inlet tract.  A pressure wave travels back and forth between the back of the inlet valve and the edge of the bell mouth at the speed of sound.  The shorter the bellmouth, the more pulses per minute you get, time this with inlet valve opening and you increase cylinder filling at that rpm. Very power full tool.

Sometimes pulse plates are fitted in the air box, to give a secondary effect, or even the lid of the airbox is used, but this isn't that efficient. 

The main purpose of an airbox is to provide still non turbulent air for the inlets, turbulent air buggers up your pulse signal. No airbox on a moving bike leaves you with all sorts weird interactions with the fairing and rider in the area of the carbs, which can cause all sorts of problems.

If you can draw the air from area of stagnant flow such as the front of the fairing, that's even better, and it will be cooler.  What  you do with the air between nose of fairing and airbox is where the bike world is fucking cluess.  Only in recent years have Gurls blouse, and Ducati started to deal with it properly, it wouldn't surprise me if the NHRA bike guys got there first though.

You also have take into account that manufacturers need to suppress inlet noise, so that becomes an important design consideration of airboxes on most road bikes.

If anyone is wondering about ramair. Don't. It's marketing bollocks, unless you can make the air entering the airbox flow supersonically.

[nlovien]

on the point of the CV carb drinking petrol versus the direct lift - yet HP figures +/- same

HP figures +/- same suggest at WOT either carb is being efficient in supplying what's needed for the given engine tune = important on a race engine that will spend time at WOT, not so important on a road based engine which spends not a lot of time at WOT, yet we often calibrate our carbs to the part that we rarely use

the bit from closed to WOT is where the road engine lives - direct lift carb we tune each primary fueling circuit with a direct relation to throttle position - on the CV its a bit of unknown as to what circuit is actually providing the juice at any given engine load other than WOT - if your drinking a load more fuel then - for me - it suggests your wetting out - possibly because the vacuum control of the slide lift is now out of sync with the engine demand ( and adding back the airbox can help to bring this back into sync ) - but the area that needs focus is the rate of slide lift via return spring and slide lift holes

trying to picture this

direct lift - easy you can see the slide lifting in direct relation to demand - fuel from here - now fuel from there - thank you

CV - I can picture, large throttle opening followed by the slide going through a fluctuating cycle from closed to full open until it finds the balance and whilst doing this its slugging fuel into the venturi versus accurately metering it  - so maybe slow the slide lift response down - i.e. damp down the fluctuation - but over do it and it will not lift quick enough, or hold open at WOT  - --  basically consider when tuning CV carbs that the slide lift response is as important ( maybe more so ) as the jetting - not something you consider with a direct lift, and hayho - any CV carb tuning kit i've seen does indeed include mod's for the slide lift - not saying the kits get it right though! 

problem is - so where do you go for a range of carb slide springs ? - ok so focus on the lift holes

[Foz]

Has anyone actually gone down the 1216,headwork, cams and modded airbox route ? Just curious to see if anyone has any settings 

[jonny1bump]

I got 162 hp out of 40 mm cvs and air box. That seemed to be most could Get them to flow. Currently before crank snapping 172 still using trusted biggest air box could make and fit.

[jonny1bump]

With stockers whole carb is in that air chamber.

[Foz]

A few hours on the dyno using the modded airbox and standard bandit carbs got me 148.3bhp and 95ftlbs with possibly a bit more in there with a bit more fettling