Airbox dimensioning

[Robert]

Hello Guy,

I would to use an airbox instead of my current filter on a plate at the end of the horns (ITG JC40).
Considering the dimensions of the bay and willing to obtain a resonance using the Helmoltz theory I arrive to a volume of the box of 4680 cc with a hose of around 600mm long with a dia of 100mm. The resonance is at 3500 rpm (if the equation I used is exact).
The volume of an intake pipe is 533 cc. Of course the unitary volume is 500 cc with the 2l TC.

Do you think the volume of the box is enough for the 2l TC ?
(in fact a friend of mine has an engine which cannot "breath" with a too small airbox on a 3l V6 and works fine w/o airbox...)

Best regards,

Merci Guy.

Robert
PS: the resonance is a plus, the other advantages to me are to reduce the noise and to breath cool air

[Guy Croft]

I don't know about application of Helmholz here but I can tell you that optimum reflected +/- waves in the inlet tract will be generated irrespective of the size of the airbox provided it operates at atmospheric pressure at all times.

That is not to say that if it's too small turbulence won't develop around the rampipes - it can. My 'rule of thumb' is minimum of 1" clear all round.

Usually airbox depression results from a feed duct to the airbox that is too small or - pressure drop in the filter wherever it's located (in the airbox or remote). An airbox at lower than atmos pressure will still generate wave action but of lower intensity because the primary relected wave that's generated at valve opening on the intake stroke and that encourages cylinder filling (goes out as negative, reflected as positive) is the net sum of cylinder depression plus the absolute pressure at the airbox, eg: 0.3 bar depression meets 1 bar at rampipe end = returning positive wave of magnitude 1.3 bar.



G

[engineerted]

Robert, I believe that with a carburetor setup, the Helmholz only applies to the intake runner volume to the point of the butterfly valve / venturi of the carb. If you want to play around with the Helmholz you need to have a common intake manifold and single throttle body. Many years ago at one of the SAE automotive show here in Detroit, I saw a prototype variable length manifold setup. The length of the intake track would change depending on the RPM on the engine. Thus trying to keep the resonance in tune with the engine. Cool stuff!


Ted

[Robert]

Hi,

Thank you for the replies.
I found that on the resonance topic and used it:
http://motorcycleinfo.calsci.com/Airboxes.html

The formula seems confirmad by the professor Hathaway's course here attached.

I confess I don't master everything but I try ...
Don't be too severe with me !

Robert

[Guy Croft]

Read it all and - sorry not convinced.

For those who don’t know about it Helmholz effect is most simply illustrated as the ‘springiness’ in a balloon when you inflate it. Imagine pressure waves in container with one (only) open port exposed to pressure behaving like the balloon, that’s Helmholz. There’s lots on the internet about it and it can be used very generally on engines but advanced software has superceded it by a flying mile as a predictive tool. I don't want to jump on Rob's thread and assert things as if to say I know everything, that’s not what the forum is about, it’s about learning, so, with that in mind and given the huge readership (if not membership..) I’m taking a minute or 60 to remark in a bit more detail. What is really under discussion here? Airbox - plenum? And does it make any difference in the context under discussion? Let’s see.

There can be a marked difference between an airbox and a plenum. An airbox is not a closed system like a balloon, it is open to atmosphere, so you can't model it pumping up and down under influence of inlet pressure waves, unless of course it's too small - and many are.
With an airbox (mounted on carbs or FI bodies) there is no effective pressure wave activity between throttles and airbox when the throttles are closed. And you don't want any, that's why the throttles are shut. And when they are open the outboard ends of the rampipes are exposed to atmospheric pressure - or at least should be. I referred to why this is a ‘good’ thing earlier. If they’re not – you’re going to get power loss, and plenty do! One might reasonably assume that since the outboard end of the rampipe 'sees' atmospheric pressure, the inlet runners are – or should be - tuned for length to take best advantage of individual pressure waves in their respective and separate pipes.

A plenum – by default – is a volume connecting the runners to a single throttle mounted on the boundary of the system that controls the pressure state inside it. That volume (typically) can be exposed to wildly varying pressure wave activity from all cylinders whether the throttle is open or closed (ie: virtually closed). Part of the function of the plenum is to prevent choking when the throttle is opened after a *closed state, ie: provide a reserve capacity of air for the cylinders until the pressure builds up in the volume – which is why they have to be quite big to be effective and the bigger the engine the bigger they need to be. There is always a lot of discussion about this and frequently it’s confused with pressure wave tuning.

Helmholz effect with wide open throttle on a plenum? Non-existent. I can see that Helmholz resonance could develop at closed throttle with the plenum but what use is it at closed throttle anyway? And there is so much complex wave activity in there (eg: pressure waves bouncing off the internal form of the plenum incl the throttle plate itself) that a Helmholz model could never fully optimise the design although it might give some useful predictions here and there. Again the danger is that if you make the plenum too small (or too tortuous) it's also going to develop significant pressure drop whether the throttle is open - or closed. And with some throttle plate locations on plenums that I have seen (esp the one below) you get powerful vortex effects in the plenum which might (?) be great for mixture turbulence but definitely penalise massflow.

I know that a plenum (such as shown below) can give a silky torque curve but it can also 'chop off' all the exciting peaks. Not always true I guess but frequently. And useful ‘humps’ in the torque curve can make you go a lot faster if they happen to coincide with the upward gear change. With a production car plenum, a lot of effort is put into deriving the right torque curve. There are numerous examples of engines with very long runners (these days) which are so-designed to assert tuning wave action but which are also joined to a common plenum with single throttle and they work very well - but one can (again) reasonably assume that there is a high degree of adverse mixing of pressure waves because all pipes connect to a chamber of widely varying pressure and only on a sufficiently large plenum at full throttle are they going behave like the separated airbox equipped ones. A tuned runner system with a nice capacious plenum (minimising viscous loss – drag) could maybe be as good as an airbox style setup. I don't unfortunately have any back-to-back dyno results to prove either way.

Worth mentioning that the most competitive cars these days are running ‘flat-shift’ gearchange systems where the throttle is never closed at all, see:


http://www.youtube.com/watch?v=2FxxVpSUAMI


G

* added a bit there for clarity after re-reading

[Robert]

Good evening Guy !

I thank you for your words "that’s not what the forum is about, it’s about learning". Because I'm considering learning as a guiding principle. It's why I looked for information as I attached to my previous post. And why also I discuss on this forum to obtain comments and explanation.
This said, I understand now that an airbox is different than a plenum (may be I'm late ;o) ).
For an airbox and separated throttle bodies the Helmoltz principle does not apply.
The only concern is to have enough volume in order to provide the atmo pressure to the pipes.

Thank you once again,

Robert
PS nevertheless the Helmoltz principle is learnt in the US college/university and seems used for plenum...

[Guy Croft]

Rob, hi

good for you! I would be keen to explore Helmholz applications so if you discover more intensive study - let me know,

G

[Christoph Thuerey]

Hi,

the Helmhotz principle is learned at german universitys too. But the only car I have seen (and driven) with a Helmholtz-Resonator is the McLaren SLR. It looks beautiful completely in carbonfibre. But the engine is supercharged, so it makes no sense at all except for the wonderful appearence.

Regards
Christoph