Air filter question

[Ian Foster]

My current thinking regarding the airbox as described above. The large diameter (circa 70mm) single pipe is  achieved by flaring out the box at the front end where the wheel arch allows.  

If I make it the right depth it can use either my existing K&N filter elements, or a remote filter at the side of the radiator.

Ian

 

[johny]

It'll look great if nothing else! Dont forget to have the good drain hole you mentioned earlier in it as this is to allow fuel to run out in case of a carb float valve failure (the overflow exit is in the inlet face of each carb)... 

[Iain T]

20 minutes ago, Ian Foster said:

I make it the right depth it can use either my existing K&N filter elements, or a remote filter at the side of the radiator.

I bought a used Vitesse air box to do just that and a cone filter to fit on the end of the tube. I thought to not use my K&N filters inside to restrict flow. It's all a bit tight for room! I think my cone has a 70mm ish fitting and good for around 250bhp...

Iain

Edited March 26 by Iain T

[Wagger]

I tried aged 16 to supercharge my moped by fitting a ram air tube so that forward motion forced air into the carb. It blew the fuel back the the tank, so useless. Gravity feed. There is no pont in having turbulence before the air is mixed with the fuel. Swirl in the intake after the carb is desirable but not exactly the same as turbulence. Of course, some 'Moderns' have the dreaded swirl flaps to mix fuel with air as it enters the cylinder. Fine until bits break off and jam valves, pistons etc.

 

[micmak]

If I make up something with tubes/hoses/pipes, is there some sort of flange or baffle or plenum or scoop at the end of the tubes?  Surely just leaving the tubes sitting there behind the front grille will not allow all that much air in.

.....Mick.....

 

[johny]

Only the clamp I posted earlier and now I think about it with the drain holes in the filter box I really cant see the hoses help much either at idle or going along☹️Perhaps the best thing is you can tell people they boost inlet pressure (seem to remember motorcycles had something similar at one stage)...

[PeteH]

The reason for "intercoolers" as I recall, is to reduce the Air Temperature and increase it`s (specific) density, allowing the air charge mass reaching the combustion chamber to be capable of supporting the "burn" of a greater volume of fuel and increasing the power?. I know, that early Turbo chargers fitted to large marine engines did not have them, and a few where actually retro fitted by the Maker. We always struggled anyway, especially in the Tropics with Engine room and by implication intake air temperatures affecting performance, ocasionally to the point where we had to reduce Rev`s to stay within safe exhaust temp; parameters. The other noticable thing was the Norton, always seemed to perform better on cold damp mornings going to work, so long as the B*gger would start!!. A couple of the "lads" early 60`s fitted flexible Exhaust type Tubing pointing forward, to the carburators, in the belief it helped "ram" air at speed. The probable unquantifiable benfit may have been Cooler Air, rather than the vast majority of motorcycle carburators in the "warmer" air behind the cooling fins?. Shaped Piston heads, where I think, one effort made by manufacturers to induce swirl in the combustion chamber?. I know the opposed piston design was far more efficient than the single piston, when it came to efficient consumption said to produce more power from a single charge of fuel air largely due to the more effective swirl and mix.

Pete

Edited March 28 by PeteH

[JohnD]

On 26/03/2024 at 15:25, johny said:

I know its a very involved subject and this is Wikipedia's take on it:

'Turbulence in the intake helps to break up fuel droplets, improving the degree of atomization. Better atomization allows for a more complete burn of all the fuel and helps reduce engine knock by enlarging the flame front. To achieve this turbulence it is a common practice to leave the surfaces of the intake and intake ports in the cylinder head rough and unpolished'

johnny,

It's important to remember that the Wiki is not an encyclopaedia, researched and written by acknowledged experts, nor are its articles peer reviewed by other experts before publication, as are papers in academic journals .    In effect it is a message board, just like this one where anyone may post.    Some who post may have the authority and expertise to teach you, others are winging it, a few are out to mislead you.   This disadvantage of the Wiki is balanced by it being open to correction, but its managers cannot monitor every entry and every line.     The Wiki is a useful source but cannot ever be a reference!

You quote the Wiki article on the Inlet Manifold: Inlet manifold - Wikipedia   and the paragraph there on "Turbulence".    It refers to breaking up pools and drops of fuel on the duct walls where a rough surface is desirable to promote turbulence.     I absolutely agree, but this refers to the 'boundary layer', the air is slowed down and made, yes, turbulent!, by contact with the duct wall.      The thinner a boundary layer the more laminar flow will there be in the duct, which is most desirable, as turbulent flow is chaotic and offers more resistance, so normally a smooth wall is best.    But in these circumstances, a rougher wall and a thicker boundary layer gives a better compromise.

If you find this subject interesting, may I offer you another online article, this time from NASA who really do do know their  aero!   Boundary Layer (nasa.gov)

John

[JohnD]

Ian Foster,

What a great design method!   You're modelling the box in foam and shaping it with filler?    How will you construct the final part - cast it in GRP?  More detail of your method, please!

You make it very clear how you can't make the box have more volume by being thicker - can you make it wider?    The double disk shape was originally to contain two cylindrical filters, but you plan to use a remote conical filter, so your box may be any shape.     The bigger the better, as the bigger the slower the air and the higher the dynamic pressure.       I'd also look at the transition between the cylindrical duct and the flat sided box - flow easily becomes turbulent in such a transition so make it widen as quickly as possible to slow the air and keep it laminar.

Lastly, I see you already have a conical filter that attaches to a 2"(?) duct.      Much better than Triumph's narrow hoses!    But 3" filters are available (see my post page 1) and resistance to flow along a 3" duct will be a fifth of that along a 2" one!  (1/2^4 vs 1/3^4 = 1/16 vs 1/81)

John

Edited March 28 by JohnD

[johny]

On 26/03/2024 at 13:05, JohnD said:

Johnny, Less turbulence, more "swirl"!  Modern engines with very lean combustion use a stratified charge, richer at the ignition point, achieved by some very clever swirl and "tumble" in the induction, where turbulence which is essentially random wouldn't do.

JOhn

What can we do to get this "swirl" in our inlet manifold/heads then John?

[Ian Foster]

6 hours ago, JohnD said:

What a great design method!   You're modelling the box in foam and shaping it with filler?    How will you construct the final part - cast it in GRP?  More detail of your method, please!

Looks as though we have thread resurrection...well done someone!

The design will require a split mould, ie built in sections and joined with flanges, to prevent it locking onto the moulded part. If I do follow the composites route, the mould could be used multiple times. The really time consuming bit will be creating the plug (the foam and filler bit) with a good enough surface finish so I'll be satisfied with the final mouldings. Being a retired Structural Engineer with well developed OCD, does not help in this regard.

I've done a fair bit in composites, from early teenage days building canoes, an E glass and epoxy sheathed foam cored yacht rudder, to various carbon fibre elements, including a mast instrument bracket, cleat bases and the heat shields for my GT6 carbs. Carbon and epoxy are much nicer to work with than regular glassfibre.

Previous research suggested that the airbox volume should be equal or greater than the engine capacity. My current design has an internal volume of approximately 3400 cc, so should be big enough for a 6 cylinder car. The opening to the box and the filter are designed around a 70mm diameter pipe. My Honda S2000 has a 80mm pipe from the airbox to the throttle body and that produces nearly 240 bhp.

I have pondered on the issue of getting sufficient air to the rear carb and have refined the latest, latest design to incorporate a ramped base as shown by this photo from the rear. 

Still work in progress and it will probably a next winter project. I would like to do it before getting the car RR tuned.

Ian

Edited March 28 by Ian Foster
grammer

[Chris A]

26 minutes ago, Ian Foster said:

Looks as though we have thread resurrection

Well it is almost Easter 😇

[Mjit]

9 hours ago, PeteH said:

The reason for "intercoolers" as I recall, is to reduce the Air Temperature and increase it`s (specific) density, allowing the air charge mass reaching the combustion chamber to be capable of supporting the "burn" of a greater volume of fuel and increasing the power?

Broadly correct - but unless you're installing some water spray or dry ice bath (which only really applies to dragsters) you're not going to cool the charge below ambiant air temperature.

The reason they are used on super/turbo charged engines is that the charger forced a volume of air into a space with a smaller volume, increading its pressure over atmospheric and, given there's no such thing as a free lunch in thermodynamics, increasing the temerature of the air in that smaller space.  This means more oxygen atoms end up in the cylinder - but not as many as you could have and also causes other temerature issues.  An intercooler lets you remove some of the extra heat compressing the charge via the charger added, meaning a cooler (then non-intercooled) charge in the cylinder so a few more oxygen atoms and fewer other temperature related issue.

[Puglet1]

7 hours ago, Ian Foster said:

Looks as though we have thread resurrection...well done someone!

The design will require a split mould, ie built in sections and joined with flanges, to prevent it locking onto the moulded part. If I do follow the composites route, the mould could be used multiple times. The really time consuming bit will be creating the plug (the foam and filler bit) with a good enough surface finish so I'll be satisfied with the final mouldings. Being a retired Structural Engineer with well developed OCD, does not help in this regard.

I've done a fair bit in composites, from early teenage days building canoes, an E glass and epoxy sheathed foam cored yacht rudder, to various carbon fibre elements, including a mast instrument bracket, cleat bases and the heat shields for my GT6 carbs. Carbon and epoxy are much nicer to work with than regular glassfibre.

Previous research suggested that the airbox volume should be equal or greater than the engine capacity. My current design has an internal volume of approximately 3400 cc, so should be big enough for a 6 cylinder car. The opening to the box and the filter are designed around a 70mm diameter pipe. My Honda S2000 has a 80mm pipe from the airbox to the throttle body and that produces nearly 240 bhp.

I have pondered on the issue of getting sufficient air to the rear carb and have refined the latest, latest design to incorporate a ramped base as shown by this photo from the rear. 

Still work in progress and it will probably a next winter project. I would like to do it before getting the car RR tuned.

Ian

Hi Ian.

 I have no experience whatsoever in working with carbon and really respect some of the engineering achievements which are carried out by people on this forum.

Being of an inquisitive mind I am wondering if a carbon airbox could be affected by resonance?

Sorry if this seems like a silly comment but I’m just being curious.

 

[Ian Foster]

Carbon/epoxy composite is just a stiffer lighter version of glassfibre.

Carbon airboxes are used in all sorts of performance and competition situations and I cannot see that resonance would be any sort of issue.

Ian

[trigolf]

You've stimulated a lively debate with filter boxes now Ian! 🤔

Gav

[Puglet1]

38 minutes ago, Ian Foster said:

Carbon/epoxy composite is just a stiffer lighter version of glassfibre.

Carbon airboxes are used in all sorts of performance and competition situations and I cannot see that resonance would be any sort of issue.

Ian

Thanks Ian.

 

[johny]

1 hour ago, Ian Foster said:

Carbon/epoxy composite is just a stiffer lighter version of glassfibre.

Carbon airboxes are used in all sorts of performance and competition situations and I cannot see that resonance would be any sort of issue.

Ian

Only one way to find out - cant wait😍 Might be a business opportunity here....

[JohnD]

Ian Foster,

Yes, I like that latest, latest box design even better!    I made a rear hatchback door for my Silverback, race Vitesse, by taking a mould off the original outer skin, but as I couldn't be bothered to copy the inner surface, I just glued polyurethane foam inside it and covered that with GRP, rather like, I suspect, your rudder.  But I found that the foam soaks up resin!    As a result, it turned out much heavier than I hoped. (the roof, in GRP and honeycomb, weighed a third of the steel original)  Did you find that with your rudder?

Do you have a heater in your car?    You can use the airbox under the intake for that, below the windscreen, as a useful source of fresh air.  I did that on Silverback, for different reasons:

 

Johnny,

Our engines don't have "swirl", they have "squish"!   The combustion chamber doesn't cover the whole of the bore, and that flat area away from the plug is designed to almost close completely at TDC, thus 'squishing' the mixture on that side towards the plug, causing a turbulent mixing of the charge.     

But this is designed to just mix the charge, like stirring coffee with a spoon.   "Swirl" is a later and more sophisticated idea, to produce a stratified charge, with a small, rich area in the middle for easy ignition, and the rest of the charge lean for economy and emissions.  It relies not only on the shape of the inlet duct and the chamber, but how the injector squirts, usually directly into the chamber.  Can't do that on our old lumps!

John

 

[Ian Foster]

5 hours ago, JohnD said:

I found that the foam soaks up resin!

John

You need to be careful with the choice of foam. I used a blue rigid foam on the rudder build, gluing blocks of foam into a thin ply structure a bit like a model aircraft wing. The foam was then shaped to the aero foil ribs and then sheathed in epoxy/E -glass laminate. Carbon reinforcement would have been classed as an exotic material under the rating rule and attract a rating penalty. It was extremely light and worked really well (hydrodynamically) replacing heavy, old fashioned glassfibre shell construction.

 

6 hours ago, johny said:

Only one way to find out - cant wait😍 Might be a business opportunity here....

I might progress along the lines of making the box fit a standard backplate, so that might be of use to others currently using a standard airbox arrangement. Business opportunity...I'm not sure I want to be knocking out multiple units, but you never know. 

Ian

Edited March 29 by Ian Foster
spelling

[JohnD]

Thank you, Ian!     "Blue rigid foam"?   Can you be more specific, please? Is that Extruded Polystyrene (XPS) or "Blue Board" to the construction industry?  That would not absorb resin as it it 'closed cell'.

And what 'glue' did you use?    I ask that  because I used polyurethane foam, because I knew, from experience, that polystyrenes DISSOLVE in the resins used with glass fibre!   Does carbon fibre use a different  class of resin?

 

On taking a mould from your model, and making it 'non-stick', covering it in body filler would work but you would risk making the mould fractionally bigger than the model.    Paint, polish and PVA?     

Or, and this seems to be little known, ordinary brown parcel tape does not stick to GRP resin!    Covering your model with tape will not significantly alter its dimensions.   A coat of release agent does no harm.

     I've found this very useful when taking a mould from rusted car panels, which would otherwise need to be virtually restored with filler.  Just bridge the gap with tape!

John

Edited March 29 by JohnD

[Ian Foster]

John

Yes blue rigid closed cell foam as used in the construction industry. Epoxy resin will bond to it, but does not soak in.

For nice parts creating a good finish on the pattern is required, as is multi layers of special release polish. I have used polish on brown parcel tape surfaced hardboard for one-off flat components. 

If you are really interested, this is a link to part 1 in a tutorial series by Easy Composites, showing pattern making. https://www.youtube.com/watch?v=XeeSsmsm9h0  Subsequent parts in the series cover mould making, laying up the part, vacuum bagging and curing.

For home produced parts it is possible to omit the vacuum bagging, but the risk of bridging and air bubbles increases. Not all epoxies need to be post cured (with heat), but this is usually required for structural components. Aerospace and other high tech industries use autoclaves which provide heat and pressure.

It's a fascinating subject, but for one off parts quite a lot of faff if you go the whole hog.

Ian

 

 

Edited March 30 by Ian Foster
grammer

[micmak]

Holy smokes Guys, My little question about missing inlet hoses/tubes has taken on a whole new life of it’s own!  I never expected such technical and detailed reactions.  All I wanted to know is if some sort of tubes/hoses were supposed to be there, and if so, would they help to keep fuel cooler in the carbs.  But you guys have taken this is a very technical direction, which is waay over my head.  I feel positively dumbfounded reading some of these responses!  Makes me wish I had taken a more technical route in life. I am sure it would have been more rewarding because even though I don't fully understand some of the responses, I DO appreciate them, and I try to understand them. It is nice to read them and try to make sense of them.

Thanks Everyone.

 …..Mick…..

[Unkel Kunkel]

Having a spare air cleaner box began led to ‘  doodling’  in metal regarding  the air box  / piping of MK 1V Spitfire.

The internals of the mk1V Spitfire air box were opened up  and reworked and welded The front was reshaped and  adapted the to accept wider intake pipes.

They are not a huge lot  wider - but  Hagen  and Poiseuille tell us that a small change in pipe radius can have a significant effect on flow.

New wider flexible piping was needed.

That came from the  garden centre  (pool  piping) .It looks very similar to the original, is quite inexpensive  and a bit smoother on the  inside than the original type pipe.

 

A more direct route for the pipes means a hole though the radiator  support and repositioning the rad water overflow/ expansion tank to the other side - the space seems more  appropriate for it that side.

To do that means unsoldering the pipe and routing  in a new copper  pipe from the rad filler neck ( 8mm central heating micro-bore  capillary solder type fittings)m supporting bracket to rad. frame.

Scrap aluminium  sheet for the air deflectors  from  cereal packet templates and similar  with pipe scrap  used to fabricate the ‘ cow bell’ intake .( This was formed on a piece of drum kit of the same name) A stainless edged  detachable ‘bug catcher’ mesh cover was formed over a wooden dolly. 

Anti vibration mounts hold the oil cooler to the radiator bracket and the the chassis rails.

I doubt whether it has any significant beneficial effect, but the process was interesting.

 

[JohnD]

Micmac,

Glad to help!  Even when unasked. Please ask questions if  needed, the only stupid question is the one you don't ask!

Ian, thanks for the link!   When I built my Silverback, as well as the hatch back door, I built GRP roof and doors,both incorporating honeycomb contributing to enormous stiffness and lightness.  EG doors 5kgs with handles and hinges.

 

John