Air box

greenamex2 · 7 Nov 2005, 14:57 (Ref:1454253)

How big, in terms of volume and intake area, should you make an air box?

Are there any clever little rules of thumb, formulae etc to flow?

Out of interest I know how big NOT to make one, it has cost me 12BHP at the top end all year!

johnw · 7 Nov 2005, 15:19 (Ref:1454274)

And I've always believed that size doesn't matter

Dauntless · 7 Nov 2005, 16:52 (Ref:1454361)

What engine, Denis? Does it have to breath through an intake restrictor? Normally aspirated or boosted?

Lot's of unknowns here for those of us who don't know what kind of car you refer to.

greenamex2 · 7 Nov 2005, 16:58 (Ref:1454365)

Toyota 1600 16 valve revving to 9500RPM. Normally aspirated.

In American speak, Formula Atlantic.

I was more after general guidelines that specific recommendations for a given engine.

Dauntless · 7 Nov 2005, 18:12 (Ref:1454453)

Under ideal circumstances (when do those occur?) one should have the walls of the airbox at least the diameter of the trumpet mouths away from the mouths themselves, in 3 dimensions.

That general enough?

greenamex2 · 7 Nov 2005, 18:46 (Ref:1454497)

Yes. Any guidelines for the intake into the air box?

Dauntless · 7 Nov 2005, 20:00 (Ref:1454572)

Yes, the usual equation is D=(CIDxVExRPM)/(IVx1130). Assuming very good preparation for your engine, that works out to:

D=(97x0.9x9000)/203,400=3.86" diameter (~98mm)

If the head work is not first class, use 0.85 for the VE instead of 0.9. The 180 in the second term is intake velocity in feet per second. 180 is a good number, but if you prefer another, go ahead and use it.

So, where does your present intake fall short?

greenamex2 · 7 Nov 2005, 21:37 (Ref:1454669)

Oh dear.

I had an intake of about 65mm ID which was strangling the engine by 12BHP.

I now have an intake of about 74mm ID, an improvement of about 30% in area but possibly still not enough. It will have do for this year. At least I haven't gone too large.

More worryingly, my V6 will need something like 6" ID. Not a hope in hell.

Interestingly when I originally ordered a filter for the engine they supplied a 95mm ID one. I now see where they got their size from.

Thanks Dauntless, another one for the list of useful things I wish I had known BEFORE I bought something!

Dauntless · 7 Nov 2005, 22:30 (Ref:1454737)

74mm is good to only about 7000 RPM, so I recommend switching to the 95mm intake ASAP.

As for the V-6, you can use two smaller intakes whose combined cross-sectional areas equals the same area as a single intake. Just use half the displacement of the engine in the same equation as above.

The trick is to feed the right amount of air at the right velocity to ensure optimal filling of the cylinders. It doesn't have to be in just one pathway. Also, the intake tubing doesn't have to be round. You can use a rectangular intake if needed for fitment purposes. Just try to round the inside corners to avoid turbulence.

graham bahr · 8 Nov 2005, 17:33 (Ref:1455362)

now, how can we work D types jag engine in to this thread??????

greenamex2 · 8 Nov 2005, 18:18 (Ref:1455406)

Quote:

now, how can we work D types jag engine in to this thread??????

Does he need an airbox for his new bigger carbs to get cold air in as the exhaust wrap doesn't work?

ian_w · 8 Nov 2005, 19:20 (Ref:1455483)

The suggestion of at least one 'pipe diameter' between the walls and the trumpet is very sound as a minimum, ideally go for more.

Not sure where the equation for intake size cames from, the result of 98mm sounds very big to me. You should remember that the peak airflow rated thro the airbox entry is the same as that for each individual runner on a 4 cyl engine.. ( It will be higher on a V8 for instance as there will be two cylinders breathing at once on certain periods of the cycle - the equation may be more appropriate to them ).

I guess that your inlet runners are probably in the range 45-50mm, I would have thought that an airbox intake dia of 75mm ( i.e. approx twice the runner area ) would be plenty.

As regards the question of volume, from my experience the best airbox volume for max power is usually the size of the engine dyno cell, i.e. no airbox! I would suggest as big as you can fit in the car as the best bet.

FilW · 8 Nov 2005, 19:27 (Ref:1455492)

Quote:

Originally Posted by Dauntless

Yes, the usual equation is D=(CIDxVExRPM)/(IVx1130). Assuming very good preparation for your engine, that works out to:

D=(97x0.9x9000)/203,400=3.86" diameter (~98mm)

If the head work is not first class, use 0.85 for the VE instead of 0.9. The 180 in the second term is intake velocity in feet per second. 180 is a good number, but if you prefer another, go ahead and use it.

So, where does your present intake fall short?

Can you tell me what CID stands for?
why is the IV 180

or am I being thick

greenamex2 · 8 Nov 2005, 19:45 (Ref:1455521)

Cubic Inch Displacement.

That is the assumed air speed going into the engine.

Dauntless · 8 Nov 2005, 21:58 (Ref:1455675)

Ian, while the exact origin of the equation is unknown to me, it is widely known and used over here. The equation is also consistent with the work of Smith & Morrison's Scientific Design of Exhaust & Intake Systems and Taylor's The Internal-Combustion Engine in Theory and Practice. If you have other equations, I'd love to see them as I am always looking for new tools.

FilW, CID is the acronym for 'cubic inch dimension'. To roughly convert liters to CID, multiply liters by 61. IV stands for 'intake velocity', referring to the speed of the air flow through the intake tract in feet per second. Sorry I didn't make that clear in my initial post. 180 feet per second is a good value for air being routed to a closed plenum feeding 3 or 4 individual cylinder intake runners, as it results in an intake velocity (Z) of 0.4-0.6 mach at the intake valve (VE maximizes at Z=0.5 for a broad range of engine speeds and inlet valve diameters, lifts and shapes).

Hope that helps clarify things.

graham bahr · 9 Nov 2005, 00:08 (Ref:1455822)

Quote:

Originally Posted by Denis Bassom

Does he need an airbox for his new bigger carbs to get cold air in as the exhaust wrap doesn't work?

yep i guess so, oh and we mustn't forget to size the air box to match Dtype's cams as well

FilW · 9 Nov 2005, 00:36 (Ref:1455851)

Thankyou to Dauntless & Dennis.

R59 · 9 Nov 2005, 06:53 (Ref:1455997)

Denis, I assume from this you're looking at a conical type filter?

Have you not considered going to a flat panel filter instead - you can therefore get a huge cross sectional area, and then feed that from a fabricated intake of whatever size you can get away with.

Rob.

quick_dry · 12 Nov 2005, 03:41 (Ref:1458604)

*probably silly newb ideas/questions*
Would a V packaged filter work better due to more filter surface area? (depending on airbox dimensions that would fit inside the car).

<tangent>
Are there any general design rules for the shape of the airbox design, particularly in forced induction setups? just plug the figures into the equation given? (my interest is in trying to build for a twin turbo inline engine which ends with one turbo inlet sitting about 8 inches behind the other) I'm concerned the front turbo may 'starve' the second mounted further back.

Should there be bellmouths protruding into the box, or should the rear wall flow into the bellmouths? I've read conflicting ideas - and seen some CFD diagrams that showed the latter arrangement being better.

</tangent>

greenamex2 · 12 Nov 2005, 09:49 (Ref:1458689)

Rob, no room for a flat panel filter. However probably not a bad idea for the V6 as there is plenty of room above the V for an air box/filter arrangement. I think the race car in going to get one of those Reverie remote air filter thingies.

quick_dry, like the idea of a V type filter (ie two flat panels), especially as I will need two inlets to the air box. I'll get me calculator!

JohnD · 14 Nov 2005, 00:03 (Ref:1459985)

Dauntless,
I'm puzzled.

A four stroke engine with a capacity of 1600cc will intake 800mls/revolution
At 9000 rpm that's (800x9000)/1000 litres/min = 7200l/m
Allow volumetric efficiency of 0.9, thats 6480l/m

A circular intake with diameter 98mm has an area of 75cm^2.
A column of air with that cross sectional area and volume will be 864metres long.
That will pass in one minute, so in one second there will pass 14.4 meters.
So the air being drawn into the 1600 engine through a 98mm orifice will pass through the orifice at 14.4m/sec, or about 43ft/sec.
(Ignoring air resistance, turbulence etc.)

Not 180ft/sec.

Is my maths wrong?

John

knighty · 14 Nov 2005, 11:33 (Ref:1460279)

dauntless, I think I have seen this equation in one of Carol Smiths books before, but it wasn’t as well as explained as you have done…..just one question…….do you know where the figure of 1130 comes from?......what does it stand for?.......

Kev_205 · 14 Nov 2005, 14:36 (Ref:1460401)

As my engine has throttle bodies is there a different equation for the size of the airbox?

I read on another site that the airbox should expand to slow the air as the faster stream of air from the higher pressure inlet will compress the slower air infront....

Seems like there are so many variables that its trial and error on design!

quick_dry · 14 Nov 2005, 15:07 (Ref:1460419)

you're not thinking of variable length runners?

THR · 16 Nov 2005, 20:58 (Ref:1462631)

johnd, i guess the equation is more complicated than that, as it would take in to account change in density of air, the pressure change would change the volume used? like the engine is sucking rather than being pushed... altho thiking about that, i bet single seaters with air boxses like f1, have ram effect and this changes the whole lot!! lol
who knows where the equation came from, being an engineer i like to know where these numbers come from too!!!
i suspect this one was done from trail and error!!