Cold air box shape - advice needed

[alolympic]

Hi Guys,
Long term lurker, and now time for starting my 1st thread.
I'm asking on here because it seems to be the best place to find people who actually know what they're doing, rather than just armchair experts
So, my question relates to a cold air box I am making for my classic car.
Short question is this, should a cold air box be of a regular shape to ensure the air in the box is less turbulent, or, is it better to have a slightly bigger box of an irregular shape?
Long question -
The airbox I am making will be fitted direct on to my Twin 45's, with the trumpets located in the box.
I am modding a Pipercross 600 box like this -

Now personally, I don't think the Pipercross box has anything like the air capacity necessary for most applications, it's just too shallow.
So, 1st step was to buy a deep base plate like this -

This will give the box approx 60mm more depth and therefore more volume.
Now, I can actually make the box deeper again with a bit of cutting and adding a band of GRP to follow the same shape.
Now, the depth I can extend to is restricted by the brake master cylinder in one place, and then the battery in the other.
So, should I add extra volume to the box by extending the box in between these parts, or, should I keep the air box to a regular rectangular shape?

Gut feel says the bigger the box the better but I'm not sure how big an influence a funny shaped box would make.
Any advice gladly received.
Cheers

[phoenix]

I don't think there is any advantage to be gained from having a larger volume "airbox".

What is critical, though, is that there is a clearance between the top of your trumpets and the 'lid' of the box. The should be a minimum of trumpet bore diameter/2 - or in your case 22.5mm. It looks like the deeper 'tray' you have bought will give plenty of clearance if the trumpets fitted inside the shallower version.

I don't like the look of the flexible pipe sold with this, and other, cold air kits. I'm only guessing, but I would think the airflow is much less than it could be due to the rough inner surface casued by the spiral wire reinforcing. I would think that substituting a combination of smooth bore silicone hoses and alloy pipe of suitable bore would give a better flow - though I could easily be wrong and the improvement might not be noticeable.

[alolympic]

Thanks for the response.
Going off what you say, I will have less fabrication than I had feared!
The size of the box will certainly be well clear of the trumpet bellmouth.
I am not going to use the cold air pipe as provided, in fact, there is little need as the front of the box will be very close to the air intake for the car.
Cheers

[knighty]

Hi Alolympic - I have designed many air box and plenum systems for professioal race engines in the past, and there are some important rules of thumb to adhere to........

1) as mentioned by Pheonix - dont let the air box cover get closer to the bell mouth by less than the bell mouths outer diameter

2) you want to achieve equal air distribution accross the faces of the trumpets - avoid the type in the image that you posted like the plague, what happens is the air rushes in, bypasses the first two cylinders and creates a head of pressure in the back of the box, the net result being the cylinder closest to the plenum inlet will run a bit ritcher, the rears will run a bit leaner........for race motors we used to install thermo couples in each of the exhaust manifold runners and aim to achieve a temperature distribution of no more than 50DegC between all 4 exhaust branches........to avoid all that polave - the simple fix for this is to run a vertical central feed air box, avoid the end feed types.

3) As yours is an air-box (pre throttle) and not a plenum chamber (post throttle) the volume of the air box should not make a great deal........but for sure I myself have noticed a difference if you run one or not, if you dont run one, expect your mid range performance to be a bit down, what happens is the helm-holtz effect, whereby the air-harmonics of the air-box basically force a bit more air down the intake runners at certain air frequencies........the best analogy is this - humm into an empty jam/coffee jar, at certain humm frequencies the tops of your lips will start to tickle.......thats the same effect inside an airbox.......as a rough ball part I would say the volume should be about 2-4 times the engine capacity........

4) if you were running a plenum chamber (post throttle) - bigger is better, I designed a 13 litre vertical central feed plenum for the proton BTCC engine back in 2003 and the thing was a missile, this was running a single 60mm throttle body, no issues with drivability either.

5) the real trick with an air box is cold air supply - colder air will make a real difference to power, so lag your exhaust manifold, and ensure the pipe work is as short and direct as possible

6) ram-air......this only takes effect above about 80mph, worth it for a race car, but not a road car, also for a carbed engine you need to take care that the fuel dosent blow out of the float chambers, hence you need to equalise the pressure in the carbs and the ram-air-box........as a guide for a 170bhp honda blackbird 1100cc bike engine, running a ram-air system is worth about 1 to 2 tenths of a second on a 1/4 mile drag strip........the design of the diffuser is critical in order that it effectivley converts flow into pressure........I ran it on my race car and quite simply my car was the fastest in a straight line, its worth about 1-2% power increase........let me know if you want to know more about this, I have some sums somewhere on how to accuratley size the inlet of the ram air box......best of luck!

[alolympic]

Thanks Knighty, loads of useful stuff to take on board there.
I currently have a base plate as in the pic, and beyond that will be fabricating the other section of the box from scratch - so it is good to know this stuff now.
My one potential problem will be where to position the inlet. The area around the box is pretty chocker, and in effect I am forced to have the inlet on one side, facing the front of the car. This isn't ideal based on what you are saying however.

I am also hoping someone can clarify about ram air. My intention was to run an intake pipe to the front of the car, behind the grille. This is partly for the ram air effect (which may be a red herring) but also, because I presume it will be a good location for finding cool air. So, even if the ram air effect won't come into play, can I presume that the air entering this area will be the coolest anyway, making it still the best option?
I plan on using a large bellmouth on the end of the intake pipe.

[knighty]

no problem.......if you are screwed for package space dont give up on the central feed concept just yet, take a look at this Duratec 1.6 formula ford engine plenum chamber

http://www.mountune.com/Racing/Engines/formulaford

you will notice that the triangular link (throttle body to plentm) is actually tapered, so it creates equal distribution accross all 4 trumpets........this was a similar trick that we employed on many turbocharged engines too, and it works very well, the slot needs to be a constant width of about 15-20mm and the length of the 3 bore centres (centre of front trumpet to centre of rear trumpet)........aim to spray the air just infront of the trumpets.

Ram-air wise.......forward facing and low mounted is best, the grill will be ideal - this is the highest pressure region - forget naca ducts.......a big bell mouth will help air entry for sure, nothing major just a half inch radius should be fine, you really shoulnt need an aperture bigger in area than a 4 inch pipe.

[knighty]

if your interested I just found the equation for calculating the minimum inner diameter for the grille mounted inlet:

CID x VE x RPM
-------------- = D
IV x 1130

Where

D = intake duct inner diameter (inches)
CID = cubic inch displacement
IV = intake velocity (180 feet/second)
VE = volumetric efficiency (0.85 for standard head, 0.9 for race ported head)

so as an example calculating for a 1600cc (97 CID) super 1600 rally engine producing 220bhp

97 x 0.9 x 9000
--------------- = 3.86 inches inner diameter
180 x 1130

best of luck ;-)

[JohnD]

knighty,
Aren't there two 'ram-effects'?
One is the Helmholtz, resonance effect, as you describe above.

The other is the apparent effect of the free-flow of oncoming air into a duct. Mounting the duct opening at the front of the car would seem to offer an advantage in that air is 'forced' down the duct, but isn't this an illusion? The apparent backwards pull on a hand held out in the free-flow is due to drag, not excess pressure.
At the car's front, there is a 'stagnation point' where the air is still in relation to the car. Your frontal intake will be nearby, so the air pressure at the mouth of the duct will be near atmospheric. Near atmospheric = no pressure differential between there and inside the duct (ignoring the engine intake of air) so no 'ram' effect.

John

[knighty]

Hi John - gawd knows mate - I'm no aerodynamacist......but what I should have added above is that a diffuser needs adding directly behind the grille mounted opening in order to efficiently convert flow into pressure......in reality that just needs to be a 7 degree (inclusive) expanding cone, the length being for about 4 times the size of the inlet ID.......if you can create 20-30 millibars of air pressure in the plenum chamber, ram-air is taking effect........be prepared for your engine fuelling to lean out a little, so you may want to be running some form of closed loop lambda fuelling system which has a plenum pressure transducer linked into the ECU.

[alolympic]

Originally Posted by JohnD

knighty,
Aren't there two 'ram-effects'?
One is the Helmholtz, resonance effect, as you describe above.

The other is the apparent effect of the free-flow of oncoming air into a duct. Mounting the duct opening at the front of the car would seem to offer an advantage in that air is 'forced' down the duct, but isn't this an illusion? The apparent backwards pull on a hand held out in the free-flow is due to drag, not excess pressure.
At the car's front, there is a 'stagnation point' where the air is still in relation to the car. Your frontal intake will be nearby, so the air pressure at the mouth of the duct will be near atmospheric. Near atmospheric = no pressure differential between there and inside the duct (ignoring the engine intake of air) so no 'ram' effect.

John

This seems true, but maybe it doesn't actually prove a concern? Either way, the result will be that the air in the airbox is a) as cool as possible based on limiting atmospheric conditions and b) the air box is full - provided through the constant flow of air through the intake, then the engine has sufficient cool air to be inducted into the carbs?
Is this as good as can be expected in very general terms, without more drastic means and expense?

[phoenix]

As you are posting in another thread about areodynamics, SHOULD you decide to fit a splitter/airdam at the front, then the area just above that will be a high pressure area so would be a good place to site your engine air intake (as long as you don't drive through deep water as a habit!)

You will notice this is where many competition cars have cooling air inlets for brakes and water/oil radiators for this very reason.

If you are not fitting a splitter, then an engine air intake at the front close to the radiator inlet will be a pretty good location and should get you air at ambient temperature into the engine.

Note that on a hot, sunny day, air near to the road is hotter than air higher up, because track (road) temperature will be a good deal hotter than air temperature.