Filter to Air Box Trunking Diameter Why so Big?

[snowyelan]

Hi Donels,

The 32mm diameter is required to create vacuum relative to intake duct pressure in order to draw fuel up from the bowl. I don't know webers/dellortos very well but on stromberg carbs the bowl vent is on the same face as the carb throat opening and would have close to no difference in pressure if the duct was 32mm as well.

[snowyelan]

Hi Hal,
I think I understand what you are writing about. In simple terms the pressure in the 2 smaller pipes is higher than in the larger filter/duct area because they are the bottleneck.

Here's a rather dry video explaining it.
https://www.youtube.com/watch?v=DUEGv8HXK8c

The amount of heat is the same based on mass but because it is a lower pressure in the bigger duct the amount of heat based on volume is lower. Cooler is generally accepted as better, but in this case, because the mass per unit of volume is lower, there is no gain in available oxygen to burn.

[HCA]

As I say Scott, I am more than happy to stand corrected!

This was all explained to me by someone way back who worked for, guess who?, K&N! He was defending the continual stick they get from people saying that their 'cold air intakes' do nothing but generate cash for them..

Despite all the negativity surrounding this product, I use one in the front of my +2. I have not a clue if it makes the air any colder, but it looks good and the engine is quite sweet (but this is probably due to the EFI than claims from K&N

[andyelan]

Hi Richard

You might be interested in my comments posted here

viewtopic.php?f=40&t=48763&start=15

Regards
Andy

[Baggy2]

Just a totally uneducated thought - the diameter of the trunking is so large compared with the inlet ports I would have thought the speed of the gas flow was rather slow - could the ducting be regarded as part of a plenum chamber rather than a duct?

[StressCraxx]

Just a totally uneducated thought - the diameter of the trunking is so large compared with the inlet ports I would have thought the speed of the gas flow was rather slow - could the ducting be regarded as part of a plenum chamber rather than a duct?

Good point. I think it may be related to the distance from the air filter in front of the radiator to the airbox opening. The flow losses in corrugated hose under slight vacuum can be significant.

The early airbox on the weber carbed cars is known to sacrifice flow to the #4 cylinder because the airbox cover is too close to the trumpet opening. It can be corrected with a deeper airbox.

[Emma-Knight]

And don’t forget about „induction loading“, not shure about that word.
A defined amount of air in move towards the inlet valve, stopped, building up pressure in the quite large duct volume, is then pressed in the now open next valve. It gives torque at low revs, especially nice for small volume engines. It „breaks“ a little at high revs (streaming resistance). Thats why modern cars have switching ducts, long high volume at low revs, short low volume for free high revs.

[HCA]

Possibly here is as good as any to post a question that has been lurking in the back of my mind... Maybe Scott or Rohan or one of the racing guys might have some thoughts.

Two complaints of the +2 breathing are, lack of depth at the rear of the airbox, and deformation of the trunking to the air filter, especially where a full width radiator is fitted.

I think a grp airbox could be fabricated to give more depth at No. 4 intake, but the only way I can see to ensure no deformation of the trunking is to cut into the body, or, use a narrower trunking. maybe 65 or 70mm diameter - but what might this reduction do to the overall performance?

[RichardHawkins]

Hal,

My thoughts behind the original question were along the lines you just mention. Fitting this trunking past the radiator without kinking or deforming seems just about impossible, whilst changing to a smaller diameter would definitely help.

Judging from memory (not my most reliable instrument) I don’t think I have ever owned a vehicle with such a large inlet tract. I was hoping to find an answer without having to actually try something smaller.

Richard Hawkins

[snowyelan]

My knowledge of automotive intake systems is limited. Best practices with flow are minimizing restrictions and any sharp transitions. I'm not familiar with the +2 duct, but my elan has a tight spot past the rad too. I wouldn't call it a kink. More of a gradual transition from round to oval and back to round.
I have access to flow modelling software but the complexity of modelling up corrugations, kinks, bends, etc aren't worth the time for what I suspect are miniscule gains. I think more gains would be made with simple stub stacks and a deeper airbox to smooth flow at the box to carb transition where air flow is perpendicular to the carb throat. I may have a go at modelling this up as the geometry is simpler.

[lotusfan]

Fitting this trunking past the radiator without kinking or deforming seems just about impossible, whilst changing to a smaller diameter would definitely help.

Richard, you have an S4 I think. Attached photo of my duct and it is not very much deformed passing the rad which is a standard S4/Sprint rad.

[RichardHawkins]

Mike,

Thanks for the photo and reply. Yes I have an S4, an early 1968 car with the wide S3 radiator. Your photo also gives me a better idea of where the air filter should be.

Thanks again,

Richard Hawkins