Spun Aluminium Donuts, And Airboxes
 

Iam considering in making a custom airbox for my 51mm weber and instead of opting for the normal ram tubes, im going to go for Spun aluminium donuts, though i dont know where to start, if anyone can give me information on spun aluminium donuts and how to use them in making an airbox to gain some good power gains please let me know, ive looked all over the net it seems donuts are something that people just dont use that much so im really left in the dark on this.

Im hoping to run a sealed airbox with donuts, and a cool air intake, or even a bonnet scoop to collect air

What length will your system be? What kind of porting do you have?

51mm weber, 13b Primary and secondary bridge port,

exhaust 2, 2 inch primarys collecting into 3" at the diff

I've been using an IDA for 11 years now, and I don't know what you mean by alum. donuts. Do you have any pictures to show what you mean?

What diameter is the top of your carb? Is it stock 48IDA (which is 51mm at the top,) or has it been machined bigger too? If you want max flow from an IDA with 51mm throttle butterflies, then you have to have the top of the carb machined much larger too, and replace the booster venturies too. If the top of the carb is not opened larger than 51mm, then you will not get the full flow advantage of the 51mm butterflies at the bottom of the carb..

This is what mine looks like without my sealed airbox around it.

Carb work performed by JayCee http://www.jaycee-ent.com/carburatorparts.html

I would think that the donuts are just a way to easily machine some sort of nice velocity stack for the top of the carb. The ones you buy for these carbs are nice and thin which means hard to copy on a lathe(not impossible though). They would also give you some nice thick material to weld to for the rest of the box construction.
The reason people machine their 48 IDA's to a 51 is to allow a larger choke size without losing the venturi action in the choke. There is a ratio of carb size to venturi size that gives the smoothest air flow through the whole carb while still giving you the needed pressure drop in the venturi.

Hempie - i don't understand how you would use a complete donut for the inlet to the weber? Cutting it in half you could use it to form 2 inlets, but they are nowhere near long enough!

http://www.speed-technology.com/efi_filters.html

Those are what people normally use. I think you are trying to re-invent the wheel. A spun alloy donut is simply a 360 degree piece of pipe that comes around a joins itself. How you would use it i don't know. And to get one small enough i'm sure either. The Pro Series ram tube is what you want. They can be cut down slightly to get the right length for most power. It has the rolled lip which i think is what you're trying to achieve. Between 70-80% of your airflow going into the ram is from under the lip and around that curled lip, so don't make the airbox to close to it.

For N/A, i'd be using this: http://www.hymee.com/philsskills/slides/Dscn6731.html

Wow hymee sure likes to custom cast aluminum. Casting has almost become a lost art.

Ever heard of Phil Laird? He's a bit of a legend over here. That's all his manifold's. A 20BT version, a 13B IDA and the 13B IPRA (N/A) manifold.

My new manifold will be a 2 rotor version of the 20BT cast manifold. Looks sexy :D All the top drag guys use them. One picked up 20mph by switching to the 20BT manifold!

Have to agree with Lukus about re-inventing the wheel comment. You will also have to pretty good with the tig joining the donut to the convergent(?) section of the trumpet to get the laminar flow you are hoping for.

A good design resource is the Vizard book "Tuning the A series engine" (I think - hazy memory!!). Basically has a lot of flow bench work on varying radiuses and different angles in the trumpet. From memory, the 270* radius was considered the best and 7* to the auxiliary venturi. I'd imagine the SB Chev book has the info as well.

Do the Injection Perfection spun alloy velocity stacks fit an IDA by any chance? That's the route I would go. Another possibility is Stewart Wilkins Rallying, he gets CNC'd stacks (in 6061) to suit DCOEs, NFI unfortunately whether DCOE and IDA have compatible mounts. Can anyone comment?