Difference between IDA & IDF Webber Carbs?

[rebelyell]

I see several ads on fleabay etc for 40mm Downdraft twin choke Webber Brand New for $70 - $90. I doubt they're really european-made Webbers but look like em to me. I do Not see similar pricing for DCOE side draft carbs.

[modok]

Generally the venturi should be between 75-86% of carburetor bore

so 36 IDFs could use a 27mm venturi, which is actually HALF the size of a 38mm venturi
So they DO make carburetors "small enough"

8 small throats might be a great idea for fuel distribution, but maybe not flow. The wall friction of 8 small ventiuris is going to be much higher than 4 larger ones.

[Geoff2]

I changed factory 40mm chokes in 48 IDF carbs to 43mm & lost power everywhere. No improvement in top end, which I was expecting.
Sometimes carb companies know what they are doing....

[BCjohnny]

Agree with mostly what's been said, one thing I'm not sure has been mentioned but ......

The IDA is physically a bigger carb than the IDF, bore centreline being about 4 3/4" / 3" respectively IIRC

One may 'fit' the tunnel ram 'better' than the other, flow wise

I know you're not running one but if you look at IR SBC manis you can see the relative 'mismatch' in bore spacing

[Caprimaniac]

When it comes ro flow numbers.... not much published.

If you got a bench + weber 48idf + various sec chokes; some numbers are wanted...

43 on the 48 idf? Ok, thr biggest catalogue ssec chokes I found was 40mm.

[frnkeore]

I now have a book on Webers, by Pat Braden. The title is "Weber Carburetors". Copy righted and printed in 1988 so, there is not much new in Weber tech, since it covers all the ones I'm interested in.

I've answered the title question, with help here and in the book. "Difference between IDA & IDF Weber Carbs?" The answer for that is, the IDF, is a more street friendly carb and has a different bore center distance. The Etube, main and air correctors, are the same. I'm adding pictures, from the book to show the bore center and bolt pattern, for IDA, IDF & DCOE. The DCOE & IDF have the same spacing of 90mm (3.543) and the IDA is 120mm (4.724). So, you can't mount IDF's on a IDA manifold.

I actually had this book, all a long and forgot about it, I've most likely got it in ~1990. I found it while looking for another book, last week. It mentions tunnel rams but, doesn't address setting a system up. I've got a lot to read and will post more info, from the book, as I discover things. I would still, very much like to hear from people that have work with any of them.

Regarding choke sizing, since they are round and removable, you can resize them yourself, in a lathe but, you have to know what your doing. You can't just bore them straight threw. You have to maintain a full, entry radius and make it tangent to a smooth draft angle, in order to maintain a venturi.

[Geoff2]

The best book on Webers is by the late John Passini. ISBN# 9781855207592. His carb knowledge is greater than any other 'guru' I have seen or read about in books or on forums. Example: what does changing the nozzle area in the aux venturi do to mixture strength, if anything. Answer is in this book.

[chimpvalet]

Please share Passini's instruction on aux venturi sizing, it's long years since my copies of his books went astray and his style of writing made it challenging to grasp the point in the first place.

Cheers

[RevTheory]

Please share Passini's instruction on aux venturi sizing, it's long years since my copies of his books went astray and his style of writing made it challenging to grasp the point in the first place.

Cheers

I have two of his books but they're too large to attach. You're welcome to PM me your email address.

[chevyfreak]

Just out of curiousity, do any of those books give the airflow ,
Ida / idf,
44 with 36 venturi
48 with 40 venturi
Searching the web gives mixed results, and dont always state venturi sizes and at what depression it is flowed.

Chevyfreak.

[Geoff2]

D. Vizard quotes some airflow #s in his A-series engine book. 48 IDA, 38mm chokes 296 cfm; Delorto 48 DHLA, 38mm chokes 283. 45 DCOE 240-245 cfm. These are for one barrel.

[chevyfreak]

D. Vizard quotes some airflow #s in his A-series engine book. 48 IDA, 38mm chokes 296 cfm; Delorto 48 DHLA, 38mm chokes 283. 45 DCOE 240-245 cfm. These are for one barrel.

Thanks.
Chevyfreak

[Caprimaniac]

So, roughly 300cfm/ barrel. So 4 48’s on a common plenum give 2400 cfm for the engine to engulf. That should be plebty....

Go on, OP- do it....

Go with smaller venturis, like 30 or something. Read Passini and otherd to find clues on the size for first try.

[Geoff2]

Capri,
It doesn't work that way...
Take a single plane intake manifold with a 800 cfm carb. Each cyl can draw 800 cfm. With a 48 IDF/IDA, with an IR manifold, each cyl can only draw about 300 cfm.

[David Vizard]

I have recently finished helping a client build a SBF with 44 IDF carbs. Cannot remember what they flowed but it was a lot less than an IDA.
Have a lot of experience with IDA carbs. Had a set of 48's on my 350 Chevy truck. Drivability was great but they have to be properly calibrated. That I guess is one of the advantages of having your own dyno.

The IDF's have a less than optimal Aux venturi - at least it is not anything like as functional as the IDA ones.

This winter I expect to do a rebuild on my clients otherwise new IDF's. The intent is to redesign the aux venturi so that it flows more and the signal is higher as well as better fuel atomization. Also the butterflies need work to flow more.
Main ventury size is critical. I did a lot of aux signal vs venturi diameter tests. One mm too big can cost a lot of output. Start small and work your way up. For the application you are concerned with here 37 or 38 for an 48 IDA seems like a good starting point for an IR system.
DV