WHY does the IDA work on a high power type 1 engine?

[wolfswest]

First of all I want to say I'm not a genius with carbs, jetting, emulsion tubes, venturi's etc...  But I always try to read some topics over here because I'm willing to learn and understand it a bit more.  I have F7 emulsion tubes and 120 idle jet holders.  I've read a bunch of topics that suggest that a F2 is a good emulsion tube and that a 110 or 115 idle jet holder could solve a hickup around 2200 rpm...   Now that's not the reason for this topic.

Now, I went with this knowledge to some engine builders / friends (non vw but high power applications) to chat.  One of them has a dyno and builds day in day out sweet italian vintage V8 and V12 stuff, so he knows "something".  Then there is this guy who builds motor cycle race engines and dyno them also, also a man with knowledge.  Then there are the gearhead friends: usa muscle, ricer rockets...   Then they start to ask questions: what's your venturi size, how much cc's, main jets...  and start to think, to calculate and they ALL have one conclusion: those IDA's will never work my son!  They are too big, switch to smaller carb, 40 or 44 and your engine will be smoother, drive better, have more HP.  They laugh and can't believe that we the hot aircooled vw people can be that stupid to run carbs THAT big.  If I mention that these carbs are regular on 1776 cc engines, they burst in tears of laughter! 

Now, we all know that the 48 IDA or even 51,5 IDA is a well known carb choice in the aircooled vw world AND it does the job quite well!  That's maybe an understatement:  It works awesome and produces a sweet amount of power, for over 40 years now.  Well the carbs doesn't produce the power but they help in the process, you know what I mean?

Okay, I do believe the hype of installing a carburetor THAT big on a small engine is part of the game, but that's not all to it.

WHAT'S THE REASON?  WHY does it work so good if it's against the "rules"  Is it some sort of black voodoo magic part from hell?     
Did some of you came across the same situation?    I mean, I don't care, I find it funny, but it makes me think...  Are we insane?  Are all the others insane?

Hope to see some reactions!

[ibg]

Once you get over 2 litres with a decent cam, 44's probably will be smoother and drive better, but they don't make more power. Don't forget Porsche put 46 IDA's on motors under 2 litres.
the other really important thing is the KOOL factor. 
I'll leave it to others to explain the science of why they work so well.

[65bug]

Not only the IDA! Hows about the DRLA? Both compact, and can flow alot of air/fuel in a relatively small area. Both made for smaller European motors.  I'm sure someone else can elaborate further.......

[modnrod]

When the V8/turbo guys have stopped laughing at all of us silly no-knowledge VW racers (the same ones I might add that have been kicking their butt whenever we try for the last 50 years..........), remind them that motorcycle manufacturers routinely use 36mm carbs on a 750cc-1000cc motor for street applications up to 8000rpm, then throw on 41mm carbs when they want to spin them up with power past 10000rpm.

It's the power output per cylinder that determines the size throttle/venturi needed, not the capacity of the motor.

[Erlend / bug66]

There is no rule you need to be over 2liters to make IDA's work

[John Maher]

What the "V8 guys" probably aren't taking into account is the fact 48IDAs are being used on isolated runner (IR) manifolds i.e. one choke per cylinder. A typical old-school V8 will have a single 4 barrel mounted on a plenum style manifold, where each cylinder takes its feed from ALL FOUR barrels rather than just ONE. On a standard VW IDA installation each cylinder is being fed by a single barrel - not four - therefore the size of each individual carb throat when used on an IR system has to be much larger than if they were bolted on a shared plenum. If the "V8 Guys" started using IR manifolds and carbs, they too would have to fit large bore carbs to make decent power. 

If an engine has sufficient airflow demand, (high flow heads, high valve lift etc), a pair of 48IDAs will continue to deliver beyond where a pair of 48IDFs will max out. I don't know exactly what the 48IDA's max power potential is but I'll guess it's somewhere around 230bhp. As we know, there are mouse motors capable of that power level, so using 48IDAs on small engines isn't necessarily overkill!

As engines get larger and head technology improves, even 48IDAS become restrictive. Hence the appearance of bored out versions (51.5mm) and completely new castings eg 62mm Terminators.

What started out as a racer's solution to making more power became a fashion item for the street crowd. If a street engine is making less than 200bhp, IDAs are overkill. In fact a smaller carb (eg IDF or DRLA) will deliver better all round performance. But for some, a pair of IDAs is an essential part of the overall look ;-)

[pupjoint]

Sorry to be out o the topic, But not many have tried Japanese carbs on vw engines.  I think JPM has tried mikunis on a type 1 but I have no details on them.

[hotrodsurplus]

What the "V8 guys" probably aren't taking into account is the fact 48IDAs are being used on isolated runner (IR) manifolds i.e. one choke per cylinder. A typical old-school V8 will have a single 4 barrel mounted on a plenum style manifold, where each cylinder takes its feed from ALL FOUR barrels rather than just ONE.

Exactly. As Shaun McCarthy once explained, a four-cylinder engine with an IR manifold is four independent engines each with its own carburetor connected by the crankshaft. If a cylinder needs 300cfm during its intake event it's going to need that whether it's fed by a plenum or an IR manifold.

And that's a good visualization. Most street V8 engines run a dual-plenum manifold. That effectively splits the carburetor in half which turns a V-8 into two four-cylinder engines connected at the crank. So a 600cfm carburetor offers each cylinder the potential flow of 300cfm. The garden-variety 350 Chevrolet runs about a 600cfm carburetor (which is overkill in most cases). So that's 300cfm potential offered to each cylinder.

To put it in perspective for your corn-rocket pals, to make that 'modest' 350ci engine perform to the same standards with an IR manifold would require four 48 IDAs each with about 42mm venturis. That combination would offer the potential flow of ~290cfm per choke. Were they all plopped on a plenum manifold then they would offer the potential flow of 2,320cfm for each cylinder filling event. That would be gross overkill.

Also explain to your V8 pals that the IDA's first really successful application was on the 289ci engines in Ford's GT40 and Shelby's Daytona racecars. Many moons ago I bought a set of sand-cast IDAs. Those are the ones that Shelby imported. That someone hogged out the 45mm venturis suggests that in stock form they were still inadequate for some application (I shudder to think what that was but I'm thinking 427).

Further compounding the issue is the rating standard. It's not consistent among all carburetors. The industry derives most two-barrel carburetors' flow ratings at 3" mercury (pressure differential) whereas it derives four-barrel flow ratings at 1.5" mercury. If four-barrel carburetors were tested at the two-barrel rating they would boast much greater (but not double) than their nominal rating. Conversely, if two-barrel carburetors were tested at the four-barrel rating then they would flow much less (again, not half).

[wolfswest]

okay, nice replies so far!  But, the italian V8's or V12's are using the same "technology": one barrel per cilinder...

[hotrodsurplus]

okay, nice replies so far!  But, the italian V8's or V12's are using the same "technology": one barrel per cilinder...

You seem to be hung up on throttle bore. Throttle bore is (almost) irrelevant. The venturi diameter is what determines the pressure differential and velocity. A smaller venturi creates a greater pressure differential and greater velocity than a larger venturi will but a larger venturi will flow more.

You can 'tune' a larger carburetor to operate on a smaller engine by using smaller venturis. It won't necessarily run well because the manufacturers design the remainder of the internal passages to operate properly within a specific range of venturi diameters but it will indeed work.

Case in point, my 42DCNFs came with 34mm venturis which is adequate to feed 540cc cylinders in an engine that produces most of its power from 3,500rpm to 5,500rpm. But I also have 28mm venturis which would suit an engine with 390ccs per cylinder that makes power between 2,000rpm and 4,500rpm. The part-throttle response and transition might not feel quite right with venturis outside of the carburetor's intended range but it would make optimum WOT power with 28mm venturis.

I hear your question: why do people choke down 48IDA carburetors to run them on a warm 1776 when in fact a pair of 40mm IDFs would suffice? They do it for the same reason that fat kids rock $300 Air Jordans: they want to be like Mike. They want the hella swagger, yo.

[65bug]

LOL...............In Like Flyn, The Cats Pajamas, BADASS,.
         This beckens me to ask this very important question: WHY NOT HAVE A BUNG FOR EACH EXHAUST PIPE. Not just one at the collector! This way you can really fine tune(my LM1) each barrel.....................


 

[JS]

Could it possibly be that the "1776" your friends are talking about make about 40hp/litre(let´s face it, most of them DO!), and that they are absolutely right in their conclusion?  

Most non-vw people don´t think that over 100hp/litre is possible with the vw flat-4. When I say that I plan 120hp/litre for this summer i get a lot of this:  

[Jon]

  .       

[wolfswest]

I hear your question: why do people choke down 48IDA carburetors to run them on a warm 1776 when in fact a pair of 40mm IDFs would suffice? They do it for the same reason that fat kids rock $300 Air Jordans: they want to be like Mike. They want the hella swagger, yo.

HAHAAAHAH!   


Most non-vw people don´t think that over 100hp/litre is possible with the vw flat-4. When I say that I plan 120hp/litre for this summer i get a lot of this: 
=>sounds bad ass Johnny!  Your engine is a 2276, 2332?  so we are talking about 260-270 NA HP here? 

[Jim Ratto]

okay, nice replies so far!  But, the italian V8's or V12's are using the same "technology": one barrel per cilinder...

Most Italian carbureted V8 or V12 are somewhere probably under 400cc per cylinder. That would equal a 1600cc VW motor. A 4.4L V12 with 39mm intake valves and 275' cams and one barrel per port is going to make max HP around 7100 rpm, and to do this requires "only" a 32-33mm venturi (in probably a 40mm carb). A 48mm carb isn't needed to feed this per cylinder capacity. Probably why these guys think 48mm carbs are taboo. Once you build a VW motor up around 2000cc and use a 320' cam, 44+ mm intake valves, etc, then you're in the arena where the 48IDA runs well. A 94 x 78 with a good 46mm head, Web 86C or FK87 type cam is going to peak somewhere around 6700 with a 39-40mm vent.
I think the reason Porsche used the 46IDM, etc, was the capability of the 692 /587 engines. Even the first 547 1500's had a 48mm intake valve and a super straight port design. And the cam patterns were nothing like what we use. Plus those cars (with the super tuned 4 cammed motors, not the tamer 40PII versions in the 356) were featehr wesgghits and geared to rev.