Sizing IR Carbs

[chimpvalet]

Hello all,

Working through selection of Weber DCOE carbs, asking whether anyone has flow ratings for 45's and 48's with various venturis?

[modok]

since a venturi is a simple shape, it will flow exactly what the math would indicate. Your just testing your flowbench. That is, if the venturi and carb are made correctly.

As a general rule, for "racing" say, venturi area 1.3x the MCSA of the intact tract
Meaning, the venturi is not supposed to be the smallest point of the system, maybe airspeed 75% of the max Although that isn't a hard rule.
In a very mild application it might be the smallest point, or in a very wild app it might be crazy big and work.

Venturi size 75-86% of carburetor bore, (above and below venturi).

Sizing the carbs to the heads should work great, assuming the heads are pretty good

[Caprimaniac]

You have probably seen the graphs/ tables with choke or bore size vs rom band for given cylinder volume?

There are some flow data out there too, but given sll the tuning done with webers, probably good enough ro follow paths already walked?

[Geoff2]

Weber 48 IDA is noted in one of DVs books as flowing 296 cfm per barrel. Factory choke/venturi size is 38mm. 45 DCOE with 38mm chokes 242 cfm per bl.

[MadBill]

In How to Build Horsepower; Vol. 2, David Vizard gave an IR sizing formula that determines the ideal choke size, which he says should be 72-80% of the butterfly diameter, preferably the latter: choke in mm = √ (CV x RPM/2600), where CV is cylinder volume in cc and RPM is that of peak power.

So for a 350 c.i. peaking at 7,000 RPM: √(43.75 x 16.39 x 7000/2600) = 44 mm, thus the ideal butterfly size is 55 -61 mm*. He says to round off choke size down, then after tuning you can try a size or two larger. *Although aftermarket copies are/were available with up to 62 mm butterflies, good luck finding something adequate to feed your 6500 RPM 502...

(Keep in mind that IR flow requirements are very different from plenum systems; since the air is only flowing <40% of the time, the former need far more flow capacity.)

[modok]

Looking at it purely in terms of cfm is not useful, the longer the tail is on the venturi, bigger bore below it, the more it will flow on a flwbench, but so what?
It will still choke when airpseed maxes out, same as a port.
Test it on an intake manifold matched to the head now the extra large carb bore not such bright idea, you sloe the air down then, speed it back up again a few inches later.
The only reason to go below 80% is for more fuel sheer, that does sometimes works better, but sometimes less torque, because you lost airspeed/runner momentum.
Size by airspeed. Pipemax can work for venturis too.

[Belgian1979]

In How to Build Horsepower; Vol. 2, David Vizard gave an IR sizing formula that determines the ideal choke size, which he says should be 72-80% of the butterfly diameter, preferably the latter: choke in mm = √ (CV x RPM/2600), where CV is cylinder volume in cc and RPM is that of peak power.

So for a 350 c.i. peaking at 7,000 RPM: √(43.75 x 16.39 x 7000/2600) = 44 mm, thus the ideal butterfly size is 55 -61 mm*. He says to round off choke size down, then after tuning you can try a size or two larger. *Although aftermarket copies are/were available with up to 62 mm butterflies, good luck finding something adequate to feed your 6500 RPM 502...

(Keep in mind that IR flow requirements are very different from plenum systems; since the air is only flowing <40% of the time, the former need far more flow capacity.)

50 mm is more than enough to get 350ci to 7000 rpm.

[MadBill]

Sure, and so is a 600 CFM carb, but 'more is better'.

The SBC engines in the 1963 factory Corvette Grand Sports were fitted with 58 mm side draft Webers.

[chimpvalet]

Appreciate the quality feedback chaps, thanks. I have Pipemax, perhaps someone can walk me through sizing carbs by way of that, as Modok suggests?

[Belgian1979]

Sure, and so is a 600 CFM carb, but 'more is better'.

The SBC engines in the 1963 factory Corvette Grand Sports were fitted with 58 mm side draft Webers.

A 58mm will make it nearly impossible to tune it at low and mid range rpm because a small opening will create a lot of airflow at those rpm. So I guess it depends on what you want. Indeed like with a carb.

[Geoff2]

Were these Corvette intakes an IR design?

[MadBill]

Yes, they were a cross-ram layout. I believe the carbs were originally developed for a 3.0 l Maserati 4 cyl racer, circa 1959.

[Belgian1979]

FWIW the grand sports had a 377 ci.

[MadBill]

Yeah, I didn't say 350 c.i., I said SBC.

BTW, for a 377" 7,000 RPM engine, DV's IR sizing formula comes up with 57 mm carbs. (But of course GM didn't have a lot of sizing choices short of building their own carbs..)

[modok]

Appreciate the quality feedback chaps, thanks. I have Pipemax, perhaps someone can walk me through sizing carbs by way of that, as Modok suggests?

Sorry it's not as fresh in my mind as it was a few years ago, but if you can get pipe max data tuned in, whatever it says for MIN port area, 1.3 times that should be well in the ballpark for venturi area.