Calculation of carburetor size/requirements (Hard core Weberphiles only)

Word from the flow bench is that the 3.2 qv ported heads and manifolds are now flowing at target, but the 40mm carbs currently used for induction are a pretty significant restriction. From what I've read, the venturi is not the principal restriction in a carburetor, but the other things (butterflies, secondary venturies) that hang out in the wind are more problematic.

Of course, for a street engine the desire is to keep a nicely responsive engine in the low ranges and not go chasing higher numbers. The current 40DCNFs have been great with the 34 mm venturis and are tolerable with 36s, and with the stock head being the restriction, they both produced the same peak hp. SO, I took this opportunity to go back and theoretically mathematically estimate carb size requirements.

The venturi size in mm is theoretically required is determined by:

20 times the square root of (V/1000 times N/1000)*

where V = size of one cylinder in cc, and N = rpm where peak hp is reached.

Sooo, using a stock 308 (American HP rating peak at 6600rpm):

20 x square root of (.3659 x 6.6), or 31.1mm.

The IDEAL carb diameter is (1.25) venturi diameter - in the stock 308 case it is 39mm. And sure enough, the 308's OEM 40 DCNF has a 40mm bore with a 32mm choke. If you use a higher rpm peak, say around 7000 as in the Euro spec cars, it comes out near perfectly to 40/32, and at a hp peak of 7700 it suggests even upgrading to a 34mm venturi!

So in the case at hand of a 3.2 liter engine with a desired power peak of 7700 rpm (redline):

20 x square root of (.398 x 7.7), or 35mm venturi, and then a 43.9mm carb body.

Ergo, I an a bit 'under-carberrated' with the more restrictive 40mm carb body, and should ideally consider perhaps a 44mm carburetor (44DCNF) with a 34 or 36mm venturi to maintain optimum ratio and response. A graph in the Pierce manifold Weber tuning manual also suggests for a 400cc cylinder volume a 37mm venturi would be the ideal choice.

So we can theoretically deduce that the car is under carb'd along with the airflow restriction shown on the flowbench, and that the car will toerate a larger carb, ideally in this case a 44 DCNF.

Using Figure 35 with the single cylinder correction from the Pierce manual, the range for the main jet will be 135 to 165, with 150 being a good start.

So, anyone want to buy my nearly new 40 DCNFs with 34mm venturis...?

 

Is it that obvious?

 

Interestingly, if you run the numbers in your case, a 450cc cylinder with power at 6500 requires about a 34mm venturi and a 42.7 mm carb...

 

One thing I do not see talked about a lot are the 'shades of gray' characteristics with large carbs that are choked down pretty far or smaller carbs that have wide chokes.

There was one FChat 3.0 qv car that had big cams and downdraft 46 or 48s I believe, but required 34 or 36mm venturis to have any kind of low end vacuum signal at all, although I am sure it was a screamer up high. I never really heard how it finally went, but always wondered about the throttle response, or possible flatness due to low velocity.

On the other side I've run my car a lot with 36mm chokes on 40mm DCNFs and it did fine, but did have a barely perceptable flat spot in transition which I attributed to velocity loss on opening. However, no real problems otherwise, and the flat spot waqs cured by increasing the velocity with 34 mm venturis which later showed no loss of hp on the roller dyno.

Of course cam profiles and overlap have a lot to do with all this, but just in regards to carbs on the outside edges of operational design, air velocity management and the management of transition between idle and main circuits must get interesting. We've all seen the Fiats and Abarths with some pretty wompus large Webers on a tiny motor....

 

it is so heartbreaking to have an unenlightened son -- he used to seem so bright.
it must be his spring love with all the blood shunting away from his brain....

 

and after such a long time hand cranking the motor....