Webers: 40 vs 46's--when??
 

When does an engine typically need the additional breathing offered by 46 mm Webers? When (if?) did Porsche use them?

How about choke sizes? I haven't seen any good info on typical choke/venturi sizes for various engine combos.

Thanks,
Matt

'73 911E

Porsche never used weber 46's on anything but a race car. I don't think you need to start thinking about 46's in street cars until you go over 3.2 litres in displacement even 3.2 litre cars work well with 40's. The car may have higher peak horsepower w/ 46's vs. 40's but driveability will suffer. A low displacement race motor w/ super high compression should benefit by the larger 46mm carbs.

I would have to think that a 2L and a 3L would have pretty different carburetor requirements. Are the choke sizes adjusted to accommodate this? I know on a sidedraft Weber (i.e. DCOE), you can get chokes from 28 to 36 mm when the throttle plate is 40 mm.

Jim hit the nail on the head. I'm going to be using 40s on my short-stroke 3.2. It's right on the borderline, but adding the 46s to this engine will indeed give it more peak power, with the sacrifice of lower-end performance. Since this car is going to be 100% on the street, then I want to maximize performance for street conditions. Anything over a 3.2 would have to go to the 46s...

-Wayne

I use 46's on my 2 liter race engine but then I'm also running very high compression (there you go JMZ). Factor in the peaky cams and you end with a motor that is practically dead until approx 5K RPM, then it becomes a screamer from then on.

Carb and venturi sizing formulae are given in B Anderson's book. Adjust those slightly for intended use. The answers above are all in line with the formulae.

I would argue that compression has a pretty small impact on the size of carburetor. The compression doesn't affect the breathing requirement of an engine like port sizes and camshaft do.

In my experience with sidedraft Webers (which are essentially the same as IDA's), the choke size has the biggest affect on low speed driveability vs power. On a 2L engine with 40mm Webers, 36 mm chokes make good power up top, but are unsuitable on a street engine. Conversely, 28 mm chokes make good bottom end torque but literally choke the engine up top.

46 MM PMO's work fine on my 1980 911SC street car. PMO has a setup for this configuration that works well. Gives you room to grow later.

My .02

On my 3.2 powered 914 I have 40's with 38mm venturis
On my 2.2 powered 911 I have 40's with 32mm venturis

both engines are responsive and pull hard from idle to redline (7200rmp and 7600rpm respectively)

You can use 46's with smaller venturis on small less extreme engines, but the initial cost of the 46's is tremendous. The 40's work fine on all but the large displacement rad cammed engines