Carbs on a 3.2 short-stroke

[78targa]

Ok, this discussion was started on another thread, but to avoid getting off-line I figured I would start a new one.

JPNovak made a point in this thread

3.0 weber engine IT RUNS!

about what sized carbs should be used on a 3.2 SC short-stroke. As I am currently an owner of this type engine and considering adding carbs, I am very interested in other people's thoughts on this. Jamie's thought was that 46mm's should be used.

Ok, just to drag Bruce Anderson in on this, he makes the recommendation for this type of engine to go with the Weber 40 IDA-3C. His point is illustrated through a quick calculation of optimal venturi size of a 3.2 displacement giving a range of 38.5mm to 43.75mm.

Does anyone have any practical experience with either sized carbs? If you do, I would be very interested in what your jetting set-up was.

[Elombard]

Waynes new book asks for 46mm on this combo. What induction are you using? CIS? What cams?

[Wayne 962]

You can go either way, there's obviously some overlap between the 46s and the 40s. If you jet the 46s down, they will work fine, if you jet the 40s up they will work fine too. On 3.2 and larger, it's a general rule of thumb that you should go with the 46s. They are more than ample for the increased airflow...

-Wayne

[Elombard]

Good to know as the 40s are much cheaper if you are going used. On that note will zenith 40s bolt up to a 3.0? or are the intakes different. Seems like I heard "d shaped" or something?

[78targa]

Well, would intake port size make a significant decision in picking one over the other? I am currently running the 34mm SC heads as opposed to the larger ones from 78-79. I would think a set of 40's vs. 46's would be much more negligable. That is a gut feel though and not grounded in any testing.

[kellzey]

I'm building one by the way with 98mm P&Cs, 3.0L bottom end, early SC cylinder heads with 39mm intakes, single plug Electomotive Ingition, 3.6L oil pump, headers, and custom exhaust.

According to Bruce Anderson's weber sizing calculation.

Venturi size = 20 x square root of (Volume of one clunder in cc x RPM where engine reaches peak power/1000000)

It would seem that a lot depends on the cam you use.

For a 3.2 motor (short stroke)... one cylinder is 527cc.

Let's say you choose a 911S cam that has a peak HP rating at 6800 RPMs.

Plug this into Bruce's equation and you get a venturi size of 37.861 mm. Throttle bore (according to Bruce) should be 10 to 25% larger than the venturi size.

10% greater = 41.64
25% greater = 47.32

So a 46mm carb would work wonderfully!

If you went to a less-aggressive cam... let's say a late 964 cam with 6100RPM at max power, your throttle bores would be 39.44 and 44.8 respectively, so you could get away with the 40mm carbs.

My engine builder says there will be no problem running a 3.2 with 46mm carbs, even for street use. He builds them all the time.

[EWPurdy]

I'm currently running a stock 3.0L motor with 46mm PMO's and have no problems. The larger PMO's do just fine and allow for future HP upgrades.

[kellzey]

The PMOs are supposed to run a bit better than the Webers (especially the 46s) as they fixed some of the idle circuit issues.

If you are purchasing carbs and have the money, then get the PMOs.

I picked up a set of rebuilt Weber 46IDAs, with manifolds, crossbar linkage, velocity stacks, and K&N watershields for about $1300.00.

New PMOs cost $3K with all the goodies.

[TimT]

Both the 40 and 46 will work. What really matters is the venturi size you use. This can vbe calculated based on the intended use of the engine. Big venturis=top end power small venturis=low end grunt..BA book has a derived formula to help chose venturis based on displacement and peak rpm