Selecting the Proper Sized Carburetor

[Zach Gomulka]

I have this old Solex book, it can be seen here:
http://vintagebus.com/parts/solex.html

On pages 8-9 it gives a formula to figure out the proper sized carburetor for your engine using displacement and engine RPM. I used this formula when I built the 1679cc engine for my ex GTV. I was planning on achieving 5,500rpm and it pointed me to a throttle plate size of 39.39mm. I used 40mm (actually 40.5mm in my case) Solex 40 P11's and the engine worked very well, great torque, very smooth and drivable power, easily pulling to 5,500 in 1st and 2nd even though I limited the engine with a stock spec camshaft.

On pages 20-21 they also give recommendations on venturi size and jetting, given a specific throttle bore. I also used these recommendations, and although I never had the opportunity to further tune it with an air fuel ratio meter, it was close enough to work well.

The relation between venturi and the throttle plate is what interests me the most. In the book, they recommend the ratio be 0.8:1. So a 40mm throttle plate needs a 32mm venturi. I rarely see this ratio on VW engines. Many times a large engine is made to work with carburetors that are too small, and venturi size is increased to accommodate, A standard IDA with 37mm vents is .77:1, and venturis are available anywhere from 32mm from 45-46(?)mm. Is this .8:1 figure relevant? How is the tuning and driveability of the engine affected if the ratio is too high, or too low?

[Erlend / bug66]

My 1915 should have had 53mm carbs (8800rpm) with 42 vents 

42x37 heads. 48IDA and 40 vents work fine.

[AntLockyer]

That's just for a single carb though right? 55.5mm for my motor.

[Zach Gomulka]

That's just for a single carb though right?

That's one thing about the formula(s) that doesn't jive for me. One formula is for one carb [venturi] feeding 1-4 cylinder engines, another for 6, and another for 8.

[modnrod]

No formula experience here, sorry Zach.
All I've ever gone by is to match the intake valve diameter to the rev range required on a selected engine capacity, then match the venturi size to the intake valve as a starting point for feeding the thing.
I have noticed over the years that the correct venturi diameter seems more important than the throttle diameter, at least in terms of "street" applications for myself.
Also, that venturi diameter seems fairly constant across the board whether you're dealing with 1 throat per cylinder, 1 throat per 2 cylinders, 1 throat for 4 cylinders, etc, as long as you aren't trying to put a 38mm venturi 1BBL Stromberg on a 7500rpm using a centrebranch.
Eg, for a 5000-ish rpm 1600 with 35.5/32 valves, I'd probably look at a 34mm venturi/41mm throttle 1BBL Stromberg  centre, or a Holley 500 (35mm venturi/43mm throttle) or Weber 40DCN with 34mm chokes on an isolated 2BBL centre, or twin Weber 40DCN or Dell 40DRLA as an I.R. set up.

As an aside, a Holley 500 with annular boosters fitted on a isolated centre manifold works WAY better than it has any right to on a sub-2L motor........but I digress, sorry! 

[hotrodsurplus]

For some reason the linked page won't load. Bummer as I'd like to read it.

1) Food for thought regarding Solex' size formulas. Think of a multiple-cylinder engine as a collection of single-cylinder engines connected by a common shaft. That's really easy to visualize on an individual-runner (IR) manifold application: each 'engine' has its own carburetor. So for IR designs divide the total engine displacement by cylinder count and calculate based on each cylinder's displacement.

Solex offered different formulas to address the overlap in cylinder-filling events in plenum-type manifolds. Think of how a cylinder fills in super-simplified terms. The cylinder can fill only when its piston is on its downward stroke (again, this is a super-simplified example). That's only 180 degrees of crank rotation, right? Well a four-cylinder engine fills a cylinder every 180 degrees (a three-cylinder engine fills every 240 degrees; a two-cylinder engine every 360; a single-cylinder engine every 720). So there's theoretically zero overlap in those cylinder-filling events. That's why Solex has one formula for one-to-four-cylinder engines. Only one cylinder at a time can use the carburetor.

It's a different story with a six- or eight-cylinder engine. Take a six-cylinder engine. It fills a cylinder every 120 degrees. That means each cylinder event overlaps the other by 30 degrees (30x2=60). Because two cylinders' intake-filling events slightly overlap each other the carburetor must flow slightly more than it would in a four-cylinder application. An eight-cylinder engine fills a cylinder every 90 degrees. That means each cylinder event overlaps the other by 45 degrees (45x2=90). Here's another way to think about it: two cylinders are open at any given time in an eight-cylinder engine with a common-plenum manifold. As a consequence, the carburetor must be that much bigger to meet the flow demands.

Again, we can use the one-to-four-cylinder formula because there's theoretically zero overlap in our engines' individual filling events. The other formulas don't apply. However, if calculating carburetor capacity for an engine with more than four cylinders it's best to divide the displacement by the cylinder count and calculate on that formula.

2) The venturi-to-throttle ratio explains that Solex designs its carburetors to work best with a venturi that's 80 percent of the throttle diameter. That's what I referenced in the carburetor thread on the other forum. A Solex carburetor will work within a broad range of venturi diameters; however, it will work best when equipped with a venturi as close to the 80-percent ratio--at least according to carburetors designed by that formula.  

But that ratio isn't set in stone. Take the 32NDIX. By that .8 ratio it should have a 26mm venturi; however, in 356 Normal applications it comes with a 24mm venturi (.75:1); in Super applications it comes with a 28mm venturi (.875:1). That carburetor works exceedingly well in both configurations despite the 'non-standard' configuration.

3) the Solex is not a P11 despite what everyone calls it. It's PII, as in Pee Eye Eye. That's consistent with Solex' alphanumeric categorization of bore diameter and series: 28PCI, 32PDSIT, 30PICT, 32PBJ, 40PBIC, etc. It's sort of like the Klingelnberg deal; most 'Murricans are too lazy to pronounce furrin' words properly so Klingelnberg got turned into Klinkenberg, Volkswagen got turned into Voltswaggin, and Porsche got turned into Porsh. And German speakers laughed heartily.

[Zach Gomulka]

For some reason the linked page won't load. Bummer as I'd like to read it.

Still works on my end... Just do a google search for "Solex Selection and Tuning of the Carburetor" Should be the first thing that pops up.

Thanks for the education. That clears a lot of things up.

I did run a single Zenith on my old original dual port 1600. I tried 28's and I could not get the thing to run right. I also tried some 25's and the engine responded well to them. What a great, simple carburetor that was.