Thanks Gordon

I thought though the "Carb Size" paragraph of your Hagerty link interesting.  In order to do the math in that paragraph I went on an internet chase:

To figure out what the cfm for my engine is, I found this:  https://www.inchcalculator.com...tor-size-calculator/

Once I had the engines total cfm, it needed to be divided in half (half for one carb, half for the other carb).

Then to see what carb is a likely fit, I compared my cfm per carb to the list found on this link:  https://shoptalkforums.com/viewtopic.php?t=128512

In the end, I became confused.  There is either a lot more to the engine cfm/carb size formula, or many of us are way over-carbureted.  I suspect the latter to be more likely.  Which makes me wonder how many of us REALLY need dual Weber 44s, (or for that matter Dellorto 48s)?

RS-60 mark posted:

 

In the end, I became confused.  There is either a lot more to the engine cfm/carb size formula, or many of us are way over-carbureted.  I suspect the latter to be more likely.  Which makes me wonder how many of us REALLY need dual Weber 44s, (or for that matter Dellorto 48s)?

I don't understand either. I once made a 2,700 cc Corvair engine run just fine with a carb off a 1200 cc Harley. 

The thing about articles and charts like this-- they're super-interesting, and can lead you in a lot of directions that may (or may not) work in the real world. 

@RS-60 mark brings up probably the very first thing a guy will find if he reads a lot of engine esoterica-- that according to the formulas, most of our engines are over-carbureted in mathematical terms. The thing is, guys who build Type 1 VW engines for a living know what works and what doesn't... and almost all of them will say something to the effect of, "yeah, the math is nice- but these engines run better and make more power with bigger carbs than you find in the charts". Obviously, there are limits, but a 2276 with 45 DLRAs is not over-carbureted.

I had a friend once who explained the smaller carb thinking like this: if the venturi in the carb is not the choke-point for the intake, then your air-speed is not going to be great enough through the carb for fuel to be properly metered. In other words, unless your carburetor venturi is smaller than every other point along the intake (intake manifold, ports in the head, back of the valve, etc.), we're all fooling ourselves with bigger carbs. It makes perfect sense when you think about it, and has almost zero application in the real world.

I have no idea why it isn't so, but I can tell you this without any hesitation- an engine with very big ports and valves in the heads, and undersized carbs is a dog. When I tried to de-fang my 2332, I ran 40 DLRAs on it for a while with great big heads. Theory aside, it was a pooch. Airspeed plays a huge role in making power on the street, and the holy grail is getting flow numbers with smaller ports (which is why the Panchito is a near miracle). It takes a defined amount of fuel to support a given HP (assuming the fuel stays atomized), and super-small carbs can't provide enough to make the kind of power I've grown to expect. 

I talked with @Pat Downs this week-- he's been getting 180-ish HP with 2332s running Panchitos (with 40 mm valves, and size-tiny ports) and bigger (CB's 2292 grind, which is a lot like an FK10) cams with a lot of overlap. My engine is out of my car, and I'm planning to step away from running a science-fair project twin-plug set-up for my car (more later as to "why"). Running a bigger cam with smaller heads and lower compression flies in the face of every formula or chart I've ever studied late into the night. But dyno numbers cannot be disputed, and the power he's making with this combo is astounding.

I'm going to give it a try- running the heads I've been high on since they came out and a cam at least one size bigger than I ever thought was reasonable. This time I'm building what a seasoned professional knows works, rather than what my theoretical mathematical chart tells me might be better.

Time will tell, but I'm trending in exactly the opposite direction from where I've been, and I've been lost in the math for a long, long time.

 

Last edited by Stan Galat

According to the Hagerty formula 40 mm Webers (or Del's) will work on all but the very biggest VW aircooled engines but we know that not to be true. A pair of 40 mm carbs  can sustain about 140 hp (there are instances of 40's making up to 15 or 18 hp more but use this figure as a general guideline- more on the exceptions later) which is really only a 2 liter-ish or 21something with ported 40x35 heads revving to 5500 or 6,000 rpm. 44's or 45's will support up to about 190? and 48's are required at anything higher.

That said, the carb sizes are close enough (which you will see if you look at the range of venturis available for each size) that applications overlap a fair bit, and either a set of 40's or 44/45's (or 44/45's or 48's for the bigger engines) will work on a wide range of engine sizes and combos.

TheMayoMachine posted:

Great article; but I thought for sure it was going to be about cutting holes to increase the HP to LB ratio!

Yeah, Mayo, it is a legitimate way to increase the hp to lb ratio. It is just a very, very, VERY time consuming way. But if you aren't going to increase the horsepower, it is the only way...

Stan wrote this:

I had a friend once who explained the smaller carb thinking like this: if the venturi in the carb is not the choke-point for the intake, then your air-speed is not going to be great enough through the carb for fuel to be properly metered. In other words, unless your carburetor venturi is smaller than every other point along the intake (intake manifold, ports in the head, back of the valve, etc.), we're all fooling ourselves with bigger carbs. It makes perfect sense when you think about it, and has almost zero application in the real world.

And that it is spot on.  That's what I found out with my recent carb "upgrade" - The carb venturii "choke point" was too big to match the rest of the engine flow so the entire system suffered.  It's all supposed to be a matched system from air cleaner to exhaust tip and that sometimes takes a little figuring to make it all compatible because of all the variables.

I didn't like math, even though my Mom was a math teacher, but I found that it wasn't what math was doing that I disliked, it was the arduous plowing through the process.  I was saved when they invented cheap, scientific calculators and then math became easy because the drudgework was done for me.  I hesitate to say it is fun, but at least it's now a helluva lot easier.

Gordon Nichols posted:

That's what I found out with my recent carb "upgrade" - The carb venturii "choke point" was too big to match the rest of the engine flow so the entire system suffered.  It's all supposed to be a matched system from air cleaner to exhaust tip and that sometimes takes a little figuring to make it all compatible because of all the variables.

An engine is an air pump, and power is based on how much air (and fuel) it will move through the engine. That's why cylinder heads are always advertised by CFM at various valve lifts.

A typical high-performance "streetable" camshaft will provide about .500 +/- of valve lift, so that's a good number to use for head capacity, and charts are always available to compare: generally using 25" of W/C as the basis for comparison, but some porters use 28". The formula to convert 25" ratings to 28" ratings is √(28 ÷ 25)  or multiply the 25" rating by 1.058. This is a mathematical approximation, not an actual tested and published flow number, so it can only be used as a baseline.

What we don't generally know is how much air a typical Weber (or Dellorto) carb will flow, because Webers and Dellorotos (and Solexs, etc.) are rated by throttle plate size, as opposed to CFM like a Holly. Enter the interweb to provide some clues. A search resulted in the following information posted in a couple of different forums and cross-checked with each other:

  • Dellorto 48 DRLA: 388 cfm/venturi at 28" W/C, or 358 cfm at 25" W/C
  • Weber 48 IDF: 340 cfm/venturi at 28" W/C or, 313 cfm at 25" W/C
  • Dellorto 45 DRLA: 329.3 cfm/venturi at 28" W/C, or 311.24 cfm at 25" W/C
  • Weber 44 IDF: 290 cfm/venturi at 28" W/C, or 267 cfm at 25" W/C
  • Dellorto 40 DRLA: 266 cfm/venturi at 28" W/C, or 245 cfm at 25" W/C
  • Weber 40 IDF: 212 cfm/venturi at 28" W/C, or 195 cfm at 25" W/C
  • Dellorto 36 DRLA: 206 cfm/venturi at 28" W/C, or 190 cfm at 25" W/C

All of this assumes stock venturis for the carbs in question. You can see by the chart that Dellortos flow about as much as the next size up Weber.

Anyhow, according to the current CB website, the following heads flow the following amount of air at .500 lift and 25" W/C:

  • Panchitos: 167 cfm
  • Wedge Ports: 201 cfm
  • Comp Eliminator: 219 cfm

Based on this information, an engine with Wedge Ports should run Weber 40 IDFs. However, in real world dyno tests, more power is made with much bigger carbs (probably 48 DRLAs or at least 48 IDFs). Math is great, but real-world dyno numbers trump them every single time. What builders will tell you is that if the carb is even close to the choke-point, you're going to leave a lot of power on the table.

The venturis have to be small enough to increase airspeed enough to pick up the fuel through the jets well, but big enough to not restrict flow unduly. Carbs are (in my mind) a miracle of the mechanical age, at least on par with a mechanical watch. That they work at all is amazing, that they work well is astonishing. Why they are as big as they are on an air-cooled engine is something I couldn't just accept until very recently. 

There's another thing that matters besides raw HP numbers on the dyno, and that is throttle response in the real world. Throttle response is a function of compression and airspeed velocity (which is measured in FPM). Higher compression ratios give better throttle response-- but there's a limit to what you can get away with, based on combustion efficiency and cooling. Bad cooling is baked in the air-cooled cake (which is why water-cooled engines have such good VEs), but a lot of strides have been made in combustion-chamber efficiency in the last 15 years.

Port velocity (FPM) information is never published, but we can make assumptions based on intake port volume (measured in CCs). This information is often published, and is pretty important. Generally speaking, when selecting heads, we are looking for the smallest intake port volume for a specific CFM rating. What we are looking for are head flow numbers that match the target HP we are looking for (there's a formula for this as well), then looking for the smallest ports available that will flow that number. One has to be careful, as hand porting varies by the nature of being done by hand. As such, I (and the tuner community generally) have become a fan of CNC (or "as cast") ports, which are consistent across individual ports.

This stuff matters in how an engine feels. It's easy to get totally lost in the math and formulas, and to start to think that smart guys who've made their lives modifying and building engines are missing something. This is hogwash. At some point, you've got to let go of the tables and multipliers, and just go with what somebody you trust says will work.

That's super-hard for a hard-headed MKA (Mr. Know-it-All) like your's truly.

 

 

If the math doesn't predict what tuners know to be true, then someone is using the wrong math, or more likely, not enough math.

For years, the experts couldn't figure out why bumblebees can fly. But it turns out they were making some bad assumptions (the experts, not the bumblebees).

In any case, when I signed up for this forum, I was told there would be no math.

 

Last edited by Sacto Mitch
Sacto Mitch posted:

 If the math doesn't predict what tuners know to be true, then someone is using the wrong math, or more likely, not enough math.

For years, the experts couldn't figure out why bumblebees can fly. But it turns out they were making some bad assumptions (the experts, not the bumblebees).

In any case, when I signed up for this forum, I was told there would be no math.

For sure.

The math (and models) are catching up with the real world, but most of the formulas published for general consumption (by simpletons like me)  are a few generations off the cutting edge.

The big-boys in automotive manufacturing know what they are doing. Builders and tuners know what works. Desktop dynos are becoming amazingly accurate. But schlubs like me are generally trying to figure it out by sifting through tables and formulas published online that are decades old.

The main problem is that this hobby is served at a rate of about 90% by impostors and charlatans, 5% who know what they are doing, and 5% who think they know better than everybody else. I'll let you guess what category I fall into. It's cost me a king's ransom over the years.

I'm turning over a new leaf. We'll see if I can keep my thumbs out of the pie.

Last edited by Stan Galat
Stan Galat posted:

The main problem is that this hobby is served at a rate of about 90% by impostors and charlatans, 5% who know what they are doing, and 5% who think they know better than everybody else. I'll let you guess what category I fall into. It's cost me a king's ransom over the years.

I'm turning over a new leaf. We'll see if I can keep my thumbs out of the pie.

I'm in the 90% of impostors and charlatans along with some VooDoo and hoodoo.

Thumbs out of the pie? LOL, you know you can't do it, my friend!

Good stuff. Learned a lot form the Vee guys last year. Have to use a stock 1200 manifold so they heat it up and push a steel ball through it. Suddenly a crappy stock intake turns into a $1500 intake. Measuring push rods down to three decimal places. Lightened rods, flywheel, etc. 

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