Carb Sizing Activity

[cr_bobcat]

So I've reconsidered some of the choices I had originally made for my carb choice. I think I was being a little optimistic about using a 1.21 venturi. So I started to do some work in a spreadsheet. I took a lot of information from the CI website. I thought I would share the excersize as others are most likely considering what carb to choose. I made the decision to go with the Autolite 2100/2150 based upon the info I've read on the annular discharge, but this analysis "should" work for any generic carb selection.

Here is the background information:

1) Needed the flow data for the different Autolite configurations by venturi size.

http://www.classicinlines.com/AutoliteCarbs.asp

2) Needed to understand Volumetric Efficiency (VE) and how my build played in.

http://www.classicinlines.com/CarbChoice.asp

3) Needed to know how to compute CFM requirement.

http://www.classicinlines.com/CFMcalc.asp

Once I had this information I decided to pull all of it into a spreadsheet so that I could try to make a decision on size. According to (2), my VE should be in the 75-85% range as I'm installing larger valves (1.75/1.50) and doing a Port & Polish. So I kind of fall in between "Mild" and "Performance" per Mike's criteria.

I then lumped the 8 venturi sizes into 6 categories (0.98, 1.01/1.02, 1.08, 1.14, 1.21/1.23, 1.33) based upon the rated CFM. In order to make a selection criteria I decided that I should make the carb selection based upon a +/- 10% of advertised flow, so I computed these values for each venturi size.

The next step was to create a matrix of CFM values as a function of RPM limit and VE. I computed CFM varying VE from 65% - 100% and used RPM steps of 5500, 6000, and 6500. Since I decided already that my VE should be in the 75% - 85% range, I put a box around these values. I also have a 264/174 *10 Clay Smith Cam, which has the powerband listed as 1900-5600 RPM. With this in mind, I also boxed the values under 6000 RPM (max of the powerband + 500 RPM). This left me with a tradespace of 260 CFM - 295 CFM.

I created 2 copies of this table: "optimize for HP" and "optimize for torque". I defined "optimize for HP" to be that the table rating for CFM must be ABOVE the 90%*Advertized CFM number (ie. slightly oversizing the carb). I defined "optimize for torque" to be that the table rating for CFM must be BELOW the 110%*Advertized CFM number (ie. slightly undersizing the carb).

When this was done I had a couple of nicely color-coded tables to look at and smackdab in the middle was my choice. So this is what I found:

"optimized for HP"
VE CFM Venturi
75% 260 1.08"
80% 278 1.14"
85% 295 1.14"

"optimized for Torque"
VE CFM Venturi
75% 260 1.01/1.02"
80% 278 1.08"
85% 295 1.08"

So I still need to make a decision between a 1.08" and a 1.14". Since the majority of my driving is stoplight to stoplight, and I think my rearend gear is 2.75, I think the 1.08" venturi is probably the choice for me. I think I would have been a little disappointed in the performance had I gone with a 1.21" venturi.

Hopefully this is helpful to someone else and let me know if you have corrections/questions.

Prost!

 

[CZLN6]

Howdy Back Prost:

This is a good analytical assessment of the Autolite 2100s and I agree, in general, with your conclusion, BUT! Don't you just hate a big But? Another variable that you may want to consider is the age and use perimeters of each carb. While the 1.08 and 1.14 are similar in total cfm they reflect internal differences based on the progression toward cleaner (by EPA standards) and more efficient (read MPG) engines. The 1.14 was used from 1964 to 1967 in 289 and 302 engines. The 1.08 was in use from 1968 - 1973 in 302s. The corporate weight of cars (in general) using these engines went down for the 1.08s. Corporate mileage went up. While many factors went into accomplishing the mileage increase, fine tuning the 1.08 was a part of that increase. The difference wasn't as much the slight decrease in cfm but, more likely in the metering in the K cluster which regulates the idle circuit, transitions circuit, squirter size and, to some extent WOT. While the 1.08 was used in 302 V8 engines, that engine might be found in any thing from a light car to a medium sized pickup, with many variations in between.

So, in general, jetting, choke and idle being equal, the 1.14 is built richer.

So, next look at the code stamped on the metal tag attached to the fuel bowl, if it's still there, or the front foot of the carb base for a stamped in ID code to determine its original engineered application. You may see only 3 digits. For a 1.08 code it may read c9 z, which would read c = 1960s, 9 = year, z = Mustang. the fourth digit may or may not be there. For our uses the "Z" code 1.08s work out pretty well. an "A" = Galaxie, may be too rich in idle and/or transitions. A "T" = Truck.

The K clusters are marked on the mounting base, but I have never been able to decipher the code for the mark. Your next question is likely, "Are K clusters interchangeable?" The answer is "yes", but without knowing the original use of a K cluster changing is a shot in the dark.

So, proceed with the 1.08 for your application. Take note of the carbs code, main jet size and accelerator pump lever positions. Then begin assessing in your application. If you need to fine tune from there, the usual external adjustments are there and usually enough. If that doesn't get the tune you want then internal variables like main jets and the power valve can be fine tuned. FYI- Holley power valves work in these carbs and they come in a wider range of use.

Well I hope I haven't rained on your objective analysis too much. I just thought you'd like to know the other variables.

Keep us posted on your progress.

Adios, David

 

[cr_bobcat]

Hey David

You didn't rain on my parade at all! This was my stab at making a sizing choice based upon what the CFM rating was. My assumption was that even if I bought "Carb A" over "Carb B" because of CFM rating, I was still going to have to make adjustments for jets and power valve. I hadn't considered the accelerator pump location. I'm aware that it's there, just hadn't occurred to me that it would be a variable to play with since I've not heard/read too much on it other than "looking at this picture, it's clear that the arm was in the wrong location".

I had considered that I could go with a larger venturi and make it act more appropriately for the small six by playing with the jets. I figured that to be the fine tuning whereas this activity got me to a "coarse tuning" point. To be honest, I don't know the first thing about the K cluster. Being an engineer by trade, I start with the variables I can compute analytically. That usually gets me in the ball park and I resort to my "hey! here's a button! I should push it and see what it does" approach to get things fine tuned.

I've read plenty of posts from seasoned folks that have said "just go with a 1.08 for an engine that size" in multiple forums. I like numbers, so I wanted to try to find how to arrive at that option by more formal methods. That being said, the very next important variable in this equation is price. If I can find a nice shiny rebuilt 1.14" for $50 less than a nice shiny rebuilt 1.08", then I'll go with the 1.14" and convince myself I did it for more HP and be completely happy with it. After all, the difference between an engineer and a scientist is that the engineer can declare "close enough!".

I also secretly considered buying a rebuilder of the other flavor on the cheap, throw an even cheaper kit at it, and hot swap 'em and measure them by my seat-o-the-pants meter.

Cheers!

 

[CZLN6]

howdy Back:

You are right on. I'm a subscriber to the butt-o-meter too. Keep us posted on your progress.

Adios, David