So I've reconsidered some of the choices I had originally made for my carb choice.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!
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!