Don't know about the transition circuit. However, I do know that when you reduce the carburetor cfm, throttle reponse improves at the expense of top end.
Here's the word from Summit. I have found the formula to be good.
The first thing to consider here is the cfm that you will need for your engine. There's a simple formula available that will put you right in the cfm ball park. The formula for naturally aspirated gasoline engines is:
Engine Size (c.i.d.) X Maximum rpm/3,456=cfm@100 percent Volumetric Efficiency (VE)
Example: 350 c.i.d. X 6,000 rpm=2,100,000/3,456=608 cfm
Approximately 608 cfm would be required for this engine. However, most street engines are capable of achieving only about 80 percent VE; a modified street engine with ported heads, headers, good intake, and carburetor can achieve about 85 percent VE; a fully modified race engine can achieve 95 percent or greater VE. The cfm number arrived at with this formula must be factored by this percentage.
Next, you need to decide if a vacuum secondary or a mechanical secondary carburetor will best fill your bill. As a rule of thumb, vacuum secondary carburetors work best on the following:
- Relatively heavy vehicles
- Street gearing
- Automatic transmission
- Engines built more for low-end torque
Conversely, mechanical secondary carburetors seem to work best on:
- Relatively light vehicles
- Strip gearing (4.11 or numerically higher)
- Manual transmission
- Engines built more for top-end horsepower
