Getting the proper Throttle Body bore size is important as bigger is not always better. Too small of a bore restricts airflow (CFM) and stifles the engines ability to make power at the higher RPMs. To large of a bore and the engine cant hold fuel in suspension when the throttle plates initially open. Were looking for Goldilocks when it comes to bore size and engine CFM requirements, but its not a simple as matching engine size to throttle bore, but its a good start.
- How many throttle bodies am I using (4 x 1 vs 2 x 1)
- How large or small are the valves
- Intake – Modified or Factory
- Engine Bore and Head Design
- Cam Design
Just looking at the list most of these questions remind you more of racing rather than getting a good running EFI engine, but with the maximum amount of top-end power with low-end torque will get off the line and cleanly accelerate to your desired engine speed. Because of all the differences, we will talk about CFM to getting the conversation to its bare bones. There are some highly knowledgeable experts in Corvair airflow and had design and I would consult them if you have a high performance engine that needs exact CFM measurements and needs.
The Basics
We will talk about standard to slightly modified engine designs and the throttle bodies that would support the airflow of these engines. Many engines with too large of a throttle bore will suffer on the low end and to small will not produce the amount of top-end power the engine is capable of. Again, these are theoretical (on paper) calculations and keep in mind, this is a guide, not the bible.
145 Engine – 2 Throttle body
- Theoretical CFM Demand: Using a standard 4-stroke formula
(CID × max RPM) / 3456 × Volumetric Efficiency, a 145 CID engine spinning to 4,600 RPM at roughly 80%–85% volumetric efficiency only demands roughly 155 to 165 CFM at peak output.
- Theoretical CFM Demand: The physical airflow requirements of the 110 hp engine are quite low due to its factory redline limit (around 4,400 to 4,800 RPM) and small valve sizes.
- Each flows roughly 70–90 CFM, giving a total combined capacity of 150–180 CFM and fills the engine’s volumetric needs efficiently.
164 Engine – 140 Engine
Theoretical CFM Demand: Despite the large factory airflow capacity, the engine does not actually use all 400 CFM at normal engine speeds. The 140 HP head has significantly larger intake/exhaust valves and dual exhaust, giving it a higher Volumetric Efficiency (VE) than the 110 HP engine, and it revs higher (up to a 5,200–5,500 RPM redline). It is able to use its larger throttle body capacity by employing staged injected (progressive linkage in Carbs) for a stable idle and low RPM. If all throttle bodies were connected together for equal opening, low RPM (tip-in) would suffer and usually used in race situations with highly modified engines.
140 Engine – 2 Throttle body
140 Engine – 4 Throttle body (Staged Injection)
3.1L Engine – 2 Throttle body
3.1L Engine – 4 Throttle body (Staged Injection)