Calculating CFM requirements for your engine

[michael hurd]

Airflow requirement for a 350 cubic inch engine with a torque peak at 3000 rpm and a power peak at an engine speed of 5500 rpm.

Torque peak:

CFM = (350 x 3500) ÷ 3456 = 354 cfm at the torque peak

Power peak:

CFM = (350 x 5500) ÷ 3456 = 557 cfm at the power peak.

These are the engine’s theoretical air requirements if there was no restriction from throttle plates, manifold runners, intake ports, valves, exhaust.



In the above, this is the calculated airflow at wide open throttle based on 100% volumetric efficiency.

What is volumetric efficiency?

In short, the volumetric efficiency is a measure of the actual airflow through an engine vs. the theoretical airflow.

For a normal street engine, you can assume around 80% VE.

If measured on a dyno, the actual airflow through an engine at your torque peak ( 3000 rpm ) in the theoretical 350 engine is ~280 cfm, then the following equation would solve for VE.

VE% = 280/354 x 100 =79.09%

What about your power peak>? If you measured 417 cfm at your 5500 rpm power peak,

VE% = 417/557 x100 = 74.86%

 

[michael hurd]

How much exhaust flow is required for my engine?

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Solving for sample 350 engine with 74% VE and 417 cfm intake flow at power peak 5500 RPM:

(1200+460) / 540 x 417 = 1281 cfm



Note that this is a theoretical maximum flow rate.

Since we live in the real world, let's derate that a bit to 1200 CFM.

With a dual exhaust, you will need about 600 cfm per side to have very low restriction at WOT at your power peak.

 

[michael hurd]

How much does that muffler flow?

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All tests via an independent lab
All tests @ 1.5” h2o

2 “ Straight Pipe 283 CFM
2 ¼ “ Straight Pipe 365 CFM
2 ½ “ Straight Pipe 521 CFM

2 ¼” Typical Bent tailpipe 268 CFM
2 ½” Typical Bent Tailpipe 417 CFM

2 ¼” Inlet/Outlet_ Glass Pack Tips- No Louvers- Smooth 274 CFM
2 ¼” Inlet/Outlet- Glass Pack Tips-Louvered 133 CFM
Same as above set for reverse flow 141 CFM
2 ¼” Cherry Bomb 239 CFM
2 ½” Cherry Bomb 294 CFM

2 ½” Inlet/Outlet Dynomax Super Turbo 278 CFM
2 ½” Inlet/Outlet Ultraflow Bullet 512 CFM
2 ½” Inlet/Outlet Gibson Superflow 267 CFM
2 ½” Inlet/Outlet Flowmaster ( 2 Chamber) 249 CFM
2 ½” Inlet Outlet Flowmaster ( 3 Chamber) 229 CFM
2 ¼” Inlet/Outlet Thrush CVX 260 CFM
2 ½” Inlet/Outlet Maremount Cherry Bomb 298 CFM
2 ½” Inlet/Outlet Hooker Aero Chamber 324 CFM
2 ½” Inlet/Outlet Hooker Max Flow 521 CFM
2 ½” Inlet/Outlet Borla Turbo 373 CFM
2 1/2" Inlet/Outlet Magnaflow 284 CFM

http://www.pontiacstreetperformance.com/psp/exhaust.html

Standard OEM 2 ¼” Inlet/Outlet 138-152 CFM
Standard OEM 2 ½” Inlet/Outlet 161-197 CFM