So What Increase Should I Expect From These Modifications?
The following is a good average of what you should expect in CFM @ .500 lift if you go to the larger valve sizes. If you decide to stay with the stock valve sizes the modified CFM readings will be about 5% lower.
The following test results were taken through the air filter, carburetor and intake manifold. They represent what your engine actually sees. So they may appear lower than some other popular ported heads (that are tested through a radius adapter, without the restriction of the air filter, carburetor and intake manifold).
The following tests were taken at 10" W.C. Test Pressure.
| Engine | Stock Intake | Modified Intake | % Increase |
| Twin Cam | 112 | 140 | 25 |
| Evolution Big Twin | 120 | 140 | 17 |
| Evolution Sportster | 110 | 134 | 22 |
| Buell | 110 | 135 | 23 |
| Shovelhead | 106 | 138 | 31 |
| Panhead | 90 | 112 | 25 |
| Knucklehead | 90 | 112 | 25 |
| Iron Head Sportster | 100 | 116 | 16 |
| Engine | Stock Exhaust | Modified Exhaust | % Increase |
| Twin Cam | 86 | 124 | 30 |
| Evolution Big Twin | 100 | 120 | 20 |
| Evolution Sportster | 84 | 110 | 31 |
| Buell | 86 | 112 | 31 |
| Shovelhead | 100 | 110 | 10 |
| Panhead | 96 | 104 | 9 |
| Knucklehead | 94 | 105 | 12 |
| Iron Head Sportster | 76 | 100 | 32 |
| To convert these results to other test pressures use the following formula: |
| Multiply the flow at the first test pressure X the square root of the second test pressure divided by the first test pressure. |
| Example: To find what the Twin Cam Modified Intake flow would be a 12" test pressure. |
| Multiply 140 (from above chart) X the square root of 12 divided by 10 (1.0954451) |
| 140 X 1.0954451= 153.36 CFM at 12" W.C. test pressure |
| Next |