Sizing ITBs

You can spend years reading the intricacies of the flow dynamics with regards to size vs velocity. In basic terms, Horsepower = Air Flow. It's as simple as that, and 10 times more complicated. As a general rule: When looking at the Horse Power per cylinder, with the butterfly to valve head distance of roughly 120mm and the engine’s maximum RPM of 9,000, the following is a good rule of thumb.

Best Bore Size:	HP per Cylinder	4 Cylinder HP	v6 / Inline 6 HP	v8 HP
40mm	40 to 55	160 to 220	240 to 330	300 to 440
45mm	56 to 70	220 to 280	330 to 420	440 to 560
50mm	71 to 90	280 to 360+	420 to 540+	560 to 720+

The above chart is a very loose fitting set of numbers to get you started, ONLY and we mean only true testing of the variables called IE: Length of runner, Taper of runner, Injector length to Valve, butterfly length to Valve, Air Box, Air Horn size, Shape, and distance of runner mouth to engine compartment components, heat, humidity, will name just a few of the factors that add or rob Horse Power! Also if your car lives at high RPMs on the track, you might choose a larger bore than if you make more use of low-end torque. Just don't make the common mistake of thinking that larger bore always equals more horsepower.

As you can see from the chart above that a typical 4 cylinder Normally Aspirated engine making 220hp to 280hp would like 45mm ITB’s if all other parts of the Manifold, and Horns are perfect, if the other parts of the system are not as efficient then the size needs to increase to compensate. Here again this is true if the other system deficiencies are possible to rectify! Sometimes it comes down to cost. Two buddies split the testing costs, one buys a set of 50’s the other a set of 45’s and they split the cost of R&D to see which set performs the best, one then sales his 45’s and buys a new set of 50’s! Forum’s, car clubs, schools, and Do-It-Yourselfer’s can all benefit from this simple cooperative testing.

The Modern Multiple Valve engines now have oval-shaped ports with a good reason: the flat roof and floor of the port flows better than a round port. Now in general most modern oval port engines have a typical 30mm x 50mm shape to them which has a certain amount of surface area (see chart below) as compared to our ITB sizes

Now you can see from the information above that an oval-port opening of only 2.02 squares inches will flow only a certain amount of air, with this said, putting 3.04 Square inches of ITB on a 2.02 Oval port will only do one thing, build maximum horsepower at the expense of low end torque. Now the opposite is not always true, but as a general rule smaller ITB’s will create a more "drivable" engine (better throttle control and low-end torque). If you are Drag Racing then all out power may be what you are looking for. If you are Road Racing, Auto-Crossing, or Drifting then you will need a certain amount of throttle control. Here again it’s not a simple Formula for any given engine!