Intake Radius Profiles - Why Small Radii Are Killing Your Tunnel Ram and Carb Offset Explained

There is a trend in tunnel ram builds that is quietly killing performance and this video is about calling it out and explaining exactly why it is wrong. We are seeing radius profiles on 4th harmonic tunnel ram intake windows coming in under 5/16 and that is too small. A tight radius at the entry point of an intake window creates a separation point - the charge cannot follow a sharp curve at velocity and when it separates you get turbulence, dead zones and a loss of organised energy right at the point where the mixture is committing to the runner. On a 4th harmonic tunnel ram where you have worked hard to get the wave timing right, a poor entry radius is throwing that work away. We also cover something that gets almost no attention in tunnel ram design - the radius is not the same all the way around the window and it should not be. On a tunnel ram the fuel air charge is arriving from above. Fuel is significantly heavier than air and under that downward trajectory the top of the window sees a different flow condition to the sides. The top radius needs to be larger than the side radii to manage that transition correctly. Routing the same radius profile all the way around the window is a shortcut that costs you mixture quality and flow efficiency. Then we get into carb offset. As the car accelerates the fuel, being much heavier than air, wants to migrate rearward in the carburettor bore under inertia loading. This creates a mixture distribution problem before the charge even reaches the intake. The fix is to offset the carburettor position slightly forward on the top plate so that under load the fuel is driven toward the centre of the bore rather than piling up at the rear. We explain the physics, how much offset is appropriate and how to think about it for your specific application. This is the kind of detail that does not show up on a flow bench but absolutely shows up on the dyno and on the track. Bain Racing - Four generations of motorsport engineering from Bathurst, NSW.