Having worked with Hondas and now a 190E, I have to wonder what the rationale is for designing in an interference fit? It would seem to me that the resulting damage risk would be a far greater concern than any design needs in regards engine efficiency. Is it impossible to improve engine performance to the same degree with an appropriate non-interference design? Steve

The quick answer is no. Interference fit engines can be designed to be more efficient with respect to specific power and emissions. All diesels are interference because of their small combustion chambers. With timing belt engines it is best to assume they are interference unless you know otherwise. The only timing belt engine I have ever had that wasn't interference was the 2.3L used in Ford Ranger pickups, and it is a fairly old design.

But why? The easy answer to keeping compression up is to increase stroke along with offset in such a way as to compensate for a larger combustion chamber. If you nee more flow, increase valve diameter. What design criterium specifically prevents designing adequate clearance.

My opinion, not to beat around the bush - it is easier to design by increment, and the designers are not too concerned about what happens 120k miles down the line, or even at 30k. They simply throw in a belt or chain swap recommendation, and cover their liability issues, and ignore the interference problem. But I will give them the benefit of doubt if there is a sound technical reason.

Steve

Flow is improved by opening the valves further. Just a bigger opening doesn't necessarily make an engine breathe better. The flow path of the combustion gases, the shape of the combustion chamber, the location of the spark are all critical elements of an engines power and efficiency. They are also interlinked, a change in one feature alters the effectiveness of another feature. Also modern engines are decreasing in engine displacement and for the most part have the same or higher compression ratios. That means smaller combustion chambers and less room to open the valves into, especially important since they are always mounted on an angle to the axis of the piston.

The timing belt replacement issues are more a product of custiomers demands for better NVH (noise, vibration and harshness) belts run quieter than chains. But belt failure is less predictable than chains which is why they recommend replacing them long before they are expected to fail.