The HF engine was made for nothing but gas mileage. I have toothpicks stronger than the rods the HF has, and as mentioned, the HF has a completely different piston cut than the other higher # valve'd heads. I also wouldn't suggest putting an Si transmission on a stock HF engine, with it's low HP and fairly high torque, you're not going to get very many benefits from lowering the gear ratios. If anything, you'd want to make that torque work for you more by going with taller gears to get more speed with less RPM, but we already know that the HF tranny has the tallest ratios that will immediately bolt up.