The one thing you guys are forgetting is a kit that runs JUST a new jug is going to have the same breathing charistics as the original jug. (for the most part) You would have better performance by getting a better head/cam on the stock jug as you would be altering the amount of air/fuel you can cycle. The HEAD is the important factor in a 4t. The jug is just something to mount it on. That's why the polini head is the same cost as the jug kit. The important measurements for the jug are the OD, and depth of the cylinder skirts. They need to clear the conrod, and fit the cases.
