Honestly a 4 turn is WAAAAYYYY TOO MUCH MOTOR in pretty much anything! It's all about what you feel you've got the most control over. I ran mine yesterday, started with an 8.5. I ended up running faster and more consistent laps with a boosted 17.5. The car felt like it was velcroed to the track and was much easier to compensate when I missed the line in certain trouble spots on the track. When you get into the lower turn motors there's a lot more "brute force" even when you adjust the radio/esc settings to mellow it out a bit. If I had my 10.5 or 13.5 with me I would have ended up with one of those in there and killed a bunch of the boost. Just my opinion but sometimes you need to slow down in order to speed up.