I enjoy the driving side more than the mechanical side so I don't profess to be very technical. All I know is that I have a 185" Boogie Scott FED (Hall of fame chassis builder) He says not to run a spool. I run 8.70"s 150 mph straight as a string.. Also, about 15 years ago I bought a Mark Williams car from a friend of mine. He never knew which direction the car would go in when he launched. I put my motor in it and took the spool out and never had an issue going straight. Like I said, I can't tell you why, I am only sharing my experience, for what it's worth.