Originally Posted by PowerHouse

Neither one really gives it great mileage. The 21 is 12.5mm shorter and will smoothen out the bottom while the 20 will give it more punch down low but will lose a little rpm on top. The stock toro gets in and around 8 minutes give or take a little depending on what pipe is on it and who's tuning it. The 007 is so close in spec with the 053 that it is hard to tell the difference between the two. I had both and on just about every engine I ran them on, they were almost dead even so I sold them as I had no need to have pipes that worked the same as the next. If you step up to my race ported toro, the engine will get 930-11 minutes again depending on pipe, tune and application and it will have quite a bit more low end torque and throttle response but with the raised port heights and intake timing adjustments, the engine will actually increase on top without losing the low end performance which will allow you to use the longer pipe which will not only keep the low end where it should be, it will also allow the engine to scavage properly increasing its efficiency giving you the extended run times needed and desired by racers worldwide.