What Redline Means

Classically (at least up until the '70s), the limiting factor in engine speed was thought to be piston or connecting rod speed. Four thousand feet per minute was considered the ceiling for street-able machines. That's piston speed. To determine this take your stroke in millimeters, double it, divide by 25.4, divide by twelve, then divide the result into four thousand to find the maximum RPM under this theory.

Now-a-days, materials are much more durable, valves have gotten smaller and more plentiful. Some high end twins have redlines/power-peaks closer to six thousand feet per minute. Small titanium valves don't float; some see revs over 20K. Pistons are short and squat; they don't stretch much. Ditto connecting rods. Air/fuel delivery starts to be a limiting factor.

Four thousand feet per minute gives a redline of 8809RPM for XV700s through 1000s. When I ran Megacycle 258X2-NS cams I regularly bumped against the stock rev limit of 8900. See the article entitled "BOTT" for information on what Yamaha did with the R series in road races. I don't know what the practical limit is for street-going XVs, but unless they are running after-market valve springs, I would hesitate to venture much past eight grand.

Just for reference: to determine "redline" for various piston speeds, for 4000FPM divide your stroke in millimeters into 609,600; for 50000FPM divide it into 731,520. For 5500FPM, divide into 804,672; and for 6000FPM, into 877,824.

More to come . . .

Topics