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Tachometer Guessing Game

Over the years I've been racing a two-stroke motorcycle, I've noticed people make some interesting assumptions about what that involves. Usually the first erroneous assumption I have to clarify is that I'm road racing, not riding a dirt bike.

The next most common inaccurate assumption is that a two-stroke revs to very high RPM. This confused me at first, since my two-stroke redlines at the same RPM as my four-stroke street bike, but I've come to accept that this is a commonly held misconecption.

In an effort to clear the air, so to speak, I came up with this Tachometer Guessing Game... have fun!


vintage racing Hondas at the VRRA Runway Romp in North Bay


Question # 1

Which one of these four tachometers belongs to a two-stroke motorcycle?


10,500 rpm redline This Honda has a 10,500 RPM redline
12,000 rpm redline This Honda has a 12,000 RPM redline
13,500 rpm redline This Honda has a 13,500 RPM redline
18,000 rpm redline This Honda has a 18,000 RPM redline

Question # 2

Which one of these four tachometers does NOT belong to a 250 cc motorcycle?


4 vintage racing Honda motorcycles
This photo was taken of myself and my friends at North Bay, the VRRA's Runway Romp event in 2009. These four happy women all race vintage Honda motorcycles, and I've given you some hints for the Tachometer Guessing Game with the model names and displaments of the various motorcycles.


Answers to the Tachometer Guessing Game...


Answer # 1

The tach reads 10,500 RPM redline on the Honda NS 250 F - the only 2-stroke in the group.

Answer # 2

The tach reads 12,000 RPM redline on the Honda CBR 600 - the only bike in the group that is NOT a 250




More information on RPM

Andrea, number 418, NS250F Honda

Andrea
VRRA # 418
1983 Honda NS 250 F
V-twin liquid cooled two-stroke
10,500 RPM Redline

10,500 rpm Honda NS250 F The general assumption that two-stroke bikes rev higher than four-strokes is not supported by this group of bikes. With a bore and stroke of 56 x 50.6 the NS has over-square dimensions. The lowest revving bike in the group is the NS250, the only two-stroke in this row of Honda motorcycles, the piston is traveling about 3,486 feet per minute at redline, which is slower than the pistons of the other 250's. Considerably slower than the CBR 250's tiny pistons.

Jen, number 343, CBR600 Honda

Jen
VRRA # 343 Honda CBR 600 Hurricane
Inline 4 cylinder 4-stroke
12,000 RPM Redline

12,000 rpm Honda CBR 600 The largest displacement of these four bikes, but with four cylinders, each cylinder on the CBR600 displaces only marginally more than the cylinders of the two twins, the NS and the VTR, even though they are 250's. The size of a piston is one of the factors limiting the speed at which it can move, hence the RPM that can be attained.

Mel, number 653, VTR250 Honda

Mel
Honda VTR250
V-twin 4-stroke
13,500 RPM Redline

13,500 rpm Honda VTR250 A liquid cooled v-twin Honda motor, but unlike my NS250, Mel's VTR is a 4-stroke. The VTR revs somewhat higher than the NS, although the cylinders are the same displacement, the bore and stroke dimensions (60 x 44.2 for the VTR250) are different, and so the pistons are different sizes. Both the NS and the VTR were originally built to be sport oriented street bikes, and have since been converted into vintage race bikes.

Laura, number 211, CBR250 Honda

Laura
Honda CBR250 Hurricane
Inline 4 cylinder 4-stroke
18,000 RPM Redline

18,000 rpm Honda CBR250, the high RPM screamer in our little group of Hondas This little Honda revs to a spine tingling 18,000 RPM. With four tiny (62.5 cc - about 4 tablespoons!) cylinders, very small pistons, and valves smaller than some body jewellery I've seen, this bike can rev high. With a bore and stroke of 48.5 x 33.8 this motors dimensions are over-square like the other 250's, but the potential for high rpm operation for this motor is far greater than any of the other motors whose tachometers are pictured above.




4 Hondas with 4 women racers - North Bay Runway Romp, VRRA, 2009
Jen, Mel, Laura and I with our four race prepared vintage racing Honda motorcycles, in this photo we've just finished our last racing round of the season, and we're all pretty happy and excited.




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