how to tune a tachometer

Engine and clutch tuning with a tachometer can be a dependable means of performance improvement- only if your tach is reading correctly. Dependingon the amount of error and where the engine powerband actually is (optimum engine speed), a miscalibrated tachometer can equate to a substantial loss in overall horsepower.

For example, an engine produces 100 horsepower at 8000 revolutions per minute. Armed with this knowledge we can clutch and gear for this figure. Hidden, however, is the fact the motor is actually spinning 8300 rpm, though the tach says 8000. The engine may be falling off the powerband, thus placing its horsepower at 95. This will vary from one tach to another and from engine to engine.

In recent years, tachs come from the factory pre-calibrated to a much finer degree. This is due to an increase in component quality and improved factory standards.

For general purpose riding, factory settings are close enough to provide good performance. If you are picky about riding, or involved in racing,accuracy is a must.

"As a general rule, if a snowmobile engine is designed to hit its peak horsepower at a certain engine RPM the tach builder will design it to operate best at that specified RPM," explains Jim Fairchild, technical advisor for Starting Line Products, located in Idaho Falls, Idaho.

Changing the power peak of the engine- higher on the scale- by porting or by adding tuned exhaust systems for example, calls for tach calibration.Hence, the more an engine is moved from its peak rpm/horsepower figure,the more the tach will read in error.

Here are the steps to calibrating an analog tachometer. The tach chosenfor this article is from a 1999 Polaris 700 RMK. As a side note, digital tachs are out of the do-it-yourself arena.

1. Remove tach from the instrument pod.

2. Remove backlighting bulbs.

3. Position tach in a "simulated" operating position.This was done by cutting a V-shaped pattern from a soft foam block. Thetach was rested to complete this facsimile position. A strong reason whythe tach was mounted in a like position is because most meter movementsare counter-balanced to reduce vibration.

The counter-balanced movement is the reason why a tach should be testedand later mounted in its original configuration; not rotated to show peak rpm at a 12 o'clock position, for example. This in and of itself could knock the tach out of synchronization. Don't assume that what works for NASCAR will work for your snowmobile.

4. Connect the calibrator's power wires to the tach. The FineLine calibrator used here has two wires ? positive (yellow) and ground (negative ? brown).

5. Position the calibrator's selector switch for the type of pulse signal that operates the tach. The Polaris 700 tach operates on a 12 pulsesignal from the rotation flywheel/alternator system. The selector switch is positioned on 12.

6. Start a calibration test by setting a target rpm on the calibrator,in this case 7800. Keep in mind when calibrating the tach, position it and calibrate it to the same viewing angle as if it were mounted in the snowmobile's instrument pod. This is a little-known tidbit that makes a difference.

First reading on this tach shows it registering 200 rpm faster (8000rpm) compared to 7800 actual rpm on the Fine Line calibrator. This much error is uncommon on the new tachs. Yet, this much of an error would pose a performance loss if the motor is being tuned for a peak horsepower figure at a given rpm.

7. To correct the slop, the tach is equipped with a small porton its back side. The calibration port is covered with tape. To gain access to the adjustment screw, remove the tape. With a small screwdriver, adjustthe tach until the needle matches that which is registering on the calibrationinstrument. Again, be aware of line of sight ? adjust for angle. Replacethe tape over the port.

8. Engine rpm can now be charted in increments from peak down to desired engagement. What this means is to record what the snowmobile's tachometer registers as compared with the calibrator's rpm reading?7600 rpm is actually 7700, etc. Calibration will vary slightly in each increment,which is normal. Never forget peak horsepower is the desired goal. Charting the variances incrementally provides information for future tuning, for clutch engagement, or just to know the error ranges.

When a tach is calibrated for the peak rpm/horsepower, it may be off by 200 to 500 rpm when the drive clutch kicks the drive belt in the pants.This is called give and take- compromise.

9. Remove the positive and negative wires.

10. Reinstall the backlighting bulbs.

11. Return the tach to the snowmobile's instrument pod.

The question asked here is how would a snowmobiler know what the peak horsepower is at a given rpm. If the motor remains stock, then consult your dealer. But, if your motor has been modified, then the aftermarket supplier should have these figures readily available.

Fairchild points out, "Tachometer calibration is not necessary forthe majority of snowmobilers, but for those who know the effects of 200 rpm differences on either side of the optimum powerband, it may just be the three to five horsepower a sledder has been looking for."