Hello. I've read that the 89981 is a newer model. But the 8998 states that you can check tach calibration and test RPM switches. The 89981 does not. I'd like some clarification as to any disadvantages from one to the other. Not considering price. BTW. I've been using a 8998 (borrowed) for some time now and I want to purchase my own and give this one back. Thank you.