The aircraft:
Recently constructed Vans RV8 with IO-360 and normal pair of brand new Slick mags. D-120 connected to the P-leads via resistors. Actually connected from the ignition switch. Short wires (6-8 inches long) connect from the switch and go to a terminal block. The D120's tach lines also go to this terminal block. The resistors are connected across the terminal block in a manner so that I can more easily change values.
The problem:
Upon full power takeoffs with the throttle wide open and the prop full forward and the RPMs hitting 2700, the tach readings will often go "all wonky" (technical term), and jump about erratically, indicating wild fluctuating numbers all over the place in the 3000+ numbers.
I know the engine is still only turning 2690-2700 because no connecting rods have departed violently thru the crankcase walls
.
At lower power settings & RPMs (cruise, etc) the RPMs read normal.
The original 30K ohms P-lead resistors would make the tach go all wonky at pretty much anything above idle rpms and I've increased the resistance values to eliminate this problem and I'm already up to 120K ohms on each P-lead. I'm afraid if I go much higher on the ohms then it will lose the ability to read at lower RPMs.
Would putting a small capacitor across the signal line, say something between 0.001 mfd to 0.01 mfd, from the D-120 side ("downstream" side) of the resistor to ground possibly help to "flatten out" these spurious spikes that are causing the erratic readings at high RPMs? Or would this soften the signal too much for the D-120 to be able to read the rising or trailing edges of the pulses that it needs to measure the RPMs across the full range?
Any other ideas? Maybe a voltage divider type of resistor network?
Recently constructed Vans RV8 with IO-360 and normal pair of brand new Slick mags. D-120 connected to the P-leads via resistors. Actually connected from the ignition switch. Short wires (6-8 inches long) connect from the switch and go to a terminal block. The D120's tach lines also go to this terminal block. The resistors are connected across the terminal block in a manner so that I can more easily change values.
The problem:
Upon full power takeoffs with the throttle wide open and the prop full forward and the RPMs hitting 2700, the tach readings will often go "all wonky" (technical term), and jump about erratically, indicating wild fluctuating numbers all over the place in the 3000+ numbers.
I know the engine is still only turning 2690-2700 because no connecting rods have departed violently thru the crankcase walls
. At lower power settings & RPMs (cruise, etc) the RPMs read normal.
The original 30K ohms P-lead resistors would make the tach go all wonky at pretty much anything above idle rpms and I've increased the resistance values to eliminate this problem and I'm already up to 120K ohms on each P-lead. I'm afraid if I go much higher on the ohms then it will lose the ability to read at lower RPMs.
Would putting a small capacitor across the signal line, say something between 0.001 mfd to 0.01 mfd, from the D-120 side ("downstream" side) of the resistor to ground possibly help to "flatten out" these spurious spikes that are causing the erratic readings at high RPMs? Or would this soften the signal too much for the D-120 to be able to read the rising or trailing edges of the pulses that it needs to measure the RPMs across the full range?
Any other ideas? Maybe a voltage divider type of resistor network?