Converting "2000 PPM" based speed signal to PPR
02-06-2016, 04:07 PM
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Car: '89 Firebird
Engine: 7.0L
Transmission: T56
Converting "2000 PPM" based speed signal to PPR
The Holley HP can't understand the T56 speed sensor so I have to feed it a 2000 PPM (pulse per mile) signal from a Dakota Digital SGI-5c speed signal converter. It's basically the cruise control output of the device.
However, the Holley ECM is assuming that any signal it receives is a speed signal (Hz) from the trans ECM, and it calculates ground speed by knowing the pulses per revolution of the speed sensor, axle gear ratio, and tire diameter. I need to fool it by putting in the equivalent "Pulse Per Rev" at the trans sensor, for given a "2000 PPM" type signal. Anybody want to tackle that math?
Note that the speed sensor in the T56 trans is 17 pulse per rotation.
However, the Holley ECM is assuming that any signal it receives is a speed signal (Hz) from the trans ECM, and it calculates ground speed by knowing the pulses per revolution of the speed sensor, axle gear ratio, and tire diameter. I need to fool it by putting in the equivalent "Pulse Per Rev" at the trans sensor, for given a "2000 PPM" type signal. Anybody want to tackle that math?
Note that the speed sensor in the T56 trans is 17 pulse per rotation.
02-06-2016, 04:46 PM
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Car: '89 Firebird
Engine: 7.0L
Transmission: T56
Re: Converting "2000 PPM" based speed signal to PPR
Gosh, I was over thinking this and making it harder than it was. Got more simplified with my approach and the answer revealed itself...
Given:
Speed signal @ 2000 pulse / mile
3.73 axle ratio
26" diameter tire
Solution:
A) Distance traveled per tire rotation
(26 inch)*(pi) = 81.68 inch
B) Tire rotations per mile
(63360 inch/mile) / (81.68 inch/rotation) = 775.70
C) Virtual speed sensor rotations per mile
775.70 * (3.73 gear ratio) = 2893.35
D) Virtual speed sensor, Pulses Per Rotation
(2000 pulse/mile) / (2893.35 rotations/mile) = 0.691
Only problem is the Holley HP software doesn't accept a value less than 1 so I will have to fake the gear ratio to compensate.
Given:
Speed signal @ 2000 pulse / mile
3.73 axle ratio
26" diameter tire
Solution:
A) Distance traveled per tire rotation
(26 inch)*(pi) = 81.68 inch
B) Tire rotations per mile
(63360 inch/mile) / (81.68 inch/rotation) = 775.70
C) Virtual speed sensor rotations per mile
775.70 * (3.73 gear ratio) = 2893.35
D) Virtual speed sensor, Pulses Per Rotation
(2000 pulse/mile) / (2893.35 rotations/mile) = 0.691
Only problem is the Holley HP software doesn't accept a value less than 1 so I will have to fake the gear ratio to compensate.
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