number of pulses per revolution from HEI tach connection?

[Heatison]

I'm setting up a PLC to run an experimental IC engine. I want to use the pulse output from a 6 cylinder GM HEI ignition. Anyone know how many pulses per revolution the HEI outputs?

Thanks!

 

[pontiacjack]

Three.
Or is this a "trick" question?

 

[Heatison]

Thanks, Jack! No, not a trick question at all. I want to program a loop in the PLC I'll be using to display real time ignition advance in terms of degrees. Will trigger count start with separate prox switch at TDC, then use a pulse count, with count stopping upon receipt of signal from induced trigger from #1 ignition wire. Then have PLC do a math function to divide counted pulses by 360. This tells me I need to install an encoder on the crank, or get a rough estimate by doing a time division instead of pulse count.

 

[MikeHalloran]

Early HEI systems that used a distributor on top of the engine, used a reluctor in the distributor base with a number of poles equal to the number of cylinders. ... and also may have included a mechanical advance that might get your PLC confused, either by moving the timing pulse without being commanded to do so, or by inducing flutter in the signal timing. So that would be 3 pulses per crank rev for a 6 cylinder engine.

Be aware that the reluctor output is not a typical logic pulse.

Also, BEWARE that GM has made a number of V6 engines on V8 production lines, with cylinder banks at 90 degrees to each other, some of which had uneven firing intervals (and tricky distributor phasing) and others of which had even firing intervals and odd-looking cranks.

Generic engine controls sold for natural gas engines are flexible enough to use a number of different inputs, but commonly take their speed signal from a magnetic prox probe looking at the starter ring gear teeth, and a #1TDC signal from a magnetic prox probe looking at a single hole in the flywheel rim.


Mike Halloran
Pembroke Pines, FL, USA

 

[Heatison]

Thanks, Mike. Great info! I can get an "off the shelf" encoder that has 360 pulses per revolution for about the same as what I can buy a prox only for.

Thought about the flywheel option also. I've got 155 teeth on the flywheel, so adds "a little hair" to the math. The large radius of the flywheel does give a lot of room for accurate timing.

I'm building my own CNG controls. Guess I'm masochistic that way

 

[MikeHalloran]

LT1 V8 engines used an opto-interrupter and a very fine-pitched tonewheel inside the distributor, which was unique to that engine, driven by the cam nose, and mounted behind the water pump and directly under the water pump's shaft seal, which consistently lasted about 30k miles. I got so I could change the cap, rotor and tonewheel (>$200 in parts) in only 8 hours (you had to r&r the water pump and the electric smog pump with its incredibly stupid mount), after way too much practice. The LS1 is much too civilized for my taste, but the electrics are not under the water pump, and don't include much 'fine pitched' mechanical stuff.

I.e., I'd be wary of using an encoder on an engine, what with the delicate glass disc and small dimension optics inside and the usual instrument bearings on the shaft. YMMV.

BTW, those natural gas engines seemed to chronically misfire at low speed and low/zero load, the only way we could test the monsters, even with super high energy CD ignition. Old timers said it was the nature of the beast. YMMV.


Mike Halloran
Pembroke Pines, FL, USA

 

[Heatison]

Thanks, Mike. Good advice. CNG (methane) has a high activation energy requirement compared to gasoline (octane). That makes it hard to light off under no load conditions due to low temps at the moment of ignition. Look at it this way; propane and CNG kits are available for Diesel engines. Apparently, the fuel doesn't pre-ignite under Diesel compression conditions, which is pretty hot. As the comedian Ron White says, "there's your sign".

 

[MikeHalloran]

Bill Engvall does the "Here's your sign" bit.

Mike Halloran
Pembroke Pines, FL, USA

 

[Heatison]

I stand corrected.

Thanks!