Crank reference angle vs. timing errors
Crank reference angle vs. timing errors
(OP)
Some time ago, GM published a chart for the (then new) LT1, supposedly showing the effects of using various crank references per revolution on spark timing error. There's a copy (5th chart down, click to enlarge) at http://www .gmhightec hperforman ce.com/tec h/0310htp_ optispark/
This chart shows an error of about 21 deg at 1000 rpm using 4 crank references, assuming 10rpm/ms crankshaft acceleration. When I try to crank the numbers using this data, the error I get is only about 6 deg, assuming acceleration starts the instant the previous period ends, and the error is the result of predicting the acceleration period based on the previous measured constant period.
The only way I can get close to GM's numbers is to assume a crank acceleration of about 100rpm/ms. Is 10rpm/ms or 100rpm/ms a more reasonable max acceleration assumption for an OEM tuned sbc? Or is my math wrong somewhere?
TIA
Bob D
This chart shows an error of about 21 deg at 1000 rpm using 4 crank references, assuming 10rpm/ms crankshaft acceleration. When I try to crank the numbers using this data, the error I get is only about 6 deg, assuming acceleration starts the instant the previous period ends, and the error is the result of predicting the acceleration period based on the previous measured constant period.
The only way I can get close to GM's numbers is to assume a crank acceleration of about 100rpm/ms. Is 10rpm/ms or 100rpm/ms a more reasonable max acceleration assumption for an OEM tuned sbc? Or is my math wrong somewhere?
TIA
Bob D





RE: Crank reference angle vs. timing errors
A typical level of torsional vibration measured at the crank nose is 0.25 degrees peak to peak - how does that tie up with your estimates? Why is that not the same as the predicted timing error?
Cheers
Greg Locock
RE: Crank reference angle vs. timing errors
Just thinking about TV levels, at low speed they do tend to rocket up, maybe as high as 3 deg pk-pk, trying to remember a graph from 10 years ago.
So, at 1000 rpm the interval between cylinders firing is 15 ms, so at an acceleration of 10 rpm/ms, or 60000 deg/s that is an error of 1/2*60000*.015^2, 6.75 degrees.
The error is proportional to the acceleration, so you only need to increase it to 15 rpm/ms to get that 10 degree error.
Cheers
Greg Locock
RE: Crank reference angle vs. timing errors
RE: Crank reference angle vs. timing errors
They could keep track of the change in time between reference marks to infer acceleration but that is more computation and cost.
RE: Crank reference angle vs. timing errors
Thanks,
PFM
RE: Crank reference angle vs. timing errors
FWIW, I am building my own programmable ignition. It also uses optical pickups but unlike GM, mine uses a window which defines the maximum and minimum range of the firing. So although acceleration can cause a delay of the intended timing, the delay is from the a known position closer to the firing point. I am doing away with a distributor completely and using sequentially fired coils. It would be easy enough to keep track of the previous two RPM calculations to get acceleration and correct the RPM before table lookup and interpolation.
Resolution is to 100's RPM and 0.1 degree firing. RPM is stored in 8 bit data giving 25,600 RPM limit. Reducing the limit would give finer RPM resolution. For example, 10,000RPM/256 = 39 degree increments in RPM data.
RE: Crank reference angle vs. timing errors
1) Manufacturing tolerance for the parts
2) Assembly tolerance
3) Location of the 'trigger wheel' or teeth. If we locate it in front, the middle and at the back of the crankshaft. The number will be slightly different. If the teeth are located at the flywheel, the number will be different too and it is also influenced by the clutch engagement and torque converter dynamic movement.
4) Firing order of the engine
5) Number of cylinders and cylinder configuration (V or inline)
6) Processing speed of the ECU
7) Crank material because the forged, machined and cast cranks also have different material strength and elasticity
8) design and construction of the trigger wheel
RE: Crank reference angle vs. timing errors
Unfortunately, it does look like GM's error figures were based on actual measurements of their early hardware, rather than "first principles". At least their error numbers are greater than expected, probably due *mainly* to the latency in their firmware. This looks likely because a "skipped period" error is larger at lower rpm.
To summarize what the errors are, acceleration causes timing to be retarted and dwell to be reduced, using 90 deg crank references the error can exceed 20 deg at 1,000 RPM.