Just for the record -
edited for accuracy 11/04/03, I was confusing DCI and Redline projects and my software co-hort was quick to correct me (nice to see there is someone keeping me straight)

My tests of the tachometer were made with the car parked. I disconnected the tach wiring and fed a square-wave into the tach lead and measured the frequency on a frequency counter, accurracy probably in the range of .01% or about 1 rpm. Using the frequency generator is what showed the error in the tach. The tach error is not necessarily related to cut-off rpm, and may well be an anomoly only associated with my car. I've not tested any other cars, not many people want their wiring harness cut up to do tach testing.

I obviously made the cut-off tests while driving the car, but none of them are made "by sound". The sound has nothing to do with it since the module stores the highest rpm in memory.

I'm pretty sure the Redline module samples the tach more often than the ECM does. While it's not as powerful a processor, it has nothing else to do - it's whole purpose is to sample and measure tach pulses, which it does about 40 times a second. That's one reason it can't work at low rpms - it samples too fast.

The Redline module doesn't really "sample" the tach pulses, it measures every one, compares it with the previous reading, averages it to eliminate jitter, then calculates the "actual" rpm based on the result. At 9000 rpm tach pulses are happening 300 times a second, so with the averaging calculation it will probably take 3 readings (1/100 sec) before the calculated value is accurate to within a fraction of a percent. The time when engine rpm changes the quickest is in first gear near redline, so I'm going to guess that it takes 2 seconds to go from 6k to 9k in first gear. That means engine speed changes by 15 rpm in 1/100 of a second (time it takes Redline to get an accurate rpm reading).

What this boils down to is the Redline module is always going to calculate a little lower than actual rpm, but I don't consider it a significant error to calculate 10 or 20 rpm lower than actual for the purposes of beeping to tell you when to shift.

The ECM must be able to operate at a few hundred rpm(200 rpm? - cranking speed), which means tach pulses are only coming at about 6 per second. Maybe it can sample faster at higher rpms, but maybe not. I can tell you this - If I instantly drop rpm from 8900 to 0, the tach takes a good 1/2 second or more to drop to 0, so it's not "instant response" by any means.

The Redline module does not have to operate in low rpm ranges, and in fact cannot operate anywhere below about 1400 rpm, because it's too fast to operate that slow. It would take a reading right smack in the middle of two tach pulses and would calculate an rpm of zero. Actually, it just runs out of bits to store the period (time between tach pulses) which is why it can't operate lower than about 1800 rpm. At those rpms the pulses are so far apart the counter overflows and the reading is discarded as unusable.

Anyway, the module does store the last rpm sample in memory, which you can play back after the fact. The playback is "rounded off" (truncated) to 2 significant digits so there can be some error there, but not more than 100 rpm (most likely (average) only 50 rpm), certainly not 300 rpm. But it also means if you hit 8999 rpm the Redline module will playback 8900, so there is a possibility for it to read as much as 100 rpm lower than actual.

As far as the "redline" goes, there is no room for argument. Look at the tach, the "red lines" begin at 8900. Just because Honda chose to put a big fat "9" at the 8900 mark does not make it 9000.

As far as fuel cut-off, show me a dyno run that goes to 9200 rpm on a stock ECM. I've not seen one that hits 9000, though there may be one or two. But 9200+? I really doubt it.

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See edits in above post

Luis - I would think your after-market tach would be very accurate but other than your readings I've not heard of anyone going past 9000 rpm on a stock ECM. And my car (that defintely won't go past 9k) is a MY2000 also. Do you have a way to test your SPA tach? Or another method of measuring rpm on the car?

I also agree the OBDII diagnostics may not be as accurate as we would like, though I don't have any specific info on it. I think I read where it typically updates the readings 5 or 10 times a second.

 

Not really. As the gauge displays in increments of 10rpm I just assumed that is what it is reading.

I'm not a professional engineer, but triggering a counter everytime the tach line goes above 5V and taking the time inverse can't be that difficult ....

Anyway, there is a small variable in all of this. My car has an F20C2 while you've got a F20C1 engine. I always thought the only difference was the OBD but maybe the gremlins lurking inside are not the same...

For reference below are the gauge specs. 0.05% at 9000 is +- 5rpm, thus below the display accuracy (99.99 krpm). You can read it all at http://www.spa-design.co.uk/SLITE2.MAN.doc

SPECIFICATIONS:-

INPUT VOLTAGE 8.0-20 VOLTS (working)
CONSUMPTION 10 mA(40mA backlit) @ 12 VOLTS
FUSE 20mm glass 250mA Fast(F).

ACCURACY:- 0.05%
CALIBRATION:- DIGITAL
DATA STORAGE:- EEPROM
WEIGHT:- 190g INCLUDING CABLES ETC
SIZE:- 93mm x 39m x 28mm DEEP
PLUS 20mm EACH SIDE FOR MOUNTING FEET

CABLE LENGTHS:- IGNITION LEAD - 120"
POWER SUPPLY -- 12"
SHIFT LAMP(S) ---- 40"

ABSOLUTE MAXIMUM RATINGS:-
INPUT VOLTAGE - 25 VOLTS
ENGINE SPEED - 39,990 RPM
INSTRUMENT TEMPERATURE 0 - 50OC

 

BTW, SPA has a much improved new shift light module:



It can now include helmet lights.




http://www.spa-uk.co.uk/design/sequential.htm

 

Luis - I would bet your tach is right and the difference is our engines, or more accurately, the ECM. That makes more sense than your tach being inaccurate.

And you're right, it's doesn't take much to measure the time between two pulses, which is why goober-heads like me make shift beepers.