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Old 10-09-2002, 05:26 AM   #1
JHSPayBar
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Default stock 2002 WRX RPM limiter

I've searched through the forums, and seen everything from 7200, 7250, 7300, 7400. Does anyone know what it actually is? Or should I just go out in third gear and take it till it stops reving to figure it out.

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Old 10-09-2002, 05:42 AM   #2
hotrod
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Default Varies slightly car to car

It appears to vary slightly car to car. You also have to consider that the numbers are no more accurate than the tach, and the driver reading it while driving, so you have at least 3 variables there.

Many of the electrical circuits that perform functions like rev limiters also depend on the accuracy of the electrical components in the circuit for the actual value they limit at. Normal electronic standard is +-20% on component tolerence with 10% +- concidered a close tolerence, and 5% +- being a high precision value component.

I was talking to a fellow that can change the rev limit on the stock ecu, and he mentioned that he cannot hit exactly a specific rpm, when he makes the modification. He can get to a value within a couple percent of what the client wants though.

I know on my car, the tach is a little behind the actual engine rpm, as I have to shift about 200 rpm faster than computations show I should for best performance. My limiter seems to trip about 7400 rpm indicated.

Larry
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Old 10-09-2002, 10:28 AM   #3
AVT
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I've logged it. The fuel cut (as there is no rpm limiter), varies from gear to gear and car to car. The lower gears tend to have a lower FC and the higher gears have a higher FC. I've found around 7100rpms for 1st, and as much as 74xx rpms in 3rd+ (typically only 73xx). This was found with a deltadash datalogger and confirmed by the avc-r peak hold.

hth
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Old 10-09-2002, 04:13 PM   #4
JHSPayBar
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thx for the info guys, gonna go out to the track here in a bit and see if I can get another 1/10th or 2 outta the car before I start modding...I like getting quicker times than the pro's
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Old 10-09-2002, 04:42 PM   #5
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Default Re: Varies slightly car to car

Quote:
Originally posted by hotrod
Many of the electrical circuits that perform functions like rev limiters also depend on the accuracy of the electrical components in the circuit for the actual value they limit at. Normal electronic standard is +-20% on component tolerence with 10% +- concidered a close tolerence, and 5% +- being a high precision value component.
Larry
This is far from correct. Your ecu measures RPM via a pulse train. It is the time between these pulses that is used to determine RPM. More than likely it is implemented as a moving average so it will take some time for the calculated value to reach the actual value (not likely more than a few milliseconds). I'd be willing to bet that if you slowly approach the rev limiter (ie a 4th gear run) it would limit at a different value than if you rapidly approached it (a 1st or 2nd gear run). This is just my guess at how they've implemented it and if I were wrong it wouldn't be the first time.

The tolerances you quote are from passive circuits. These tolerances are easily improved where desired. The vast majority of resistors sold today are 1%, Capacitors used in power supply filtering are typically 20% but those used in sensitive signal filtering applications are almost always 5% or less, the same goes for inductors (used far less than the others).
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Old 10-10-2002, 10:59 AM   #6
hotrod
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Default Haven't analyzed the circuit

D_REX:

Ooops, Your right about the resistors, I havn't bought resistors for many years, obviously the surface mount components are much higher tolerence than when I worked as an electronic tech.

As far as the rev-limit detection scheme, as I havn't seen the schematics for the ECU, or a detailed description about how it implements rev control, we are both just specualting.

The speed sensor circuit for the speedometer is a pulse counting circuit, 2548 pulses/km, off of a hall effect pickup which generates 4 pulses /rotation of the manual transmission output gear. The Auto Trans pulls its signal off the front diff as 16 pulses per rev, and down converts that to a 4 pulse / rev input to the ECU.

I can find nothing that tells what the rev limit circuit uses for input, however it is obviously not the speedometer speed sensor as it can rev limit when your in neutral, or clutch in.

The most logical is the pulse train from the crank position sensor, (cam position sensors have problems due to cam windup so would likely not be used). This data would be used either as a raw pulse train which is integrated to a dc voltage across a capacitor, or a derived digital ouput computed by the ECU from the crank position data stream. Subaru documentation does mention that the fuel Injection duration is computed using engine rpm info from the crank position sensor.

The crude schematic of the tachometer drive implies a classic RC circuit integrating an input to a voltage across a capacitor, but gives no specific details.

The impression I got was that the rev limit was using a simple RC circuit to integrate a voltage across a capacitor. When the voltage gets to some preset value it trips the rev limit. That type of circuit has been a very common way to do that sort of limit for several decades.

Modern ECU's might have a more accurate way to do it. The question of course is if that capability is used.

The Corvette EFI system (for example) has a high resolution crank position sensor ( which the WRX lacks) that allows it to actually monitor the changes in the angular acceleration of the crank shaft as each cylinder fires. This is how it detects misfire.

I would assume that if the rev limit on the WRX was a true digital counting, or interval measuring circuit, measuring the time interval of the pulse train, someone modifying the rev limit should be able to hit any rev limit value +- a small fraction of a percent.

My understanding that the rev limit can only be adjusted to a value +- a couple percent leads me to believe the ECU is still using a simple RC voltage across a capacitor.

Of course third party reverse engineering based on best guess info is not the most reliable way to determine whats happening.

Someone with delta dash could give us a much more precise idea of the variability of the rev limits on the WRX ECU.

Even if the ECU rev limit was absolutely accurate, people are by necessity judging its limit, based on the output of an analog tach, which have been repeatedly shown to vary brand to brand, unit to unit, and depending on rate of change of RPM. Not to mention the variability of individuals reading the swing of an analog needle while driving.

If you have access to the actual process used by the ECU to detect and control rpm it would be nice to know the details.

With the low resolution crank position sensor system used, I would expect a digital system would measure the elapsed time between successive firing of a specific cylinder to determine rpm, so when that pulse width drops to a critical value, the engine would be over reving, and it could activate its rev control strategy.

Larry
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Old 10-10-2002, 11:18 AM   #7
D_REX
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Quote:
With the low resolution crank position sensor system used, I would expect a digital system would measure the elapsed time between successive firing of a specific cylinder to determine rpm, so when that pulse width drops to a critical value, the engine would be over reving, and it could activate its rev control strategy.
This is how I would implelment it. I'd add just a little complexity though and average across some range of values. The RC PWM to voltage converter would, but it'd be pretty imprecise, they are already using the CPS signal in the controller I can't imagine adding extra circuitry to do this in the analog world.

Like you said we are just speculating at how we would implement the rev limiter, but either way there will be a window of values, whether created by the tolerance of the components used or inherent in the algorithm, it's there.

Later,
Dustin
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