I know what some of you are thinking:
"Here's that great Spanish beardy poof banging on about that dammed throttle snatching problem again".
"Anyone knows that with total concentration and precision throttle and clutch control it's not an issue".
"Why doesn't he spend a King's ransom in a lovely aftermarket Tuneboy or Power Commander and software and spend the rest of his life breathing fumes in some God-forsaken tuning emporium remapping his ECU?".
Ever since in an old post I suggested a mechanical throttle damper device to help with the gearchanging and possibly cure the throttle snatching inherent in these EFI systems, I became obsessed with the idea of solving it. It's not a great problem, but I find it annoying and want it cured. The bike runs so well otherwise that it's a shame about this one glitch.
I believe I've found the answer. This is long and boring but bear with me.
It seems that our EFI systems are rather old fashioned, they're of the "SPEED-DENSITY" variety rather than the "MASS AIR FLOW (MAF)" type as used on virtually every car nowadays.
The Speed-density types accept input from sensors that measure engine speed and load (manifold vacuum), then the computer calculates airflow requirements by referring to a pre-programmed look-up table, a map of thousands of values that equates to the engine's volumetric efficiency (VE) under varying conditions of throttle position and engine speed. Engine rpm is provided via a tach signal, while vacuum is transmitted via an intake tract Manifold Absolute Pressure sensor (MAP). Since air density changes with air temperature, an airbox-mounted temperature sensor is also used.
The ECM also utilizes oxygen sensors mounted in the exhaust downpipes. The computer looks at the air/fuel ratio from the O2 sensor and corrects the fuel delivery for any errors. Other sensors on a typical Speed-Density system usually include a motorised idle-air control valve to help regulate idle speed, a throttle-position sensor (TPS) that transmits the percentage of throttle opening and therefore the rate of throttle opening and closing, a coolant-temperature sensor,(in our case the oil temp.sensor does the job) and a knock sensor as a final fail-safe that hears detonation so the computer can retard timing as needed.
Our bikes lack some of these: no knock sensor and no motorised idle air by-pass system. We just get a manual idle air control in the shape of the "choke" for setting a fast idle, and a mechanical idle speed setting knob under the left throttle body , no knock sensor either and the possibility of damaging knocking is probably taken care of by programming a conservative maximum advance figure that is safe, but robs us of some power.
Although Speed-density EFI systems can run without a TPS , the maker has fitted one, not to control ignition timing like on carburetted bikes. This is done by the previously mentioned sensors and maps. It's there to speed-up throttle response, in our case this is so fast that it creates snatching problems and the revs drop too fast on closing the throttle.
The purpose of this component is, I've found after much research, to supply the computer with information of the RATE of throttle position change, or how quickly or slowly the throttle is opened or closed. The transitions from one engine speed to another, however absolute throttle position is one factor that the ECM uses to trigger closed loop fuel trim and idle speed stabilisation. However, the computer also switches to a TPS referenced table at anything over 6% of throttle.
After much searching and finally inspired by, of all things, a device to damp the movements of the TPS in a Honda Insight Hybrid car to improve the MPG, I have fitted what amounts to an electronic damper to my TPS.
It's an extremely simple device, costs a few cents, and the snatchy throttle on my bike has disappeared. It now behaves just like a carburetted engine with a soft throttle on-off feel. The only snatching left now is that caused by normal chain-driven bike driveline slack. No cure for that so far, but I'm thinking of fitting a spring-loaded chain tensioner to deal with that. Watch this space.
A surprising by-product of fitting this device is that the gear changing has improved a lot. This is due to the slower throttle response which means the revs take a bit longer to drop during a cog swap.
You might think that the lighting throttle response of these engines is now ruined, but in practice it's hardly noticeable and can be adjusted in a few seconds with the device's built-in variable resistor or indeed put back to standard altogether by setting this resistor to zero.
I have yet to do more tests for a longer period of time but it's looking good so far. I'll post the details of further tests for you good people to approve or tear apart.
Credit is due to Lachlan Riddel of ChipTorque for the original suggestion (although for a different application), and to Julian Edgar editor of Autospeed WEB-based magazine. Both Australians by the way
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Below a series of annotated images detailing the mod:
This last image taken with an oscilloscope illustrates what happens when you close the throttle abruptly. The signal that goes to the ECM from the TPS decays relatively slowly, whereas as standard it drops like a stone:
Linear Mode
