TPS, up or down.

Hi, 

I put my TPS to 850 (stock around 700) and now the bike runs great, but i dont know exactly what going up or down really does.

Can someone explain for my

2006 250X

I'm not sure anyone outside of Honda knows exactly what the result of that is, but in general, the TPS when fully active modifies the ignition map so it's on a curve rather than a straight line.

Increasing it means the ECU thinks the throttle is open further than it is, so it would be getting into the flatter part of of the curve.  Exactly what the curve looks like though and what it means to set it at 850 vs 700 I don't know.   

I think there's some other functionality there as well, because even when you disconnect the pink wire, that's only one leg of the TPS, and the ECU is still receiving input from the other two legs, so I don't think the shape of advance curve is the only thing the ECU and TPS deal with (i.e. when it starts might come into play).  I know Chuck has said that he has totally disconnected the TPS and there is a difference (don't remember what off-hand), so it does do something else.

Chuck or CRF Doc may have more info, so hopefully they'll see this.

Jim.

Edited by Jim Dettman

Thinking it would make for a simple 'mapping switch' last season I installed a

toggle switch next to the fuel petcock that would bypass the center TPS wire.

 

I tried it on/off during various situations (constant cruising, accelerating)

and throttle openings but honestly could not tell any difference in power characteristics.

 

Switch and wiring since removed to un-clutter the bike.

As Jim said only Honda knows exactly. My understanding from researching DIY EFI/Ignition is the ECU uses the TPS as an indirect way to sense intake vacuum and then advances ignition lead, very much like the vacuum advance on cars. This optimizes ignition timing for different throttle positions.  At part throttle there is high manifold vacuum resulting in less air entering the engine, which results in lower compression pressure, and a low pressure mixture burns slower than a high pressure mixture. So beginning ignition sooner during part throttle helps throttle response, gas mileage, and power. Also starting a slow burning mixture earlier allows a more complete burn before the exhaust cycle which means less heat in the exhaust; classic examples are red header pipes during engine warm on bikes with only 2D ignition.  A 2D map uses RPM to determine ignition lead, 3D adds a third parameter. The ECU has a three dimensional map for ignition timing; it looks like a table with  RPM and TPS values on the X and Y axis, and timing values in the cells.   Here is a thread in the Jetting forum; https://thumpertalk.com/forums/topic/959149-tps-on-keihin-fcr-necessary-or-not/

My TPS is set at the mid point of the Honda specs;  when at the high end of the spec it will slightly reduce added ignition advance, vice versa for lower settings.  The Honda Service manual has the procedure for setting the TPS.

I've disconnected the TPS on my X and it does soften throttle response and that helps in low traction situations when in 1st and 2nd gear.  What is interesting is there are several other things that also effect low speed throttle response and power delivery such as CCC mods, jetting, and exhaust. I currently have nice smooth low end throttle response and power with the TPS and pink wire connected by fine tuning exhaust and jetting.

 

The TPS is used as a reference of assumed manifold pressure, at a given opening based on static air/fuel mixture and changing vacuum  when the venturi is opening. "advancing" the tps would initiate an earlier timing advance on the set ignition map. The pink wire mod raises the rev limiter slightly, and uses a slightly more aggressive timing map that is shared with the CRF250R ecu. Advancing the TPS can (and will) have an effect on performance, but will still be dependent on correct A/F ratio to actually make or lose power. Just adding timing to the same manifold pressure does not always = more power. 

 

Honda's factory suggested TPS settings are a very good baseline to having the timing map in the sweet spot providing jetting is close to correct. 

Thanks for the replys guys, now i understand how it works.

 

Cheers

The TPS itself is a potentiometer (i.e. variable resistor).  When you adjust the TPS, you are changing the resistance "seen" on the output leads, which translates to a voltage seen by the ECU, which in turn modifies the ignition curve as others have said.  Lower resistance (e.g. 700 ohms) equates to a lower base voltage.  Higher resistance = higher base voltage seen by ECU.  In other words it feeds an analog voltage to the ECU which varies based on throttle position.  If you increase/decrease the static resistance (throttle closed), you are simply shifting the range of voltages seen by the ECU per a given throttle position.  If I remember correctly the service manual to adjust the TPS to 0.13 to 0.15 of full resistance.  This means measure the full resistance across the leads and set the TPS to 13% to 15% of that number.  This accounts for variations in TPS resistance from bike to bike.  For example, mine reads from 600 ohms to about 4.5kohms if I remember correctly.  Whereas another bike could be 550 ohms to 4.2kohms.  What matters is you adjust it properly for your given bike, not strictly based on a number provided.  You likely won't notice much difference if you are off by 50 to 100 ohms though, so it isn't too critical.

Edited by mossman77

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