Overrun-fuel cut, Necessary?
Discussion
Pupp said:
Well, TVR didn't on some chips in the 14CUX set up, but it's kinder on the wallet and the environment to tick the box... I've been playing with the idle control threshold to try and 'cure' the transitional abruptness from trailing throttle to throttle-on that sometimes occurs...
Care to expand on that some more ??Mine judders from fuel cut to getting back on the throttle.
Well, mine sometimes ends up with the rpm taking a dip below what would be the natural idle speed, typically when dipping the clutch approaching traffic lights or a junction after trailing the throttle. I think this is really just a function of single plenum/big cam disease and due to charge robbing so I am just masking symptoms but lifting the point at which the idle control cuts in, both in terms of rpm (currently 1500 rpm) and throttle angle (sorry, can't remember the figure but it's 'some' throttle as opposed to 'none') does seem to have minimised this with no other adverse effect. I am using an idle valve.
Another trick you may be aware of is to table some ign advance just below wherever you want it to idle, so that it gets a kick if the revs fall below that point. Only thing I'd mention is I run slightly higher idle than most given the cam profile (as much to try and prevent lobe scuffing as to stop it dying) so I perhaps have more scope for playing with these features. I have also rescaled the load axis to give more resolution in the lower third of the throttle range (recommended).
Still not glitch free, and probably never will be on a plenum, but much more drivable than when I started
Another trick you may be aware of is to table some ign advance just below wherever you want it to idle, so that it gets a kick if the revs fall below that point. Only thing I'd mention is I run slightly higher idle than most given the cam profile (as much to try and prevent lobe scuffing as to stop it dying) so I perhaps have more scope for playing with these features. I have also rescaled the load axis to give more resolution in the lower third of the throttle range (recommended).
Still not glitch free, and probably never will be on a plenum, but much more drivable than when I started
The key to successful over-run fuel cut off is to have the activation point above the rpm range used around town, and the reactivation point about 500 rpm lower. This avoids always activating/deactivating when trundling around.
Also adding more fuel when the over-run fuel cut off deactivates helps too. This redistributes fuel in the inlet ports that has been drawn out when the over-run fuel cut off was active.
Another thing to think about is the ign timing in the area where the over-run fuel cut off is operating is very critical to the part throttle drivability. People are surprised how much advance you need with 0-5% TPS and 2000 rpm upwards. This transition area is difficult to tune. Adding more load sites in the low load areas helps with resolution. I used 3, 6, 9, 12, 15, 20 successfully.
Tune the area 0-10% load 1000-2000rpm first then play with the over-run fuel cut off setting would be the best way.
Steve
Also adding more fuel when the over-run fuel cut off deactivates helps too. This redistributes fuel in the inlet ports that has been drawn out when the over-run fuel cut off was active.
Another thing to think about is the ign timing in the area where the over-run fuel cut off is operating is very critical to the part throttle drivability. People are surprised how much advance you need with 0-5% TPS and 2000 rpm upwards. This transition area is difficult to tune. Adding more load sites in the low load areas helps with resolution. I used 3, 6, 9, 12, 15, 20 successfully.
Tune the area 0-10% load 1000-2000rpm first then play with the over-run fuel cut off setting would be the best way.
Steve
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