Tune resistor - Australia - No shunting ???

[DaveG]

I have been having great problems tuning my Griff 500 since an engine rebuild. The main problem has been the familiar 'shunting - hesitation' on light-steady throttle between 1200 and 2300rpm. I have spent many hours playing with all the variables (Timing, AFM calibration, fuel pressure, coolant senders, etc), but have not managed to fix the problem. Reducing fuel pressure down to 1.5 bar helped but did not fix. As a last resort I changed the tune resistor from the standard 3900 ohms down to 180 ohms. Hooray!, a much smoother engine. It also idles much better without the small amount of 'hunting' I was having.
Can any of our member tuners tell me what is different about the Australian mapping? Why is there a separate tune resistor for down-under?

[cinquecento]

it's tuned for the sunshine...

I suppose that's not a useful answer....

[blitzracing]

Its possible its a non cat map of some sort- running the non cat (UK spec) map certainly makes the engines run better low down as it does not constantly sweep the mixture. Shunting is normally occurs during the period the engine is running a bit lean.

[blitzracing]

Did you ever try the 400 ohm map (UK non cat)? Apart from that I can only think that there may be fuel trim for avarage higher temperatures, or lower / higher fuel octanes. If I have time tommorow Ill give it a try on my car and watch what happens to the fuelling.

[spend]

It's odds on it will be a LR (3.9?) map for temps & fuel quality ,not the TVR map. If it works better you may have mistakenly set up your timing like a truck?

[David Beer]

My 500 was really smooth with the non cat map' mark Adams said on here that it could catch fire!! It did but not because of the tune resistor!

[blitzracing]

I think a fundemental problem is down to the basic age and design of the engine as it was never designed to run catalysts, and Id suspect a fixed air fuel ratio of say 14:1 for idle and pulling away is a lot more suited to the engine than the 14.7 the catalysts and emmisions require. In addition the whole lambda feedback system is pretty sloppy, as the ECU waits for the lambda probes switch state at 14.7:1 and then applies a fueling offset to make them switch the other way. If they do not switch it then applies as series of additional off sets until the probe switches, but this takes some time, so the engine can remain richer or leaner for some period. In an ideal world if the engine is perfect across all 8 cyclinders, and the mapping is really good, then very little off set is required, so the lambda control is very tight, and reasonably rapid. However as the engine wears (example cam wear) then the mixture across all 8 cylinders will vary so the avarage signal from the Lamda probes is skewed, so required fueling offset is increased before the probes switch. The best way to describe this is the system goes into saturation either rich or lean, and then takes longer to switch the other way that leads to a rough idle and poor low speed performace. Once the ECU goes open loop at 3400 rpm the ECU then simply applies all the fuel needed as per the mapping, so things then run smoothly from that point onwards. This is not the whole story, as some cars still shunt on the non catalyst fuel map, but Ive not got to the bottom of that one!

[DaveG]

Thanks for your input Blitzracing . Would be very interested to see effect on your fuelling.(if you have time)
Some more testing today. The problem is still there, but is much 'softer'.
I will try and get the CO checked today to see if this needs tweaking and
I will post as results unfold.

[blitzracing]

The 180 ohm map is definatley a non cat set up, and the mixture does not sweep to keep the catalysts working. The low speed mixture will be fixed by the CO trim pot on the side of the AFM, so the car may well be running a bit richer all the time so it no longer shunts. I could not say any more than that about the map. Do you need the catalysts (legally)? If you dont you could run one of the non catalyst maps (after removing the cat's themselves) but you would need to get a TVR / Mark Adams chip to go with it, as the non cat map you are trying may well run lean at higher RPM, as its likely to be for a Range Rover / Land Rover.

Mark

[DaveG]

Mark, many thanks for taking the time to test fuelling and your input.
Yesterday I did a couple of runs (4000 rpm in 1st gear) to see what he plugs looked like at the top end - seemed ok and no weak mixture. I also got the garage to check the AFM/CO, which appears to be ok ! I might also try one of the other non cat tune resistors.
I have a wideband and Techedge so should strap this on with the laptop.
When I rebuilt the engine I fitted a trigger wheel and have now just received a Megasquirt ECU and EDIS 8, plus coil packs. I originally planned to do this mod after I had got the car working normally, but should probably now just get on with it. (Only issue is I have rented my house and garage while I renovate a Victorian cottage.)
I will no doubt be back on here with a few questions once new installation gets going - take note Cinqucento!.
Once again - thanks.