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 Post subject: Cold starting - something to check
PostPosted: Sat May 27, 2017 9:55 am 
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Joined: Wed Jan 28, 2015 5:42 am
Posts: 57
ECU Model: S40 Pro
Distributor: Lotusbits
Firmware Version: 75
Ever since I had my DTA S40 fitted, I've had problems with cold starting. I think (provisionally, as it's Summer) I've found the problem. Cold start ignition timing is set using the teeth on the crank sensor to provide timing. Double check that this is set close to where the static timing on your engine might be. My sensor wheel has 36 teeth and timing had been set to 2 teeth BTDC which is equivalent to 20 degrees. It would often backfire during starting and require several attempts - the backfiring almost certainly trashed my original starter motor. I've changed the setting to 1 tooth BTDC which is close to the static timing for the engine (around 10 degrees) and starting is now much cleaner, and the engine normally fires on the first attempt.

My cold start fuelling table is set with up to 400% fuelling uplift for the first 4 turns at low temperatures to ensure the inlet surfaces are sufficiently damped with fuel to allow a decent mixture to reach the combustion chamber. I just need to tune the table a little more to achieve clean idling during the first minute or two of warmup.


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 Post subject: Re: Cold starting - something to check
PostPosted: Mon May 29, 2017 4:22 am 
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Joined: Wed Jul 27, 2011 5:30 am
Posts: 562
Location: New Zealand
ECU Model: S80 Pro
I usually end up with 1 tooth on cranking.

Sounds like you're on the right track.

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 Post subject: Re: Cold starting - something to check
PostPosted: Sat Nov 25, 2017 3:05 am 
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Joined: Wed Jan 28, 2015 5:42 am
Posts: 57
ECU Model: S40 Pro
Distributor: Lotusbits
Firmware Version: 75
Now the cold weather's back, I've been able to do a little more testing of the cold start parameters. I mentioned before that idling was uneven and lumpy during warmup. It turns out that additional ignition advance had been configured in the base idle advance map. I zeroed this out and the engine idles a lot more smoothly during warmup.

As an aside, can anybody explain why an engine might need more advance in cold conditions? My only guess is that, with carburettors, fuel atomisation is not as good as with injectors giving an uneven flame path.


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 Post subject: Re: Cold starting - something to check
PostPosted: Sat Nov 25, 2017 4:51 am 
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Joined: Thu Jul 28, 2011 4:10 am
Posts: 813
ECU Model: S100 Pro
Its to do with the thick oil just slowing the engine down, more ignition helps speed the engine up.
I think Ive mentioned this to you before but you should target 4 to 5 degrees timing at idle and then adjust the butterfly to get the idle speed you want.
This way the engine is more responsive to ignition to maintain your desired idle speed.

Also the firing tooth on start up is not just for cold, its at all temperatures. This is covered in the book.

I should add that if you are using ignition for idle speed control the ignition timing shown in the main ignition map is ignored, the timing is controled by all the settings in the idle control window, so long as the <rpm and <throttle conditions are met, these idle windows also show you the current ignition timing being used and control loop adjustments, as does the real time display and chart.


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 Post subject: Re: Cold starting - something to check
PostPosted: Sat Nov 25, 2017 11:30 am 
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Joined: Wed Jan 28, 2015 5:42 am
Posts: 57
ECU Model: S40 Pro
Distributor: Lotusbits
Firmware Version: 75
Thanks very much Rob. I do recall your earlier suggestions re idle speed, but I'm planning to install smaller throttle bodies to improve low aperture throttle progression and I'll install an idle control valve at the same time. Doesn't this just show how usage profile affects what you want to do to a car?!

I'm just going by the standard Lotus static timing for the engine which is 10 degrees advance. The idle speed is controlled by timing at the moment and it tends to retard the timing to achieve the target speed. Again idling settings will be properly set up when I change the tbs and install an idle control valve.

I do realise that the tooth position is used warm or cold on startup so as it is set to the Lotus recommendation I'm happy with that for the moment.

I still don't really understand how adding timing advance improves cold idling. I accept the fact there is more drag on the engine when cold, but the relatively rich cold start/running mixture should support a slightly faster flame path anyway. Surely the optimum static advance setting (either factory or tuner's setup) should always be used and advancing (or retarding) that setting will move the timing away from optimum, hence degrading performance? All I can say on my engine is that it starts and runs far better cold without any advance beyond the factory 10 degrees.

Thanks, Richard


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 Post subject: Re: Cold starting - something to check
PostPosted: Mon Nov 27, 2017 5:52 am 
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Joined: Wed Jan 28, 2015 5:42 am
Posts: 57
ECU Model: S40 Pro
Distributor: Lotusbits
Firmware Version: 75
I think I may have found the answer to my question on ignition advance and cold idling.

When an engine is set up for optimum performance, additional advance won't help. From the 1980s or so, emissions regulations mandated that NOx emissions be reduced. NOx emissions in particular are a function of high combustion chamber temperatures such as those found in an engine optimally tuned for power. Before ECUs were widely used, in order to meet NOx emissions targets, manufacturers fitted devices to engines that limited vacuum advance at normal operating temperatures, thus reducing combustion chamber temperature. Looked at from the other end of the telescope, this translates to adding advance at cold temperatures, as the cold running advance is the optimal advance for maximum power.

Thus if a DTA equipped engine is tuned to meet NOx emissions standards, the ignition map may well be set up to provide less than optimal advance for power. In order to mitigate the negative effect of this slightly retarded timing on smooth running at lower temperatures, the correction maps enable the tuner to add advance in these conditions.

I'm sorry if this is rather apart from the normal application of the DTA ECU, but if I'm right it does clear up the riddle of adding advance for cold idling!

Richard


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 Post subject: Re: Cold starting - something to check
PostPosted: Mon Nov 27, 2017 8:53 am 
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Joined: Thu Jul 28, 2011 4:10 am
Posts: 813
ECU Model: S100 Pro
I still think there is something wrong with how you have the ecu set, changing the TB will only make a small difference.
I have an 1800cc engine on 42mm TBs that makes 190+ BHP and at low rpm (1000-1500) and <2% throttle drives like a ***** cat and would mate to an auto just fine.

Oh and did you ever get a wideband fitted? If you don't have one you will find it very very useful, but on an S40 you can only log with the PC attached.


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 Post subject: Re: Cold starting - something to check
PostPosted: Mon Nov 27, 2017 12:38 pm 
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Joined: Wed Jan 28, 2015 5:42 am
Posts: 57
ECU Model: S40 Pro
Distributor: Lotusbits
Firmware Version: 75
As I don't use much above 5500 rpm I'm going for 40mm TBs to replace my 48mm ones (the original Dellortos had 37mm chokes.) If my maths is right, you get around 30% less area which means 30% less flow for any particular butterfly angle, meaning finer throttle control. The 48s are definitely too big for my engine. Your 42s would be as good, and sound right for your engine. Another potential benefit from the smaller TBs is better mid range torque from the increased flow velocity. I should be able to sell the 48s on so it won't be too costly I hope.

Wideband lambda is there on the list with the TBs and idle air control valve!

Thanks, Richard


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