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 Post subject: Re: BMW 745i conversion
PostPosted: Sat Nov 05, 2016 10:52 am 

Joined: Wed Sep 29, 2010 5:01 pm
Posts: 2849
Location: Yorkshire
I know this is an old thread, I've just converted another BMW745 myself and done a thread on it (link below).
On this conversion I fully addressed and overcame all of the issues I mentioned above... but by using a method of compensating fuelling depending on mode of engine operation (normally aspirated / Valvetronic) that is not possible with other LPG ECU's.

Pleased to read you've had next to no problems.

I think it unlikely that misfires/poor running on petrol will be due to issues with petrol injectors due to lack of their use, more likely that fuel trims have been learned to fuel the engine correctly on gas but these trims are now set differently to what they should be for petrol. I'm always dubious when I read about vehicles running better on gas than on petrol - it is unlikely we manage to do a better job of getting the fuel system correct than the vehicle manufacturer (BMW in this case), we can only really hope to achieve indistinguishable running on LPG compared to petrol not to better it. In cases where better running on LPG than on petrol is claimed it is far more likely that aspects of the LPG system have upset how the vehicle runs on petrol, i.e. fuel trims being steered so that they become incorrect for running on petrol. Where it is believed petrol injectors need cleaning and someone uses a special fluid to clean them we might expect them to drive on petrol for a short while to allow the cleaning fluid to do it's thing... In cases where fuel trims change to different values (when running on petrol/running on LPG), during such 'cleaning time' we might expect fuel trims to be relearned towards correct figures for running on petrol (unlearned for running on LPG) while actually running on petrol and to be relearned for running on LPG (unlearned for running on petrol) while actually running on LPG. This could account for a vehicle seemingly having fewer and fewer engine light issues while running on LPG while at the same time seemingly getting worse at running on petrol, and this could also seem to validate suspicions that petrol injectors were in need of a clean, when actually the real issue is (and always was) incorrect LPG system calibration. In the case of a Valvetronic BMW this would most likely be due to the LPG system not being capable/set up/calibrated to compensate LPG fuelling properly for the different modes (normally aspirated/Valvetronic) of engine operation. Your results are pretty much what I would have predicted and I might further predict that the vehicle will go through periods of the engine light coming on again a bit more often (and then less often again etc), partly affected by: how much you drive on petrol / when your petrol ECU decides it's time to check engine operation while running in normally aspirated mode (check the throttle) / if any pending codes are detected for such as cam sensor issues (even phantom pending codes).

There are only two ways of achieving correct results on a Valvetronic: 1. Fit a system where the ECU compensates for the effective difference in petrol pressure (relative to manifold peressure) when the engine switches between normally aspirated and Valvetronic modes / 2. Fit a system where the hardware components (mechanical components i.e. injectors and reducer) effectively provide the same level of compensation without the ECU having to do any of that compensation. In case 2, what we might usually consider are less suitable components (e.g. injectors that don't respond as quickly as others or respond less quickly at higher pressures / reducer that isn't as pressure stable as others) can actually provide better results than components that we would usually consider a better spec or more consistent over the range of operating dynamics. Case 1 is the better way of addressing the issues, for case 2 to be viable you've either got to know a lot about the real world spec of components or be lucky that components have the necessary dynamics. With an ECU that is not capable of compensation for manifold vacuum (or not set up to compensate for manifold vacuum) with very linear injectors and pressure stable reducer, fuelling will be incorrect to the extent of around 20% when a Valvetronic engine switches between modes. So, say if someone had best intentions regarding converting a Valvetronic and thought they had carefully considered all the tech aspects and selected components that were hyped as among the best in terms of performance, but they hadn't thought to address the variable petrol pressure aspect of the Valvetronic... Now if they had actually managed to select components that performed as well as they were hyped the engine would now see around that 20% difference in fuelling depending on which mode the engine was running in. So it might be just as well if the components didn't actually perform as well as they were hyped, because latency of injectors might help bring down that 20% range of fuelling error. Though these injectors might not reduce that 20% as much as with other injectors that the person had overlooked (overlooked because they were not hyped as much as the components they fitted). In cases where good results seem to have been achieved, this might lead someone to believe that they had made all the right choices for the right reasons, when actually results may only be as good as they are because the real world spec of components they fitted differ from the hyped claims - and meanwhile if they had known about and propelry addressed all of the truly relevant issues and seen through the hype they might have acheived perfect results. It is possible to achieve good results by luck for the wrong reasons but it is very unlikely to achieve perfect results without properly addressing relevant issues and choosing components for the right reasons.


Full time LPG installer
Servicing / Diagnostics / Repairs to all systems / DIY conversion kits supplied with thorough tech support
Mid Yorkshire
2 miles A1, 8 miles M62,
Twitter https://twitter.com/AutogasSimon

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