LPG ignition advance

[Quagmire]

Guys,

Trying to figure out the timing i should use for my LPG map in megajolt. I have been using my petrol map as a base and tweaking a bit here and there but havent really done any serious tuning as i have been trying to read up on it. Trouble is that there is lots of conflicting info.

Some people say- "whack on an extra 8-10 degrees across the board"

Other people say- "Max out at the same full advance as petrol, but bring max adv. in sooner"

Yet others say- "retard the timing more than it would be on petrol at high revs"

Now of the three i prefer the sound of a mix between statements 1 and 2.

Option 3 makes no sense to me at all. Surely for a given mixture the combustion process will occur at the same speed. This principal must therefore surely apply at all times for that fuel and so retarding timing would make no sense at higher revs when you need to ignite things sooner.

Surely if LPG burns more slowly then it will do so no matter what rev range/ loading the engine is under.

Does anyone have any hard science (rather than hearsay) about how an LPG ignition curve should deviate from a petrol one?

Thanks

[mickeyw]

When running 'standard' ignition you have few options really. Advance the timing at the DZ manually, either optimising for one fuel or the other, or a compromise setting somewhere in between.

Alternatively you use an advance/retard device such as the one RPI sell. This at least removes the compromise situation.

From what I have read in various places it seems that optimum settings for LPG are not linear in relation to those for petrol. (Yes I know this is hearsay, but I'm sure I heard it from a good authority) This is where I think MJ or MS can help in that one ignition map can be created totally independant from another.

MJ is on my list of projects and I've started gathering bits so I'm expecting to learn more about LPG ignition maps in the near(ish) future.

I'm sure there are a number of others on here and other forums (fora?) that are already running MJ and LPG, so with luck someone may pipe up and offer you a map to get started with. It's got to be worth asking. Try here for example.

Good luck, and I'll be interested in what you turn up.

[geoffbeaumont]

I never finished sorting out a proper LPG ignition map on the Range Rover before I sold it, mainly because I never got table switching working (I had petrol and LPG maps, but was normally running a compromise one).

However, the experimenting I had done suggested that at low revs you need a good deal more advance on LPG than petrol (about 13^o versus 8^o), but that at high revs you didn't actually want any more advance than petrol (so steeper curve, 2 above). I can't remember if it reached max advance earlier, but given that it wasn't a finished map I probably wouldn't want to follow it in that much detail anyway.

I couldn't find any information about ignition maps for LPG when I did mine, other than from companies selling timing advancers for dizzy systems who all said that it needed advance across the board (which, strangely, was all their boxes of tricks were capable of...). I found advance across the board resulted in very lacklustre high revs performance on LPG.

[RangeyRover]

Interesting discussion for petrol here http://www.britishv8.org/Articles/Ignition-Tuning-201.htm which is all the info I could ever find on setting your own ignition timing. It does seem to be firmly in the black art category.

[Bull Bar Cowboy]

The point of ignition (timing) for any fuel is governed mainly by the combustion chamber shape, the compression ratio, and the cam / valve characteristics.

LPG burns far slower than Petroleum and therefore needs to be ignited earlier (i.e. the flame front travels much slower). It is also 10% less efficient due to the different calorific value which is why you get less MPG on LPG (but it cheaper).

With Rover V8 engines running LPG I think its safe to generalise and say the engine would benefit from 10 advance above that of petroleum. However, once you get into the higher rev ranges then you are up against the mechanical limitations of the engine with regard to the speed of the flame front (i.e. the slower burn time).

[ThreeSheds]

The most authoritative source I have found is this. which states that mor advance is needed at low revs and less at high revs! I can't find it now, but I am sure that I have seen this advice confirmed on another technical site...

and loosely based on that, I came up with this for my 3.9:

Now - CAVEATS!

The above map is not optimised.

The above map works for mine (very well in fact ) but who knows how accurate my timing marks are and hence what the actual timing is - I have been told that they can be upto 10deg out and that getting TDC from the piston is the only really accurate way.

[nas90]

I use 16 degs BTDC on LPG and 8 degs BTDC for petrol, has worked like this for some years.

[Bull Bar Cowboy]

Time prevented me from completing the above post and it is too late to edit it………..

Whilst it is possible to calculate the ignition point in order that the combustion of the propellant is complete with maximum pressure at a fraction past TDC the normal method is to use a dyno (rolling road) and set the timing for MTBT which is translated as Minimum Timing Best Torque. This is further shortened to MBT and called Minimum Best Timing. This is also done in conjunction with the correct fuelling (lambda).

In practise the timing is advanced until the best torque is produced and that is the optimum setting. This is done at various points throughout the rev range to give the correct timing curve.

HTH

[geoffbeaumont]

I use 16 degs BTDC on LPG and 8 degs BTDC for petrol, has worked like this for some years.

But the question here is what the whole ignition curve should be like, not just the static idle timing With a megasquirt/jolt and similar you can completely remap the ignition curve instead of just offsetting the whole curve which is the best you can do with a dizzy, even if you have one of the aftermarket timing advancers.

[bobtail84]

This work's for me.

HTH

jeff