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Pwyll
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Hi there, complete novice time again.

I need to reset the timing on my 3.9 disco (N reg).

The problem is that Haynes give a variety of setting points depending on the compression ratio and wether it has a catalytic conveter or not.

The Cat thing I can check, but how do I know if the engine is high or low compression ratio?

Thaks in advance,

Paul

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The fact that LPG is fitted makes a big difference. LPG burns at a different rate than petrol does, so you have to alter the ignition timing for LPG (no knock sensors present on the 3.9).

Unless you have a fancy gadget that alters the timing depending on whether or not you're running on LPG means you'll need to find a compromise. This means the ignition timing won't be quite right for either petrol or gas, but it'll be close enough to run on both fuel types. It does however mean a drop in performance and increased fuel consumption rates.

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With respect to Rich, I'm not sure I agree that you have to live with the worst of both worlds in terms of performance.

You've got LPG because its half the price. I have as well, so I make sure that I run very few miles on petrol, sometimes going for a couple of months without using the expensive stuff at all! Usually, I will only need to go to petrol when I make a long motorway journey and even then, if I plan properly, I will locate an LPG source about every 100 miles.

LPG burns slower than petrol. for that reason, the spark has to take place sooner in the ignition cycle, so that the maximum energy is being generated by the fuel explosion when the piston is at TDC.

This means that if you want to get the most power from your LPG, you need to advance the timing. For my Disco, the recommendation (for petrol) is 7 degrees before TDC at idle. I have mine running at 11 degrees on LPG. As the revs speed up, the vacuum advance and the distributor weights will advance the timing even more.

Having run like this for months, I have decent fuel consumption and power when I want it.

What I do notice is that if I am for any reason on petrol, the power curve above 4000 revs degrades. The car will, however, run fine on petrol apart from this.

I would also suggest that amplifying the spark will boost LPG performance.

If you want to discuss, please PM me with a phone number and I will call you over the weekend. :)

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