3.5V8 - Dual Fuel Timing

[101RRS]

The troublesome 24v ignition system on my 101 has been ripped out and has been replaced with a holden bosch dissy with an early electronic system fitted. The engine (3.5 carby V8) also runs LPG and when installing the new dissy I set up the static timing at the listed 3 degrees BTDC for petrol as I do not know what to set it up on for dual fuel. It runs OK just a little down on power.

So first question - what is the static timing setting for just LPG and what should it be for dual fuel. If I were to set it up to the 'just LPG' setting what would the impact be if I had to run on petrol?

Second question - ultimately I want to buy a more modern ignition system for the dissy, one that will switch ignition timing settings when switching from petrol to lpg etc. Any suggestions on something that might be suitable would be appreciated - also how hard are they to program or do they come pre-programmed.

Thanks

Garry

[Hardchina]

Mate i don't reckon there will be a perfect dizzy setting for lpg. I muck around with mine a bit. I can set it for WOT power but lose big time with cruise economy, or i can set it for cruise which causes it to bog down when flat to the floor. I'm running a comprimise between the two at the moment - If i can find the timing light i'll get a idle figure for ya - but i have a bit of a cam and different manifold, so may not suit yours

The only proper option i reckon is going coilpacks and aftermarket ecu.. search here for megajolt. you could get the dizzy regraphed to suit just lpg but you'll pay and it will still be a sucky dizzy.

[101 Ron]

Lpg requires more advance earlier than petrol , but less overal advance than petrol.
Using a petrol distributor for LPG and a LPG distributor for petrol is always a compromise and a timing light and static settings are useless in the real world for a Petrol distributor on a motor running LPG.
Road timing is the way to go and is a compromise and most people or people who lack experience tend to add to much advance when road timing.
My personal thing is to set the motor up with a petrol distributor on petrol to petrol specs and timing light ect and just run the LPG on those settings.( when required to)
A petrol distributor converted to LPG usually has a weaker advance springs on the fly weights and the fly weight stops brazed up for less overal movement for less advance overall.
To do this correctly you need a dyno to test what the real world ignition requirements are for the pacific motor and fuel.
From the readings and information gained and distributor is then set up and and adjusted in a distributor machine.

[101 Ron]

The only way to have your cake and eat it is some form of ignition mapping and adjustment electronically and there are systems out there that can do this and can have two different ignition maps , one for petrol and one for LPG.
The problem is those ignition maps can be a guess unless the engine has been dynoed and the exact engine mapping requirements are known.
There have been some distributors around in the past that had a dual ignition curve set up in them and would be hard to find today.
The real answer is digtal constant engine management and knock sensors ect as per modern cars where the car adjusts itself to the qualities of the fuel the engine is burning.

[101RRS]

My personal thing is to set the motor up with a petrol distributor on petrol to petrol specs and timing light ect and just run the LPG on those settings.( when required to)

Thanks Ron - that is how I set it up. 3 degrees BTDC. Getting reasonable fuel economy on gas but seems to lack a little power. Was thinking 4 or 5 degrees might have been a little better overall but maybe not.

Cheers

Garry

[101 Ron]

Fitting a Hodlen distributor to a Rover motor is wrong as both the petrol and LPG ignition timing will be out of specs to engine needs instead of just the LPG.
If you want to ditch the 24 volt distributor, fit a early range rover one with the early drive on the end as the petrol advance curve will be set for 3.5 litre rover motor and if it is a points early one , pertronix and a few others make a electronic pointless up grade kits.
Ron

[101 Ron]

The slight lack of torque on LPG is not only ignition timing but compression and valve timing not being totally correct for LPG.
LPG motors should have higher compression and slight different valve events to suit the fuel.
Thats another reason why LPG is a compromise on a motor designed for petrol.
Modern car engines with variable valve timing under electronic instant controll run well on LPG.

[101 Ron]

More base advance on the distributor will tend to improve low to mid torque on LPG , but while the motor may not ping, it will run harsher at higher revs and loads and chew slightly more gas,
Better to run less overall advance with a better top end on LPG as this is where the real power is if you needed it and lose out on torque at lower speeds and power settings , like cruising revs on the flat and the loss of torque matters little.
I if doing a trip with my 101 tend to use the LPG on the flatter and better parts of the road and swap to petrol for the hard work for the hills ect and this tends to save engine valves harden seats or not.( the base timing for LPG can be advanced more if this strategy is used)

[101RRS]

What I currently have came out of a 80s rangie - it works fine and will do at the moment. The 24v system had degraded to the point that it was missing badly even with replacement coils and new points, rotor and condensor. The 101 failed to proceed in the desert 80km east of Tibooburra after backfiring and blowing the muffler apart and had to be trailered into town. Two changes of the coil failed to fix the problem, nor did redoing points etc. Once the system was pulled out and my backup bosch dizzy, coil and amp was fitted the problem was fixed - just not sure what the timing should be.

What I am looking for is a dual map amp etc and if that means a change of dizzy so be it.

Interestingly just changing the coil from the original 24v unit to a bosch GT40R (still with the 24v dizzy) improved my fuel economy on petrol from 10.5mpg to nearly 15mpg - regularly getting less than 19l/100km now. LPG was still giving only 10.5mpg but with the full change over to electronic ignition and the bosch dizzy it is now up to just under 15mpg on gas as well as petrol.

Garry

[101 Ron]

Could be condenser on the 24 volt distributor......it will still throw a spark, but it will be weak (in the combustion chamber)and any slight defect in the ignition system will cause a miss fire and points tend to never be working well and sparking on with a faulty condenser.
The original 24 volt leads would best be served with old fashion copper core HT leads replacing the 35 year old items......suspression is not a issuse because of the outer supression cover.
The 24 volt dist should work as well as a 12 volt versions ect expcept for slightly more fuel burn at cruise power settings as it doesnt have vacuum advance like the other versions.