LPG and Ignition mods.

A few basics first of all...

Regardless of the type of LPG system installed, LPG as a fuel has different characteristics to petrol vapour. It's a dry gas not a vapour and theoretically offers better combustion if it's mixed with the air charge, hence lower emissions and better efficiency (not more power).

A dry fuel runs hotter, as less energy is absorbed in the dissociation and ionisation of the fuel. Hydrocarbons are good electrical insulators, much better than air so the tendency is for the breakdown voltage at the spark plug gap to increase, typically 20-50% for LPG-air mixes.

LPG as a gas burns quite a bit slower and has a lower calorific value than petrol hence gas engine produce less power at a given rpm, however, mechanical stresses are lower. LPG as a gas compresses more easily than air so stoichometric ratio is more difficult to determine than petrol vapour and air and varies according to compression ratio.

LPG has no inherent lubrication properties.

LPG has no warmup time because there's no vapour to drop out of suspension, so if the engine can be started on LPG, the warmup is considerably cleaner and quicker than with petrol. Note that most LPG systems can be set or forced to start on gas and should start OK except under subzero conditions although you may be short of full power until the vapourizer is warmed up.

The stress on ignition components is much higher and consequently, standard components may be unreliable or prone to failure. I'll come back to this later. Cooling systems run under a higher continuous load and need to be in good condition.

New engines should not be operated on gas until they are fully run in as bore glazing can be an issue due to the low level of abrasive particles from combustion. However,contamination of engine oil is greatly reduced and it should be possible to extend intervals between oil changes.

Engines designed to run on leaded petrol may suffer from valve seat recession at a significant rate. Nearly all Rover V8 engines should cope without problems though.


How to set the V8 up for Gas

Assuming the installation is done for you, check your cooling hoses, radiator, thermostat and pump for signs of aging and replace them if you aren't sure. If you have electric or viscous fans, make sure they are working correctly and at the right temperature.

Ignition system

Make sure your distributor is in perfect mechanical order and the vacuum advance does what it says. Clean the distributor cap and coil top until they shine, do not spray anything on them. If you have manky old leads with 7mm insulation, sling them and buy 8 or 8.5mm silicone racing leads or you will suffer constant insulation failure and backfires. Magnecor leads work very well and there are probably others out there which work well too. The amount of and quality of insulation is very important as your HT voltages could exceed 50KV at times. The various spark plugs as recommended by LR or TVR are entirely suitable. The fancy copper ones don't make any difference and may not last.

The TVR ones seem to be the best, especially on high performance and high compression engines. (I'll stick the part no in later). Avoid the silver and platinum types as they track down the outside of the plug at high voltages. Some people recommend reducing the plug gap by a few thousandths of an inch to reduce the stress on the HT system. This may be an effective way of negotiating defects in your ignition components but will also reduce the spark energy and possibly engine performance. Because of the different combustion rates for gas and petrol, you may want to consider altering ignition timing to suit the fuel you use most, or seek a compromise between the two.

As a general rule, if you set up your petrol timing so it's just off pinking (knocking) under heavy load, that's the best compromise. If you want to get the best from gas and are prepared to drive around (not through) the odd bit of pinking on petrol, then you can advance your ignition timing up to 8 degrees if you have a 9.35:1 compression ratio engine. If the engine becomes difficult to start on petrol then you''l have to set it back a bit but most will tolerate 16 degrees of initial advance. If you want the best of all worlds then you'll need to consider an ignition advance/retard module from RPI, Iwema or others. These detect which fuel you are running and modify the advance accordingly.

Backfires

LPG engines are notorious for this due to the presence of a quantity of LPG-air mix in the intake and plenum on EFI engines, which can produce enough of a bang to destroy inlet hoses and airboxes.

If this is a persistent problem or happens fairly often then you have a fault, most likely in the ignition system. Backfires are in my experience extremely rare on a properly set up system and have only happened to me when I have been trying to run on an empty tank or messing about with the ignition system.

Performance

Generally mixer based LPG systems suffer from performance limitations due to the necessity of generating a pressure drop across a venturi on the engine inlet to draw gas in and a roughly 10% loss caused by the calorific value (energy content) of the gas itself.

Overall performance drops usually run at 15-20%.

The newest LPG Injection systems are significantly better and can reduce thoses losses to about 5-8% overall with calorific losses compensated for by efficiency gains. See the FAQ about improving performance on LPG if you want to boost the engine output significantly or want to run a high performance or racing engine on LPG.