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Vapour LPG Injection Tuning

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Hey all, just wondering if anyone here has alot of experience tuning LPG vapour injection systems?

Mainly what are your Target AFR'S, I know Stoich is 15.5:1 but how long do you hold that for and what are your full load targets.

Also what effects it has on Ignition tuning compared to Petrol?

Anything else to consider/look out for

Ive got a Torana with a 355 holden on LPG vapour injection I want to change the ECU on so will need to retune it.


I was involved with a Bonneville LSR Streamliner running a turbocharged big-block Ford engine on Compressed Natural Gas (CNG).


We tuned for typical (lean for a turbo) lambda values of 0.87 to 1.0, and pretty typical ignition timing (same as a high-octane petrol fuel).

Interesting things -- gas injectors leak. We ran like 170 psi gas pressure. With the engine shut off, the injectors would leak and basically flood the manifold / cylinders -- startups were terrible (crank, crank, crank... no start) until we learned to shut off the engine by turning off the fuel pressure solenoids (not a fuel pump, because our tanks were pressurized to 3000-4500 psi) and let the engine run out of fuel. To start up, we would crank the engine and then flip on the fuel solenoid and it would fire up immediately.

Another interesting thing -- you need to enlean on acceleration as opposed to enrichen, since there is no fuel wall film that changes with the manifold pressure change.

Thats a cool project David to be involved in , CNG is a totally different to LPG though and needs different fuel and ignition strategies compared to LPG , also it needs a lot more fuel to do the same job , were the injectors you used the Bosch 1600cc ones ? i am guessing you were using a high delivery pressure to save using more than 1 injector per cylinder , normal del p is way lower on run of the mill motors.

Viper , what LPG ECU are you using for control ? , i convert mostly V8 Dodge and GMC vehicles with the odd F150 Ecoboost , but both my son and i run VAG 1.8 20V motors on LPG , we have also done supercharged V8's , i always work in Lambda as its easier with less conversion needed , on WOT at a bar of boost we are looking for 0.78 / 0.8 Lambda , basically tune as you would pump fuel , ignition wise both our own vehicles are running factory ignition maps as the LPG is piggy back , in theory you can run more advance as the higher octane rating of LPG should be more knock resistant but because vapour is dry it will cause increased cylinder temps possibly , its worth tuning for MBT on the dyno and see what results you get , keep us posted with your progress this is interesting.

Currently it has a "profire" which is a ECU a guy over here created from scratch... its half decent as its running the car now satisfactory but tuning it is so painful.

for example if you make a change to the fuel/ign map it doesn't instantly update the cars map, you have to choose "burn" and it takes a second or so. making steady state tuning horrible.

Also I have alot of trouble just connecting to the ecu, the features are very basic and it used a old phone cable as the tuning cable (literally has a plug like you have on a landline phone (in Australia)

I have a Megasquirt 3X here which i think ill use. Otherwise Id look at a Haltech Elite but the price is prohibitive for this project considering its already has a ok ecu. My only concern is I do plan to sell the car in a year or so and I know it would "look" better if it had a Haltech but not 3k better

Clint (Viper) when tuning for LPG you need to be aware of a few things that make it different to tuning petrol.

1. LPG likes leaner tunes, if you tune LPG rich you increase the chances of burning valves/seats/pistons. This is because LPG is a dry fuel and it doesn't remove any heat at all from the combustion chamber/process. The more fuel you add the more heat it will develop so staying leaner than you would with petrol is a good policy. I, personally, wouldn't go lower than 0.92 - 0.90 (lambda) on a naturally aspirated engine unless dataloging was able to verify if any increase in richness is actually benefiting the engines output.

2. LPG has different burn rates. With petrol you have a slower ignition advance compared to LPG but LPG advance stops about 2500-3000 rpm whereas with petrol you can keep advancing at higher rpm if the engine responds. So the same engine will have totally different timing maps for petrol and LPG.

3. Megasquirt is a good ECU for LPG injection. The firmware and tuning software has the required formula already in it, and has been for a few years now, so the information supplied by the sensors in the convertor/regulator help the ECU calculate required injector pulse width as things like cooling system temps change (remember LPG is converted from a liquid to a gas by coolant temp and flow and as these things change so does the amount of LPG gas available).

Hey David/All

I would love to pick your brain about CNG not trying to hijack the thread but I have questions.

1. Are there cooling problems on CNG because of the lack of heat of vaporization of the fuel as it atomizes in the cylinder?

2. Interesting comment on the injectors leaking, I see a lot of after market injectors are rated for CNG are you saying they all leak? Also notice that most CNG systems are retrofits that use a single gas input to the manifold (yuck) and swap from petrol for start up. what other design considerations are there with CNG that you ran into?

3. Enlean instead of Enrich on acceleration can you expand on the concept. I guess its lack of knowledge of fuel wall film because it sounds like that is a consideration you don't have with CNG which is normal with liquid fuels.

You should start a separate topic about CNG tuning -- perhaps one of the admins could move a couple of these posts to a new topc.

1 - CNG doesn't atomize as it's already a gas -- it just needs to mix with the intake air. We ran a dry-ice /(alcohol I think), intercooler to help cool the charge air after compressing with the turbos. Bonneville is a different beast, typically the engine only runs for a few minutes (total distance 8-9 miles, 12 sec/mile at 300 mph), and we had no radiator, but did have a water tank that was heated by the engine.

2 - Our injectors were from a company that specialized in CNG engines & conversions -- I can see how a lower pressure would have less leaking, but ours maintained the high-pressure and thus given time, would flood the engine making it difficult to start.

3 - I got this clue from one of the MoTeC guys, who told me that feature of the ECU to enlean on acceleration was specifically added for gaseous fuels. Now that I understand about wall-wetting, it's clear that you don't need to add any extra fuel if it's not a liquid.

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