My coupe runs a 351c straight gas via a sprintgas twin convertor system through a modified 780 holley edelbrock performer manifold 302 heads, ive just had the ignition upgraded to electronic and tuned on a dyno but i still have trouble with pinging depending on where i get the lpg fromand depending on how hot it is.
any suggestions on how to combat this?

how retarded is the static timing with the dizzy, with my xd i run at about 8-10 degrees btdc. and thats with 351 open chamber heads

yes thats comon problem that ppl here have identified that depending on were the the lpg was bought from
has somethign to do with that the govermnet has no set rules for teh octane rating of LPG

lpg just isnt as good as it use to be. to run lpg safely, you shouldnt have a compression ratio much higher than 9.2:1. this pretty much means having to run open chamber heads unless you are ALWAYS going to by directly from the supplier

I built a 351 with over 12.5 compression ratio years ago to run on gas - now i have to retard it so much it just isn't worth driving.

Timing for LPG is normally more initial less total advance, retarding the initial will only make it doughy.

LPG in Australia is Propane/butane mix, the regulations between manufacturers and batches etc should be regulated, and this would reduce the problems of pinging etc in highly tuned or high compression LPG engines. It used to piss me off to no end having the engine tuned to one batch of fuel only to find it run like crap the next different batch of fuel.

The LPG suppliers are making the mix too high in Butane; they have to use butane somewhere so they decided to burden the automotive industry. The same does not apply for domestic household and BBQ gas; it has to be 100% propane to prevent rich mixtures in confined areas, causing affixation. Propane is the good stuff, slightly lower BTU rating but much higher Octane rating than Butane; Butane also has more problems with freezing in cold conditions, unlike Propane.

One important thing with LPG is to keep it cool, cold air induction and any way to reduce intake temperatures will make a difference i.e. insulated throttle body spacer, intake oil splash tray, air gap intakes etc. Otherwise LPG looses its octane rating, LPG normally enters the engine as a gas, not a liquid, it has no way of absorbing latent heat, unlike petrol where it absorbs heat when entering the engine as a liquid and when changing to a gas in the combustion chamber, so LPG needs the coolest intake temperatures you can achieve.

Too many wanker installers could not be bothered with setting up the most important thing, cold air induction, instead preferring to sell something they stock, is easy for them to tune and easy to install i.e. Sprintgas, then they will s**t that it is not important, the NSW distributor/installer in Newcastle would say this.

A simple way to check how bad hot engine bay air temps are is to tune the Sprintgas setup with a gas analyser, leave the engine and gas analyser running and close the bonnet, watch it change, it is much less dramatic if a CAI LPG setup is fitted.

The Sprintgas mixer is pretty good, but it is not available with cold air induction, you could build an enclosure for the mixer to fix this problem, you could set it up in a Shaker or many other designs picking up air from the bonnet vents, windscreen cowling, or piping to the front grill etc

so fuzzy you saying all lpg owners need bbq gas ie propane? then the pingin issues would be gone???

Well it would be less likely to happen, 100% Propane is over 104 (can be much higher) octane, but butane is around 93 (if pure, lower otherwise), doesn't take much to work out the octane rating would be all over the place with different propane/butane LPG mixtures.

Butane produces slightly higher British Thermal Units (BTU) than Propane does, and slightly lower BTU than petrol, BTU is a measurement of heat energy produced, Layman's terms: Produces more bang for the buck.
But butane's normally much lower octane rating gives Propane the upper hand in an engine built and tuned for it, a much bigger hand.

Wow thanks for all that i didn't expect such a huge response i've got a few things to work with now , thanks guys

[quote="Damage"]Timing for LPG is normally more initial less total advance, retarding the initial will only make it doughy.

Too true ,I found the easiest solution was to regraph the dizzy advances to almost the heaviest springs and depending on the engine limit total to around 26deg with initial timing being critical to be advanced to around 15deg, a lot of people have trouble starting the car when advanced this far but I found by installing a very light vac advance unit and limiting the amount to a few deg (5-7) you can set the timing at 8- 10deg without vac hooked up ,and when she starts the timing will advance to the set amount , it sounds like a lot of work but it is worth doing to prevent low down boggieness (is that a word!)
And with differing quality gases, look for servo's selling BORAL gas ,I found this to be of the highest most consistant quallity

what to forklifts use then? butane or propane? or they use lpg?

Propane ,these bottles are the same as gas heaters use
I don't think a great deal of gas powered cars anymore, simply because nobody seems to know how to get em right, after countless dyno's I could still improve on the performance & economy once it came back , for instance I took the ol' XD in for a tune on the rollers and when it came back it had less than when I took it there??? the dyno guy said it was tuned for max eficiency "like its on gas mate why would I want better economy" coincidently it wouldnt rev over 5000 without dying in the a**, but also used more litres per 100km and overheated till I screwed out the convertor regulators, I hope you have better luck than I did!

im hearing ya my old xd had 11 1/2 : 1 it got that bad i sold it lol uhh the good old days

i just parked mine up in the shed - no avgas - s**t LPG
what is the world coming too???

I know that you can buy better quality LPG from country areas than in the city areas. My previous XF was a dual fuel and it would fly when filled up in the countryside. I've also had some mechanics I know who enjoy four wheel driving also state that country LPG is usually higher quality.
I wish I knew why it was better LPG for them than here in urban areas but I just don't know.