Thread: Is injected gas worth the extra cash

Originally Posted by Tank

Still nobody has come up with an explanation of why the Backfires, I've never had one on any vehicle I owned that was fitted with LPG (dedicated or dual fuel), why the big Bang, Regards Frank.

Hi Frank,

You asked a good question and I started to dig around to find out more. Here's something that I will cut and paste from some snooping around on the internet - hope it helps!

Lawrance


<start quote>

How can a modern engine backfire? It should not be possible on a fuel injected motor. Most LPG conversions use a single point injection system. LPG in it’s gaseous form is simply injected into the inlet manifold, like a rudimentary carburetor. When switched to gas the petrol injectors are switched off (this is the main reason they clog, they don’t get used enough)

A number of conditions make backfires happen. Because of the high temperatures, there can be hot spots in the cylinders, namely eroded valve seats, valves and spark plugs. The gas mixture entering the cylinder ignites prematurely before the inlet valve closes and the resulting explosion travels back through the inlet manifold.

Incorrect mixture is also another prime cause of backfires. Pumping the accelerator on starting will do it as will an incorrect installation. Some systems, particularly those on factory fitted new cars won’t start the engine on gas. The motor is started on petrol then switches over. An antibackfire flap can be fitted but there effectiveness is pretty limited. Some people in desperation have cut holes in the inlet manifold and put rubber plugs in them so they blow out or stick on masking tape so it blows before the meter.


<end quote>

Originally Posted by tempestv8

Hi Frank,

You asked a good question and I started to dig around to find out more. Here's something that I will cut and paste from some snooping around on the internet - hope it helps!

Lawrance


<start quote>

How can a modern engine backfire? It should not be possible on a fuel injected motor. Most LPG conversions use a single point injection system. LPG in it’s gaseous form is simply injected into the inlet manifold, like a rudimentary carburetor. When switched to gas the petrol injectors are switched off (this is the main reason they clog, they don’t get used enough)

A number of conditions make backfires happen. Because of the high temperatures, there can be hot spots in the cylinders, namely eroded valve seats, valves and spark plugs. The gas mixture entering the cylinder ignites prematurely before the inlet valve closes and the resulting explosion travels back through the inlet manifold.

Incorrect mixture is also another prime cause of backfires. Pumping the accelerator on starting will do it as will an incorrect installation. Some systems, particularly those on factory fitted new cars won’t start the engine on gas. The motor is started on petrol then switches over. An antibackfire flap can be fitted but there effectiveness is pretty limited. Some people in desperation have cut holes in the inlet manifold and put rubber plugs in them so they blow out or stick on masking tape so it blows before the meter.


<end quote>

Lawrance, thanks for that, I can see where their coming from, but as I said I never had a problem with it and it seems to me anyway, that it's a recent thing. I am tossing up whether to Chip my 4L V8 (93 efi and dissy) and fit a 200L gas tank across behind the back seats or find or swap the V8 for a 300TDi and auto, a dilemma, Regards Frank.

Originally Posted by Tank

Lawrance, thanks for that, I can see where their coming from, but as I said I never had a problem with it and it seems to me anyway, that it's a recent thing. I am tossing up whether to Chip my 4L V8 (93 efi and dissy) and fit a 200L gas tank across behind the back seats or find or swap the V8 for a 300TDi and auto, a dilemma, Regards Frank.

I've got my old venturi system for sale......

In referance to the backfiring, I do have A answer but not THE answer. An this relates to the old style lpg systems (as that it what I have gathered from most posts people are asking about?)

A few years ago a friend had one of those brilliant (for their time) gas research systems fitted to his worked 351 clevo. Now the car ran like a dream, and had no power loss whatso ever from the crappy holly 650dp that was removed. But it had developed a backfire problem, this after 6-7months got beyond a joke as the finer filters were 30 clams each and he's destroyed enough for the founding members of finer filter corp. to retire on. Sounding familier so far.
Well being both mechanics, and well researched in lpg we decided to fix what no gas expert had been able to rectify.
We altered the timing, regraphed the dizzy, changed the plugs untill the correct temp plugs were found, had the gas jets changed, to no avail, giving up we decided one drunken night to do some durability testing on some tires, ironically enough this was when we solved the problem.

We invertably had the bonnet up when the engine backfired in total darkness, now this was quiet spectacular to say the least, especially in our inhebreated state!
This is what we worked out.
Electricity (the spark) will always take the easiest path of resistance, a cylinder under compression has a much much higher resistance between the plug gap as a cylinder that is undertaking the intake stroke. This resistance is magnified with old plugs. Now when you have standard plug leads (HT leads) that are old and have been mal-treated they have a substansual amount of resistance. And in our case the HT leads were touching between two cylinders that just happen to overlap with their strokes, the electrical charge was jumping leads!!!! and going to the cylinder on the intake stroke. Very speky we were quiet amused.
The rest as they say.....is history.
New Gas rated leads and lead seperators totaly solved the problem.

Hope this may help at leas one of you guys out there. an I hope it hasn't bored those that already knew.

Cheers

Originally Posted by p38arover

I've got my old venturi system for sale......

What brand, usable capacity of tank, how much? Regards Frank.

Originally Posted by Larns

In referance to the backfiring, I do have A answer but not THE answer. An this relates to the old style lpg systems (as that it what I have gathered from most posts people are asking about?)

A few years ago a friend had one of those brilliant (for their time) gas research systems fitted to his worked 351 clevo. Now the car ran like a dream, and had no power loss whatso ever from the crappy holly 650dp that was removed. But it had developed a backfire problem, this after 6-7months got beyond a joke as the finer filters were 30 clams each and he's destroyed enough for the founding members of finer filter corp. to retire on. Sounding familier so far.
Well being both mechanics, and well researched in lpg we decided to fix what no gas expert had been able to rectify.
We altered the timing, regraphed the dizzy, changed the plugs untill the correct temp plugs were found, had the gas jets changed, to no avail, giving up we decided one drunken night to do some durability testing on some tires, ironically enough this was when we solved the problem.

We invertably had the bonnet up when the engine backfired in total darkness, now this was quiet spectacular to say the least, especially in our inhebreated state!
This is what we worked out.
Electricity (the spark) will always take the easiest path of resistance, a cylinder under compression has a much much higher resistance between the plug gap as a cylinder that is undertaking the intake stroke. This resistance is magnified with old plugs. Now when you have standard plug leads (HT leads) that are old and have been mal-treated they have a substansual amount of resistance. And in our case the HT leads were touching between two cylinders that just happen to overlap with their strokes, the electrical charge was jumping leads!!!! and going to the cylinder on the intake stroke. Very speky we were quiet amused.
The rest as they say.....is history.
New Gas rated leads and lead seperators totaly solved the problem.

Hope this may help at leas one of you guys out there. an I hope it hasn't bored those that already knew.

Cheers

Larns, that may explain why I never had a problem with backfiring, every vehicle I owned fitted with LPG always had the correct plugs, the best leads available, high performance coil and on LPG only vehicles I always had the dissy regraphed to suit LPG. I am also wondering with modern fuel injected vehicles, esp. V8's with Plenum chambers full of tuned length air horns or bunch of banana inlet runners that there is some residual LPG hanging around waiting to be detonated by a flame from an overlapping inlet and exhaust valve esp. with the advanced timing used on LPG engines, something to ponder, Regards Frank.