Exploding airbox

[mushy_lx92]

Hey, Got my car put on gas a couple of months ago and the airbox keep blowing up. The gas fitter warned me of this but i have no idea how to stop it from happening. It hasn'nt done it in a while but the box is in pieces with s**t loads of silicone holdin it together and a warped filter. Any help would be good thanks

[Boonz]

cut a 2 inch hole in the front top half of the air box, drill 2 6mm holes around the hole you cut, make an aluminium plate that is 2 1/2 inch round, thread 2 30mm long 6 mm bolts thru the 2 holes, drill 2 holes in the 2 1/2 inch plate so it slides over the bolts.... find 2 compression springs and 2 nyloc nuts and put the springs infront of the nuts and you should have something that resembles a one way blow off vlave

[r_i_d_a_h]

hrmm ive heard a couple of stories just like this and some of resolutions to this problem was getting the engine tuned so it works well with the LPG.

[daron]

lol nice work penut
mine had 2 holes drilled in back with 2 bath plugs wedged in and rire/string holding onto them so if ya get a backfire they blow out and dangle rather than blowing box to bits
the more tighter you make the box with silicone the bigger the bang and mess
dont silicone
ps get car retuned for gas and it would backfire

[pandaman]

hrmm ive heard a couple of stories just like this and some of resolutions to this problem was getting the engine tuned so it works well with the LPG.

Exactly, get it tuned so that it's not running lean and backfiring through the intake.

[mushy_lx92]

Thanks for that tip peanut, ill pick up a box and give it a crack. Definatly needs a tune tho, rebuilt motor 2 years ago and still havn't tuned it, got no time or money to spend on it

[Stretchomatic]

only time i heard of this was when revving on start up...lol...it'd be fun if u had a few boxes...

[mushy_lx92]

only time i heard of this was when revving on start up...lol...it'd be fun if u had a few boxes...

Done the start up one couple of times, funny for onlookers especialy at servos and near large crowds

[pandaman]

only time i heard of this was when revving on start up...lol...it'd be fun if u had a few boxes...

Yeah, fun, right. Main problem is that if there's enough explosive power being released in the inlet tract to demolish an airbox, chances are it could eventually damage something more vital.

[mushy_lx92]

Done it again, on the way to work, took off from a set of lights at a fraction over 1000rpm and BOOM! blew the whole front off and sent shards of plastic everywhere very nasty, ill post up pics later, funny what gas can do when it travels the wrong way

[Not_An_Abba_Fan]

There is a valve that can be fitted to the intake tube that stops this from happening. Anti blow back valve or something its called.

[pandaman]

There is a valve that can be fitted to the intake tube that stops this from happening. Anti blow back valve or something its called.

That's treating the symptom and not the actual problem, if he'd just get the bloody mixtures adjusted it wouldn't do it. At least nowhere near as often anyway.

[woteva]

I remember seeing this happen years ago. Was behind a VN on gas at a red light, lights changed to green, the VN went bang and stalled. I helped them push it to the side of the road. Walking back to my car I saw the round carbon canister thingy laying on the road

[samuels]

That's treating the symptom and not the actual problem, if he'd just get the bloody mixtures adjusted it wouldn't do it. At least nowhere near as often anyway.

there wouldn't be a symptom at all, if the fitter had done his job properly
i sure as hell wouldn't be paying $2500 for a gas conversion,to find out it blows up my airbox,you pay that money to have gas made available to your engine,it's the job of the fitter to install it correctly and tune it correctly,otherwise it's just a half kocked job isn't it?

[Not_An_Abba_Fan]

That's treating the symptom and not the actual problem, if he'd just get the bloody mixtures adjusted it wouldn't do it. At least nowhere near as often anyway.

The gas kit you get for one of the Subaru models comes with that valve as part of the kit. So it must be a regular occurrance for the manufacturers to supply it. In the exhaust shop where I used to work, we had templates made up for a lot of vehicles for the intake tube to be made from exhaust pipe to stop that from splitting. 2 of the gas fitters in town got us to make the pipes for them.

[pandaman]

The gas kit you get for one of the Subaru models comes with that valve as part of the kit. So it must be a regular occurrance for the manufacturers to supply it. In the exhaust shop where I used to work, we had templates made up for a lot of vehicles for the intake tube to be made from exhaust pipe to stop that from splitting. 2 of the gas fitters in town got us to make the pipes for them.

Not so much a regular occurrence as something that may happen once in a blue moon on a well tuned vehicle. If it's backfiring through the intake it means it's running very lean. Subaru would likely have included the valve as a well thought out safety measure, but I doubt they'd be expecting it to happen often.

[jules]

There is a valve that can be fitted to the intake tube that stops this from happening. Anti blow back valve or something its called.

that would push the pressure back the other way and risk damaging the TB wouldn't it?

my airbox (petrol) blew up once and the replacement one had ribs on the top. the parts guy explained holden started building them with ribs to strengthen them. if you're still running a box with smooth surface, consider getting one of the ribbed ones.

i don't know if that makes it strong enough to survive a backfire in practice though.

[pandaman]

that would push the pressure back the other way and risk damaging the TB wouldn't it?

my airbox (petrol) blew up once and the replacement one had ribs on the top. the parts guy explained holden started building them with ribs to strengthen them. if you're still running a box with smooth surface, consider getting one of the ribbed ones.

i don't know if that makes it strong enough to survive a backfire in practice though.

The idea of the one way valve abba hater refers to is that it gets whacked on the side of the intake pipe and if the intake becomes significantly pressurised (ie. by a rapidly expanding cloud of combusting gas) the pressure is released via the valve rather than being released in the form of explosive force destroying the airbox. It's not directing pressure back into the engine it's releasing it to atmosphere. It will "cushion" the rest of the inlet tract, although I'm still concerned about the idea of what the backfire is doing to the valvetrain.

The ribbing is a fairly crude method of protecting the airbox, and probably not a brilliant idea. When it backfires through the intake there is a fair amount of explosive force being released. Better that it destroys a $30 airbox lid than anything else in there. I prefer ford's solution, on some of their gas packs (i think it was from AUII onwards they started doing this) they had spring loaded airbox clips, so if you got a backfire it'd just stretch the springs and hopefully snap back into position. At worst you had to reseat the bastard next time ya stopped.

[nsladden]

I heard this nearly ALWAYS happens if you depress the accelerator upon starting it up. The worst case ive heard of is an EF Falcon blowing out the headlight when it backfired....

[pandaman]

I heard this nearly ALWAYS happens if you depress the accelerator upon starting it up. The worst case ive heard of is an EF Falcon blowing out the headlight when it backfired....

Can happen, depends on the vehicle. My old LPG XE needed throttle on startup otherwise the bitch just wouldn't start. The only time it backfired through the intake was when the mixtures somehow walked from where they should have been and the bugger ended up too lean. Then it was doing it at random, basically whenever it felt like it. Paid the local mechanic $50 to put the exhaust gas analyser on it and adjust the mixtures and it never did it again till the day it died.

[Not_An_Abba_Fan]

Until a direct injection gas system has been developed, backfiring through the airbox will never be stopped. Because the gas is mixed before it goes through the TB, the heat in there will ignite it and cause the explosion. If you want the same power and response from gas as you have from petrol, you have to increase the volume being pumped in, this will result in a build up of gas in the intake, add heat and bang. When it doesn't happen, the volume of gas is decreased to stop the backfiring.

[pandaman]

Until a direct injection gas system has been developed, backfiring through the airbox will never be stopped. Because the gas is mixed before it goes through the TB, the heat in there will ignite it and cause the explosion. If you want the same power and response from gas as you have from petrol, you have to increase the volume being pumped in, this will result in a build up of gas in the intake, add heat and bang. When it doesn't happen, the volume of gas is decreased to stop the backfiring.

Lean running will also cause this as I have said numerous times, in exactly the same manner as extremely lean running will cause it on a petrol engine, but as you have said, in a gas engine the mixing occurs before the TB and so it will ignite more readily. I still maintain that on an engine with properly adjusted mixtures the rate of intake backfire will be negligable, I know this as:

a) I owned and drove such a vehicle for three years with the only occurrence of intake backfire being due to stupidly lean mixture.
b) My Father owned and drove two such vehicles for the better part of 10 years with NO occurrence of intake backfiring because he doesn't drive as hard as I do and he's meticulous about keeping his car in AAA condition.

So, if the vehicle is tuned correctly it will not backfire through the intake, except maybe on the rare occasion when it's running hot, lean and being belted at the same time. Of course this will happen occasionally, which is why safeguards such as the anti blowback valve and the airbox spring clips are included. Fact is the OP's problem would most likely be greatly reduced by having his mixture adjusted, he's already said he hasn't had it tuned for gas. So doesn't it seem llikely that may be at least part of the root of the problem?

[flyen]

this is one of the reasons i havent run my car on gas since it blew up the first motor.

no way that bbq **** is getting injected in my replacement 5L

[samuels]

Can happen, depends on the vehicle. My old LPG XE needed throttle on startup otherwise the bitch just wouldn't start. The only time it backfired through the intake was when the mixtures somehow walked from where they should have been and the bugger ended up too lean. Then it was doing it at random, basically whenever it felt like it. Paid the local mechanic $50 to put the exhaust gas analyser on it and adjust the mixtures and it never did it again till the day it died.

where'd they walk off to,your mixtures? they're buggers for that aren't they

[dufus]

In the 10 months ive had my car, running on gas nearly every day i have never experianced this. After this post it will happen knowing my luck. BTW its an impco system if that makes any difference