Huge backfire

[Nick Read]

Can someone explain all the possible reasons for a backfire big enough to lift the bonnet and blow an airbox apart? Its a 1.8 16v, obviously multipoint injection, running fast road cams with slightly more overlap and duration than standard but not exactly wild - 264 degrees inlet and 256 exhaust. The plugs all looked sooty/black, if anything had been running rich rather than lean.

The details are all on a thread on the main forum but it was on a rolling road run using nitrous, too much for the lowish revs I think and the loading of the rollers bogged down the engine, it started pinking and as soon as the throttle was let off, a huge backfire and a messy engine bay! So any ideas appreciated. A lean-out on one of the cylinders and then the inlet valve opened with still-burning fuel inside it? Pooling of fuel/nitrous in the inlet manifold and an open valve just as the plug was firing? I cant figure it out!

 

[cliocup]

Does it still run? Are you sure it was only a backfire or has it dropped a valve? Is it possibel to get contaminated/bad batch nitrious oxide from whoever sells it to you?

Dont know why it would do that, not really into setting engines up, surely the best person to ask it the guy running the rolling road...?

 

[BUTRE]

The thing that I can think of is the auto-detonation of the mix on the inlet.

 

[Nick Read]

Oh yeah, it still runs, but as soon as you start it up, clouds of steam start pouring out of the bottom end... the nitrous oxide is fine, Ive used it on the road no problem.

What I really need to know is why backfires happen, just in general. Pops and bangs from the exhaust I can understand, but surely the inlet manifold is nowhere near hot enough to ignite an air/fuel mix? So the only way anything in the inlet could be ignited is by an open inlet valve somewhere at the same time as the cylinder is fired?

I think the reason why it made such a mess of things was because a mix of nitrous and fuel is hugely combustible whereas normally there is nothing there really to burn. But *what* can trigger a backfire?

 

[pete bann]

When a car backfires, especially on heavy acceleration, the main reason is that too much fuel is being pumped in (too rich). Running a car too rich causes carbon build up in the cylinders which also causes backfires. the carbon deposits heat up and cause the fuel to preignite before the intake valve is closed.

Afterthought; was your filter and plugs clean? was your radiator blocked? Have you checked the timing recently? Noticed any misfiring?

Found this on a webpage aswell; may be of some use?

The process of combustion forms several gases and vapors; many of them quite corrosive. Some of these gases get past the piston rings and into the crankcase. If left in the crankcase, these substances would cause all kinds of bad things (rust, corrosion, and formation of sludge), so they have to be removed. Back in the old days, they used to be dumped out into the atmosphere through a tube. Once we realized what a problem pollution was in the sixties, the PCV (Positive Crankcase Ventilation) system was developed to take the place of the old "dump tube." The PCV system uses a hose connected between the engine and the intake manifold to draw these gases out of the engines crankcase and back into the cylinders to burn with the regular fuel. The only problem to solve is how to keep these gases from going willy-nilly into the manifold and upsetting the required air-fuel ratio. The solution to this problem is the PCV valve. The PCV valve controls the release of crankcase gases and vapors to the intake manifold. The valve is kept closed by spring action when the engine is at rest. When the engine is running normally, the low vacuum it creates allows the valve to open and release crankcase vapors and gases into the intake manifold for burning. If the engine is idling or you are slowing down, the vacuum level rises and pulls the valve plunger into the valve opening. This partially blocks off the opening so that only a small amount of vapors and gases can be drawn into the intake manifold. One really comforting feature of the PCV valve is its behavior in the event of a backfire. If your car backfires in the manifold, the pressure makes the spring close the valve completely. With the valve closed, there is no chance that the flame can move into the crankcase and cause an explosion.

Dont know whether any of this helps mate?

pete

 

[Ben j]

bugger! overfueling i thought, thats why u can make flames on certain cars

 

[2 live]

maybe if the valve seats a little pitted or summat and not making a true seal when closed.

i lost a fast mini throuhg a backfire through the carb resulting in a very hot twisted piece of metal where my car once stood..........not good

not saying thats whats gunna happen nick mate but thats what CAN happen

ps by the way the mini was on twin 1 1/2 in sus without filtersand the backfire ignited the soundproofing on the bulkhead (and blew the top off 1 off the float chambers igniting all the petrol)

jon

 

[Nick Read]

Yeah I had thought about that, the valves not closing perfectly...I suppose we wont know until the heads off. Well I think if its bottom end damage Ill be going 2.0 with lairy cams. Whether thatll be enough to hold off yours for next year, who knows? you still thinking of throttle bodies as a winter project?

 

[JJ172]

Nick - I recall some time back a guy with a NOSd Ford Ka who put it on the rolling road, activated the NOS and ended up blowing the air box to pieces! He also suffered a HUGE backfire (the likes of which the Space Shuttle rocket boosters would be proud of!) and a plethora of singed engine bay components! The result was (a) trying to obtain a power run with an inexperienced r/r operator and (b) activating the NOS in 4th at low revs on the rollers.

 

[JJ172]

QUOTE:-

The nitrous boys were expected to do the best today. Unfortunately Dave Flannagans Ka didnt make it to the end.

In dyno testing the throttle is opened fully. This activated the gas and at low revs there wasnt enough fuel or air. The pressure built up very quickly and basically it exploded. The Ka gave a huge backfire and flames shot from the exhaust. With a bright flash of flames the inlet manifold and plenum chamber were blown into pieces, throwing shrapnel all over the testing area. The foam air filter was left with only its gauze showing and the air was filled with silence as everyone recovered from the shock. Luckily nobody was hurt and we hope that the damage doesnt go deeper into the engine for Daves sake.

Alex J Bogges decided not to run his nitrous on the dyno after seeing Daves but I can tell you it was very fast as I got a test run in it after the event and it struggles for traction in second gear. However we are not condemning nitrous systems on cars as it is a good form of tuning. You can find details on how to run it correctly by clicking http://www.highpower-nitrous.co.uk/ face=Arial color=#ffffff UHERE/U



http://www.kaklub.co.uk/events/meets/2001dynoshootout/meets2001dynoshootout.htmhttp://www.kaklub.co.uk/events/meets/2001dynoshootout/meets2001dynoshootout.htm

 

[Fred@BTM]

If only that had been my exs KA, the b**ch!!!

 

[BenR]

um.....initial mixute might be lean, when lean in a cylinder the burn is not complete. Thus when the inlet open there is still lightly burning mixture inside which ignites mixture in the ports and dollows back up.....

When on a carb you get a spitback up through the carb. If this occurs on your car, it could possibly ignite the N20 and fuel mixture in the plenham IF it was in a broken down state and then highly combustable.

Its higly improbable that it was a spark when the inlet was open.

 

[Nick Read]

So the backing off would lean the mixture and the inlet valve would open up again when the burn hadnt finished. Normally this would be no problem but because of the highly volatile mix coming down the manifold the whole thing blew?

Anyway, the diagnosis is complete - 4 cracked pistons, the head and the rest of the bottom end is fine. So what happened then?

JJ thats pretty much what happened to mine but with it floored you could hear it pinking, so he let off the accelerator and thats when it blew.

 

[BenR]

so it was running on nitrous before it blew?

hmm....there should be enough fuel in teh N20 suspension for all teh O2 to be used up, so leaning would be a weird thing as you were running on already. Pinking would simply be the advance too high.....as i suppose you were running optimax already?

then the only thing which could crack the pistons would be detonation in a mild mannered way. is it a serious crack as in half or a slight crack on the edges, along the face, under it etc etc?

few ques:

when he let off the gas, was it enough to stop N20 injection?

Is your r****d module throttle operated aswell or a simple on off switch?

 

[M.C.]

The key thing is you said that it started bogging down, and revs dropping. When running nitrous, the lower the rpm, the more nitrous goes in per cycle increasing torque and compression. Combine this with an engine under load (enduced by rollers) and the pressure would be far too great. SOmething has to give, and in this case it was your pistons. It would be an absolute mirracle if the bores are not marked in someway. Make sure they are perfect b4 a rebuild as it will save time and money in the long run.

Have a look at http://www.nitrous.infowww.nitrous.info and check out the torque curves for the nitrous runs.

There is a temporary hold on my DIY nitrous kit. I traded the Clio in for a V6 4-motion Golf. Im currently working out fitting for a R5 GTT.

 

[Pysco]

Yeah I think M.C. has got it. Never a good idea to use NOS on a RR unless you got a long build up time, NOS hates being held back and the RR was doing just that, if you where bogged down to 2500 then your poor engine would be making approx. 320 ft/lb of torque on 100bhp jets, somethings gonna give.

 

[BenR]

that could possibly explain the damaged pistons.....as it could only be a massive pressure increase.

BUT, that wouldnt explain the igniting of the contents of the inlet manifold...or even since it (N20) was off, the huge spitback experienced, and the detonation would of damaged the pistons, motor stopped, and no way for the flame to extend up past the inlet valves.....odd....

damage has to of been done whilst the inlet valve was open....

 

[Pysco]

Nick did the cam belt survive? If the huge increase in torque had snapped it then the inlets could well have been open when it fired, just an idea.

I dont think detonation could have happened when your on N2O, as when it breakes down nitrogen is realesed which is a great at supressing detonation, efectivly raising the octane levels.

Im stumped!

 

[BenR]

detonation happens VERY quickly with N20 ON mate........

and im stumped too..why would the belt snap only when the inlets were open?....surely a larger pressure rise would occur when both inlets and exhaust were closed.....but all our assumptions are based on one cylinder! there are 3 other doing different things! haha

didnt you meantion a leaning of one cylinder nick during the run?

 

[Pysco]

OK, I mistaken, I thought N2O supressed detonation because of the extra nitrogen, learnt something new

I just meant the belt could have snapped and it was just a fluke that it happened to snap when the inlets where open, so theyd stay open.

If one cylinder was leaning and then detonating surely only that one would have been damaged? Seeing as all 4 are bust it must have been the huge pressure incease casused by it bogging down.

The huge increase in cylinder pressure would have raised the in cylinder temp. to something astronomical right, or if one was running lean it would get danm hot too. So is it possble that it was so hot the the plug tip was hot enough to light the incoming mixture when the inlet opened causing the spitback. This happend on my mates Mini once when he jetted the carb incorectly and was running very lean.

 

[BenR]

pre ignition could happen of many points....hot carbon buildup on vavle seats might of done it.......although it might of burnt of if it was running lean. But since valves cool through their seats, it might of stayed cool enough not to burnoff, but hot enough to pre ignite....

damn my brain is scrambling!

and seeing as you pointed out it happended on each cylinder....DOH!! (to me) the same thing happend 4 times in a row!!!

oh, and when N20 breaks down, its endothermic, clooling the charge as it does. But the detonation is purely through too early a pressure rise. weather its pre ignitied off anythign but teh spark or through too much advance.....its a right killer! and what killed nicks pistons......

i cant help but feeling were goin over the same stuff haha!

 

[Pysco]

So detonation cuased by the bogging down on NOS killed all the pistons, s**t son, thats ill!

 

[Nick Read]

well I think youre all just about there. I thought I was being thick not being able to figure it out. All I know is that Ive used nitrous loads and when I use it on the road its perfectly safe, not a chance of any problems. Even if I decided to go mental and arm the system then floor it from 2500 revs, it would come in slow and the revs would have somewhere to go. Plus as soon as the controller starts pulsing the solenoids, Ive wired it so that another unit retards the ignition by half a degree per thousand rpm. Kind of back to front I know, it should be more r****d at low revs and less at high, but it better than leaving it on the standard ignition timing.


So on the rollers I still think it definitely bogged down, the revs had nowhere to go cos the rollers were doing their job, trying to load up the wheels the more power was laid down. Until eventually the cylinder pressures got too much and the damage was done. That idea about a super-hot plug nose igniting the mix as it came in sounds right. Even though the nitrous and fuel at the throttle body injector would have switched off the instant it was off WOT, there would have been a certain quantity of nitrous/fuel still making its way down the tortuous 16v manifold, and on its way it met the flame coming back up that had ignited the fuel coming from the normal fuel injectors. And then BANG. What do you reckon to that theory?

 

[Pysco]

Yep, sounds about it Nick. Though Im not sure it the N2O/Feul mix in the manifold would have lit, dont think the pressure/heat would have been high enough to break the N2O down, but either way its the same end resault.

Id go with rebuilding it as a 2.0l, get the pistons from Renault, a crank from a scrappie and use you 1.8l block, I may be able to get the machining done for you when I get mine done.

 

[BenR]

i thought you would run more r****d at high rpm as the natural ignition setting increase advance as rpm incease......to get the mixture fully burning before TDC.

so, your setting sounds about right............BUT, then some will probably argue that the charge is smaller at high rpm......

you engine will be running advancing ignition up to about 3500rpm then itll stay constant usually.....but i dont really know how the F7 ignition system works, its it a normal dizzy with weights and vacuum advance?

 

[Nick Read]

The advance is taken care of in the ECU. My chip is not a standard Hill Power one, but basically it runs full advance at WOT, whatever the revs (or at least thats what I think it does!)

Yes, the division of the charge was the reason why I said more r****d would be needed at the bottom, cos in my experience you can dump any amount of nitrous in at high revs without a problem.But low revs is the danger zone, it doesnt feel right and as I found out to my cost, its *not* right! Who knows?

Well all I know is that on the road during a lot of road testing and emptied 11 lb bottles, the 0.5 degree/1000rpm works best, any more r****d and it really struggles at the top end, any less and its just plain slower...making me think that without any dialled in r****d there, the ECU wakes up and pulls the timing anyway?

 

[Pysco]

Nick,

I doubt it runs full advance at WOT regardless of revs this would mean if you floored it at 1500rpm youd be running something like 66 degrees of advance, pinking and detonation would be pretty bad. I suspect it just runs are more agressive advance curve if that makes sense, ie removing the fail safe margin.

Dunno about the r****d on gas though, need to thik about it, Ben you know anything about this?

 

[Nick Read]

Yes thats what I mean by full advance. It would have been a bit of a mouthful to give the complete explanation as you have done Simon. But what I meant by full is as far as it will go by removing the safety margin.

 

[Pysco]

Oh sorry mate, wasnt trying to imply your stupid.

 

[Nick Read]

No thats alright I know what you mean, best to clear up what we all mean on here!Otherwise wed all end up like this

But it might be interesting to see 66 degrees of advance though...

 

[BenR]

well, generally you run little advance at low rpm and lots at high (well, up to about 3500-4000 as it doesnt really make a diff after that) as the time margin for the burn get shorter, youhave to start the burn earlier.

Thus as advance gets high @ high rpm, you need the total r****d to be more, But as it works on deg/1000rpm, its a bit weird. And it all depends on how much advance your running and when it stops. if it stops advancing at 5000rpm, then your just loosing power after that.

since youve got a progressive system and a r****d module, you could run far more N20 at high rpm! get another N20 solenoid to pump in another 25-50 at higher rpms hehe.....

 

[Pysco]

Ben,

Am I right in thinking the Emerald M3D EMS has the option of moding the ignition maps when you on gas? So you could program in the correct amount of r****d, etc.

As for pumpong in more N2O, why get more solenoids, just get some bigger jets. Be warned though, dont wanna melt your pistons again.

 

[BenR]

so you can get the 2nd solenoid to pump in only at higher rpms thus no need for huge jets in the 1st solenoid and to pulse it so rapidly...smoother

and i guess you could map for on gas but that means you will have a map for on gas ignition settings all the time. Im not sure if the M3D has the ability to support the swapping of maps with no laptop attatched.

 

[Pysco]

So youd need an extra feul solenoid too. You could set up the second pair with an RPM window switch, would be a good idea to find out when the map stops advancing first though.

 

[BenR]

well, yeah......or you could double teh fuel solenoid jet and half the pulsing.....but that would defeat what you achieved on the N20 side haha.....

keep N20 simple......on or off....fine for me, and 50bhp is enough, makea mad motor instead.

 

[Pysco]

Why only 50bhp when you can build a mad 200+bhp motor AND put 100bhp of N2O on it?

And depending on how you map it, you put put on extra 50bhp on top at high rpm, 350bhp would be pretty cool, but would the lump/gearbox take it?

 

[BUTRE]

Dont know, but its starts to sound like greedy.

 

[BenR]

becasue whe its £40 a refil and it lasts you 30 secs.......you;ll know haha.

i run twin tanks and it lasts me about a week if im sensible, and i dont really use it that much anymore.....no need. if you had 350bhp on N20...how would you contemplate getting ti on teh road? i mean, if you can use it below 4000rpm and with 200bhp you well infront of the guy at the lights and you its not the best thing round corners...whats the point!?

and if you were mapping for 350bhp on N20...you certainly wouldnt have 200bhp when its running off.....again, a compramise.

 

[Nick Read]

I think you said it all there Ben. 200 bhp I reckon is perfect in a Clio, its all there in the midrange to blow everything away, then once you get up in the revs you get your peak hp. The power/torque curves for the BB turbo job show something like 155bhp (calculated flywheel not at wheels) at 3000rpm. Thats a lot to be going on with! Putting any more through fwd is a bit of a waste of time really?

 

[Nick Read]

Well obviously not a waste of time for 1/4 miling but for normal road use its excessive and theres the cost of nitrous oxide at those kind of jettings which would be a real killler - compare that with compressed air which is free!