Pinking Detting Knocking Preig questions and stuff

[andyf]

Hello.
Following on from the interesting RR session where several people, S13's and S14's, seemed to suffer various forms of the above, I was wondering what are the main causes, and what are the main fixes. So I'm going to list off a few things and if anyone can enlighten me, well that would be just superb!

1. Under-fuelling. I take it in this case you have a bad mixture, too much air, too little fuel, and the result is a much hotter combustion process, which damages the pistons / cylinders? Would it also cause detonation by igniting the mixture too early and thus leading to detonation problems - such as broken crank / cod-rods and things.

2. Detonation. This is definitely bad. But how much different is this to pinking/preignition ? Is it just a more severe case? With this I believe the shockwave hitting the pistons from the mixture igniting too early and trying to push the pistons down when they shouldn't will knacker something expensive. So how can this be fixed. My own thoughts are: Underfuelling, so raise fuelling (see above, is this a valid fix?), too high boost for ignition, so retard ignition, compression ratio too high so lower compression ratio, inferior quality fuel, use a higher octane.

Question is this. If people are getting detonation at under 280bhp but not underfuelling, how can you attempt to plan for 300/350/more ? What do you do to really stop detonation from occuring. I imagine a custom chip or whatever, retarding the ignition as you come on boost, and having a lower compression engine is great, but how effective will that be.

I'm going to stop writing now. getting confused already!

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Site Admin of Silvia-S13.Net and BB
Big Garrett powered S13
229.5 bhp flywheel, 187bhp wheels, loses boost top end
Spits flames, breaks exhausts

[andyf]

eh!!

[Jezz_S13]

I dunno Andy, but some answers would be cool.

[Kristian]

I have also been suffering detonation problems.
Detonation can be classed as a more severe form of preignition but is in actual fact different. Preignition is caused by 'Hot spots' in the combustion chamber. This can be caused by such things as carbon deposits that glow red hot or spark plugs which are not 'cold' enough for the given application and so again become very hot and result in a pre-ignition of the mixture. Pre-ignition is not usually as damaging as detonation but can result in damage to valves and valve seats most commonly.
Detonation results when again the mixture combusts earlier than it should in the combustion cycle. However, detonation is caused by the pressures present in the combustion chambers. If the mixture is too lean or the internal temperatures are too high (poor intercooling or a very hot summers day can be contributory to this) the high combustion pressure can cause the mixture to ignite and the result is a powerful flame front and shock wave, which can damge the engine severly. The most common by-product of detonation is piston damage and repetitive prolonged detonation will result in a hole being burnt through the piston. Piston damge can also occurr to the rings. If this has occurred to a bad degree you will/can see smoke being produced, in severe cases a lot, or misfiring and poor idling/general lumpy running of the engine etc. The other damage resulting is as you say to crank etc due to the shock waves hitting the pistons hard and putting strains on engine internals.

How to eliminate Pre-ignition:
1) Use cold plugs
2) Retard ignition, generally only if it has been advanced too much, std ignition should not result in preignition. However, tuning could effect this.
3) If carbon deposits are severe, they may need to be removed. This is done by removing the head and removing carbon deposits from the piston crowns, valves and combustion chambers on the head.
4) A way to do this simply is to run the engine say down the motorway at high revs for a few minutes and then lift off abruptly. This can result in the carbon deposits being blown off. You will see a small to largish cloud of smoke released. Note the smoke will not be prolonged or re-occurring, any signs of constant or repetitive smoke is something else!

How to eliminate detonation:
1) You are correct in that adjusting the fueling i.e. richining the mixture will reduce and eventually eliminate detonation. A turbo charged car should really run slightly rich. The extra petrol helps to cool the combustion temperatures and therefore eliminate detonation. The extra fuelling is also needed at high boost high in the rev range.
2)You are also correct about retarding the ignition. The ultimate way to do this as you say is with a chip, specifically designed to your set up. However, you either need the equipment and knowledge to make a chip or the money to pay somebody else to do it. The best way is to retard your ignition manually (turn crank angle sensor clockwise). If you increase the boost you should ALWAYS retard the ignition. This is where a chip comes in because you ideally want to run the engine at std ignition or even slightly in advance while off boost, as it improves bottom end performance. However, once on boost you need to ideally gradually retard the ignition and when at full boost the engine needs to be fairly substantially retarded (obviously dependant on the boost you are running), but as much as 5-7 degrees when running higher boost.
3) Improve your intercooler. The most important and effective measure is to reduce intake temperatures. This will probably not counteract a mixture that is too lean though. Note that on rolling roads your intercooler is pretty much ineffective so you may detonate on a rolling road because you are not cooling your intake temperature enough. Often a cause of detonation on rolling road tests.
4) Use a metal head gasket which will lower the compression ratio and hence reduce combustion pressures and result in detonation being less likely. These are generally only needed in high boost applications. They are also more resistant to damage.
5) Use an air fuel ratio meter to indicate that you are running sufficient fuel. These are good because atmospheric conditions can alter boost. I.e. there is quite a substantial boost increase in winter as opposed to summer which could warrant fueling changes accordingly.

There are other problems that can cause or be contributory to detonation but these will need extensive diagnosis. For example a worn head gasket can cause 'dirty' air to pass between combustion chambers and cause increased internal temperatures. However, the above remedies should cure most detonation and preignition problems.
I hope this helps a little. Unfortunately my Main fuel lines have sprung a leak due to corrosion so I'm off the road for a couple of days.

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Grey S13
HKS Hiper Muffler
HKS super power flow
HKS FCD & AFR
Hybrid Turbo
Lowered and stiffened
Boost at 1.0 bar
Evo VI Intercooler
R33 GTR Fuel Pump

[Jez]

I think Kristian wins biggest post of the year...

He should also claim the prize for his explaination of pre-ignition and detonation - it's spot on.

One this he's missed is fuel and octane. The higher the octane the less likely detonation (not pre-ignition) is. So get filled up on Optimax.

Another point is that oil fumes that are sucked into the inlet air from the cam covers increase the likelyhood of detonation. This is due to oil fumes in the air and also the relative high temperature of these fumes - get an oil catch tank.

Cheers
Jez

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Red S13
T28 hybrid, 300ZX AFM with my own ECU chip to suit, Mongoose, K&N, Saab FMIC, Bosch fuel pump, Koni adj. dampers & stiffer springs, 300ZX TT 4 pot brakes and front strut brace
264bhp @ 14psi (before 300ZX AFM)

[This message has been edited by Jez (edited 04-11-2001).]

[Kristian]

Cheers Jez!! Yeah foregot to mention the octane rating. I'm using Optimax now every fill up, seems good, although most is leaking out onto the floor now!!!! Hopefully can fix fuel lines soon.

[Billy]

Spanner in the works time...

If you are getting pre-ignition caused by glowing deposits, retarding the spark is not going to help. It may stop you hearing the pinking sound, but it doesn't help the problem. The pinking sound is caused by the flame front of the pre-ignited fuel, colliding with the flame front from the spark plug, the resulting shock wave rattles the piston around in the bore. Retarding the spark changes the point at which these flame fronts collide and hence changes the sound. Unfortunately, retarding the spark also raises combustion chamber temperatures and makes glowing deposits all the more likely. Catch 22.
Detonation is generally a more extreme form of pre-ignition, but strictly speaking not the same thing at all. If your mixture detonates before the spark fires, then yes, it is pre-ignition by definition, but detonation can happen as a result of the spark, in which case it is not pre-ignition. I know that sounds a bit pedantic, but there is a lot of confusion between these terms, and if we don't know what each other are talking about, then enlightenment will not follow.
Detonation refers to the manner in which the mixture burns. Actually, burns is not a good choice of word, explodes is much more accurate. Retarding ignition is not a particularly good way of curing this ailment either. It is caused by excessive temperature or pressure causing the mixture to become unstable and explosive. If you retard the spark, the piston is nearer TDC and so compression is higher still (unless you fire after TDC!) not to mention increased exhaust temperature.
The problem with all this is that it is very easy to say the only way to cure detonation is to reduce compression or increase intercooling, but unfortunately it is true. Anything else is a bodge. There is a theoretical ideal curve of combustion time against pressure which would indicate less advance for more pressure. Sadly, such graphs are not available, as they are different for every head/piston/camshaft/fuel/RPM combination(plus a lot of other variables) so you are left with empirical methods of trial and (hopefully not too much) error. Retarding the spark is the only thing you can do short of engine modifications, as increasing the fuel only helps as a side effect, in the same way as water injection works. It stops your detonation, but it also loses power.
What all this comes down to in the end is fuel quality. You can run as much boost as you like as long as 1) your engine will hold together, and 2) your fuel will hold together! They got 1000+ bhp out of 1500cc in F1, but the engines only had to last 200 miles (they often didn't last 20 miles) and the fuel they were using was not 97 octane refined creosote like we suffer from. (That's what it smells like anyway.)

The other problem with retarding ignition is the lack of engine responsivness that it induces. Of course, this is usually made up for by the boost increase, but the other side of the coin is that to do big boost properly, you need to drop your static compression, and no amount of extra spark lead is goimg to make up for the loss of off boost performance or economy, plus the only way to get big boost is to use a big turbo, which helps even less.

It's a question of what you are prepared to put up with in the quest for big numbers.
Personally I prefer a fast car to a steep dyno graph....

Hope you can make some sense out of all this, as it is possibly the most mis-understood aspect of tuning (camshafts and aerodynamics coming joint second). And if you don't, don't worry, I've read books by respected tuners who get it all arse over tit too.

[Jez]

Hi Billy - A lot of what you right I agree with but Im sure a couple of your points arent right - please correct me if you know Im wrong!

1) "The pinking sound is caused by the flame front of the pre-ignited fuel, colliding with the flame front from the spark plug, the resulting shock wave rattles the piston around in the bore." - Im sure this isn't correct. The pinking sound (in pre-ignition and detonation) is down the fact that the fuel / air mixture doesn't burn - it explodes. Im also fairly sure its got nothing to do with the piston rattling in the bore.

2) "If you retard the spark, the piston is nearer TDC and so compression is higher still (unless you fire after TDC!) not to mention increased exhaust temperature.
" - The pressure at the point of ignition is higher later in the stoke but the reason retarding the ignition stops detonation is because the *peak* cylinder pressure is reduced. With the ignition advanced too far the mixture is ignited and then conpressed further to create really large pressures and temperatures. The fuel/air explodes and you hear the pinking sound.

As I said before - if Im wrong tell me! I agree with everything else especially: "It's a question of what you are prepared to put up with in the quest for big numbers.
Personally I prefer a fast car to a steep dyno graph...."

Cheers
Jez

[Billy]

Hi Jez,

Like I say, it's a bit pedantic, but pre-ignition is a cause rather than a symptom. Whether it causes detonation or not is down to other factors.

1) I'm quoting A. Graham Bell on the causes of pinking due to pre-ignition, largely on the grounds that he is the only person (that I've read) who seems to understand the whole picture. It makes sense to me, though ultimately I can fit a lot of other scenarios to the evidence too. I would take (minor) issue with your statement that pinking is caused by exploding fuel in pre-ignition as well as detonation, for the pedantic reasons above. It might be the cause a lot of the time, just not all. Moot point.

2) The point I was trying to make is that detonation becomes more likely as the mixture is compressed more. I would agree with you completely about the peak pressure aspect.

The problem with all this is that there is pontentially more than one chain of events leading from the same start point to the same end result, and unless you have some way of identifying exactly what is going on, then you are back to trial and error. Of course, there is a way, but who (outside F1) is going to pull the head off after every power run to look at the piston tops/combustion chambers?

I wouldn't like to give the impression that I am stating any absolute truths about any of this, since none of it happens in isolation and there are countless other variables to take into account such as combustion end gasses, atmospheric conditions, mixture distribution, con-rod length/stroke ratio, the list is endless. In fact, I would be very wary of anyone claiming to know exactly what is going on, who didn't qualify their statements with reference to Schrodinger/Heisenberg et al.

Now, what were we discussing?

[Jez]

Hi Billy

So you're saying that pre-ignition can be a cause of detonation? If thats what you mean I fully understand and agree.

I got the book by Graham Bell - are you talking about his 4-stroke performance book?

I see what you're saying now - detonation can occur due to a huge number of variables.

I should be able to get into Schrodinger and Heisenburg if required, got an MSc in physics. Snag is, I've forgotten everything...

No idea what we were discussing...

Cheers
Jez

[Billy]

Yep, that's the long and short of it!

"Four stroke tuning" and "Tuning new generation engines" both give slightly different angles on the subject.

Interestingly, even Mr.Bell manages to contradict himself, saying at one point, "Both excessively rich and lean mixtures burn more slowly...", but then goes on to say, "Weak mixtures burn in a rapid, uncontrolled manner...", so who knows?

I think the moral of this is don't take anything for granted as even God makes mistakes...

[Paul_S13]

I'vr found tuning books by the engine god David Vizard a damn interesting read, also the magazine Car & Car Coversions is pretty damn good too.

[Jez]

CCC is a really good mag - just got the lastest edition today

I race my golf in their Speed Championship too! http://www.barc.net/ccc.htm

[This message has been edited by Jez (edited 05-11-2001).]

[SteveSadler]

Originally posted by Jez:

O
Another point is that oil fumes that are sucked into the inlet air from the cam covers increase the likelyhood of detonation. This is due to oil fumes in the air and also the relative high temperature of these fumes - get an oil catch tank.

Cheers
Jez

[/B]

Correct jez

which is why u see loads of Cossie and RS's with them fitted

Therea doddle to make if u wanna knock a cheap n nasty one together

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Bayside Blue '94 S13 - now fitted with a Std T28 being run in at 11psi

[Paul_S13]

Yeah i've made my own one.

[Jez]

nice one paul - im going to be making one myself soon - when I can find some shiney stuff...

[andyf]

Well BIG THANKS for the very informative answers to the questions I feel somewhat more enlightened yet still determined to find The Big Book Of Engines that surely is out there and read it till it's sunk completely in. Strange thing is, my dad is well clued up on this stuff, and my grandad helped setup MIRA or something, but it seems to have skipped this generation completely :\

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Site Admin of Silvia-S13.Net and BB
Big Garrett powered S13