Air/fuel ratio and detonation question:

[Tom R]

I'm a fairly new TR owner and I must admit I don't read as much as I can. However, the search tool on this site, while decent, still provides way too many pages to sift through after making an inquiry. So...I have a question and please feel free to direct me to the resource material instead of repeating information already available...unless you're really bored.

I'm trying to understand why detonation occurs assuming all systems are functioning properly. What I mean is this...If there is enough fuel available, the ecm/chip are good, spark and all sensors are operating properly, why would knock occur when turning up boost? It seems that the all these components should adjust accordingly and make corrections as needed under whatever boost one might be running at any moment.

 

[bison]

Do a google search. It has more to do with the fuel being used than the engine or engine management.

 

[my1stgn]

Do a google search. It has more to do with the fuel being used than the engine or engine management.

Please excuse my friend Bison, he has enough time to tell you to do a search, but not enough time to explain anything. Very busy man! LOL

But to make a long story short, knock is when the engine senses a vibration in the bottom end. This could be a false reading (as in something hitting, loose/bad motor mounts, etc) or it can be a true reading as in the piston was trying to travel up, in the compression stroke most likely, and something was pushing it down, usually a detonation, which cause the vibration a knock sensor would read.

Now when you increase the boost, you increase the amount of air entering the engine by compressing it (air is compressable, liquid is not). when you compress air, you also create more heat.

Detonation is cause usually by the air/fuel mixture igniting prematurely (as in when the piston is still coming up). This can be cause by too much timing, too much heat in the combustion chamber, not enough fuel or not enough octane. There can be various other reasons also.

Even though you may have enough fuel for the air in the combustion chamber, the heat created may be too high and the fuel would be ignited by the heat prematurely instead of by the spark plug.

In order to reduce this "knock" you can try adding higher octane fuel (which burns slower so as to not self ignite prematurely) or you can add methanol which would also increase octane and cool off the incoming air too!

Hopefully this makes sense, if it doesnt, then DO A SEARCH!!!! J/K, just ask more questions and if I can get to them I will! My goal is to NEVER TELL ANYONE TO DO A SEARCH. I believe in that if you dont know the answer, then dont post, and if you do know the answer (and have time) then post the answer or a link to the answer. Hope this helps!

(No offense Bison, its just me pet peeve when people post the infamous "Do a search" reply.)

 

[Tom R]

Thanks my1stgn and you too Bison. I did ask for direction to resource material so I guess a properly performed Google search would do the trick.

Prior to getting your response my1stgn, I did find some good information on just this topic. I discovered that heat is not a good thing. It seems that the more you can do to control engine heat, the less likely detonation will occur. I DO NOT want to blow my head gaskets off or cause any other internal damage if it can be avoided. So...if I am interested in adding more boost safely, I have to make sure that several things are up to par before I turn it up.

I don't think I want or need to upgrade my stock intercooler, so I better check several other potential problems and ensure they don't exist before moving forward. I do have Rick aka TurboBuRick from this site that has helped me tremendously and I guess I'll be visiting him again soon. I was thinking about adding alky but it doesn't seem to make sense until eveything else (vacuum leaks, egr, compression, timing etc.) are all in prooper working order first. Still learning...thanks.

 

[DailyDrifter]

Do a google search. It has more to do with the fuel being used than the engine or engine management.

I'm going with Bison on this.

But long story short, a low octane fuel will ignite easier, and is more susceptible to knock. Fuel with a high octane rating are more stable and dont ignite as easily. (yeah I even threw a big word in there for ya )

 

[bison]

Please excuse my friend Bison, he has enough time to tell you to do a search, but not enough time to explain anything. Very busy man! LOL

But to make a long story short, knock is when the engine senses a vibration in the bottom end. This could be a false reading (as in something hitting, loose/bad motor mounts, etc) or it can be a true reading as in the piston was trying to travel up, in the compression stroke most likely, and something was pushing it down, usually a detonation, which cause the vibration a knock sensor would read.

Now when you increase the boost, you increase the amount of air entering the engine by compressing it (air is compressable, liquid is not). when you compress air, you also create more heat.

Detonation is cause usually by the air/fuel mixture igniting prematurely (as in when the piston is still coming up). This can be cause by too much timing, too much heat in the combustion chamber, not enough fuel or not enough octane. There can be various other reasons also.

Even though you may have enough fuel for the air in the combustion chamber, the heat created may be too high and the fuel would be ignited by the heat prematurely instead of by the spark plug.

In order to reduce this "knock" you can try adding higher octane fuel (which burns slower so as to not self ignite prematurely) or you can add methanol which would also increase octane and cool off the incoming air too!

Hopefully this makes sense, if it doesnt, then DO A SEARCH!!!! J/K, just ask more questions and if I can get to them I will! My goal is to NEVER TELL ANYONE TO DO A SEARCH. I believe in that if you dont know the answer, then dont post, and if you do know the answer (and have time) then post the answer or a link to the answer. Hope this helps!

(No offense Bison, its just me pet peeve when people post the infamous "Do a search" reply.)

It was too vague of a question to answer properly and could have many answers that would be considered correct. If he searched he would be able to get a great understanding without waiting for partial responses and be able to come back and ask more specific questions about it. Ill say this though. The most common reason for detonation which is an uncontrolled combustion event in the cylinder is that not enough octane is more often than not the problem.

 

[TurboBuRick]

Tom, some good reading here. Vortex Turbo Buick Performance

Check out the tuning section.

 

[my1stgn]

It was too vague of a question to answer properly and could have many answers that would be considered correct. If he searched he would be able to get a great understanding without waiting for partial responses and be able to come back and ask more specific questions about it. Ill say this though. The most common reason for detonation which is an uncontrolled combustion event in the cylinder is that not enough octane is more often than not the problem.

I agree with you on that point. I guess the only reason that those "do a search" responses are a pet peeve of mine is because when I was a young LCpl in the Marine Corps I didnt know **** about fixing gear. And those little people that did know mostly kept their knowledge to themselves. Very few shared knowledge but eventually I figure stuff out and I became a better tech. But I always remember those a-holes that didnt know crap would pretend to know crap and then tell you to "look it up" when u asked them a question. And once I became a supervisor I vowed never to tell my Marines to "look it up". I would either A) explain it to them and how it worked or B) show them were to find the information if I didnt have time to look it up myself. But it wasnt a "look it up" answer, it was a "Lets grab the pub and see what it says" answer.

And I know you have TONS of knowledge cause Ive read your posts. And a lot of times you are right on point about advice, so Im not aiming this towards you, just gets on my nerves when people say "do a search". But you are right, there is tons of info on detonation and pre-ignition that you can read for days.

 

[jasjamz]

The search function is very useful. I can say that because I use it frequently. I see so many questions posted on here that could have been answered by using the search. Sometimes it needs to be said to avoid "intercooler type" debates. Then you have those that would rather you tell them everything vs. doing some homework, learning, or searching of their own. This is not directed in anyway at the thread starter because he is doing things the right way for sure, and I commend him on his efforts! I wish more of the guys new to TRs were like him!

 

[Pronto]

Doing searches can be time consuming. Many times I search for things and end up reading threads that have nothing to do with what I'm looking for; they caught my interest and then I'm off on tangents. Anyway, sometimes a quick question is good to ask if it's very specific to get a quick answer but a question about what causes knock like others said is not a one sentence answer. When, where and how I've gotten knock are pretty typical. Downpipe hitting, bad gas, wrong tune, knock on shift. Bad gas was very real and scary. Hitting downpipe and knock on shift, intermittent and went I found the issues they were resolved. Tune is big one. It's easy to get overzealous with boost and not have things right. That's why I try to follow the golden rule, only change one thing at a time. Unfortunately there are many members that are way to impatient and want to run fast with out knowing what makes the lil' 6 so great. Next thing they go boom and are all pissed off. Make the time to do your reading and learn. You'll save a ton of money in the long run.

 

[Razor]

I use the analogy of increasing boost increases cylinder pressure. Like raising compression on an engine.

If you increase compression you need to increase octane. This is why you cant run a 13:1 motor on 87 octane.

So basically, when you increase boost you need to slow down the burn by increasing octane.

There are other factors like quench, cylinder head design, timing, air temperature, air density, etc etc etc. As to the Buick, having a clean combustion chamber, efficient intercooler, and properly sized turbo.. you can get away with a lot more boost than on a stocker.

Lastly, your factory computer doesnt know what boost your at. All it can do is feed fuel into the motor based on maf, load, and rpm.

HTH