Relationship between knock, AFR, timing, and EGT?

[jspaeth]

Can someone explain to me how knock, AFR, and timing are related to EGT?

I understand how AFR and timing can influence whether there is detonation (knock) or not.

However, my real question is:

Are knock and EGT independent of one another?


The reason I ask, is that when tuning a motor's fuel and timing, typically the tuner pays attention to AFR and knock, and adjusts fuel and timing appropriately.

However, high EGT can also cause damage.......but many times, a tuner doesn't monitor this....


Are high EGT directly related to knock? In other words, can you get unsafe high EGT without having lean AFRs or high knock counts? Or do high EGT come as a RESULT of lean AFRs and high knock counts?



Thanks

[johngriff]

This should probably Get stickied.

First, your direct focus on EGT itself is a little misguided.

EGT

EGT can be seen as the byproduct of cylinder temperature, though there are quite a few different exhaust devices, such as headers, turbochargers, downpipes that tend to maintain a higher temperature than an aluminum combustion chamber, because they are typically iron/steel.

Because of this, EGT is not ALWAYS the most reliable tuning tool, and in theory, the closer you can get an EGT sensor to the head, the more accurate a reading of cylinder temp you will have.

Temperature is EVERYTHING

This brings the tuning discussion to the principal tuning factor, Cylinder temp, and attempting to avoid excessive heat.

Traditionally, gasoline is looked at as the "combustible" and air as the facilitator in the internal combustion process. In effect though, gasoline should really be looked at as the accelerant in the process of combusting air.

Taking this approach, the mixture of gasoline to air determines rate at which the combustion takes place, and more importantly the temperature of the combustion.

Traditionally, in 4 stroke gasoline tuning, the lean spectrum of AF/R will produce hotter cylinder temps, and the Richer spectrum of the AF/R will produce cooler cylinder temps.

Octane and Cylinder Temps Relationship

Octane ratings of Gasoline are an indication of the heat/temperature point at which the Gasoline will ignitie. The lower the octane rating, the lower temperature at which the gasoline will Ignite. The higher the octane rating, the hotter temp it will take to ignite.

This is why we traditionally prefer higher octane rated fuels for performance applications, because the likelyhood of "pre-ignition" of the fuel is less likely the higher the octane rating.

Cylinder Temp and Knock

With the above taken into account, it is important to control the Grade of fuel (octane) and the mixture of fuel (AF/R) to maintain acceptable cylinder temperature per your application. If the octane rating of the fuel is too high, or there is too high a mixture of fuel to Air, the combustion process will burn slowly, so slow that it typically will not burn all fuel, with that unburnt fuel exiting the engine through the tailpipe (Black Smoke Rich Condition).

On the other side of the equation, lower grade fuel, and higher cylinder temps result in the pre-ignition of fuel. This is known as Knock. Knock is bad because it is defined as an ignition of fuel mixture BEFORE the desired rotational point in the 4 stroke process. So when knock occurs, the energy of the combustion is met with the opposing force of the mechanical rotating assembly. At this point, the weakest material surrounding the combustion, or any gaps available will absorb the combustion process. This is why you see knock producing aggressively worn / broken rings, sleeves, valve seats and head gaskets, even taking pieces of pistons apart.

Boost and Heat and Knock

Going back to the rule that Air is the principal to combustion, adding more of it makes more power. The increased pressure of forced induction increases the likely hood of gasoline igniting. Typically this is the reason you will see "on boost" fuel maps AF/R result richer than "off boost" maps.


Summarizing the Tuning Process

First the engine must be able to adequately cool itself through fans, radiators and oil coolers. Once these basic systems are functioning, the tuner first adjusts the AF/R to a desirable ratios. This assures cool cylinder temperature throughout the map. The tuner then tunes ignition timing against tractive effort of the dyno so that for each degree of timing adding, at leas 2.5ftlbs of tq are gained. When additional timing nets less than this tq, ignition timing is very close to its knock threshold, or igniting the fuel/air mixture at a mechanically inappropriate period.

Fin

In the End, EGT is not a primary concern, if the foundation of the tune is correct. EGT is also a very poor indication tool to tune ignition timing against, though it can work as a very good alarm mechanism if a mechanical failure occurs.

[jspaeth]

Thank you kindly John.

Your responses follows closely what Steve told me during a recent conversation, mainly, that if timing is set reasonably and AFRs are in check, that EGTs should also be safe as well.

[burnsauto]

God I hope this doesn't get lost in all the other crap. Very nice explanation john :)

Its times like this I miss rep.

[jspaeth]

The main reason I posted this, is because I know that high EGTs can kill the motor, but I don't have an EGT sensor.

I was wondering if monitoring AFR and knock is sufficient to INFER that the EGTs are in check.

[johngriff]

AF/R's and knowing your timing map is tuned off a tractive effort dyno is all you need. Don't even need / trust the oem knock sensor. (My Opinion)

[burnsauto]

yeah the oem knock sensor kinda gets thrown out the door..lol. Its about as useless as check engine light on a modified engine.

[let5l1de]

This Should Definitely Get Stickied.

Nice explanation! I myself, couldn't have explained this much better than John.

When everyone has thoroughly read this writeup and has had time to soak in the info, I may chime in with a part II "The Effects of Anti-detonant Injection & Methods Used to Cool Combustion Chambers".

yeah the oem knock sensor kinda gets thrown out the door..lol. Its about as useless as check engine light on a modified engine.

I'm sure (conservative) OEM knock sensors are used for more than just controlling spark. In theory, knock sensors also reduce the return rate of engines...

~m

[dongoesby]

to johngriff, i actually searched through zilvia and tried to find a proper egt reading for sr20s, and i couldn't find any. Even from your explanation, i still don't know an answer. For your expertise on tuning, what would you say a proper egt range for sr20?

I hope i'm not jacking the thread, but try to clear this up. I have a stock s14 sr20, and the egt is mounted on the downpipe (<1in next to turbo). I am getting 1200F 70mph driving, 1500F under boost. So what would you say if i'm in the safe zone of egt?

[dongoesby]

anyone interested answering this question?

[iwishiwas-all*]

Knock is dependent on spark advance, octane level and principally intake manifold pressure although comp ratio can effect it as well. Egt is dependent on afr and spark advance. Knock occurs when a portion of unburnt fuel reaches it's sit= self ignition temperature. This is why you run higher octane fuel and then can run a higher cr= higher er= higher efficiency. The more spark advance= you run , the higher peak press and temp u will reach in the ignition cycle. That is good because u reach a higher pressure (more total work possible) but have more time and heat losses ( less work) and like I said higher temps which means a possiblity of knock . Knock has to do with how the flame propigates. As gases burn, they reach higher p and t, and thus heat and pressurize the adjacent unburnt gases, which if t=sit causes knock.

Best efficiency spark advance is the spark advance , for a given afr , that makes u the most work and thus power . It behaves like a parabola y=x^2 with a low point just rich of stochimetric. So as u go leaner u run more sa to gain max eff. You do this because of lower flame speeds so u need more time to burn.

I got pleanty more but I hope this helps