knock

[brandons 95]

Ok I'm new to the turbo cars and i see in alot of forums about knock, can someone please explain to me what this is?

 

[DGajre777]

Knock is Detonation when the air/fuel mixture in the cylinder explodes, instead of burning rapidly. (To be really technical, detonation is when the flame front moving through the cylinder exceeds the speed of sound.)

This is bad for two main reasons:

1) The sudden shock load can break parts - the repeated shock loads of constant
detonation is like running a jackhammer in your engine.
2) The temperature inside a detonating cylinder is much hotter than normal, and can get hot enough to melt aluminum.

Symptoms of detonation:

1) A loud pinging or rattle/knocking noise. It sounds like a bunch of marbles inside a
tin can. (This is why detonation is commonly called "knock")
2) Higher than normal exhaust gas temperatures
3) Little "flyspecks" of aluminum on spark plug center electrodes and the center
insulator. This is the dead giveaway indicator for detonation. Examine your plugs with a
magnifying glass. Little balls of aluminum clinging to the plugs means that you are detonating.
(It's aluminum from the pistons being vaporized and condensing on the plugs)
4) Engine damage (holes in pistons, melted spark plug ground electrodes, etc.

Causes of detonation:

1) Low octane fuel
2) High static compression ratio
3) High boost levels
4) Excessive spark advance
5) Lean fuel mixture (may be insufficient fuel pump capacity/pressure, insufficient
injector flow, air flow sensor out of range)
6) High inlet air temperature / low air density

For a given fuel octane rating, an engine can only stand so much boost, spark advance,
compression ratio, and mixture leanness before it begins to detonate. Therefore, as increasing boost, spark advance, compression, and leaning the mixture tend to increase power, _the onset of detonation is normally the limiting factor on the amount of power a given engine configuration can produce_ Collorary: _to produce maximum power, an engine should be running right on the limit of detonation

In a non-computer-controlled car, this is a very difficult thing to do. (Do a pass, pull the
plugs, check for the flyspecks, change something, go faster, pull the plugs, check for the
flyspecks etc. ad nauseum) However, the DSM CPU has a knock sensor that "listens" to the engine to detect knock. If the sensor detects knock, the ECU first cuts back the ignition advance, and if it still knocks, cuts the boost levels. Result: a drastic loss of power, and a safe engine.

If you have somehow limited the ECU's ability to retard the spark (unlikely) or cut the boost (easy) then it is still possible to get into a destructive detonation scenario. It is still a
good idea to pull the plugs every couple of runs and inspect them for signs of detonation. Keep an eye on the boost and EGT gauges!

Fixing a detention problem

1) Use higher octane fuel
2) Reduce the level of whatever it is you have increased (boost, advance, etc.
3) Reduce inlet air temp with a bigger intercooler
4) Confirm proper ECU/knock sensor operation.

OK, if I can easily adjust boost, spark advance, and mixture ratios, (and to a certain extent inlet air temp) what should I have more of to make the best power?

Excellent question. I don't know. I suspect that boost is worth more than spark advance, with mixture ratio somewhere in the middle. If I had a PMS, a datalogger, and a dyno, I think I'd set spark advance to about 2/3 of the possible range the computer provides, set the mixture a little on the lean side, and then run the boost up to the detonation point, and back off a little. Then I'd play with the mixture to see if I can get more power, or get the motor to stand more boost. Once I've got a good WOT boost level and mixture ratio, then we see how much more advance it can take. Maybe.

Note that we haven't taken inlet air temp or density into account here, nor have we allowed for real-world factors like: how fast can the wastegate react to boost spikes? This is just an educated guess. Bottom line? Lots of testing. Lots. And you can't get the inlet air temp too cold.

Credits: Dennis Grant, DG, [email protected]

A FMIC reduces the chances of knock. A coolant additive like Water Wetter that reduces coolant temp by 30 degrees helps too.

 

[brandons 95]

Ok, thanks for the info.

 

[oldman]

http://www.streetrodstuff.com/Articles/Engine/Detonation/index.php

 

[x_wolf90GSX]

This sounds like soemthing the car I just bought does. Only at high RPM under boost, I hear a "rattle" noise. The article you quoted says it sounds jsut like that so im thinking thats what it is. Whats the best way of combatting something like that? All great info

thanks

 

[oldman]

This sounds like soemthing the car I just bought does. Only at high RPM under boost, I hear a "rattle" noise. The article you quoted says it sounds jsut like that so im thinking thats what it is. Whats the best way of combatting something like that? All great info

thanks

Turn your boost down.

 

[x_wolf90GSX]

doesn't have a boost controller on it. Why would something thats otherwise stock and runs fine at lower RPMs do that? Could a boost leak cause that? I'm researching this stuff as I type this too, so I may stumble upon it, but interesting to know none the less.

 

[oldman]

How much are you boosting? Just because you hear some noise at wot doen't mean you're knocking, only logging can tell you if you're knocking.

 

[x_wolf90GSX]

hmm maybe i'll have to get some logging software/cable then

I'll have to tell ya in a couple days what the (currently) stock boost guage shows. outta town at the moment but I'll definately look into it.

thx

 

[oldman]

stock boost guage shows.

Damn, you really are stock.

 

[x_wolf90GSX]

insanely stock I bought it for that reason. I want to do the work me self And learn all I can about it. I know a fair amount about the good ole chevy SBC 350 -400s but am DYING for knowledge on these cars. So forgive me if my questions seem stupid now, I am learning just like alot of others are here

Just wish I could tell what the rattle was. The guy that owned it before said that he had heard/seen the timing belt covers rattle, so maybe its soemthing as simple as that

Thanks

 

[droppinbottom]

Could be any number of things. We are talking about a 15+ year old vehicle. Have you tried it by slowly increasing the rpm's without building alot of boost.

 

[x_wolf90GSX]

Hey your from Vancouver! Nope not yet. I'll be gettin back to it on Monday and will run it through some tests thanks

cheers

 

[AVC-R]

Yes we have another domestic guy go dsm. Jk, I would say your knock sound is something simple, so just get out the trusty 10,12,or 14mm socket wrench and a flash light and look for loose items. I had a problem about like that where my intake manifold bracket was loose and i could only hear it at high RPMs. Not saying thats what yours is but just check around