One thing that really kills me, is that nobody really seems to know the difference between detonation and preignition. Even so called performance shops commonly reference them as the same thing. This should clear things up a bit.

First of all both are equally harmful, and will detroy your motor with a quickness.
How they do that, is slightly different. Basically everyone with basic engine knowledge should be familiar with the fact that the air/fuel charge does not explode upon ignition, but burns in a slow (relatively speaking) controlled manor. with that said:

Preignition, in the simplest form, is when your air/fuel charge is ignited prematurely. Commonly caused by over advanced ignition timing, or sharp edges on pistons which create hotspots causing premature ignition. What happens here is that the burning fuel mixture starts expanding before the piston reaches the top of the stroke. In other words, the combustion gasses are pushing down while the piston is pushing up. This commonly leads to bent rods and cracked ringlands. Preignition is not as detromental as detonation, but it will eventually weaken the rings and pistons to the point of failure.

Detonation, put simply, is uncontrolled combustion. Instead of the nice steady burn we want, the air/fuel mixture ignites violently causing massive damage. Detonation results most commonly from overly lean conditions. The combustion process happens so fast, the resulting pressure has no where to go. You can expect the resulting shockwaves to crack your ringlands, bend/break your rods, damage your valves, etc.

For those people who listen to their motors:
>Preignition makes more of a wump-wump-wump-wump sound, and can sometimes be felt through the floorboard if you have a mount kit. It almost feels like an outof round tire, except more subtle.
>Detonation sounds like "BBs being shaken in a coffee can" as somebody posted. Also can sound like a very fast ticking sound. Of course every motor is different, so it all depends.


I've missed quite a few key points, but this should clear things up for the newbies

 

 

Nice i did not know the difference

 

Pretty cool but I think you have the sounds backwards. The preignition I have always experienced has the pinging/ bbs in a tin can sound. My car will do it under accelaration if I run 87 octane or if I advance the timing up to high.

 

I learn something new everyday..good info

 

Some rely on their hearing to sense detonation, and adjust their ignition accordingly. Valvetrain noise is loud enough to hide it.


Just wanna add my .02..

In neutral, when you perform a leakdown test, the piston must be at true TDC as the compressed air is put into the chamber. If the piston is not at TDC ,then the motor will crank over.

I have found that on low r/s ratio (longer stroke than rod) it is much harder to get it to true TDC (its a b!tch. Ive had more luck with high r/s ratio motors (B16's). So, I still have this theory, but dont take it as a fact:

Stroked motors are edgy when it comes to tuning them.
They are more likely to suffer from detonation/pre-ignition at a larger scale compared to high r/s motors.


[Modified by Quick 200k Mile Motor, 11:47 PM 11/8/2002]

 

Cool post!

and after reading this I realize I must have blown my moter by way of detonation instead of preignition.

WHY?

cuz it went

KABOOM!!!

and not just smoke smoke cough cough

Time to get those 450's cleaned


[Modified by T-sohctec, 6:44 AM 11/9/2002]

 

Good info.

 

Good info, and I listen to my engine ALL the time when driving.... radio? whats that? oh you mean my cd-tray-holder thingy....

I hear all kinds of noises but nothing along those lines... having a 3" exhaust makes things harder to hear.... and when accelerating its almost IMPOSSIBLE to pick out any sound cept the "air rushing in" sound, and likewise the super loud 3 "exhaust sound.....

 

Article written by a GM engineer:

http://www.streetrodstuff.com/Articl...e_Basics_I.php

 

Good post RioNinja and good article John. This is why I love H-T.

 

There can be some confusion in the statement: Location of Peak Pressure (LPP) is always 14 ATDC for any engine. It is not! Take a look at this graph...

Taken from a Mercedes. Same motor; 5 different combustion events. Notice that LPP can be well past 20 ATDC ..and if its past 14 ATDC, there is less Peak Pressure.

The author in that streetrodstuff article needs to revise. LPP @ 14 ATDC is the target (our goal), and it does not occur @ 14 ATDC for every motor.

 

 

this great thanks guys

 

unfortunatly for me. I have to mix 100 octane with the shitty 91 here in california. If i dont, i start hearing random "ticking" at high rpms. The ticking isnt violently fast. its spaced apart pretty well, but it scares the **** out of me when I hear it.

 

Ok, so who is right? Unless I completely misread the article or the post, they are saying opposites. ???

 

http://www.jandssafeguard.com/tech.html
http://www.avweb.com/articles/pelperch/pelp0043.html


[Modified by iloveracing, 11:25 PM 11/9/2002]

 

people like you that take the time , are much appreciated

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by rioninja &raquo;</TD></TR><TR><TD CLASS="quote"> Preignition is not as detromental as detonation, but it will eventually weaken the rings and pistons to the point of failure.
</TD></TR></TABLE>

I believe you had it the other way around. Preignition is the dealiest among all especially when the forces of the combustion is going against the piston's travel direction. Preignition that occurs early enough at the compression stroke is what you commonly see rods bending and **** basically exploding in pieces

It's great that you took the time to bring this subject up I am also very annoyed when guys throw these terms without knowing exactly what they mean.



Modified by Tony the Tiger at 9:22 AM 11/21/2004

 

I disagree on the preignition vs. detonation.

Preignition IS more harmful. Not by a little, but by a LOT. Preignition is actually the uncontrolled and rapid burn of the mixture while the piston is still traveling up. This can be looked at as you running towards someone who is throwing a baseball at your head.

Detonation in the uncontrolled and rapid burn of the mixture after the piston has passed TDC. The piston is moving down with the force of the uncontrolled burn. This can be viewed as someone throwing a baseball at your head as your running away from them.

 

Nice post. To add some failures due to pre ignition and detination are streatched headbolts cuasing blown headgaskets, and smashed/overworn bearings.

It is important to know the failures of pre ignition and detination but knowing the difference between the two can better help avoid having them.

 

Someone described detonation to me as "pins dropping on glass", I think its quite accurate.

 

thanks for the info i think it happens when i get to i high rpm

 

Two means to the same end, uncontrolled combustion event.

Funny how so many 'reputable' people will argue over which one to label with what name lol

 

what do you listen for for preignition?