Detonation vs. Pre-Ignition

[Quijinn]

Ok you all are both right to a degree. A spark plug heat range is simply this, how fast can it move the absorbed heat into the head. It absorbs heat because it protrudes into a 1500 degree oven so given enough time the plug will rise to 1500 degrees. By haveing more internal contact area, the plug can pull the heat it absorbs and displace it into the head. To get hot it has to absorb to cool it has to displace. The colder the plug, the more heat it can displace before it starts to glow, thereby pulling heat from the combustion chamber.

Ok now remeber the 1500 degree number being toss around is only at and around peak torq rpm and only during the 0-15* ATDC in normal burn so the rest of the time the rings, combustion chamber, and cylender walls are releasing heat into the coolant, also so is the plugs. The plugs are now trying to cool back down due to the contact area of aluminum so as the coolant pulls the heat away The plug is still in the chamber absorbing some heat. given not as much as the walls but more then the piston rings, more contact area with the head.

During the duration of the flame font the plug is heating up, just like everything else, it can only do this if it is in the heat area. Plus you should look at this, the center of the combustion chamber is the hottest part, the outter rim is cooler because of the coolant to help control detonation. A hot plug will glow because it cant disipate heat fast enough and preignite, it can also cause detonation......

Anything that can start a 2nd font will cause detention, gas's are not sitting there wating there moveing, so as gas moves over a hot plug it can cause a 2nd flame font. Cooler plugs can help with detination, but it can help more with preignition and preignition caused by detination. Detination heats up the plug, the plug glows and preignites the fuel. Colder plug will resist this much more, by releaseing the heat into the head.


My $.02

[Drweldin]

So how important is it then to run colder (than stock) plugs on just a street car? For example, a 13G car running 17 psi? I've always wondered if it made enough of a difference, Vs. the fouling potential....?
Edit: I'm referring to controlling pre-ignition by the plug as a heat source...not knock or detonation....

[mjannusch]

One heat range cooler doesn't seem to increase the risk of fouling at all. For any car that's BPU I'd go cooler just for the safety factor if for no other reason.

[Drweldin]

OK - stupid question: If Detonation is cause by risidual mixture in the chamber, why do people automatically assume they have to richen the mixture to cure it?? It would seem the opposite wouild be true - right?

[bluemax_1]

Originally Posted by Drweldin

OK - stupid question: If Detonation is cause by risidual mixture in the chamber, why do people automatically assume they have to richen the mixture to cure it?? It would seem the opposite wouild be true - right?

It's not quite that simple. To understand it, you need to understand the actual mechansim of detonation. A fuel-air mixture has tendency to explode under pressure and heat. How resistant a particluar fuel is to exploding under pressure and heat determines its octane number.

In the combustion event in an ICE, the fuel-air mixture is ignited by the spark from the sparkplug and should follow the path of a controlled burn that expands outwards igniting the fuel in a widening sphere from the kernel at the sparkplug.
this flamefront of igniting air-fuel though tends to pressurise the unignited or yet to be ignited air-fuel mixture as the combustion gases expand.

Detonation/knock arises when the pressurisation of the as yet unignited air-fuel mixture detonates and explodes of its own volition before being ignited as part of the controlled burn. The force generated by the detonation of the fuel-air under severe pressure from the expanding combustion gases is much greater than the force generated from the normal controlled burn of a regular combustion event. That's why knock/detonation breaks engine parts. I don't think they've yet managed to find a material that can withstand the extreme violence of detonation, otherwise, they'ld simply build an engine to run on detonation since the energy/force generated per unit fuel is so much greater.

Detonation can be caused by a number of things, but they all boil down to pressure and heat. If your mixture is too lean, it reduces its ability to withstand pressure and heat, so richening up the mixture serves to avoid detonation in 2 ways. The higher fuel percentage reduces the inclination to knock and the added fuel also reduces the heat in the cylinders.

Using colder plugs as explained in the previous posts reduces the heat factor. Boost pressure is limited on a specific octane gasoline because of the threshold pressure (at a certain heat) afterwhich knock is encountered. A better cooling system (more efficient turbos + more efficient intercooler) reduces the heat of the intake charge, allowing more boost pressure before knock is encountered. Water and alcohol injection also reduce heat for more knock prevention.

Retarding timing advance reduce the pressure factor, as does running lower compression pistons.


Max

[Drweldin]

Originally Posted by bluemax_1

It's not quite that simple. To understand it, you need to understand the actual mechansim of detonation. A fuel-air mixture has tendency to explode under pressure and heat. How resistant a particluar fuel is to exploding under pressure and heat determines its octane number.

In the combustion event in an ICE, the fuel-air mixture is ignited by the spark from the sparkplug and should follow the path of a controlled burn that expands outwards igniting the fuel in a widening sphere from the kernel at the sparkplug.
this flamefront of igniting air-fuel though tends to pressurise the unignited or yet to be ignited air-fuel mixture as the combustion gases expand.

Detonation/knock arises when the pressurisation of the as yet unignited air-fuel mixture detonates and explodes of its own volition before being ignited as part of the controlled burn. The force generated by the detonation of the fuel-air under severe pressure from the expanding combustion gases is much greater than the force generated from the normal controlled burn of a regular combustion event. That's why knock/detonation breaks engine parts. I don't think they've yet managed to find a material that can withstand the extreme violence of detonation, otherwise, they'ld simply build an engine to run on detonation since the energy/force generated per unit fuel is so much greater.

Detonation can be caused by a number of things, but they all boil down to pressure and heat. If your mixture is too lean, it reduces its ability to withstand pressure and heat, so richening up the mixture serves to avoid detonation in 2 ways. The higher fuel percentage reduces the inclination to knock and the added fuel also reduces the heat in the cylinders.

Using colder plugs as explained in the previous posts reduces the heat factor. Boost pressure is limited on a specific octane gasoline because of the threshold pressure (at a certain heat) afterwhich knock is encountered. A better cooling system (more efficient turbos + more efficient intercooler) reduces the heat of the intake charge, allowing more boost pressure before knock is encountered. Water and alcohol injection also reduce heat for more knock prevention.

Retarding timing advance reduce the pressure factor, as does running lower compression pistons.


Max

Thanks...everything you said makes sense, but I'm a little confused as to how you can have 10.xx AFR's, and still have knock...is it simply too much advance / boost....? Assuming you have decent gas (92 or higher)...or will colder plugs help? Background: I have 13G's, 450, DSM smic, good tune, etc, 17 psi....car runs good (stock platinum plugs), but I always have knock here and there (1-8 counts)...Ive already retarded base timing to 4 degrees, and that was am improvement...in my scenerio, can I ever cure it? Do I need to cure it? Should I retard back to 3 degrees?
Also, what's your guys take on the NGK Iridiums? I may pickup the BKR7EIX-11's (one range colder)....sorry, I didn't intend to make this a "how to tune my car" thread, but hopefully it'll help someone else too....

[bluemax_1]

Originally Posted by Drweldin

Thanks...everything you said makes sense, but I'm a little confused as to how you can have 10.xx AFR's, and still have knock...is it simply too much advance / boost....? Assuming you have decent gas (92 or higher)...or will colder plugs help? Background: I have 13G's, 450, DSM smic, good tune, etc, 17 psi....car runs good (stock platinum plugs), but I always have knock here and there (1-8 counts)...Ive already retarded base timing to 4 degrees, and that was am improvement...in my scenerio, can I ever cure it? Do I need to cure it? Should I retard back to 3 degrees?
Also, what's your guys take on the NGK Iridiums? I may pickup the BKR7EIX-11's (one range colder)....sorry, I didn't intend to make this a "how to tune my car" thread, but hopefully it'll help someone else too....

I'm using the Denso iridium plugs and like them. IMHO, iridiums are definitely better than Platinum plugs.

As far as getting knock, do you have a thread somewhere that details what your problems are eg. datalogs that show where the knock occurs while logging rpms, O2, knock, TPS etc.? Knock could be due to lots of things but one of the first things to try to rule out is phantom knock caused by things like worn motor mounts or a nut/bolt that was accidentally dropped in the vicinity of the knock sensor/lower intake plenum.

Can 13g's hold 17psi to redline? I didn't think they did well at higher than 15psi in the higher rpms.


Max

[Drweldin]

Originally Posted by bluemax_1

I'm using the Denso iridium plugs and like them. IMHO, iridiums are definitely better than Platinum plugs.

As far as getting knock, do you have a thread somewhere that details what your problems are eg. datalogs that show where the knock occurs while logging rpms, O2, knock, TPS etc.? Knock could be due to lots of things but one of the first things to try to rule out is phantom knock caused by things like worn motor mounts or a nut/bolt that was accidentally dropped in the vicinity of the knock sensor/lower intake plenum.

Can 13g's hold 17psi to redline? I didn't think they did well at higher than 15psi in the higher rpms.


Max

No, I dont have any logs posted....Ive never learned how to transfer to excel or anything....all I can say is everytime I get it the WB is reading rich if anything...the 13Gs should hold that to redline....close if not....its been a while since Ive tried to be honest, but if everythings buttoned up tight I think they will...
I know going from 7 degrees to 4 degrees base timing helped alot, but its still there....probably more of a fuel issue than anything...even though its supposed to be 92, who really knows..I think my next step is to change plugs...if that doesnt do it, lower timing to 3 degrees ??

[mjannusch]

You've probably got the car tuned too rich. 10.xx A/F ratios are much richer than where you need to be for a safety factor. Overly rich mixtures have a higher percentage of unburned fuel, and on cars like ours with lots of exhaust backpressure there is some exhaust gas reversion back into the cylinder at high loads and rpm. When that happens, you get a puff of hot exhaust gas into the intake charge before the exhaust valves close at the beginning of the intake stroke.

So you get fresh intake charge, with fresh fuel as the primary charge in the cylinder... But you've also got some hot exhaust gas that contains hot vaporized fuel that didn't burn in the last cycle because there wasn't much oxygen left in the mixture. When that goes through the compression stroke there is a higher probability of detonation. Sometimes that's what we'll see as knock even though the mixture is very rich.

I would shoot for wideband O2 sensor readings more towards the 11.2-11.7 range on pump gas. 11.2 to be extra safe, 11.7 to get a little more power.

[Dakken]

I found two nice things on spark plug heat dissipation:


1st article:
http://www.iridiumpower.com/heatranges.htm

Heat Dissipation in a spark plug. Of the 100% heat generated from the combustion, 20% is absorbed by fresh air from the intake of the following stroke. 58% of the heat is absorbed by the walls of the cylinder head which hold the spark plug in place. Twenty percent is absorbed by the insulator and side walls of the plug, the remaining 2% being absorbed by the spark plug wires.

2nd article:
http://www.centuryperformance.com/spark.asp


I'm sure there are more on the web but those two were at the top of my search.