Quote: Originally Posted by TJ1376
EGT's are NOT the culprit.. UNLESS the EGT's are causing the incoming air to be much hotter than normal, which may lead to more detonation.
This car has no way to measure EGT's.. The computer had no idea EGT's are hotter than they should be.
Just to make sure everyone has a clear understanding on how EGT's relate to knock, they have nothing to do with the compressor side and its charge temps. Basically none of the heat on the turbine side will directly transfer to the compressor side. The heat of the charge on the cold (compressor) side is developed from the act of compressing the air. The compressor wheel draws the air into the inlet and via centrifugal force throws it outwards (in a radial motion) into the diffuser. The diffuser is the inside machined area of the compressor cover housing. The air coming off the compressor wheel is expelled at a high rate of speed and the diffuser slows it down. When it does this you can kind of think of the air as stacking up against it which builds pressure. And heat. When you compressor air it's going to get hotter due to friction of the molecules that are being forced closer together and against the surface of the container (i.e. compressor cover/diffuser) combined with friction of the air moving across the surface of the diffuser and out into the compressor's outlet on the way to the engine. The efficiency of the compressor wheel is determined in part by how well it can draw the air in, accelerate it, and then disperse it evenly over the diffuser.
The high exhaust gas temperatures are influenced by a wide range of variables but it comes down to the fact you're burning fuel in the combustion chamber and
fire = heat. If the timing advance is low (such as when it gets retarded/pulled) the ignition is lighting off the burn later. It's safer because it lowers cylinder pressures but you also don't get a complete burn and some fuel will be left over and burn later when the exhaust valves opens. (On a side note that's similar to how an anti-lag "bang, bang" system used on rally cars works to keep boost up and the turbo spinning. When the driver lets off the gas the system dumps a ton of fuel, pulls a ton of timing, and slightly opens the throttle. This essentially causes fuel to go throw the combustion chamber and burn in the exhaust manifold. It can keep it spinning and boost up but is
extremely hard on exhaust components.)
The cylinder head cooling and spark plug heat range also affect EGT's. If they can't effectively scrub off heat it starts to build up in the chamber. That's one of the reasons there are piston oil squirters in the block. They spray oil on the bottom side of the piston to help draw off heat. The theory of applying a coating to the combustion chamber on the head to reflect heat downwards is also another example of trying to prevent this heat from building up (but comes with it's own issues such as how that heat will affect the charge pressure). Related to the previous timing example is the valvetrain and camshafts used. If they allow the exhaust valve to open early some of that fuel and heat from the burn will escape into the exhaust and heat up the manifold and head.
Then there's the stock manifold and turbocharger itself. It's small by design to minimize lag and works great on near stock cars. But when you turn the boost up you burn more air and that air has to get out. It also happens that the exhaust gasses from that burnt air/fuel mixture are pretty hot. The longer they stay in the exhaust system the more heat that transfers into the manifold and also back into the head via the exhaust ports. To compound it even worse, when there's so much backpressure it's like compressing already hot gasses, which in turn creates more heat. The turbine is always going to act as a restriction to exhaust flow but it's a compromise to be able to maximize the energy transfer back into turbine rotation without excess restriction to flow. A more efficient turbine/housing that flows better (i.e. less back-pressure) while still being able to spin the compressor in it's desired operating range is what you'd look for. If we could do this on a stock turbo it would help top end but hurt lower end response.
So those are some of the sources and things that affect the EGT's. So how does that relate to detonation and knock? The hotter a mixture of air gets the more it expands. This increases pressure. In the cylinder and combustion chamber you have a piston that's already compressing the intake charge on the compression stroke. Combine that with a turbocharger that's pressurizing the chamber before the piston can physically compress it even more and the cylinder pressures go up higher when the piston reaches the top of its stroke right before the ignition system lights the air/fuel mixture off for burn.
With hotter temps in the cylinder head and combustion chamber it's going to increase this already high pressure even more. That can have an effect of making it more likely to light off early and/or more violently. If you read the other linked thread on timing you'll see that timing advance before top dead center (which is what you're watching on the gauge/scan tool) means the ignition is starting the burn before the piston is all the way to the top of its stroke and on the way down again. So if the mixture lit off too violently and faster because of the higher pressures then the piston would be driven back down in the opposite direction it's trying to move (which tries to stop and reverse the rotating direction the crankshaft is spinning) and it has to fight that high pressure to make it to the top of the stroke before it can continue downwards. That causes the bearings to take a beating and is hard on the piston, rod, and crank assembly. The piston also starts to resonate in the bore during this, the computer hears that via the knock sensor (a small piezoelectric microphone tuned to that frequency), and it pulls/reduces timing advance. That essentially means the PCM and ignition system is firing the spark plug later and closer towards top dead center. The EGT's go up and you don't get as efficient transfer of power from the burned mixture expanding to push the piston down.
Quote: Originally Posted by Lagzilla
...with the turbo spinning that fast im sure EGTs cause the intake temps to sky rocket. the amount of heat generated on that turbo is much higher than the amount of heat generated by my GT40 so my intake temps arent going to be the same as say billys 19 psi running TD04, watch the boost billy.
Like just mentioned the heat from EGT's work a little differently. For the compressor on a stock turbo car we don't have the exact compressor map that shows how efficient the reverse-rotation "16GK" wheel is. But we do know it's similar to a "small 16g" compressor and Mitsubishi Heavy Industries has published that. If you look at the map you'll see the compressor side isn't as bad as you'd think. Sure the charge temps coming off it will be high (which a good intercooler can effectively overcome), but the main culprit and problem is the much smaller 6cm² TD04 turbine on the hot side. It spins the compressor up fast for better boost response but puts that big restriction on the exhaust trying to get out when the volume at higher boost and rpm's increase.
The stock/Stage computers try to combat this by injecting a lot of fuel and running rich under boost. Gasoline in and of itself isn't that great at absorbing heat which is why water injection can be so effective without having to inject as much fluid. But the fuel will work to an extent and it also slows down that combustion process. When it slows down it's less likely to ignite prematurely and create detonation. The high timing advance that those computers run can then be used and that will decrease the EGT's. But it's a balancing act between getting too aggressive on the advance for the air/fuel mixture. Sure it helps lower EGT's and creates good power, but if you lean it out or increase the cylinder pressure (
like what happens when you put a bigger turbo on a car with the stock/Stage computer) it can and will detonate creating knock.
By the way I did a quick search and here's some physics explanations to go along with this:
http://hyperphysics.phy-astr.gsu.edu...ic/idegas.html
http://jersey.uoregon.edu/vlab/Piston/index.html
http://wine1.sb.fsu.edu/chm1045/note...as/Gases04.htm
While air is not an 'ideal gas' it is close enough to work with the gas laws. And who thought they'd never find a real-world, practical application for that physics stuff?
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