^^^^werd. Knock kills the motor. At least for drag racing and shorter pulls, EGT's are more a timing gauge than anything. I seem to know of a few turbo cars that run in the neighborhood of 2000* and have perfect looking valves. I might even take some pix of them some day and show off what they look like. I would definitely like to see Danl's though because I can compare them to the umpteen DSM heads that are around the shop.

 

On a side note i have seen engines run with EGT's around 1600-1650 for short periods and looks like it just re-entered the atmosphere...i too have pictures.


But good point on to control boost at around peak torque. I need to show you how my Tables look. Because im thinking a EBC would be more suited then my MBC. But yah if your ever planning a vegas vacation Bryan.

 

Aluminum starts to soften and melt when exposed to temps around 1650*F too so thats a big reason to limit prolonged exposure to those temps.

 

I don't know the exact alloy that our cylinder heads are made of, but pure Al actually melts at 1200F. But, AL has such a high heat capacity and high thermal conductivity that a block of Al can withstand temperatures well above it's melting point without being affected. It depends on the mass of Al, the temperature, the time, and our cooling system.

 

Most Al alloys melt at around 1100F.

 

Boundary layer stops it melting, unless you have detonation.

 

What specific alloys? Usually Al alloys have a higher melting point that pure Al, not lower. So, I would expect most alloys to have a melting range above 1200F.

Actually, all alloys technically have a melting range, not one specific temperaure, like a pure element, depending on the percentages of the alloying elements. It's dictated by the phase diagrams.

Whatever the range, though, the cylinder head is protected for the reasons mentioned above. It's the exhaust valve that must be made of a higher temperature alloy because when open, it's small mass will have a low heat capacity and when open it will not be in contact with the cylinder head for thermal conductivity.


Eric

 

6061, 5052, and 2024 all melt at around 1100F. Al alloys generally have relatively low minor element content, so the effect on melting point is not large. Adding minor elements can actually reduce the melting temperature, similar to how salt lowers the melting temperature of ice.

Key item to remember when talking about these high temperatures is that the strength of metals begins to go down well before reaching the melting point. As you said, the ability of the piston to conduct away heat is the key to its survival.

Valves are likely a stainless steel, maybe sodium filled. SS has a much higher melting point but is a lousy heat conductor. Sodium, esp liquid sodium, is a very good heat conductor.

 

and Inconel valves too

 

I hit 1650*F EGTs. I think i have peaked around 1680 car seems fine to me. From what i am told advancing the timing should actually cool the EGTs since there is more time for the cylinder walls to absorb the heat

 

i was just gonna ask if you were reading this thread.

 

Absolutely....it just depends on the composition of the alloy/alloying elements and the resulting phase diagram(s). Usually, though, phase diagrams are only presented for binary or tertiary systems, such as Fe-C for carbon steels, since it becomes extremely complex to try to show multiple alloying elements. You can look at the binary systems for all elements that Al is alloyed with to get an idea of what will happen to the melting range, such as the Al-Fe, or Al-Si, etc. If the diagrams are eutectic systems, then the melting range actually does drop, as you mentioned, to the composition of the eutectic, at which the melting range then increases again.

It's been such as long time that I have looked at the properties of Al alloys, that I forgot what kind of melting ranges and systems they were. I used to work in a failure analsys lab as well as in an investment casting foundry, but I never had the chance to work with Al alloys. I worked mostly with iron alloys, carbon steels, stainless steels, etc, and some copper.

Since I have been removed from my materials engineering background for so long (working for IBM and the IT field for the last 8 years), I tend to forget some things, but metallurgy and materials engineering is still a big interest for me.

Yes, the piston can conduct heat away as well. For the valves, though, it is the seat with the cylinder head that I would imagine provides the most protection. Obviously, when they are open, this protection goes away, which is why exhaust valves are usually made of more exotic alloys, like Inconel. They have to be able to withstand the heat of the exhaust flames directly passing over them, especially during high EGT situtations.

The cylinder head itself, even though it melts at temperatures well below the temps in the chamber, is well protected by the alloy's properties for heat capacity and conductivity, not to mention our cooling system as well.


I think the whole point of what we are saying is that higher EGTs should be no problem, up to a point. It's knock/detonation that is more important to consider for cause of damage to the engine. When EGTs do get too high or for too long, it's the exhaust valve that will take most of that load.

 

I concur, and man... I have to say that I can really appreciate the sound engineering found exclusively on this forum. At any rate, I don't have an EGT probe, but this weekend at CMP I melted my turbo gasket running 25psi, 11ish AFR, and peak torque timing of 1-0 degrees on an 88 degree day. Needless to say, low timing is definately causing high EGTs. I'm convinced that the only reason I had gasket failure is because my stupid Alcahol pump must have an internal thermal overload switch which means halfway through my hottest session my water/alcahol injection went away. Knock started to climb, but I think the bigger problem was that my EGTs must have rocketed.

I was fortunate enough to be able to limp the car back 6hrs home... but I've learned a valuable lesson.

 

And what was your peak timing up top? I would imagine peak torque timing has little to do with EGT's as it's there for the shortest amount of time compared.

From some testing I have done with an EGT gauge, is that as long as you have a good advancement spread from peak torque to the rev limiter than EGT's will stay in check better. Meaning, 8*-10* advancement over the powerband, 2* at peak torque to 12* at 7600, or 1* at peak trq to 11* at 7600, etc etc

 

So a good piston also conducts heat to the cylinder walls well. Thats actually a major job. Also similiar to you jumping over a campfire, the piston isn't exposed to heat constantly. The boundary layer is also important, and when the detonation pulsewave breaks it up heat is more easilly conducted to the piston.

The valves rely heavily on the cylinder head and more specifically the seats to get rid of heat. If you look in your FSM you'll see that if you cut the valves you must retain a certain amount that touches the heat to help wick heat away from the valve.

Trust me guys I pull the same stunts on my baby the evo IX as I do my DSM. I wouldn't be putting this out there if I didn't thoroughly test it on both my cars.

Detonation will **** your **** up bad. EGT's, don't even like the gauges unless to measure cylinder to cylinder equalization, and even then I think I'd like individual widebands not EGT gauges.