pyrometers..

[jauguston]

Hypermax says you can run a 6.9 - 7.3 IDI "All day long" at 1250 EGT. There is a lot made out of the fact that supposedly aluminum melts at 1250 degrees. That may or may not be true for the alloy used in pistons but one BIG thing is always missed in the discussion. The gauge we are talking about is a Exhaust Gas Temperature gauge, it is not a piston top temperature gauge. The pistons are cooled by the cylinder walls being cool and by oil streams that are sprayed up under the piston crown.

Jim

 

[BigRigTech]

The big "C" , usually stands for big CASH....

 

[Cheaper Jeeper]

Hypermax says you can run a 6.9 - 7.3 IDI "All day long" at 1250 EGT. There is a lot made out of the fact that supposedly aluminum melts at 1250 degrees. That may or may not be true for the alloy used in pistons but one BIG thing is always missed in the discussion. The gauge we are talking about is a Exhaust Gas Temperature gauge, it is not a piston top temperature gauge. The pistons are cooled by the cylinder walls being cool and by oil streams that are sprayed up under the piston crown.

Jim

Good points Jim. On the other hand, although it isn't actually measuring the surface temp of the pistons, it is measuring the temp of the gasses that have been in direct contact with them a millisecond or two earlier. Given the fact that gasses cool so quickly (even quicker than aluminum) it is at least possible that the temp of some spots on the top surface of the pistons could be slightly higher than the escaping gasses.

Although the cylinder walls are cooler, and the pistons loose some heat to them through contact (conduction) that is only happening around the outside edge of the piston top. The farther from the cylinder wall you get, the less conductive heat transfer is going to happen. The real concern then is for the very center of the piston - the hottest spot and (suprise) exactly where they end up with a hole melted in them when you slag one.

Then there is always the margin for error on the guage to consider. I'd be willing to bet that any of the affordable EGT guages are going to have around a +/- 5% accuracy limit. 5% of 1250 is almost 63 degrees, so if your guage reads 5% low then you'd be at 1313 degrees when it says 1250. Probably wouldn't want to sustain that high of an EGT for any extended periods.

I think that the 1250 "limit" is a pretty good number to use, and yes, you should be OK at 1250 - as long as your guage is spot on. But I certainly wouldn't want to exceed 1250 for long, and would rather err on the side of caution and keep it to no more than 1200 for sustained periods - just in case the guage is reading a little low.

The instructions that came with my Banks kit say not to exceed 1150 degrees (which I take to mean "for extended periods"). They are no doubt attempting to err on the side of caution as well - that and compensate for the cooling of the exhaust gasses as they travel the 5 feet or so down through the Y-pipe and back up to the input of the exhaust snail. Seems reasonable to me....

 

[G. Mann]

Something that's not being mentioned here is pyrometer hysterysis, that is the time it takes for the pyrometer probe to heat up and send the signal to the gage. During that lag the temp can rise enough to slag a piston if you are near the limit anyway and hammering up a hill with a load.

This is one reason that Banks set their kit limit to 1150 degrees. The actual temp spike at the piston top can be higher since the pyrometer sets down stream "Post" turbo. The turbo takes heat energy and converts it to work energy so "post" temps are always lower than combustion chamber temps with a built in lag in getting the reading. A little margin means a little more life in real terms pretty quickly.

 

[icanfixall]

Now thats a fine looking inline 6. Gees... A seperate valve cover for each cylinder too. Someday I'll be upgrading my truck to one of those when they are better designed. I'm not starting a war here, I just usually wait till something is built the way it should be. Or I do it for them. I'm not out there in mega hp area. Just like a more efficent fuel mizer type motor that has enough hp and torque to get the everyday job done. Just my thoughts... Not everyones either.....

 

[Cheaper Jeeper]

Something that's not being mentioned here is pyrometer hysterysis, that is the time it takes for the pyrometer probe to heat up and send the signal to the gage. During that lag the temp can rise enough to slag a piston if you are near the limit anyway and hammering up a hill with a load.

This is one reason that Banks set their kit limit to 1150 degrees. The actual temp spike at the piston top can be higher since the pyrometer sets down stream "Post" turbo. The turbo takes heat energy and converts it to work energy so "post" temps are always lower than combustion chamber temps with a built in lag in getting the reading. A little margin means a little more life in real terms pretty quickly.

Actually the old Banks I have doesn't have the probe "post-turbo" - it is on the exhaust INLET side of the turbo snail - just barely "pre-turbo", but still pre-turbo none the less. That is why their recommended temp compensation factor is only 100 degrees. For most true "post-turbo" installations the temp compensation figure I've seen most often recommended is 300 degrees - precisely for the reason you stated. The turbo is converting (and/or absorbing) roughly 25% of the exhaust heat energy into work energy to run the turbo compressor.

That's also why insulating the exhaust pipes all the y up to the turbo will increase boost and decrease spool time. You are conserving more heat energy and delivering it to the turbo - which still converts roughy 25% of it to work.

More goes-in-ta' = more goes-out-ta'....

 

[BigRigTech]

6 Valve covers are a pain in my world....Take one big one off and get it out of my way. Mercedes has 6 valve covers on the 4000 truck engines....6 individual heads too with 4 headbolts per head....Make a good paper weight they do.

 

[jauguston]

Cheeper Jeeper,

Did you ignore the oil cooling for the pistons for some reason?

Jim

 

[Cheaper Jeeper]

Cheeper Jeeper,

Did you ignore the oil cooling for the pistons for some reason?

Jim

Nope, just forgot to mention it.

The oil cooling no doubt helps, but much like the heat conduction to the cylinder walls, it isn't as effective in the center of the pistons where the rod & wrist pin block the bottom of the piston from the direct oil spray. Also while it helps dissapate heat from the piston, the cooling effect is on the bottom side - but he concentration of heat from exhaust gases is on the top. So the top surface can actually begin to melt while the metal just under the surface remains unmelted - but only up to a point.

If you've ever soldered you know that once the surface gets hot enough that it starts to melt the heat transfers through the metal and the melting action spreads through the entire metal mass pretty quickly. The oil spray on the bottom side of the piston will slow that process to an extent, but I sure wouldn't count on it delaying the melt-through very long if the temp on the hot side doesn't start dropping pretty darned fast. Remember, there is also the extreme high compression inside the cylinder pushing against the over-heated metal, which becomes softer the hotter it gets.

The combination can blow a hole through the top of a piston pretty fast if the temp threshold is exceeded by too much or for too long.

How's that?

 

[sassyrel]

and to add on gauges--darrin seen mine on the post--at the one gtg we went to in his area--mine are autometer---he said they looked nice--got two of the three on ebay way reasonable---