Optimum clyinder temp

[g zero]

Lycoming gives you the high and low temp range for cylinders , what would be the target temp for cruise (100 LL) ?

[BillL]

Good question. I will get the snowball rolling. Cool enough to have long fatigue life, and hot enough to still operate combustion efficiently. Higher surface temps in the cylinder will lower heat rejection and allow better hot gas expansion and fuel efficiency. That is probably not as big a factor, or "knob" as leaning and A/F.

So, since the tensile strength (drop) gets steeper around 300F and steeper yet at 400F. Somewhere in the 300-400F is good, but closer to 300F is probably better. This theory is just based on head material strength, not "MPG" or valve guide cooling and lubrication, or the like. That, in reality, could easily change the "best" range.

If the temps could be kept in the 250F range, all the time, then compression ratio, timing etc could be reoptimized to take advantage of the extended detonation range, and material strengths. Since we still have to operate hot in some weather, those options are not viable.

[jthocker]

Originally Posted by g zero

Lycoming gives you the high and low temp range for cylinders , what would be the target temp for cruise (100 LL) ?

Why not just go with the manufacturers recommendation!

Max service life at 400 degrees or less for max continuous Ops!

That would mean climb at Redline temp or less and cruise at 400 or less.

Telling new builders to shoot for 300 degrees or 250 causes them to land prematurely to trouble shoot constantly, with the unintended consequence of improper cylinder break in and cylinder glazing.

[g zero]

Originally Posted by jthocker

Why not just go with the manufacturers recommendation!

Max service life at 400 degrees or less for max continuous Ops!

That would mean climb at Redline temp or less and cruise at 400 or less.

Telling new builders to shoot for 300 degrees or 250 causes them to land prematurely to trouble shoot constantly, with the unintended consequence of improper cylinder break in and cylinder glazing.

Why ? I can control my CHT's by a variable exit and looking for info on optimum cylinder head temp . I can can cruise in 75 degree (F) with the hottest CHT- 300* , if it winter ever comes to Fl I'm not sure what I'll see .
There has too be an optimum temp to shoot for not a 200* range .

[BillL]

Originally Posted by jthocker

Why not just go with the manufacturers recommendation!

Max service life at 400 degrees or less for max continuous Ops!

That would mean climb at Redline temp or less and cruise at 400 or less.

Telling new builders to shoot for 300 degrees or 250 causes them to land prematurely to trouble shoot constantly, with the unintended consequence of improper cylinder break in and cylinder glazing.

I thought we were talking about how cool they could run and not cause trouble on the low side. There are a wide range of conditions from season to season, latitude and altitude and the factory range provides the definitive numbers. I would be an extreme tradeoff (and likely impossible) to try and get such low temps on a climb on a hot day. Cooling is always a tradeoff with other design/operating parameters, cooling drag being the one that comes to mind. This is in no way a recommendation for a new builder to attempt to set anything but the factory numbers as limits.

Any talk about 250F is in the context of a design parameter for the engine designers, not for a user selected limit.