Thermal Coating Piston Crown

[lindy 911]

I was talking recently to a respected engine builder who said that thermal coating the piston crown on a 911 was a bad idea. His logic was that the piston needs the heat to grow large enough to fit the cylinder properly. With thermal coating, too much heat is rejected and the piston won't expand enough.

I had my pistons coated during rebuild with the idea of reducing engine temps. I thought that if you reduce piston heat soak the oil would see a reduction in temperatures. I think it works but now wondering about the piston fit.

What do the experts think?

[Flieger]

I think they should still get plenty hot. I think the benefits for thermal efficiency will outweigh any small increase in piston slap.

[BoxxerSix]

He's correct with that thinking but the benefits outweigh that though by far. Some will say you can run a couple tens closer on the PTWC due to the reduce thermal expansion of the slug, but I wouldn't worry about it one bit

Almost every motor I build I have the piston thermal barrier coating applied, as well as have the first ringlands hard anodized. Charles @ LNE now offers all these optional coatings to every piston he sells with his nickies

Pictures of the custom JE's I had made for a 996 Turbo build by charles @ LNE. Moly skirt(break in coating), thermal barrier, hard anodized first ringlands.

[lindy 911]

My thoughts were that if the thermal coating would keep more heat in the combustion chamber it would yield more efficiency. The thermal coatings rejection of heat soak through the piston crown would force the heat to exit the engine through the exhaust port. This seems to be a win-win situation to me as long as the piston expands to close to where it should be in overall diameter.

I also did the skirt coating in anti-friction material that I think is more permanent than just break in. I can definitely hear a difference with internal noise levels from cold to hot. I guess the only thing that really gets substantially bigger is the piston accounting for the noise reduction as the engine warms.

[BoxxerSix]

The skirt coatings do little to nothing once the motor is running and oil is flowing. They are simply there to provide a lower friction surface during startup, and during initial break-in especially in motors with certain ring/bore types that require you apply virtually no oil during ring installation. You will find that most of the coating will be worn down after the first 1K miles or so and what will be left is what is impregnated into the skirt material. They really do help prevent piston skirt galling in the event of a very hard bore contact.

The thermal barrier coatings do exactly as you thought. Less thermal transfer to the piston means it runs cooler, as does the pin bushing and rod bearings. How much so is very difficult to determine, and thus why it's best to not mess with changing piston clearances unless you're very adept with thermal dynamics and the forgings you are using. Most if not all people would never notice the minute change in PTWC that will occur in the motor due to the reduced expansion rates. Were talking one or two TEN thousandths of an inch.

You won't notice any power improvements with the coatings until you start coating combustion chambers, valves(face and exhaust backside), and exhaust ports in the heads but you should certainly notice slightly lower oil temps as suggested. Another very good side benefit of most of those coatings is that carbon deposits have a hard time forming and sticking to the surface.


What company did you use for the coatings?

[lindy 911]

Poly-Dyne in Houston.

[Henry Schmidt]

Thermal barrier coatings are often generically referred to as ceramic coatings, and they establish a thermal barrier that prevents heat from passing into a part. For instance, a thermal barrier coating on a piston dome prevents combustion heat from dissipating through the dome and into the piston, which would normally cause the piston to expand. Since the piston won’t grow as much as before, the builder can run a tighter piston-to-wall clearance. Because Air cooled Porsches are already designed to run a very tight tolerance this becomes far less important an issue. More clearly stated, air cooled Porsche pistons are designed to control expansion.

Also, if the combustion area is coated with a thermal barrier, the heat of combustion stays in the chamber, promoting more efficient use of thermal energy. To aid in this, the valve faces can also be coated to help seal the heat as well as to prevent excess heat from traveling up the stems. Coating the exhaust path contains the combustion heat inside the exhaust path, preventing loss of thermal energy and preventing adjacent areas from being subjected to excess heat. Coating the exhaust ports and outside of the exhaust manifold also creates a scavenging effect, contributing to a more efficient exhaust “pull.” In essence, the combustion energy and heat stay in a contained path, doing their job, instead of being wasted into surrounding areas.

Anti-friction coatings, which are applied to components such as bearings and piston skirts, not only provide a much lower coefficient of friction (when and if physical contact between two moving surfaces takes place), but also serve to retain surface oil between moving parts. In this regard, anti-friction coatings serve a dual role.

[lindy 911]

Thanks everyone. I was hoping you would chime in Henry; did you notice the cylinder studs in my photos above?

If I'm understanding correctly, the 911 piston is designed to resist expansion from thermal transfer. The use of a barrier coating will only help reduce engine oil temps because the oil cooling of the underside of the piston via squirters will have less heat to absorb. I would also assume that the cylinder wall temps would be reduced because of the reduced skirt temps.

Is there a down side to using the thermal coating?

Thanks again.

[Henry Schmidt]

Originally Posted by lindy 911

Thanks everyone. I was hoping you would chime in Henry; did you notice the cylinder studs in my photos above?

If I'm understanding correctly, the 911 piston is designed to resist expansion from thermal transfer. The use of a barrier coating will only help reduce engine oil temps because the oil cooling of the underside of the piston via squirters will have less heat to absorb. I would also assume that the cylinder wall temps would be reduced because of the reduced skirt temps.

Is there a down side to using the thermal coating?

Thanks again.

The only down side I can see is cost and if improperly installed the coating can flake off. This flaking can cause damage to part down stream in the exhaust. (cats and turbos)

[tadd]

Id like to toss a few other questions into the ring to see what pops up. Ive had answers, but like other things there was no overwhelming "yea or ney".

1. Are the thermal coatings truly thermal coatings? In other words, they are only a few microns thick. Even taking into account the small thermal conductivity of ceramics, the film thinkness dominates and makes a big number small. Especially given that our engines reach thermodynamic equillibrum while running. Is there actual data from something like an RFID thermocouple on the underside of the piston?

2. Our air head motors need their oil to dump heat, so why restrict a heat flow pathway? Wasnt that the idea of the oil squirters to lighten the thermal load on the heads since they reach saturation at about 170 hp? Add squirters and big coolers, and bring it on... Heck, the squirters get bigger as the hp went up.

The only thing I can rationally accept (so far) till someone educimates me different is that the piston failure requires local softening (over heating) of the piston before a shock event can pit the piston. The local softening seems to be from short term hot spots in the burn. Given the speed of these events, I could see how the coating protects the piston from the kinetic heat event, thus the material isnt soft enought to pit under detonation shock.

I've always been curious if these heat barrier coatings are truly quanitifable improvments rather than just 'it solved our problem'. It may work great, Id just like to know why.

t

[lindy 911]

I think lower oil temps would be a pretty good indicator of whether or not the barrier coat works. I can tell you that I have a 10.5:1 motor that I run pretty hard and I don't have an aux cooler other than the one mounted on the engine. I think that says a lot all by it's self.

As far as softening goes; I think it depends on where the hot spot is in the combustion chamber that's causing detonation. If it's a high point on the head that heats up enough to create an ignition source then the piston won't be involved as much as if it were a hot spot on the piston itself. I've seen iron heads on a blown motor that broke in half, pulled the head bolts out of the block and started a lifter valley fire that was a site to be seen. JE pistons looked like new, not a scratch. Go figure.

[tadd]

Lindy:
But is that really the goal, to have lower oil temps? I would think the goal is to have lower head/piston temps, and if the oil is getting too hot, then you need more radiation surface (bigger cooler).

That heat has to go somewhere. I'm not sure that I buy its being contained and going out the exhaust instead of into the piston. Some of what went into the pistion is now going into the heads instead.

It seems counter intutive to protect (keep cooler) the cheap, replaceable thing (the oil), rather than striving for lower head temps.

Im certinatly not one of the porsche does no wrong freaks, but an external cooler on anything over a 2L is pretty well accepted... or is that a myth that needs busted?

t

[Turbo_pro]

Originally Posted by tadd

Lindy:
But is that really the goal, to have lower oil temps? I would think the goal is to have lower head/piston temps, and if the oil is getting too hot, then you need more radiation surface (bigger cooler).

That heat has to go somewhere. I'm not sure that I buy its being contained and going out the exhaust instead of into the piston. Some of what went into the pistion is now going into the heads instead.

It seems counter intutive to protect (keep cooler) the cheap, replaceable thing (the oil), rather than striving for lower head temps.

Im certinatly not one of the porsche does no wrong freaks, but an external cooler on anything over a 2L is pretty well accepted... or is that a myth that needs busted?

t

Thermal barriers are really more about better combustion not cooling. "promoting more efficient use of thermal energy". When properly done (money is no object) the combustion chamber and valve faces are also coated.

[gtc]

I suppose coatings might help reduce pre-ignition caused by hotspots on the head or pistons, but I suspect that detonation caused by lean mixtures, high compression, etc would rip right through the coatings. And even if the piston face survived, your ring lands probably wouldn't last long.

Like Jim said, heat lost through the face of the piston (and then dissipated by coolant) is energy which otherwise could have been used to expand the combustion gasses and drive the motor.

[tadd]

gtc:
I've heard that response before, but that doesn't vibe with what I remember from stat mech... We know heat flows from hot to cold. We also know that it's exponential with the delta. Greater the difference in temps the faster the system moves towards equilibrium.

So if we make the combustion gases hotter, they are going to push heat harder where it can... to the heads (unless they are coated too).

If we want to improve efficiency, then we need to keep the heads hot. Hence volvos (and others) attempts at ceramic, no oil engines.

Has anyone coated the pistons then monitored head, oil, and exhaust temps?

I'll see if I can chase up where I came across the micro pitting thing. The experiment involved changing the head or cylinder temps in a water cooled motor by restricting flow then tweaking the fuel mix ratio (toluene and heptane iirc) to induce knock. So they would run mostly toluene to avoid knock, establish a set block temp, then start substituting heptane for toluene to induce knock.

They saw a step function in piston damage with respect to cylinder/head (piston) temp.

t

[lindy 911]

I agree that pulling heat out of the combustion chamber through the piston is good for head cooling. I also agree that keeping as much heat in the combustion chamber as possible elevates combustion efficiency. My original question as to whether or not the piston will grow enough to properly fit the cylinder has been answered.

I'm now curious as to how the head gets maxed out thermally at 170 hp. Is there heat transfer from the cylinder to the head? If so, cooling the piston before it has to transfer heat into the cylinder via skirt and ring lands seems logical. But is it more a function of the heat produced by the combustion process directly?

I think I'm making about 100 HP more than the maxed out number for the head and really can't say that the heads get all that hot, even when run hard. I've been involved with some crazy air cooled engine builds where the head pipe of the exhaust runs cherry red hot for hours without any apparent damage to the head or it's components. My 911 does not do that (that I'm aware of) and is of similar design. The photo below adds some scale to how big the heads are (cooling wise) related to the cylinder.

[ChrisBennet]

Originally Posted by tadd

gtc:
I've heard that response before, but that doesn't vibe with what I remember from stat mech... We know heat flows from hot to cold. We also know that it's exponential with the delta. Greater the difference in temps the faster the system moves towards equilibrium.

So if we make the combustion gases hotter, they are going to push heat harder where it can... to the heads (unless they are coated too).

If we want to improve efficiency, then we need to keep the heads hot. Hence volvos (and others) attempts at ceramic, no oil engines.

t

It's been a couple of decades since I took Thermodynamics but I think the what you are referring to is something to do with the Karnough (sp?) Cycle. The gist of it is that the greater the difference between the heat inside the engine and outside the engine the more efficient it will be. In this case you want to keep the heat in the combustion chamber but I don't think you gain anything from having the head hot. (I'm too lazy to look it up.)

[Flieger]

French, not English

Carnot cycle - Wikipedia, the free encyclopedia

[RoninLB]

beyond the basics this subject gets over my head fast so I'm a little more that worthless here

when I did my re-build many moons ago i thought about heat coating etc and wound up looking into EGT

what I came up with is that a non turbo 911 engine EGT can stabilize easily below a damaging high EGT with proper tuning and stay there. [i use 6 EGT probes full time] That no ill affects or lowering of engine life will happen if the EGT window isn't breached. That turbo EGT's can become hotter that desired for a brief time due to various circumstances. That a heat barrier on a turbo engine is a good safety measure to accommodate that brief higher than desired EGT.

also that head temps is a function of power output [my heads with e-cams and carbs runs around 25F hotter that CIS stock]

that's all I know

[afterburn 549]

Cave man science - if you have ever worked with a oxy torch,..the flame has to either get the spot hot enough to cut, or find a edge (heats quicker).
The ceramics are also a good insurance policy to prevent this "hot spot" from occurring.