Piston thermal coating

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pamotorman
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Re: Piston thermal coating

Post by pamotorman » Fri Dec 08, 2017 10:04 pm

the reason i did that was i was told by the engineers at GM racing at the tech center that they had found the engine made max HP right away if the coating was left on.

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Re: Piston thermal coating

Post by John Wallace » Sat Dec 09, 2017 7:53 am

Possibly from the compression ratio increase?

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Re: Piston thermal coating

Post by 4vpc » Mon Dec 11, 2017 5:16 am

Jeff Lee wrote:
Fri Dec 08, 2017 6:37 pm
I’ve used Tech-Line Coatings many times (applied at the machine shop) and it is similar to apply; glass bead parts for adhesion, tape, shoot and bake in typical kitchen oven. Coated a lot of parts and I can not say it made HP or did not make HP. However, you should use it with the idea “it won’t hurt” in making HP, may ad a slight amount.. But more importantly, it is good on piston crowns should you go lean on jetting or some other issue while racing. It will protect the crown and can take a considerable amount of leaning out that an unprotected crown will take.
Now I do know that coatings to the exterior of the intake manifold are worth some power. Coat base of the intake to repel oil, coat the top to dissipate heat.
Glass bead is the wrong kind of media as it leaves a smooth surface, read the instructions or call them up, but I believe fine sand or similar is what you should use.
I came to the conclusion that if it's going into moderate det' it'll kill something, if it isn't the piston crown then it''ll be something else possibly more expensive, this lowered my need for it.
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Re: Piston thermal coating

Post by kimosabi » Mon Dec 11, 2017 5:34 am

I wouldn't coat a piston unless it was a titanium piston.

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Re: Piston thermal coating

Post by dhidaka » Mon Dec 11, 2017 9:25 pm

Contact Eric at Rebco Machine. He can coat top and sides. Priced right. Plus he is a real master at custom piston machine work. Nice guy and only a couple of miles from me.
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Re: Piston thermal coating

Post by Walter R. Malik » Tue Dec 12, 2017 10:02 am

pamotorman wrote:
Thu Dec 07, 2017 8:38 pm
i remember people coating piston tops with hi temp header paint. never found out if it worked
The white VHT "ceramic" header paint will last until you mechanically take it off if it is cured correctly.

I have used it successfully in the past but, now I use Tech-Line.
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Re: Piston thermal coating

Post by David Redszus » Tue Dec 12, 2017 11:32 am

The first question to ask is: Why are we considering coating a piston? What are we trying to accomplish?

Are we trying to keep the piston cooler? If so, we should recognize that the surface and chamber will become hotter.
The ability of a thermal barrier to prevent heat transmission is a function of coating thickness in addition to its physical properties. A thick coating is often subject to cracking due to thermal expansion incompatibility with piston crown material.

A hotter combustion chamber may increase flame speed but may also lead to pre-ignition.

Surface strength is best accomplished not with coatings but with surface conversions such as hard anodizing.

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Re: Piston thermal coating

Post by pamotorman » Tue Dec 12, 2017 12:55 pm

the engine is a heat pump and the more heat that is contained in the combustion chamber the more HP you make. that is why uncoated aluminum heads require 0.5 increase in CR to get the same HP a CI head engine with the same parts

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Re: Piston thermal coating

Post by needforspeed66gt » Tue Dec 12, 2017 2:49 pm

The only coating that I have found to stay put is the Techline Polyphen Gold stuff - all the others are cheap shit that burn off quickly, scratch with even the dial indicator tip you use to check piston protrusion, etc. I've pulled down our blown gas and high boost diesel engines to find the coating still looking like new.
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Re: Piston thermal coating

Post by Walter R. Malik » Tue Dec 12, 2017 6:27 pm

David Redszus wrote:
Tue Dec 12, 2017 11:32 am
The first question to ask is: Why are we considering coating a piston? What are we trying to accomplish?

Are we trying to keep the piston cooler? If so, we should recognize that the surface and chamber will become hotter.
The ability of a thermal barrier to prevent heat transmission is a function of coating thickness in addition to its physical properties. A thick coating is often subject to cracking due to thermal expansion incompatibility with piston crown material.

A hotter combustion chamber may increase flame speed but may also lead to pre-ignition.

Surface strength is best accomplished not with coatings but with surface conversions such as hard anodizing.
I coat the crown in order to hinder the transfer of heat to the underside of the piston; which keeps the oil temp lower in an oval track & road race situation.
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Re: Piston thermal coating

Post by MadBill » Tue Dec 12, 2017 6:34 pm

Any feel for how much the temperature drops Walter?
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Re: Piston thermal coating

Post by Walter R. Malik » Tue Dec 12, 2017 6:40 pm

MadBill wrote:
Tue Dec 12, 2017 6:34 pm
Any feel for how much the temperature drops Walter?
From about 250 to near 225 in the sump; and that is with a wet sump and Harrison air/liquid oil cooler.
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Re: Piston thermal coating

Post by MadBill » Tue Dec 12, 2017 6:56 pm

That is substantial! It also means the piston is running much cooler.
Quite apart from combustion effects, I think many people fail to see all the implications for piston coatings, e.g.:
o With piston alloy's steep drop in yield strength vs.temperature, gives a wider safety margin and/or the opportunity for reduced mass.
o Less mass = more RPM and/or lighter rods and CWs.
o Narrower top ring lands for reduced crevice volume.
o Less heat to dissipate through rings, thus maintaining durability with narrower lower friction rings.
o Tighter P-B clearances for better ring stability/wear.
o Smaller, lighter oil cooler.
o I'm sure there's more..
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Re: Piston thermal coating

Post by David Redszus » Wed Dec 13, 2017 12:45 am

pamotorman wrote:
Tue Dec 12, 2017 12:55 pm
the engine is a heat pump and the more heat that is contained in the combustion chamber the more HP you make. that is why uncoated aluminum heads require 0.5 increase in CR to get the same HP a CI head engine with the same parts
Heat is used to raise combustion chamber gas pressure. It is the change in gas pressure that produces the downward force on the piston. A glance at the pressure curves of a PV diagram will clearly show this.

An increased inlet air temperature will not change the compression pressure at all. It will change the compression temperature and the resulting combustion flame speed leading to a higher combustion temperature and combustion pressure. This may or not be beneficial to performance.

By far, the best way to cool a piston is to use oil spray jets directed to the underside of the piston.

The ability of a metal to conduct heat is given by its thermal conductivity value.
For cast aluminum it is 92 BTU/hr/ft^2/F/ft.
For cast iron it is 23 BTU/hr/ft^2/F/ft.
Aluminum will conduct heat at a rate four times faster than cast iron.

As far as insulating properties of materials the conductivities are:
Alumina (aluminum oxide) is 6-12 BTU/hr/ft^2/F/ft. This is often called hard coat anodize.
By far the best insulating coating would be zirconium dioxide at .6-1.0 BTU/hr/ft^2/F/ft. (if you could get it to stick)

Thermal conductivity is affected by the material, time in hours, area in square feet, temperature and thickness in feet.
Piston coatings are not usually applied to a depth measured in feet but rather in a few thousandth of an inch. Which is why the ceramic tiles on the space shuttle were nearly an inch thick.

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Re: Piston thermal coating

Post by GARY C » Wed Dec 13, 2017 1:08 am

lemons racer wrote:
Thu Dec 07, 2017 7:20 pm
Has anyone had any experience with Cemkote V136 piston thermal coatings, it is self applied with a small detail gun & baked on. It's less than $40.00 for 4oz. which will coat 8 pistons, compared to paying over $26.00 each with full service company.
I'm putting a cheap Chevy 350 together for the 24Hrs of LeMons, It should make about 330 HP & won't go over 6000 rpm.
I have used their products on chambers, valves and piston for nitrous engines, they recommend aluminum oxide blasting so I bought a small dedicated cabinet for that only to prevent contamination.

I never had a problem with it.
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