Ceramic coated pistons & bearings are in w/ pics
02-08-2004, 01:57 AM
#16
Three Wheelin'
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Tony,
as far as your question regarding the crown coating thickness. You can barely see it in the pic...its a lighter silver.....it at the top of the piston only and it is very thin. They also coated the underside of the piston with the stuff.
as far as your question regarding the crown coating thickness. You can barely see it in the pic...its a lighter silver.....it at the top of the piston only and it is very thin. They also coated the underside of the piston with the stuff.
02-08-2004, 02:40 AM
#17
Nordschleife Master
I talked to a machine shop/guy that does a lot of work for the mechanics in the NE area...I pointed out the scuffing on the side of the piston and asked if the coating the factory used had worn off. He said nah, it goes deeper than that.
89k motor with track time, and 3/4 pistons out of spec.
89k motor with track time, and 3/4 pistons out of spec.
02-08-2004, 04:17 AM
#18
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“Tony: I hear two stories.
One being that the coatings keep the heat in the combustion chamber not allowing it to escape through the body of the piston itself...rings...skirts..oil contact etc etc. Heat is energy..HP..but at what expense with the coating?
higher heat in the chamber to a point increases the onset of detonation.
The other is that the coating keeps the surface of the pisotn crown cooler and prevents it from getting hot....and acting almost like a heat sink.. Keepign the piston cooler lowers the amount of heat transfer from it into the combustion chamber ..lower temps keeps detonation at bay.”
You can’t have both ways. The first explanation is the right one. Yes it will get slightly more prone to detonation, but if you have control over the timing, you can compensate by slightly retarding it. Power is only a weak function of timing, while detonation is a strong function of timing.
The net effect of thermally coating the piston top is typically considered to be a 1-2% power increase. Other benefits of coating the piston top are less strain on the cooling system, slightly cooler oil and less tendency for coking (carbonizing) the oil on the underside of the piston.
I am much less of a believer in anti-friction coating the piston skirts, after I saw most of the coating (by HPC) had worn off my Espada pistons after only 10k miles (failed rebuilt). It is probably good for dedicated racecars with frequent teardowns. It may be good for bearings though (have not tried it) since the protective oil film is much thicker here (compared to piston cylinder) and only gets compromised at startup after having sat for a while.
Laust
One being that the coatings keep the heat in the combustion chamber not allowing it to escape through the body of the piston itself...rings...skirts..oil contact etc etc. Heat is energy..HP..but at what expense with the coating?
higher heat in the chamber to a point increases the onset of detonation.
The other is that the coating keeps the surface of the pisotn crown cooler and prevents it from getting hot....and acting almost like a heat sink.. Keepign the piston cooler lowers the amount of heat transfer from it into the combustion chamber ..lower temps keeps detonation at bay.”
You can’t have both ways. The first explanation is the right one. Yes it will get slightly more prone to detonation, but if you have control over the timing, you can compensate by slightly retarding it. Power is only a weak function of timing, while detonation is a strong function of timing.
The net effect of thermally coating the piston top is typically considered to be a 1-2% power increase. Other benefits of coating the piston top are less strain on the cooling system, slightly cooler oil and less tendency for coking (carbonizing) the oil on the underside of the piston.
I am much less of a believer in anti-friction coating the piston skirts, after I saw most of the coating (by HPC) had worn off my Espada pistons after only 10k miles (failed rebuilt). It is probably good for dedicated racecars with frequent teardowns. It may be good for bearings though (have not tried it) since the protective oil film is much thicker here (compared to piston cylinder) and only gets compromised at startup after having sat for a while.
Laust
02-08-2004, 07:34 AM
#19
Race Director
"Power is only a weak function of timing, while detonation is a strong function of timing.
"
Laust is correct, due to the inefficient 2V heads in these cars, timing doesn't affect power very much. Our GURU chips provide over twice the power-gains of the APE chips (+50-60hp vs. +20-30hp) and I run less ignition timing across the board in every single point on the maps than the APE chips. As an indication of efficiency, the 4V heads actually run at only 1/2 the ignition advance as the 2V heads.
On the anti-friction coatings, we also have to consider the surfaces it will rub against. Traditionally, aluminum pistons (rough surface) slides inside an iron bore (very smooth). Adding a slick coating to the rough surface will probably help it slide better against the smooth bore. However, in our cars, the situation is reversed. We've got a rough Alusil surface for a bore with a smooth iron-coated piston sliding on it. Now if we coat that hard smooth iron-coating with a lower-density, softer coating, how is it going to last against the same rough Alusil surface as before?
I'm experimenting with the HPC coatings too, but on my sleeved engine, so the coatings should last longer than inside a stock 951 block. I'll pull them out at 10-20K miles and inspect since I might go with higher-compression pistons later.
BTW, the coating is supposed to fill the clearance between the piston and bore!!! It acts as lubricant and takes the place of the oil that's normally in that gap. So when machining the cylinders, you want to maintain the same 0.003" clearance as before minus the thickness of the coatings. So you machine the cylinders to spec. first, then coat the pistons to take up some of that clearance.
"
Laust is correct, due to the inefficient 2V heads in these cars, timing doesn't affect power very much. Our GURU chips provide over twice the power-gains of the APE chips (+50-60hp vs. +20-30hp) and I run less ignition timing across the board in every single point on the maps than the APE chips. As an indication of efficiency, the 4V heads actually run at only 1/2 the ignition advance as the 2V heads.
On the anti-friction coatings, we also have to consider the surfaces it will rub against. Traditionally, aluminum pistons (rough surface) slides inside an iron bore (very smooth). Adding a slick coating to the rough surface will probably help it slide better against the smooth bore. However, in our cars, the situation is reversed. We've got a rough Alusil surface for a bore with a smooth iron-coated piston sliding on it. Now if we coat that hard smooth iron-coating with a lower-density, softer coating, how is it going to last against the same rough Alusil surface as before?
I'm experimenting with the HPC coatings too, but on my sleeved engine, so the coatings should last longer than inside a stock 951 block. I'll pull them out at 10-20K miles and inspect since I might go with higher-compression pistons later.
BTW, the coating is supposed to fill the clearance between the piston and bore!!! It acts as lubricant and takes the place of the oil that's normally in that gap. So when machining the cylinders, you want to maintain the same 0.003" clearance as before minus the thickness of the coatings. So you machine the cylinders to spec. first, then coat the pistons to take up some of that clearance.
02-08-2004, 01:52 PM
#20
Originally posted by Danno
BTW, the coating is supposed to fill the clearance between the piston and bore!!! It acts as lubricant and takes the place of the oil that's normally in that gap. So when machining the cylinders, you want to maintain the same 0.003" clearance as before minus the thickness of the coatings. So you machine the cylinders to spec. first, then coat the pistons to take up some of that clearance.
BTW, the coating is supposed to fill the clearance between the piston and bore!!! It acts as lubricant and takes the place of the oil that's normally in that gap. So when machining the cylinders, you want to maintain the same 0.003" clearance as before minus the thickness of the coatings. So you machine the cylinders to spec. first, then coat the pistons to take up some of that clearance.
Alright Danno. You got me here. Your running iron sleeves (ductile iron I assume), and you are running a 4340 piston of some brand, and JE suggests a normal .003 clearance. So if the pistoins would go in without film, whatever the specis on the bore, we would go to .003 on the piston to wall clearence, right? So then if the coating on the piston was .0015 (hypothetically), then we would run a total piston to bore clearence of .0015?
Just checking. I will have sleeves (iron) and JE pistons (aluminum), so I need to find out as much as I can now.
Thanks.
02-08-2004, 11:47 PM
#21
Three Wheelin'
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Just some measurements that I took today for you guys.
Piston to bore clearance was 0.0005-0.0007"(cylinders aren't perfectly round anymore). Before they were around 0.0007-0.009" so that would indicate a 0.0002" coating thickness for the side skirts.
For the rod bearings, with new bearings coated my rod bearing clearance was 0.034mm(note this measurement was in mm). Unfortunately I didn't take a before measurement so I don't know how thick the coating was here.
In either case it looks like my clearances are good and I've begun the assembly process.
Piston to bore clearance was 0.0005-0.0007"(cylinders aren't perfectly round anymore). Before they were around 0.0007-0.009" so that would indicate a 0.0002" coating thickness for the side skirts.
For the rod bearings, with new bearings coated my rod bearing clearance was 0.034mm(note this measurement was in mm). Unfortunately I didn't take a before measurement so I don't know how thick the coating was here.
In either case it looks like my clearances are good and I've begun the assembly process.
Last edited by Mike S; 02-09-2004 at 01:22 AM.
02-09-2004, 12:03 AM
#22
I'm far from the authority on 944 pistons, but the coating Porsche uses is not a sacrificial coating like the one that quoted article talks about. Porsche/Mahle uses an iron coating that basically gives the reverse of a traditional aluminum piston in a iron bore. Now you have an iron piston in an aluminum bore. The coating will wear off in time but it is not sacrificial by any means.
Originally posted by Tony
hope this helps a bit...
hope this helps a bit...
02-09-2004, 01:27 AM
#25
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Mike, How are you measuring clearances in the piston bore?
I'm very interested to see how yours turns out. I'm putting mine back together with just normal factory stuff.
Dan
I'm very interested to see how yours turns out. I'm putting mine back together with just normal factory stuff.
Dan
02-09-2004, 11:27 AM
#27
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Originally posted by Mike S
The only way I have been able to do it is with feeler gauges and the piston rings not installed. Is there another way to measure?
The only way I have been able to do it is with feeler gauges and the piston rings not installed. Is there another way to measure?
Bore Gauge
02-09-2004, 04:07 PM
#28
I sent my pistons to Swain on saturday, why Swain. After talking to both Swain and HPC it sounded like Swain had the advantage of non commercially available coating. Does it matter? who knows, but I paid 80 bucks extra for some piece of mind. But if my budget alows, I'll be coating the bearing surfaces and maybe the header to the down pipe.
02-09-2004, 05:33 PM
#29
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To measure bore, the best way is to use one of these to find exactly go/no go clearance (micron accuracy):
http://www.use-enco.com/CGI/INSRIT?P...PMAKA=240-0034
And then not to rely on that gauge's readout, but to mic its length with one of these in the 3-4" size:
http://www.use-enco.com/CGI/INSRIT?P...PMAKA=600-0038
Also use the micrometer to mic the piston OD. Not cheap, but very accurate.
Why Swain? Because Swain rocks!
Sam
http://www.use-enco.com/CGI/INSRIT?P...PMAKA=240-0034
And then not to rely on that gauge's readout, but to mic its length with one of these in the 3-4" size:
http://www.use-enco.com/CGI/INSRIT?P...PMAKA=600-0038
Also use the micrometer to mic the piston OD. Not cheap, but very accurate.
Why Swain? Because Swain rocks!
Sam
02-09-2004, 05:43 PM
#30
Remember, when you get down to measurement accuracy like that, temperature starts to have a factor, especially on a large piston/bore like a 944's. I could calculate out the change in diameter over a temperature range from 0-100 F but I'll let someone else have the fun.
I will say that I once was checking a 944 cylinder head for flatness. It had sat out in my car all day in 20 F weather. It was warped .015" and would actually rock on the granite block I had set it on. Once warmed up to 70 F it was flat within .001". Might make you think a little more before you go bolting on your resurfaced head during the dead of winter in an unheated garage.
I will say that I once was checking a 944 cylinder head for flatness. It had sat out in my car all day in 20 F weather. It was warped .015" and would actually rock on the granite block I had set it on. Once warmed up to 70 F it was flat within .001". Might make you think a little more before you go bolting on your resurfaced head during the dead of winter in an unheated garage.