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Old 08-01-2005, 11:22 AM   #1
SaabTuner
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Default Forged Pistons, Ceramic Coatings, and clearances ...

So I got to thinking ... everyone knows forged pistons require more clearance in the bore when cold than cast pistons. But how much has always been up for debate. Big hot rod muscle cars, because of relatively low power/displacement, seem to work well with smaller tolerances than high pressure turbo 4-cyllinder engines. Obviously, this is primarily because the pistons in the high hp/liter engines get much hotter and expand more.

Now, many of the people on this forum with forged pistons complain of piston slap. So I got to thinking, what could one do to the piston to keep it cooler and allow smaller tolerances and reduce piston-slap, and how effective would each change be?

The obvious two choices for cooling the pistons down are ceramic coatings on the tops, and oil-cooling jets. I'm not sure if oil-cooling jets can be installed on EJ motors which did not originally have them. But, since the EJ22T had them, it might be worth discussion how piston/bore tolerances on that engine work compared to similar EJ motors without the cooling jets.

Also, slightly less obvious, is the thickness of the bore. Thin bores conduct more heat from the piston into the cooling system, keeping them cooler, but also are weaker and easier to distort.

Anyway, anyone here have any ideas or experiences to share?

-Adrian
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Old 08-01-2005, 11:46 AM   #2
n2xlr8n
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Default

Quote:
Originally Posted by SaabTuner
... everyone knows forged pistons require more clearance in the bore when cold than cast pistons. But how much has always been up for debate.
The same rules apply; there is really no debate, other than between the gurus on the wondernet.

Quote:
Originally Posted by SaabTuner
Big hot rod muscle cars, because of relatively low power/displacement, seem to work well with smaller tolerances than high pressure turbo 4-cyllinder engines. Obviously, this is primarily because the pistons in the high hp/liter engines get much hotter and expand more.

We won't go there, but alot of it has to do with application, and the fact that a large percentage of "tuner" engines are aluminum castings, which require more clearance. Parting shot: When our JA friends can get 1350hp out of pushrod, two-valve per cylinder normally aspirated 8.2L engine, then I'll believe the hype. Even F1 friends. I'll agree to disagree to save the hijack.


Quote:
Originally Posted by SaabTuner
So I got to thinking, what could one do to the piston to keep it cooler and allow smaller tolerances and reduce piston-slap, and how effective would each change be?
The obvious two choices for cooling the pistons down are ceramic coatings on the tops, and oil-cooling jets.

The less obvious would be better ring packages, and a revised cooling system.

I've used ceramic coatings with success in the past; I'm using ceramic coatings on my EJ257 now, skirt and crown. The piston skirts were going to yield .007" clearance, so I had them coated to make up .004" (before honing).

S.
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Old 08-01-2005, 01:57 PM   #3
SaabTuner
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Quote:
Originally Posted by n2xlr8n
The same rules apply; there is really no debate, other than between the gurus on the wondernet.
Nah, I know at least three different local engine-builders who have been in the business for 25 years or more who all have different oppinions on piston-clearance on the same application. (Ford Cortina 1.6L in the Formula Fords) It's a hotter topic than just gurus on t3h intr4w3b.

What I meant was that in different applications the pistons will be subjected to different amounts of heat. Since the extra clearances are from increased expansion/contraction, the more heat under peak load, the more clearance which is needed to avoid scuffing. (Also, 4xx.x series forged alloys expand less than 2xx.x series alloys.) And more clearance means more cold-start wear and noise.

I agree that better ring-packages and cooling setups could help as well. There's no doubt about that and I appreciate it being mentioned since I hadn't thought of it when I made this thread. If anyone has any information relating to this topic about those, feel free to add!

Any more thoughts anyone wants to add?
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