Concerning forged pistons that is. Why are engine with forged piston that comes from the factory (eg. s2k, suburu wrx, sti, mitsubishi evo, srt-4, etc. etc.) piston to wall clearance are tight - about .015-.030mm, compared to the aftermarket slugs where it required about .060-.070mm clearance?

 

Bump for a good question.

 

My guess is that different amounts of silicone in the pistons lead to different expansion rates upon warm up.

 

correct, aftermarket pistons expand more than stock pistons, therefore needing more piston to wall clearance.

 

ever heard of the term "piston slap"

its when you just start your car and the forged pistions are moving around slightly.

like was said before, they expand more than the cast pistons so when the car is warm the PTW clearance is tight. close to that of the other cars you mentioned.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by SkRiBLaH &raquo;</TD></TR><TR><TD CLASS="quote">ever heard of the term "piston slap"

its when you just start your car and the forged pistions are moving around slightly.

like was said before, they expand more than the cast pistons so when the car is warm the PTW clearance is tight. close to that of the other cars you mentioned.</TD></TR></TABLE>

Yes, however the s2000, wrx, and many other vechiles come with forged pistons from factory. The question is, why do our aftermarket forged slugs contain such high silicon content relative to factory forged pistons? What is the reasoning behind this?

I'm currently running forged wisceos in my b18c1 and my **** slaps like a mother ******. I'm also runnin eagle rods so I don't know if some of the sound is coming from the connecting rod moving around on the pin(bottom of piston).

 

the rod wont move... its centered by teh crank... if the rod is moving you have big problems.

piston slap is common, but it shouldnt be terribly noticable.

different ammounts of silicone make for different expansion rates and different strengths. aftermarket forged pistons are stronger than OEM forged pistons

 

I've built 2 engines with wiseco pistons/eagle rods and one with JE pistons/eagle rods. None of them had piston slap.

High silicon 4032 aluminum alloy pistons require .0022"-.0028" for a Honda-sized bore.

Non silicon 2618 aluminum alloy pistons require .0030"-.0036" for a Honda-sized bore.

You can go from .001" to .003" over, depending on how your engine is going to be used.


If you are using 2618 pistons at .07mm, then that's a bit tight.

If you are using 4032 pistons at .07mm, then that may be fine,depending on your application.


Note - I'm not an engine building expert, so check it out for yourself, but I think I got the numbers right.