The factory evo 8 piston sits .070 below the deck. Add the .041 for gasket and you get a whopping .0110 squish band. This is a unusually large amount of deck height. I have found over and over that mitsu engineers are smart little fockers and they did this for a good reason. For comparison sake the early 4g63 pistons sit about .015 below the block. or about .058 squish band.

Most aftermarket pistons are near 0 deck height. they usually set them .005 below the deck to allow for a shaved deck. Now just because everyone does it this way to get a better squish area doesn't make it a good idea. I am looking for a aftermarket piston that was a copy of the evo8 piston and maintained this loose deck height spec. Anyone know of one? I know the new not yet availabe Mahle pistons are zero deck height. I think the wiseco is near zero but I cant confirm.

 

Forgive the newbie question, but why is more deck height good? What's a squish band?

 

the squish band is the outer edge of the piston that comes closest to the cylinder head. the gases get squished toward the center of the piston casing faster burn rates. faster burn chambers are less likely to detonate.

you would have to ask mitsu engineers why they run virtually no sqish band with the evo8. if I had to guess i would think its becuase the high compression flat top piston itself produces excellent burn rates. addition swirl from the squish may hurt more than it helps. I do know mitsubishi knows there **** when it comes to tweaking this 4g63. they have been doing it for over 15 years.

I am not happy about aftermarket piston manufactures not paying attention to this detail. most are designing the piston by simply taking the 2g piston specs and reversing the valve cutouts.

 

Not to be the math police but .070 and .041 = .111

 

Off topic my post is , but correct you are (Dave Chappelle Yoda Voice)

 

Talked to Ross today. They gave me the numbers for the compression height of the different pistons. the numbers are consistant with what I measured. The compression height is measure from the center of the rist pin to the top flat of the piston.

the early 4g63 89-92 compression height is 1.375. this puts the piston .015 below the deck
the late 4g63 from 93-99 is 1.400 , these pistons sit about flush with the deck.
the evo 4g63 from 2003 has compression height of 1.320. this puts the piston .070 below the deck.

couple of things learned today from Ross. they dont have a piston designed for the evo8. if you are buying a ross piston from a vender it is a custom spec piston they had made up. Or they are using the early pistons from the 4g63 with 1.375 compression height. The pin in these pistons are not offset so you can simply flop the piston to get the valve reliefs happy.
They were also very curious as to why mitsu abandoned the squish zone setup. they had no answers why they did it.

will call someone else tommorrow. likely get the same lack of answers.

 

The only piston manufacturer that I have seen that has a part number specific to the Evo 8 is Wiseco. As far as I know this was a piston designed specifically for the evo. I have not seen anybody else do that yet. All of them use a 2g part number. Give Wiseco a call tomorrow Jerry. Their catalog list the same compression height as all the other 2.0 4G63T pistons though.

 

Thanks Geo, the reason wiseco has a separate part # for the evo 8 piston is because they are the only piston with an offset pin. All the others use a symmetrical pin , so all they have to do is mount the piston reversed in the bore to accomodate the cutouts.

It seems no one has done there homework on this one. Shame.

 

Now my cogs are turning...