club.liberty.asn.au

Okay so, I've order a brand new stock EJ257 short block from Subaru and I'm planning on using my stock EJ20Y heads. Now generally I know I'd have to machine my EJ20 heads to fit the EJ25 bore however I've come across a set of JE forged pistons (Part number #291060) that apparently have an extra 10cc of dish to compensate for the ej20 head.

Just curious if anyone has used these, or can clarify that I won't have to machine the EJ20 heads if these pistons are used?

Cheers for any help!

There isnt much to take out of the head, the valves run to close to the deck height, I'll take some picks of mine when I get home friday.

Ej20y heads have a tiny combustion chamber (48cc from memory). Even with a 10cc dish on the je pistons, and assuming a 0.8mm quench area and 0.7mm compressed headgasket thickness comp as follows

Total capacity 2457cc
Or 614.25cc per cylinder

99.5mm bore
95mm stroke

Bore radius 49.75mm

Working backwards:
Head Combustion chamber volume 48cc
Plus area of squish (49.75x49.75x3.142x0.8mm) = 6.2cc
Plus compressed head gasket area (49.75x49.75x3.142x0.5mm) = 5.44cc
Plus 10cc piston dish

Total compressed area in cc at tdc is 69.66cc

Static comp = 614.25:69.66
= 8.817:1


Mick

8.8:1 Seems like a reasonable compression ratio to me? Not the I know anything about building motors of course...

Pre die grinder. In hind sight I probably wouldn't of done it

Get the heads ported

dr20t wrote:Ej20y heads have a tiny combustion chamber (48cc from memory). Even with a 10cc dish on the je pistons, and assuming a 0.8mm quench area and 0.7mm compressed headgasket thickness comp as follows

Total capacity 2457cc
Or 614.25cc per cylinder

99.5mm bore
95mm stroke

Bore radius 49.75mm

Working backwards:
Head Combustion chamber volume 48cc
Plus area of squish (49.75x49.75x3.142x0.8mm) = 6.2cc
Plus compressed head gasket area (49.75x49.75x3.142x0.5mm) = 5.44cc
Plus 10cc piston dish

Total compressed area in cc at tdc is 69.66cc

Static comp = 614.25:69.66
= 8.817:1


Mick

Okay, trying to learn here and unsure of a couple of things....

I understand that you divide the cc per cylinder by the cc of the combustion chamber (614.25/69.66=8.817:1)

Probably going to sound dumb but there's only one way to learn!

Why do we times the bore radius by itself to calculate the headgasket/area of squish? (I'm sure this is just basic math :S )

Also I thought the stroke was 79mm?

I was wrong aswell, the extra 10cc of dish on these JE pistons is on top of what the standard dish would generally be (22cc IIRC?) Should start to look like a closer CR to stock.

And finally how do we actually calculate the CC rating of the headgasket? 49.75x49.75x3.142x0.5 (Should be 0.7 shouldn't it?) comes up with something completely obscure for me.. :S

A few things (and I realized I made a mistake above too)

1. Volume for a cylinder is area x height. Area of a circle is radius^2 x pye (3.142).

So volume of a cylinder is radius^2 x 3.142 x height

2. Static compression ratio takes the total combustion chamber volume as a ratio of the compressed chamber volume when piston is at top dead centre (tdc)

Radius^2 x 3.142 x stroke will give you volume per cylinder
I didnt show this in my calculation but an rj257 has 99.5mm bore and 79mm stroke

So cylinder volume is 49.75 (radius) x 49.75 (radius) x 79
= 614.35 cc

3. To get an accurate total chamber volume reading, you need to factor in the volume of the space in the cylinder that the compressed headgasket occupies, the head chamber volume and the squish volume. Squish volume is the gap from top of piston to the top of the bore. On an ej257 I'm pretty sure its 0.8mm.

Yes the oem headgasket is 0.7mm thick, but when compressed (head bolts / studs torqued down), the thickness reduces. This volume therefore needs to be factored into the compression ratio calculation.

In my calculation above I forgot to add the combustion chamber volume back to the total swept volume. So it should read:

(614.35 + 69.66) : 69.66
= 684.01 : 69.66
= 9.819:1

However if piston total dish is 32cc then calc is :

(614.35 + 91.66) : 91.66
= 706.01 : 91.66
= 7.7:1

Something doesn't seem right there to me. 7.7:1 is way too low. And 9.8:1 is probably too high for an alloy block EJ motor unless you're running e85.

You need to verify:
1. Piston total dish
2. Quench / squish area of ej257 including compressed gasket thickness

Mick

Had a little search and trying to wrap my head around this all ahahha.

So the "Proper" quench for a turbo ej engine with 2618 pistons is 1mm apparently. (http://6starspeed.com/sumybu.html)

The actual piston dish for these JE pistons is 25.5cc.

Sooo... 48cc for the head volume
Area of squish is (49.75x49.75x3.142x1mm)=7.7cc
Compressed head gasket thickness (49.75x49.75x3.142x0.5mm)=3.8cc
Plus the 25.5cc of actual piston dish

So total compressed area is 85cc

Static comp= 614.35+85:85

=699.35:85
=8.2:1

Soo.... Correct....? Or no....

Correct and makes sense as ej257 factory cr is 8.2:1 with 56 or 58cc heads

The extra 10cc dish in pistons is offsetting the drop in combustion chamber from 58 to 48cc

Finally learning some technical shit!

Just been talking to someone else and they've said there's two main reasons to machine the head, obviously the correct combustion chamber but he also said to smooth out the shape edges on the EJ20 heads as they tend to cause detonation on the 2.5L blocks.... Anyone knoe if there's any truth to this?

You'll find a full heads FAQ on nasioc.
Or rather a bunch of related threads:
I started by googling "nasioc d25 heads" or similar...
Not sure what the code for the 2l heads is...
Ports:
http://forums.nasioc.com/forums/showthread.php?t=903873
Scroll down for links to the other good threads inc:

Dual avcs ej20y heads are z20

Definitely get the chambers machined out. But if you're doing that I wouldn't go with the JE pistons you initially referred to with the additional dish. Go with normal ej257 forged pistons or if not chasing more than 230-240awkw then just leave the bottom end untouched

dr20t wrote:Dual avcs ej20y heads are z20
Definitely get the chambers machined out.

Thanks Mick, would there be a cnc machining option for that route, if eg cost was no object?

bigBADbenny wrote:You'll find a full heads FAQ on nasioc.
Or rather a bunch of related threads:
I started by googling "nasioc d25 heads" or similar...
Not sure what the code for the 2l heads is...
Ports:
http://forums.nasioc.com/forums/showthread.php?t=903873
Scroll down for links to the other good threads inc:

Heads sectioned to show differences in the length and shape of the ports:
http://forums.nasioc.com/forums/showthr ... ?t=1856856
This is one of the best threads on the subject of hybrid engines, pity it's a little hard to find:
Hybrid engines inc various heads on various bores and observations pertaining to quench & compression...
http://forums.nasioc.com/forums/showthr ... ?t=2141800
Almost everything you need to know.

Will be going for forged pistons for peace of mind anyway, I've really only heard bad things about the 257 pistons.

I was thinking of just leaving the heads and going with those pistons and leaving the heads un-machined as cost is an object. Don't really want to be going over 10k including manifolds/turbos etc