1point3liter

I'm calculating the compression ratio of the engine I want to build before I start ordering parts. Help is much appreciated.

I'm shooting for 8.5:1 but so far have come up with 8.7:1. Anyone care to check my math?

specs:
3.905 bore
3.622 stroke
6.125 length rods (eagle h-beams)
71cc heads (#317 casting 6 liter truck heads)
-8.6cc Diamond 3.905 bore pistons
.060 4.0" head gasket (8.8:1 with .054 gaskets)

What bore head gasket should I use with this setup? I believe I read 4.0" when using stock #317 heads. Who sells a 4.0" .060 gasket? Is there a thicker gasket I can use that will get me down to 8.5:1 CR? I've heard rumor of cometic .078 gaskets. Is there a downside to running this thick of a gasket?

Anyone using a set of custom 3.905 bore pistons (more than -8.6cc) to achieve lower compression? What brand did you choose and aproximately how much did they cost?

I guess what i'm trying to figure out is what are you guys doing to achieve 8.5:1 with a 3.905 bore? Are you increasing your chamber volume?

TIA, I'm still learning this piston stuff.

y2khawk

iTrader: (5)

Here's what mine's at, i spec'd 8.75:1 when i ordered the pistons. I went with JE pistons. Valve reliefs with inverted dome to maintain quench area. $70 to $80 a slug with pin fit. Had them teflon coated on the skirts.

Bore 3.91 (inches)
Stroke 3.622 (inches)
Rod Length 6.125 (inches)
Cyl. Head Vol. 71 (cc)
Deck Height -.006 (inches)
Head Gasket Bore 4.04 (inches)
Head Gasket Thickness .051 (inches)
Piston to Cyl. Wall Clearance .004 (inches)
Top Ring Land Height 0.25 (inches)
Piston Dome Vol. -11 (cc)


Compression Ratio:
8.78 : 1

Bore / Stroke Ratio:
1.08 : 1

Rod / Stroke Ratio:
1.691 : 1

Total volume:
804.423 cc's

Compressed volume:
91.618 cc's

1point3liter

3.905 bore
3.622 stroke
6.125 rod length
71cc chamber volume
-0.006 deck height
4.04 gasket bore
0.060 gasket thickness (still need to decide on a gasket)
0.004 piston to cylinder wall clearance
0.25 top ring land height (is this pretty standard?)
-8.6cc diamond pistons

8.803:1 CR

1point3liter

Also, what is quench?

y2khawk

iTrader: (5)

It's the area between the flat portion of the piston and the flat part of the combustion area in the head. The piston basically squishes the air/fuel mixture in that area and forces it in to the area near the spark plug. I'm no combustion expert, but the turbulence from this squish promotes better combustion and help to ward of detonation.

Production is about .046 piston the head clearance at TDC, and the general suggestions seem to say .050 to .040 is about right.

My pistons maintain the flat portion of the piston nearest the head to keep the quench area intact instead of a compete dish.

http://harlan-engineering.com/eviltwin/short_block.jpg