Piston Upgrade Guide for the 3000GT/Stealth 6G72 Engine

by Jeff Lucius

Our factory cast aluminum pistons are what Mitsubishi calls "autothermic type with steel struts cast into it". The steel struts provide structural support and help control thermal expansion. The stock pistons are fine for stock engines. However, when engine modifications lead to power output over 400 HP and boost values over 15 psi, forged aluminum pistons should be considered.

Forged pistons are stronger and often a little lighter than cast pistons. Our stock cast piston weighs about 400 grams; a forged piston replacement can weigh between ~350 and 400 grams. The densely-packed molecules in forged pistons allow them to conduct heat away from the piston top quickly so that forged piston crowns can run 75F to 100F cooler than cast piston crowns (450F instead of 550F typically). However, forgings often have much less silicon content than cast pistons and expand more when heated than cast pistons do. This requires slightly larger piston-to-cylinder wall clearances (0.005" instead of 0.001", for example) and oil consumption and blow-by may be higher with forged than with cast pistons. The higher clearance can also cause forged pistons to be a bit noisier than cast pistons until they warm up. Forged pistons, with pins and rings, can cost more than cast pistons ($660 to $1200 for forgings instead of $600 or less for a set of six castings; list prices for factory parts: pistons are ~$67 each, the ring set is ~$184 [~$589 total]).

There are two common alloys used in forged pistons, 4032 and 2618. Silicon-aluminum alloys, such as 4032, have great wear characteristics because the silicon particulate hardens the alloy and reduces the thermal coefficient of expansion. However, silicon-aluminum alloys can turn brittle and become prone to fracturing when subjected to extreme stress. With a piston made of a silicon alloy once a crack starts, it doesn't stop until the piston suffers a catastrophic failure. Low- or no-silicon alloys, such as 2618, may wear a bit faster but provide better strength and durability. In the rare case of a crack in a 2618 piston, the crack will migrate to an area of lower stress and stop. 2618-alloy pistons keep their shape under extreme pressures and high RPM's.

Some owners have considered hypereutectic cast pistons as an alternative to forged pistons. Hypereutectic cast pistons could be an upgrade for normally-aspirated engines. However, A.Graham Bell in his recent book "Forced Induction Performance Tuning" (published in 2002 by Haynes) says they are a poor choice for turbocharged engines. Hypereutectic cast pistons have about twice as much silicon in the aluminum alloy as regular cast pistons (15-20% instead of only 7-8%). According to Bell, the added silicon leaves them "quite brittle and, as such, prone to breaking when subjected to detonation."

A question often asked when rebuilding an engine is "How much can I overbore the cylinders?". From the service manual's engine cross sections I guessed that the cylinder wall thickness might be 0.25". Joe Gonsowski (Mitsubishi Engineer) had his block sonic tested (top, middle, and bottom of bore) during his engine rebuild. Average wall thickness was 0.288" with a minimum value of 0.231 inches. Ray Pampena (Mitsubishi Master Tech and race engine builder) measured wall thickness for each cylinder in four places with a sonic tester and found an average wall thicknes of 0.300 inches. Ray notes that blocks sometimes core shift, which might reduce wall thickness to as little as 0.150". Always have a block sonic tested to measure wall thickness before boring the cylinders. An average wall thickness of 0.200" is considered by many as safe for our engines. However, Ray says that wall thickness can be as little as 0.100", but this greatly increases the risk of cracking if severe detonation occurs.

The stock bore is 91.1 mm (3.5866"), and Mistubishi provides factory pistons up to 0.040" over factory bore. I had my engine bored 0.050" over stock, which increases the bore to 92.38 mm for a displacement increase of 84 cc to 3056 cc from the stock 2972.3 cc. This reduced average wall thickness to about 0.238-0.250" (acording to Joe's and Ray's measurements; I do not know if my engine builder sonic tested my block). After twelve thousand miles I have not encountered any problems, but the increase in wall deflection and stress may reduce the durability of the engine somewhat over the long term. Some of our alloy-steel block 6G72 engines have been bored 0.075" over stock resulting in a 93 mm bore, which increases displacement by 126.1 cc to 3098.4 cc and reduces average wall thickness to 0.213-0.225". Ray Pampena bored his block 0.100" over, which increases displacement 5.67% to 3141 cc (see the table below for overboring comparisons) and leaves an average wall thickness of 0.200" for his engine.

The pistons listed below can be purchased from the manufacturers and from many speed shops and parts supply stores. I suggest contacting the speed shops that specialize in our cars listed on my Garage Page. You can also discuss how much to over bore your cylinders with these shops. Some piston manufacturers, like Ross, have worked with 3000GT/Stealth owners to incorporate design improvements.

Note: Because our engine blocks are alloy steel, care must be taken when honing the cylinders. This should be discussed with your engine builder.



Pictures and Terminology

The Venolia pistons pictured below are a close copy of the stock pistons as far as valve reliefs in the crown, ring positions, and compression height. If you want to reduce the compression ratio (to compensate for the compression ratio increase from the larger bore) then the compression height can be decreased to lower the piston in the cylinder or the crown possibly could be dished a little deeper to increase the combustion chamber volume.

Venolia pistons sides

Venolia pistons top/bottom

Piston terminology


Over-boring and Compression Ratio

STOCK (turbocharged 6G72):
stock bore: 91.1 mm; 3.587 in.
stock stroke: 76 mm; 2.992 in.
stock displacement: (bore/2)^2 x PI x stroke
    = 3.587/2 x 3.587/2 x 3.14159 x 2.992 = 30.235 CI (total=181.41 CI)
    = 91.1/2 x 91.1/2 x 3.14159 x 76 => 0.495382 L (total=2.9723 L)
stock CR = 8.0:1

The total volume (TV) equals the swept volume (SV, or displacement) plus combustion chamber volume (CV) which together are the 8 parts (in the 8 to 1 ratio). At TDC, the CV represents the 1 part (in the 8 to 1 ratio). Therefore, the SV equals 7 parts.

TV = SV + CV
At BDC, volume = TV = 8 x CV
At TDC, volume = CV
Stock CR = (8 x CV) : CV = (SV + CV) : (CV) = (7 + 1) : 1 = 8 : 1
So, CV = one seventh of the SV (or displacement).
Stock CV = 30.237 / 7 = 4.319 CI (or 0.070769 L)

BORE CHANGE ONLY (no change to piston crown, compression height, or stroke):
I'll use my car as an example. New bore = 92.38 mm; 3.637 in.(0.050 in. over stock)
New displacement (or SV)
    = 31.084 CI (total = 186.5 CI)
    = 0.50940 L (total = 3.0564 L)
New CR
    = (SV + CV) / CV
    = (31.084 + 4.319) / 4.319 = 8.197:1
    = 8.2:1 (close enough)
The displacement with 0.050" overbore represents a 2.8% increase over the stock displacement.

6G72 Block Overboring Comparison
Overbore Bore (mm) Bore (in) Total CI Total cc % Larger CR Wall thickness
0.000" 91.11 3.587 181.41 2972.3 0.00 8.00 0.288-0.300"
0.010" 91.36 3.597 182.42 2989.4 0.58 8.04 0.278-0.290"
0.020" 91.62 3.607 183.44 3006.0 1.14 8.08 0.268-0.280"
0.030" 91.87 3.617 184.46 3022.7 1.70 8.12 0.258-0.270
0.040" 92.13 3.627 185.48 3039.5 2.26 8.16 0.248-0.260"
0.050" 92.38 3.637 186.50 3056.3 2.82 8.20 0.238-0.250"
0.060" 92.63 3.647 187.53 3073.1 3.39 8.24 0.228-0.240"
0.070" 92.89 3.657 188.56 3090.0 3.96 8.28 0.218-0.230"
0.075" 93.01 3.662 189.08 3098.4 4.24 8.30 0.213-0.225"
0.080" 93.14 3.667 189.59 3106.9 4.53 8.32 0.208-0.220"
0.090" 93.40 3.677 190.63 3123.9 5.10 8.36 0.198-0.210"
0.100" 93.65 3.687 191.67 3140.9 5.67 8.40 0.188-0.200"

MILL ONLY:
The combustion chamber opening is circular at the top of the block so milling will only remove a small cylinder from the CV. Let's assume we mill at the service limit of 0.2 mm (0.008 in).

Removed volume = (bore/2)^2 x PI x thickness
    = 3.587/2 x 3.587/2 x 3.14159 x 0.008 = 0.0808 CI.
New CV = 4.319 - 0.0808 = 4.2372
New CR
    = (SV + CV) / CV
    = (30.235 + 4.2372) / 4.2372 = 8.1356
    = 8.14:1 (close enough)

MILL (0.008") and BORE (0.050 in. over)
Removed volume
    = 3.637/2 x 3.637/2 x 3.14159 x 0.008 = 0.0831 CI.
New CV = 4.319 - 0.0831 = 4.2359
New CR
    = (31.084 + 4.2359) / 4.2359) = 8.3382
    = 8.34:1 (close enough)


Acknowledgments

Some of the information presented here was gathered from various email lists, message boards, and vendor web sites. I would like to particularly thank Ray Pampena, Jeff Crabtree, John Adams, Trevor James, and Joe Gonsowski for their contributions.


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Page last updated December 25, 2006.