Austarion

I am building a 4G64 SOHC 8 Valve and can't find an off the shelf Forged Piston, have contacted several piston manufacturers in USA and have been advised that will have to be a custom build.
4G64 SOHC 8 Valve has 72cc combustion chamber and OEM flat top pistons give 8.5:1 C/R, which is specification in Mitsubishi Engine Manual (1986-1990). Later 4G64 SOHC 8 valve had 8.9:1 C/R. (1990-1992)
The OEM 4G64 piston has 3 valve reliefs (1 Inlet, 2 Exhaust for alternate valve position on Cyl 2/3)
The regular Twin Cam 4G63 piston has -23cc dish and 4 valve reliefs (8.5 C/R 4G63 DOHC - 6.9 C/R in 4G64 SOHC) and is unsuitable for targeted compression ratio as car will do competition hillclimbs as well as sprints.
ACL offer 2 Cast piston types for 4G64 SOHC , one at 8.5:1 C/R the other at 8.9:1 C/R
Has anyone sourced forged flat top piston for 4G64 and or know of supplier / part number?

Is the motor going to be NA or turbo ??

Sounds low compression, so highly likely to be turbo.

Comp N/A motors rarely run less than 10.5:1, sometimes upto 13:1

Yes the engine will use a TD05 16G turbo, so needs to keep compression around 8.5:1.
4G64 has 3 mm bigger valves than 4G63 SOHC and larger Inlet and exhaust ports.
Other people I know go straight for twin cam conversion, but some of us swim against the tide!

Wiseco have an Eagle Talon 88-92
piston in 85,86 & 86.5 with a -10 dish and a 9 to 1 cr in a 47cc chamber
You will have to do some calculations to see if they are close to the cr you want along with a little shaved off the head it may come close. Good luck :beer

Interested to know how this works out...

can you not just use DOHC 4G63 forged pistons?

OK Pistons finally arrived from CP pistons in USA.

After talking with CP pistons they understand the necessity of customised pistons in this 4G64 SOHC application.

I got custom SOHC pistons after talking to USA speed shop who had an inside connection with CP pistons and wanted to produce custom pistons as a challenge, and I was already committed to 4G64 SOHC. Starion is in original condition and wanted to keep it looking like factory fitment…4 runner inlet manifold, custom Exhaust manifold and 20G will be only difference...
Needed 87.5mm pistons as Block is 1mm overbore, most DOHC pistons only go to 86.5mm (+1.5mm) which is STD bore in a 4G64

You can actually use the DOHC pistons in SOHC engine as it is a non-interference engine, as many in the forum have used them, but was trying to build engine with 8.5 : 1 compression and more importantly to have correct valve reliefs so valves are not shrouded by piston dish/crown which improves flow.

Using DOHC pistons with -13cc dish leave compression around 7.2 : 1 and the valves are shrouded by piston crown and inadequate valve reliefs in DOHC pistons. Using -23cc dish leaves compression around 6.9 : 1 which is great for very high boost applications but shrouding of valves still limits flow.

The piston crown with DOHC -13 cc dish just misses 4G64 SOHC valves and obviously DOHC valve reliefs are not used in SOHC application.

End result is a 4G64 piston with - 4cc dish and correct valve reliefs. 4G64 SOHC has 69-72cc chamber so is much larger than DOHC and needs a much smaller piston dish. ACL make 2 OEM pistons, flat top at 8.9:1 compression and -4cc dish for 8.5 : 1 compression for atmospheric engine. This being turbocharged I have gone for 8.5 : 1 compression using -4cc dish.

There are actually 3 valve reliefs per piston 1 for inlet and 2 for exhaust as the exhaust valves are staggered for two cylinders.

Have attached pics and am dummy assembling engine for final assembly.

Will post Dyno figures after tune…

Piston Crown:


Show Link : http://www.fr33z3.net/starions/Apr06/dsc03007_copy1.jpg

Inlet Valve Relief:


Show Link : http://www.fr33z3.net/starions/Apr06/dsc03019.jpg

Piston Skirt:


Show Link : http://www.fr33z3.net/starions/Apr06/dsc03008.jpg

Forged Stamping with extra thickness to accomodate valve reliefs:


Show Link : http://www.fr33z3.net/starions/Apr06/dsc03012.jpg

what is wrong with the stock pistons ? Very impressive SOHC 64 build posted here running them

quest wrote:what is wrong with the stock pistons ? Very impressive SOHC 64 build posted here running them

At 20-30PSI STD pistons won't last, much less what this JOKER has in mind for them. :beer

Ausminx wrote:OK Pistons finally arrived from CP pistons in USA.

After talking with CP pistons they understand the necessity of customised pistons in this 4G64 SOHC application.

I got custom SOHC pistons after talking to USA speed shop who had an inside connection with CP pistons and wanted to produce custom pistons as a challenge, and I was already committed to 4G64 SOHC. Starion is in original condition and wanted to keep it looking like factory fitment…4 runner inlet manifold, custom Exhaust manifold and 20G will be only difference...
Needed 87.5mm pistons as Block is 1mm overbore, most DOHC pistons only go to 86.5mm (+1.5mm) which is STD bore in a 4G64

You can actually use the DOHC pistons in SOHC engine as it is a non-interference engine, as many in the forum have used them, but was trying to build engine with 8.5 : 1 compression and more importantly to have correct valve reliefs so valves are not shrouded by piston dish/crown which improves flow.

Using DOHC pistons with -13cc dish leave compression around 7.2 : 1 and the valves are shrouded by piston crown and inadequate valve reliefs in DOHC pistons. Using -23cc dish leaves compression around 6.9 : 1 which is great for very high boost applications but shrouding of valves still limits flow.

The piston crown with DOHC -13 cc dish just misses 4G64 SOHC valves and obviously DOHC valve reliefs are not used in SOHC application.

End result is a 4G64 piston with - 4cc dish and correct valve reliefs. 4G64 SOHC has 69-72cc chamber so is much larger than DOHC and needs a much smaller piston dish. ACL make 2 OEM pistons, flat top at 8.9:1 compression and -4cc dish for 8.5 : 1 compression for atmospheric engine. This being turbocharged I have gone for 8.5 : 1 compression using -4cc dish.

There are actually 3 valve reliefs per piston 1 for inlet and 2 for exhaust as the exhaust valves are staggered for two cylinders.

Have attached pics and am dummy assembling engine for final assembly.

Will post Dyno figures after tune…

Now you just have to learn how to Tune it & then DRIVE IT rofl

So have I got this right...ACL made a cast -4cc piston that would have suited if it was forged. So the CP forged pistons are a copy of the ACL one.

What sort of cost? Do they use standard over-the-counter rings? What will sprt of material and thickness will you be using? What is it's intended use and what sort of boost will it see? Conrods?

[quote="enthuzed"]So have I got this right...ACL made a cast -4cc piston that would have suited if it was forged. So the CP forged pistons are a copy of the ACL one.

Yes the CP pistons are basically a copy of ACL cast piston.
The Mitsubishi pistons were designed for non-turbo application, but Mitsubishi had 2 compresion ratio's for different applications.

[quote="enthuzed"] What sort of cost? Do they use standard over-the-counter rings? What will sprt of material and thickness will you be using? What is it's intended use and what sort of boost will it see? Conrods?

The CP pistons use racing moly rings 1.00mm top, 1.2mm 2nd, 2.8mm Oil.
CP custom pistons were around $800
I chose forged pistons as I intend to run 25-35psi boost and have Eagle rods.

That's good info. So it was easy to get the 40 thou over rings?
Who are you dealing with in Queensland (or did you take your business interstate)?

enthuzed wrote:That's good info. So it was easy to get the 40 thou over rings?
Who are you dealing with in Queensland (or did you take your business interstate)?

Internationally, its also cheaper that way :D