4g13 owner here... currently on 74mm pistons, if not mistaken those are 4g12 over-sized pistons. ... U can bore it safely up to 74mm. Better pick up than a stock setup. But if u aren't in tight budget, go for 4g15 block with 78mm piston. You will never regret as that gonna be my future project.
For 4G13 engine block the maximum oversize pistons limits is 74 mm, which is originate from 4G12 engine. Original 4G13 pistons size is 72 mm and is high compression. You need to bore your block cylinder another 2 mm which is acceptable. Some of the user install 4G15 pistons and the size is 75.5 mm, but you need to take the risk due to cylinder side wall become very thin, and is really depends on luck whether the side wall crack in high rev. Even the cylinder side wall is not crack you still need to face high temperature issues due to 4G13 radiator is small in manual car.
if you use 4G12 pistons in 4G13 block, due to it pistons is low compression it is good for turbo setup and you will gain extra boost, but for NA setup you will face problem in loss pick up and loss hp. I can feel that after i install the 4G12 piston in my 4G13 engine, the power is worst than original 4G13 pistons. Plus high fuel comsumption due to your engine loss power. Therefore, for my opinion go for 4G13 73 mm high compression pistons.
If you insist want to install 4G12 pistons in your 4G13 engine, you need to skim your engine head for another 1 mm. This will lower down your engine head for more compression, and regain the power loss. But you will face tempreture issues due to extra heat, you need to change your radiator to wira 1.6 double layer size radiator to stand the extra heat.
After you have skim the engine head for 1 mm without any problem, you can try to skim another 0.5 mm. But I'm not sure whether the pistons will hit the valve or not. For my experience i buy another 4G13 head to skim for 1 mm, and after they have install everything and before they starts my car i was sweating and praying hopefully does not having any problem. Therefore, to skim another 0.5 mm is really a challenging, and if the pistons hits the valve please don't blame me if your valve become banana size or your piston surface has crack. Even you have pass this, you still need to see whether or not the water temperature raising with your big size raidator can stand the heat or not. Therefore, if you face any 1 of the both issues that i mentions, you can say bye bye to your engine head, once skim there is no going back. So try at you own risk.
For my humble opinion, get the 4G13 73mm oversize high compression pistons. You will get less hassle, less worry and less money spend. Even if you do all the works for installing the 4G12 pistons in your engine, from my estimation if your put 4G13 73 mm oversize pistons the compression ratio and the hp will be the same or maybe higher.
Therefore, compression ratio is very important for NA car, below is the compression ratio for 4G13, 4G12 and 4G15 for your preference:
Piston size 72mm
Piston surface raise + 1.9mm - 2 mm
Compression ratio 9.5:1
Piston size 74mm
Piston surface down - 1.9mm - 2mm
Compression ratio 8:1 or 8.5:1
Attach for my dyno result for proofs. Hope fully nothing goes wrong for further skim another 0.5mm, target 110 HP.
From the chart:
Red line (Lowest part) is when i install 4G12 pistons in my 4G13 engine. (Is like Shit)
Blue line (Middle part) is when i have skim engine head of 1 mm, before ECU tunning. (Powers Back YeH)
Green Line (Highest part) after ECU tunning. (Extra 4HP Gain)
Incidentally - 4G13 block max size it can go is 1.5L size. Erm.. wait - that didn't come out right. 4G13 can be bored to 4G15 size max. Cheaper to buy a 4G15 block and bore it from there. With a stroker you can hit 1.7L long stroke. Torque galore!
Hi Izso, I did my car dyno the wheel is still intact, how can you said is on crank?
---------- Post added at 06:46 PM ---------- 6 hour anti-bump limit - Previous post was at 06:39 PM ----------
Hi, poeman gtr, just my personal preference. I dont like upgrading camshaft, ekzo sound like shit. I prefer as normal as possible. I know that I spend a lot and the money I spend can purchase a greater than my 4g13 engine. But that is my hobby, unless my 4g13 has hit the limit in n/a setup. I will still continue to find room for improvement