Forged pistons compared: CP, JE, Wiseco and Manley

Forged pistons are the part people buy first when a build gets serious, and the part they read up on last. Over the counter it is always the same question in different clothes: which brand, what ratio, and do I need rods too. Here is how we answer it.

The short answer

• Any of the four brands we stock will hold together a boosted street build. Whether a set exists for your exact engine, bore and compression ratio matters far more than the badge on the box.
• Pick the compression ratio from the fuel you will actually run and the boost you plan to hold.
• If the engine is coming apart anyway, buy the rods now. Doing them later means paying for the same labour twice.
• Order the ring set, bearings and head gasket in the same hit. Those are the small parts that leave a build sitting on the stand for a fortnight.

What forged actually means

A cast piston is poured into a mould. A forged piston is squeezed out of a solid billet of aluminium under pressure, which lines the grain up and makes it much harder to crack. That is the whole reason people buy them: a forged piston survives detonation and rpm that a factory cast piston will not.

There is a trade off. Forged aluminium grows more with heat, so these pistons are fitted with more clearance in the bore. A cold engine can be noisier for the first minute on a frosty Manawatu morning, and that noise is normal rather than a fault.

The product title tells you most of what you need. Read CP RB25DET Pistons .5mm OS 8.5:1 SC7308 as three facts: the block gets bored half a millimetre over, the piston gives 8.5:1 static compression, and SC7308 is the number your machinist will want.

What each brand is good at

CP

CP covers more of the Japanese turbo engines we build here than anyone else in our range. The CP Pistons collection has RB25DET sets in 8.5:1 and Neo 9.0:1, an RB30 block with RB25 or RB26 head hybrid piston at 8.2:1, a 2JZ 10:1 and 1JZ 9.0:1 set, and SR20DET at 8.5:1. Evo builders get a real choice of ratio in half a millimetre over: 8.5:1, 9.0:1 and 10.0:1, plus a 1mm oversize and a 100mm stroker set. Subaru people get an EJ20 8.5:1 set for AVCS engines. CP is American, built in Irvine, California, and the CP forged pistons page is the quickest way to see the range.

JE

JE is the one to check for engines CP does not cover. The JE Pistons range includes a Barra BA to BF XR6 Turbo set at 9.0:1, 3SGE Beams pistons at 9.5:1 and a 12.5:1 version for a naturally aspirated Beams, plus SR20VE and SR20VET at 9.0:1. JE list their ring sets as separate lines, so add the matching ring set to the order rather than finding out on assembly day.

Wiseco

Wiseco is narrower here, but it owns a couple of engines nobody else looks after. If you have a Mazda B6T, the 8.7:1 set in half a millimetre over or the 1mm oversize version is the answer, and there is a 4G93 8.9:1 set for the older Mitsis. The Wiseco range also carries an LS set and a ring set. Wiseco has been making pistons in Mentor, Ohio since 1941.

Manley

Manley is where Subaru and Honda builders should start. The Manley piston range has EJ205 sets at 8.5:1 in standard stroke 92mm and 92.5mm, stroker versions on a 79mm crank in 92.5mm and 93mm, an Evo X 4B11T set at 9.0:1, and B18C1 sets from 9.0:1 through to 12.5:1. One practical difference: the Manley sets here are listed with rings included, which is one less line on the order.

Getting the compression ratio right

The ratio in the title is what the piston gives you in a standard chamber on a standard gasket. Change gasket thickness or skim the head and it moves, so tell your engine builder which set you are ordering before anything gets machined.

For a boosted street car on pump fuel, the lower ratios give you room to move, and 8.5:1 is the safe default for a car that will see real boost. Step towards 9.0:1 or 10:1 when the car runs good fuel and you care more about how it feels at 3000rpm than about the number at the top. The 12.5:1 Beams set is a naturally aspirated part, and that is worth saying out loud because people order it by accident.

Oversize is a machining decision, not a shopping one. The block gets measured, bored and honed to suit the pistons in your hand, so order the set first and hand it over with the box unopened.

Rods, and when to buy them

New pistons on tired factory rods is a half job on the engines known for snapping them.

Eagle is the value end and covers the usual suspects: RB26DETT and RB25DE or DET, SR20, 2JZ-GTE, EJ20 and EJ22, 3SGTE and several 4G63 variants. Watch the pin size on the Mitsi ones. The Evo 1 to 9 set uses a 22mm pin and the E39 VR4 set uses 21mm, and mixing those up costs you a week.

Manley H beam rods are the next step up and cover a wider spread, including RB26, RB30, Barra XR6 Turbo, SR20, EJ20 and EJ25, and 4B11 for the Evo X. There is also an H Tuff set for the WRX and STI when a standard H beam is not enough for the plan.

What to buy with pistons

Bearings go in at the same time, and the ACL Race Series sets cover the same engines as the pistons above. You need a head gasket that suits your final bore, which is what the Cometic range is for, and the head should come back down on ARP head studs rather than stretched factory bolts. An ARP ring compressor in your finished bore size makes assembly day much calmer.

Where NZ Performance fits

We hold pistons, rods and the assembly parts in Palmerston North and ship them the same week, so a block does not sit at the machine shop waiting on freight from overseas. The forged pistons page and the forged rods page are the fastest way to see what the range covers.

If you are pricing a whole bottom end rather than one part, our RB rebuild kit guide and the 4G63 rebuild guide walk through what a full package includes. For the boost side of the decision, head studs, gaskets and pistons for big boost covers what fails first and why.

FAQ

Are forged pistons worth it on a stock turbo engine?

If the car stays near factory boost and gets tuned properly, factory pistons usually survive. Add boost, a bigger turbo or E85 and start chasing a number, and forged is the sensible call. Forged pistons take detonation and rpm that cast pistons do not.

What compression ratio should I pick for a turbo build?

Start with the fuel you will run and the boost you plan to hold, then pick from the ratios that exist for your engine. Lower ratios like 8.5:1 leave headroom on pump fuel. Ratios nearer 9.0:1 or 10:1 feel sharper off boost, and suit a car on good fuel that is not chasing the top of the map.

Do I need forged rods as well as forged pistons?

Not always, but the engine is already apart, so the labour is free now and expensive later. Factory rods in a few families are the known weak point once power climbs. If you are unsure, buy the rods while the block is on the stand.

What does 0.5mm OS mean on a piston set?

OS is oversize, so a 0.5mm OS set is half a millimetre larger than standard bore and the block has to be bored and honed to suit. Do not order an oversize set before your machinist has measured the block, because the final bore size is their call.

Which piston brand is best for my engine?

Ask which brand makes a set for your exact engine, bore and ratio, because that usually narrows it to one or two. CP covers most Japanese turbo fours and sixes, Manley is strong on Subaru and Honda, JE covers Barra and Beams, and Wiseco has the B6T and 4G93.