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Rebuilding Engine

GFunk

By GFunk,
in Engines & Forced Induction

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GFunk Enthusiast

GFunk

Posted
Posted

I'm in the process of putting my baby together (imaculate 32 gtst)

My new turbo has been made up, intercooler, exhaust as well as a few other things, clutch, pod filter, greddy boost controller, short shift box, body kit, carbon fibre hood and new paint....

But my engine is still in pieces in my mates workshop (rb20). I am sending to an engineer i know this weekend to start working on it. The turbo i have is rated at 400-450hp, and im only going for a bit over 300hp at the wheels.

So long story short - Do i need forged pistons???????

....and what about cams and shit too, thank you

(550cc injectors are going on as well as bosch 040 fp)

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browny Proficient

browny

Posted
Posted
So long story short - Do i need forged pistons???????

I pinched these from Warspeeds posts over at CT. Hopefully it helps....

The big thing about rod stress is piston speed and piston weight. Piston speed is proportional to stroke times Rpm, so the longer stroke of the RB30 will always put it at a disadvantage at any particular Rpm to the RB25. Piston weight is entirely up to you, but forged pistons are always going to be heavier because of the shape limitations of the forging process.

Cast pistons can be made very thin and can have complicated internal ribs and re-entrant shapes. The casting mould can be constructed in several parts so it can be removed from inside the cast piston once the metal has solidified. So there is no real limitation on what shape it can be cast. Material need only be added where it adds directly to strength, so cast pistons will be the lightest pistons possible to make.

Forged pistons are made by forging dies. In other words it is a sort of stamping process. A male die is slammed into a female die and squashes the aluminium into shape. The dies must be solid and very strong, so the shape of the die inside the piston must be a very simple and slightly tapered, so it can be withdrawn after forging.

Forged pistons always have extra material around the pin bosses simply because of the way they are made. They may be stronger if the pistons are made thick, but a lot of the extra material only adds weight without extra strength. Forged pistons may be stronger, but if they are, they will always be far heavier.

The exception is very short slipper racing type pistons which are not really suitable for long life street engines.

Forged or cast, refers only the manufacturing process, it says nothing about what the material is. The TYPE of aluminium, mainly silicon content has a lot to do with strength and thermal expansion. So there is a lot more to think about than just forged versus cast.

Too many guys say "I want to fit some really cheap forgies" without knowing what they are really doing. The belief that any forged piston is superior to every cast piston for any engine is simply not true.

There are exceptions, but most manufacturers still use cast pistons, and for very good reasons. If forged pistons were far better, Ford or GM could probably mass produce forged pistons just as cheaply as cast pistons if they wanted to.

If you are planning a high Rpm street engine, and you expect it to last for a very long time without throwing a rod, think seriously about piston weight as well as just conrod strength.

I would ask them two things, what is the forged piston weight compared to a stock factory cast pistons, and what is the recommended bore clearance of the proposed new forged pistons ?

The increased weight (if any) will tell you a lot about increased conrod stress at high Rpm.

The recommended bore clearance will tell you rather a lot about piston expansion and the piston material.

If low expansion high silicon (hyper-eutectic) forged pistons are used, they will run very similar to original factory cast piston bore clearance. This is both good and bad. The good part is that they will not rattle when cold, they will seal better for both oil and blowby and the rings will be far happier for a long life street engine. The bad part is that they are made from IDENTICAL material to cast pistons, and are just as brittle. Detonation will still kill them fairly easily.

Being forged they may be a little bit stronger but only because they are heavier (more material up top) but the tradeoff is more weight.

There is a completely different material used for some forged pistons. This is low silicon aluminium. These aluminium pistons are SOFT and they expand considerably with heat. They resist detonation wonderfully well, just like rubber pistons would (hehehe). The down side is expansion, they run huge clearances when cold, they rattle and carry on, and the rings flop about and they leak bad. In a street engine you will most likely have oiling and blowby problems after a few months and a few thousand Km running.

But if you are building a short life mega horsepower drag race engine, they would be the strongest pistons available.

Just realise that the "HOT" superstrong genuine racing pistons may be a very poor choice for a long life street engine.

Find out what these "forgies" actually are.

A lot of people disagree, but I personally believe cast high silicon (factory type) pistons are best for long life street engines. Spend the money on engine management and tuning, and keep it well out of detonation.

As to what particular pistons are, It is difficult to know. Companies like Arias and Venolia make identical pistons in both materials in a large range of styles.

Cubes Grand Master

Cubes

Posted
Posted (edited)

No forged pistons required.

The std. comp ratio of the rb20 is good.

Spend the money on some head work to get the intake and exhaust flowing better.

Deshroud the intake and exhaust valves, polish up the combustion chamber and smooth out ALL sharp edges. This improves low lift airflow and discourages detonation.

Port match and port/polish the exhaust port, smooth out the exhaust port stud lump. Suitable valve job.

Port match and port/roughen the inlet port, 3 angle valve job.

I don't know a whole lot about head work, basically spend a good 1-1.5k on headwork including the rebuild cost.

Much more spent than this goes to waste unless you are running huge 10+mm lift cams.

OR... Grab a R32 RB25DE (is able to run of the stock R32 ecu and wiring loom without mods, bolts up to everything the rb20det used), slap a set of cast RB25 pistons in to it to drop the comp ratio, leave the head stock and it will make 300hp at the wheels on 11psi with a stock rb25 turbo. ;)

Best part is, it looks 100% like a RB20DET. You cannot tell.

Edited by Cubes
wheels_24 Collaborator

wheels_24

Posted
Posted

see how much the guy is going to charge you for standard pistons as i did and i found a guy that sells aries forged pistons so it isn't like they are no good and they were a pretty good price. I am right in the middle of a rebuild myself i am hoping to reach 250rwkw not sure of bhp and there were a alot of people that were not going to rebuild it with out them in the end it only cost me about $400 more to get the forged piston and ring so it was a pretty good investment in my mind up to you really. If you want send me a PM and i will give you his number he is in melbourne but he does ship out of state i think

Cubes Grand Master

Cubes

Posted
Posted

Providing you are up for a set of pistons because your old ones are mashed or stuff then the forged items is probably worth it.

Thats what I did.. If I didn't have to buy new pistons I wouldn't have worried about forged and stuck with the stock items.

Generally, if the power you are making warrants forged pistons, it also warrants forged rods. ;)

Cubes Grand Master

Cubes

Posted
Posted

Blueprinting is really just a word which means you check things instead of simply slapping it together without measuring it.

Blueprinting is simply double checking everything, setting up everything perfectly, i.e squish, bearing clearances, piston/bore clearances etc.

Mumbo#5 Collaborator

Mumbo#5

Posted
Posted

balancing is a good idea it will iron out some roughness when getting into it.. it has on mine and i got the pistons, rods, crank, balancer and flywheel balanced.. feels like it revs much quicker. I also got the crank linished/polished which just gets rid of some of the sharper bits on the running surfaces on the crank. I used arias pistons and crower rods.

R31Nismoid Grand Master

R31Nismoid

Posted
Posted
Generally, if the power you are making warrants forged pistons, it also warrants forged rods. :P

I CRY FOR A MOTION OF DISMISSAL... thats a shocking comment lol :P:P:P

Forgies with a set of GTR rods pumps 400rwkw safely...

And they arent forged rods...

Generally if you wanna change and go higher rev limits then rods come into it

all hail my new fav icon :):):)

browny Proficient

browny

Posted
Posted

Basically the assembly procedures (which include all the checking & measuring, resizing, surface finishing, balancing etc) should be consistant with the rest of the build. ie if its just a quick and nasty slap together job then you are probably not going to take it too far, however if its a full on no expense spared high rpm race engine then you are going to be pretty thorough.

Your engine builder should be able to tell you what is appropriate for the engine you are building.

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