4032 forged pistons for rb25/30?

[Desean Strickland]

Hey so I’m building a rb25/30 and I’m after some 4032 forged pistons but I’m struggling to find any.

Has anyone here used 4032 forged pistons for a 25/30? If so where did u order them from

Second question is people who went with 2618 forged pistons, have u had any piston slap on cold starts or anything like that? Iv been told it’s a possibility when using 2618. 

It’s not gonna be a track car and I wana run tighter pistons to wall clearance to suit a daily driver which is why I’m after 4032 pistons but keen to hear people experiences with both types of forged pistons.

[Murray_Calavera]

It would be worth getting in contact with Supertech, they do 4032 pistons. 

Would also be worth contacting the big players in the forged piston game. The modern coatings used on the piston skirts allow for tighter piston to cylinder wall clearance, you might find that your happy to run a 2618 piston with a modern coating. 

Also, how much power do you expect to make and what fuel will you run?

[blind_elk]

This is my understanding - feel free to correct me if I'm wrong - but forged pistons need to run hotter (work harder) than cast pistons so they expand more to fill the cylinder. Basically they run in a competition engine.

Given that understanding, if you are only building a daily driver, why do you need forged pistons? And why do you need a 25/30 - what is wrong with a plain old 25?

[Duncan]

1 hour ago, blind_elk said:

This is my understanding - feel free to correct me if I'm wrong - but forged pistons need to run hotter (work harder) than cast pistons so they expand more to fill the cylinder. Basically they run in a competition engine.

Given that understanding, if you are only building a daily driver, why do you need forged pistons? And why do you need a 25/30 - what is wrong with a plain old 25?

In my experience that is a requirement decision not a forged vs non forged but rather a usage decision. You can run forgies with pretty tight clearances in light use but would still run them loose for racing.

Having said that, my street stagea got big clearances and the bloody thing burns and blows by plenty as a result.

[GTSBoy]

Yeah, the clearance difference between cast and forged is because of the material difference and the fabrication method difference, not because of the usage. The alloys used and the forging process leads to a piston that has a higher coefficient of thermal expansion than the cast material does. This is  fundamental property of the finished piston itself, not something that is desired.

How you then deal with that depends on the state of other technology, the depth of your pockets, and perhaps the rules in any racing class that may or may not have things to say about such things. Before the advent of coatings, and with the materials that were originally used for forged pistons, the only real option was to go for wider clearances to allow for the expansion. And then you had to warm the engine up carefully, etc etc. Or, if you ran tighter clearances for a "street" engine, then you perhaps couldn't lean on it as hard as you might if it were built looser and treated properly (like a proper race engine would).

Nowadays, with materials that are a little better (on the raw material thermal expansion coefficient front), plus better design/machining to control expansion (mostly around keeping it even instead of being non-isometric) and especially with coatings to control heat input on the crown and friction on the skirts, you can get away with a lot that would have been "against the rules" in the old days.

These days there is absolutely no reason to fear running sensibly tight piston/wall tolerances on a street forged piston engine. You need the forgies because you will be giving it curry from time to time, but you want tighter tolerances because 99% of the time you're not bashing on it. Modern tech gets you there.

And if you're building an actual race engine that will get leaned on hard all the time, then you still run wider tolerances because the pistons will definitely run hotter than your street engine will.

Edited April 10, 2024 by GTSBoy
typo

[Desean Strickland]

1 hour ago, blind_elk said:

This is my understanding - feel free to correct me if I'm wrong - but forged pistons need to run hotter (work harder) than cast pistons so they expand more to fill the cylinder. Basically they run in a competition engine.

Given that understanding, if you are only building a daily driver, why do you need forged pistons? And why do you need a 25/30 - what is wrong with a plain old 25?

Cast pistons have a lot higher silicone content, 2618 has less than 1% silicone content and 4032 forged is somewhere in between. You wouldn’t use 4032 in a 1000hp engine since the extra silicone makes it more brittle but for my application, wanting a strong piston that will run close to factory piston to wall clearance running probably under 20psi boost it’s perfect. 
 

and the need to go forged is more to do with needing a 86.5mm or 87mm piston since I probably won’t be able to keep the stock 86mm bore size. And even if I could it probably wouldn’t be a great idea to put 30 year old scored stock pistons back in it. 
 

as for going with a 25/30, it gives a lot more low end torque and power, hits full boost about 1-2k rpms earlier and should make a lot better street car than a 25det would, the downside being it won’t rev as high which dosnt really matter for a street car.

[Desean Strickland]

2 hours ago, Murray_Calavera said:

It would be worth getting in contact with Supertech, they do 4032 pistons. 

Would also be worth contacting the big players in the forged piston game. The modern coatings used on the piston skirts allow for tighter piston to cylinder wall clearance, you might find that your happy to run a 2618 piston with a modern coating. 

Also, how much power do you expect to make and what fuel will you run?

I’ll be running 98 since there’s no e85 where I live and my power goal is around 450hp

[joshuaho96]

On 4/9/2024 at 10:21 PM, Desean Strickland said:

Cast pistons have a lot higher silicone content, 2618 has less than 1% silicone content and 4032 forged is somewhere in between. You wouldn’t use 4032 in a 1000hp engine since the extra silicone makes it more brittle but for my application, wanting a strong piston that will run close to factory piston to wall clearance running probably under 20psi boost it’s perfect. 
 

and the need to go forged is more to do with needing a 86.5mm or 87mm piston since I probably won’t be able to keep the stock 86mm bore size. And even if I could it probably wouldn’t be a great idea to put 30 year old scored stock pistons back in it. 
 

as for going with a 25/30, it gives a lot more low end torque and power, hits full boost about 1-2k rpms earlier and should make a lot better street car than a 25det would, the downside being it won’t rev as high which dosnt really matter for a street car.

I think you're thinking of silicon, not silicone. Silicone is a polymer of siloxane which is Si-O-Si with some hydrocarbons hanging off of the silicon.