Pistons For Your Rb30

[AngryRB]

Is it possible to get between 9.0:1 and 9.5:1 when using all stock parts and a Rb25DET head with poncams. Goal is 300kw, 14psi,gt30 using E'Flex

Is there a proven combination that someone has used? ive most likely missed it somewhere.

Piston?

decking?

gasket used?

shaved head?

[Glynn82]

You can get 300rwkw with an internally stock rb25det, 300 will be easy with a 30det

[Glynn82]

Sorry I missed the actual point of your post, I don't know about the combination of parts to get 9.5 I think it would be pretty easy with a neo head

[Scando]

Is it possible to get between 9.0:1 and 9.5:1 when using all stock parts and a Rb25DET head with poncams. Goal is 300kw, 14psi,gt30 using E'Flex

Is there a proven combination that someone has used? ive most likely missed it somewhere.

Piston?

decking?

gasket used?

shaved head?

RB30 bottom end with RB25 pistons and the block decked 30 thou will give you around 9.5:1 with an RB26 or RB25 (non-neo) head and a standard gasket. That's what I've built as a trial engine.

Edited December 10, 2013 by Scando

[Glynn82]

I thought the rb30na pistons had a higher deck height than the rb25? The crown of the two look almost identical

[AngryRB]

RB30 bottom end with RB25 pistons and the block decked 30 thou will give you around 9.5:1 with an RB26 or RB25 (non-neo) head and a standard gasket. That's what I've built as a trial engine.

are you sure 30thou is enough? on the commodore forums they say

"Using the stock RB30E pistons will give you a CR of ~8.2:1. This can easily be increased to 9.5 - 10:1 (by shaving the head, ~2mm shaved) depending on fuel availability for regular street use."

[Scando]

RB30 na pistons have a slight dome but they also have valve reliefs which gives them effectively a 0cc dome. RB25DET (non-neo) pistons have a 14cc dome with no valve reliefs. They have a 31.3mm pin to deck height vs 32mm for the RB30 which is why you need to deck the block 30 thou.

[AngryRB]

Would RB26 pistons be ok in an RB30 block with decking? would they work or are the RB25 pistons more sensible?

This is using a non-neo RB25DET head with tomei cams, trying to get minimum 9:1comp

Edited July 7, 2014 by AngryRB

[Scando]

Rb26 pistons have a 30mm pin to deck height vs 32mm for the rb30 pistons. So you would need to deck the block 2mm which is likely to cause issues.

[AngryRB]

ah ok, finding it hard to decide what is best for my application, as a street car with some time at the drags

The availability of stock rb25pistons seems limited, RHDjapan sell them for $570 plus delivery so after delivery and buying some rings im almost at the price of forgies. The issue with Forged pistons

is that they seem a little overkill for a street engine with 300kw, and they also can suffer cold start slapping.

Is there a forged piston that can run a tightish clearance and doesn't have piston slap, im assuming that at about 3thou piston slap starts to occur and from there on it will accelerate wear in an engine that see regular cold starts and used as a daily.

Thinking of the possibility of reusing my RB25 pistons or wether they would likely be a weak link from the many years and km's of previous abuse, this is supposed to be a budget build.

Edited July 8, 2014 by AngryRB

[GTRPSI]

Why are you worried about piston slap with forged pistons?

Once the engine has been running for a minute or 2 its all gone.

If your planning on drag racing and street with only 300Kw a stock bottom end will do just fine.

The RB30 bottom end will give you the bottom end power you need without resorting to higher compression ratios, just dont put large duration cams in.

[AngryRB]

If its only for a few minutes that would be ok, but i would imagine that constant rocking over time for a street engine will lead to accelerated wear, scuffing, even development of some

cracks in the skirts with a 4032 being brittle compared to the 2618.

The engine rebuilder i spoke to says the stock pistons crack up the skirt and the top ring landing also cracks from heat and is weak, although a lot of people run big power fine with them.

what pistons and clearance do you have? has the slap got any worse over time or same?

SCANDO- how is that trial engine running, any updates , how much power did you run through it etc..

Edited July 9, 2014 by AngryRB

[R32Abuser]

Why are you worried about piston slap with forged pistons?

Once the engine has been running for a minute or 2 its all gone.

If your planning on drag racing and street with only 300Kw a stock bottom end will do just fine.

The RB30 bottom end will give you the bottom end power you need without resorting to higher compression ratios, just dont put large duration cams in.

As much as I'd like to agree, its not necessarily the case. If the builder/machinist doesnt set the clearances correct, you will end up with a bottom end which will rattle all the time (operating temperature or not). My forged bottom end is loud on startup and even when up to temp, has the slightest of taps maybe every 1 or 2 seconds.

Yes, you can say the builder f**ked up but that doesnt do much good once the engine is built and in the car.

Find the right builder who will make sure it wont happen? The builder I used was reputable for building tough RB30's (amongst other RB's) down here.

If it were me, I would have stayed with a stock bottom end given the power I've made. Knock-free and easily replacable if it lets go. IMO engine reliability/longevity is all in the tune...

Edited July 9, 2014 by R32Abuser

[Scando]

If its only for a few minutes that would be ok, but i would imagine that constant rocking over time for a street engine will lead to accelerated wear, scuffing, even development of some

cracks in the skirts with a 4032 being brittle compared to the 2618.

The engine rebuilder i spoke to says the stock pistons crack up the skirt and the top ring landing also cracks from heat and is weak, although a lot of people run big power fine with them.

what pistons and clearance do you have? has the slap got any worse over time or same?

SCANDO- how is that trial engine running, any updates , how much power did you run through it etc..

I haven't had it going yet. It's going in a 240z so still lots of jobs to do for the conversion. Another popular option for daily drivers is hypereutectic pistons.

[GTRPSI]

As much as I'd like to agree, its not necessarily the case. If the builder/machinist doesnt set the clearances correct, you will end up with a bottom end which will rattle all the time (operating temperature or not). My forged bottom end is loud on startup and even when up to temp, has the slightest of taps maybe every 1 or 2 seconds.

Yes, you can say the builder f**ked up but that doesnt do much good once the engine is built and in the car.

Find the right builder who will make sure it wont happen? The builder I used was reputable for building tough RB30's (amongst other RB's) down here.

If it were me, I would have stayed with a stock bottom end given the power I've made. Knock-free and easily replacable if it lets go. IMO engine reliability/longevity is all in the tune...

If the clearances are not setup right, dont blame the pistons for being forged, blame the machinist.

In our case we friction coated the pistons too, not for slap, more to help side loads under higher boost pressures.

If running them on the loose side expect piston slap issues, some brands are more prone to it than others, i have not looked into if it was the recommended clearances being loose from the piston manufacturers or the design/contruction/material of them or skirt length/design.

[R32Abuser]

If the clearances are not setup right, dont blame the pistons for being forged, blame the machinist.

In our case we friction coated the pistons too, not for slap, more to help side loads under higher boost pressures.

If running them on the loose side expect piston slap issues, some brands are more prone to it than others, i have not looked into if it was the recommended clearances being loose from the piston manufacturers or the design/contruction/material of them or skirt length/design.

Not at all blaming the pistons. If done properly, there should be no slap when up to operating temperature. Only noting that even reputable machinists/builders don't get it right and AngryRB should be aware of the possible pitfalls if he were to use forgies.

[chairman]

cp flat tops best

[AngryRB]

Think ill be going soft and opting for some cast pistons, just want a quiet torquey 300kw's, I think it should be decent with the E85 etc..

If I went forged I'd have to change my goals and push for 400kw's, tempting but expensive.

thanks all for the good advice.

Edited July 10, 2014 by AngryRB

[Simon-S14]

if your doing that why rebuild at all? stock nissan pistons will do that easy!

[AngryRB]

The block ive bought is worn, if I rebuild it then I can hopefully avoid any issues such as uneven comp, low comp, odd knocking noises, blow by etc etc

I don't plan to take the engine out again after this so don't want any suprises or failures, i hate piston slap and noises from worn out engines too

if it were for track then id take the chance.

Edited July 31, 2014 by AngryRB