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Rb30 Neo Piston Selection

R34GTFOUR

By R34GTFOUR,
in Engines & Forced Induction

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R34GTFOUR Mentor

R34GTFOUR

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Posted (edited)

Okay so I’m starting to crunch some numbers now and I just wanted some opinions on what I was thinking. Basically I’m going to have a built rb30det with a gt35r pushing 22+psi and hopefully pump out 500+awhp.

I’m going to use a NEO head and as most of you know it only has a capacity of 50-51cc compared to the normal rb25 head of 60-64cc, obviously this will raise compression. From what I understand the stock rb30 piston has a compression height of 1.260in with a 0.040 deck height (so the piston sits roughly 1mm below the top of the bore at TDC). I would like to have a zero deck height, and I’m still unsure on what piston size to run, either std 86mm or 87mm because at the end of the day most rb30s out there would of done at least 250,000kms and a hone might not cut it.

So this is what I was thinking:

CP Pistons (8.2 CR in the normal rb25/30)

Piston Compression Height: 1.280

Deck Height: 0 (shave .020 off the block)

Piston volume: 0 (flat tops)

Head gasket thickness: 1.6mm

Head gasket bore: 88mm

Head chamber capacity: 51cc

Bore: 86mm

Stroke: 85mm

According to the calculations that will achieve 9.13: CR, and if I needed to run 87mm pistons that would bump up to 9.32:1 CR which is starting to get a bit high for my likings, but I could drop that down to 8.75 and 8.93 respectively by having a deck height of 0.020in but im not sure how much that would affect the quench.

OR

ACL Race Series Forged RB30ET

Piston Compression Height: 1.261

Deck Height: 0 (shave .040 off the block)

Piston volume: -4.5 (dished)

Head gasket thickness: 1.6mm

Head gasket bore: 88mm

Head chamber capacity: 51cc

Bore: 86mm

Stroke: 85mm

With that combo I would be looking at 8.56:1 CR and with a 87mm bore it would have a 8.72:1 CR. I just don’t have any experience with the ACL pistons, how do they rate compared to CP?

Also I’ve noticed that most aftermarket head gaskets that I’ve seen come with a bore of 88mm, has anyone experienced trouble running a head gasket like that on a 86 or 87mm cylinder bore?

Any input would be great

:3some:

Edited by R34GTFOUR
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R34GTFOUR Mentor

R34GTFOUR

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Where did you get that info from?

Cheers

Gary

i've read it a few times on the forum, but its also listed in the rb30 PDF guide on here. Spoolup also mentions it on his site www.spoolimports.com that the combustion chamber in the head is smaller, and different pistons need to be used.

R34GTFOUR Mentor

R34GTFOUR

Posted
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Im using the ACL's in a NEO 25/30 im building. Why do you want to run a 1.6mm HG? I would run a 1.0 or 1.2 and higher comp.

Mainly to be a bit more boost friendly, but i just did some quick calculations:

With the 86mm acl pistons it will run close to 8.86:1 CR with a 1.2mm hg and 9.01:1 with a 1mm hg

With the 87mm acl pistons it will run clsoe to 9.04:1 with a 1.2mm hg and 9.20:1 with a 1mm hg

So depending on the condition of the bore (i havent got a block yet) if i run a std piston the 1mm hg seems the go, and with the 87mm piston the 1.2mm hg seems the go

Whats some feedback on acl race series pistons? Anyone heard good/bad things? Anyone know of any high hp cars running them on a daily basis?

:down:

R34GTFOUR Mentor

R34GTFOUR

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The ACL distributor here told me the ACL race pistons are made by Ross.

As we all know Ross make pretty sharp pistons so i think they look to be a good option.

didnt know that :down:

i can get acl at trade price so that looking better and better

thanks

R34GTFOUR Mentor

R34GTFOUR

Posted
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If your in sydney, you can come and have a look at a set if you like.

nah im in perth cheers anyway, coventrys over here sell them and i can get trade throught them so it should be sweet. I looked at their website and it shows a set, but it has valve reliefs so im not sure if its the correct picture.

Whats the general opinion on running a head gasket bore of 88mm on an engine with a bore of 86-87mm? Is that going to cause a problem? 90% of the gaskets i've seen are 88mm for the rb25/26

drgnball34 Rising Star

drgnball34

Posted
Posted

mentioned to my workshop from the very get go that the NEO head have smaller cc. But they still went with the cp pistions for a r33 rb25. Dont know what comp ratio its going to end up with or how but i would like around 8.5. Will be whacking 2bar from a t04z.

R34GTFOUR Mentor

R34GTFOUR

Posted
  • Author
Posted
mentioned to my workshop from the very get go that the NEO head have smaller cc. But they still went with the cp pistions for a r33 rb25. Dont know what comp ratio its going to end up with or how but i would like around 8.5. Will be whacking 2bar from a t04z.

hey mate,

it all depends on wat height they decked your block, your headgasket size etc but regardless i dont think you'll get it down to 8.5:1 CR

The lowest i can see it going it 8.7:1 and thats with no decking wat so ever, running a big 1.6mm headgasket and std size pistons (86mm)

Worst case senario, if they zero decked the block, with a 1mm hg and 87mm pistons your CR would be as high as 9.85, which i wouldnt think the engine would like 2 bar boost that much

Spoolup Mentor

Spoolup

Posted
Posted

Decking your block only increases your compression ratio as you are reducing your piston to deck volume ( if you have one ) and or making your piston sit proud of the deck, hence making your gasket volume smaller.

My NEO head ( from memory ) had 56cc comb chambers.

Swiper the Fox Grand Master

Swiper the Fox

Posted
Posted
The ACL distributor here told me the ACL race pistons are made by Ross.

As we all know Ross make pretty sharp pistons so i think they look to be a good option.

they also have a thicker top ring than the CP's, so they form quite a good seal...reducing blow-by.

good piston for the $$

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