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Fastrotor

Piston Comp Ratio & Deck Clearance 26/30

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Need some clarification, 

I have an RB25 piston sitting in the RB30 block and the flat of the piston is 1.1mm below deck.

In the RB25 block the RB25 piston is sitting 0.3mm above deck..

So how much shall i deck the RB30 block, looks like 1.3mm will immitate a stock RB25 comp..

Edited by AngryRB

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Hmmmm. Have you watched any of the Motive DVD Platinum Tech videos about RB blocks? Such as, https://www.youtube.com/watch?v=KF1iTh-2u3E

I would be MOST hesitant to slash 1.3mm off the top of an RB block, unless I had had it tested and everyone involved was confident that there was going to be enough meat left.

The alternative is custom pistons.

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I wanna use the RB25 pistons, in a few threads ive read its been suggested as a good option, i just need to know how much to shave off to get 9.0:1 or better... 

I thought they were testing cylinder wall thickness not deck thickness..

Edited by AngryRB

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How much power are you aiming for? Rips sells short blocks with stock RB30 internals and says they are good for 450KW and 7000 rpm...or stock crank, forged rods and pistons for up to 900KW and 9000 rpm.

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47 minutes ago, KiwiRS4T said:

How much power are you aiming for? Rips sells short blocks with stock RB30 internals and says they are good for 450KW and 7000 rpm...or stock crank, forged rods and pistons for up to 900KW and 9000 rpm.

I'm aiming for around 250rwkw so its not going to be pushed that hard, possibly 300kw later if i build the auto.. ( car is a stagea)

Edited by AngryRB

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any reason you dont want to use the stock 30 pistons?

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4 hours ago, AngryRB said:

I'm aiming for around 250rwkw so its not going to be pushed that hard, possibly 300kw later if i build the auto.. ( car is a stagea)

The RB30 in my C34 had stock internals stock R33 head and a GT3540 and made 320AWKW with room for a lot more development. You can easily make 250AWKW with stock RB30 internals (just rings and bearings and rod bolts) if there is no actual damage.

NB if its well worn you should fit oil restrictors and drill and tap (and plug) a couple of half inch holes high up in the sump in case you need them later for oil control if you are going to be tracking it.

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7 hours ago, azzada1 said:

any reason you dont want to use the stock 30 pistons?

Because they drop comp down to about 8.2:1, i have access to E85 also

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Id take the lower comp over machining the deck, In a budget build (Stock pistons/rods, No disrespect ment by this) I wouldnt have though the comp wouldnt matter all that much, block integrity on the other hand.

 

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19 hours ago, AngryRB said:

I wanna use the RB25 pistons, in a few threads ive read its been suggested as a good option, i just need to know how much to shave off to get 9.0:1 or better... 

Mark you'd probably want to speak directly with the guys who suggested RB25 pistons and see how they went, what problems if any occurred.  And talk with engine builders who do this stuff with any regularity.  See what they say.

Can confirm RB30E pistons are a no brainer, cheap, and do a great job.  Mine ran over 8 seasons, track only without problems.  Actual CR I achieved was 8.38:1, measured by cc, not calculated.  It had plenty of off idle torque.

If you're thinking E85, we did back to back dyno test and from 98 to E85 went +75hp at the same boost.  So even if at 8.2 - 8.4 CR range you will achieve gains.

 

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Here's my torque and power with stock block and GT3540 (you could probably use a smaller turbo for your auto):

 

2013.11.21 Power & Torque 001.jpg

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I was measuring the difference between the RB30 and RB25 pistons.  The cut outs in the 30piston are worth 5cc and the 25piston sitting 1.1mm low in the bore is also about 5cc.

The 25 piston has alot of meat on the dome part so i dont know why it would be lower comp than the RB30 piston.. i measured both using syringe..

I wont decking the block anymore because the water gallery is too thin on the top.

I already have good RB25 pistons with good ring end gap so its cheap to do, 

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20190605_165325.jpg

20190605_165425.jpg

20190605_165822.jpg

20190605_165827.jpg

Even though an RB30 piston will do the job fine, the RB25 piston sitting down in the bore doesnt lose much, infact i reckon it will gain , could be 5cc extra in the dome at a guess.

If a neo head and stock RB30 gives in the nines (9.3-9.5)  then that 14cc smaller head bumps it up 1 point, so 5cc is say 0.3-0.4, so it could end up being 8.8ish.. 

The RB25 piston looks nicer , it has a thin top comp ring 1.1ish compared to 1.5 on the RB30 but they both make good power.

Doesnt matter really but it saves buying new rings as the RB25 pistons and rings have very little wear so seems a good choice for this cheap slapper build.

 

Edited by AngryRB

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Keep us posted mate! if you can get away without decking the block and still manage a decent comp ratio thats fantastic, the fact that you have measured it all your self and are happy with your conclusion is great.. 

 

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I have results, comp test shows 197, 195, 200, 185, 205, 200.

All timing marks line up and no decking or shaving and i used old 25 pistons and rings.  Its better comp than i got on the RB3025neo but that has poncams.

 

 

 

 

Edited by AngryRB

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Thats awesome mate! good to know. Ive now got mine running with the rb30E pistons so i should do a comp test just to compare.

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