Getting A 9:1 Or As Close To Comp Ratio By Head Shave

[menthol369]

Hey guys, in the middle of building a 25/30

what i want is a completely stock bottom end, so no higher comp pistons or deck shaving etc. and get the preferred 9:1 CR by shaving the head.

so basically an rb30 achieves the 9:1 ( excluding block dimensions) through a combustion chamber of 55cc. the rb25 has a 63 cc combustion chamber.

has anyone gone to the effort of working out how much the head needs to be shaved to reduce the combustion chamber of the rb25 to 55 cc?

or know the method in which you would work this out? i assume youd need an engine shop that has the tools for this option though. ( i somehow dont think squirting 55cc out of a syringe and measuring the gap to the head surface will be accurate enough )

[The Mafia]

don't even try. You want that compression you are going to need new pistons.

With 20thou from the block and head I managed 8.45:1 (Up from 8.3).

If you grind the head back too much, you are going to ruin the shape of the combustion chamber, and there will be very little meat left for the exhaust studs.

[Ben C34]

Unless you use a neo head. They have almost the same cc as rb30 head.

But I suppose you already have head.

[menthol369]

ahh yeah i didnt factor on the shape of the combustion chamber.

is that 20thou = x amount off head + x amount off block
or 20 thou from block and 20 thou from head.

is that all stock pistons etc? also what HG did you run.?

[GTRPSI]

Dont forget to mock up your build with a head gasket and some play dough on the pistons and check your valve to piston clearance before you start to deck the head, especially if your running a big lift cam and oversized valves.

[AngryRB]

I'm interested in this too, was planning to take 2mm off the RB25DET head I have and bolt it down with a stock head gasket to get

close to a 9.5ish to 1 comp.

Does it really ruin the chamber shaving a couple of mm off or would it hardly be noticeable as spending $1300 on pistons is

expensive.. I prefer the stock pistons with a 2thou clearance for quietness.

Does anyone know how much clearance there is generally in this situation ?

Edited February 26, 2014 by AngryRB

[menthol369]

chamber design plays a huge part in the way the combustion process burns, hence why hemis are actually pretty rubbish

[GTRPSI]

Depends on what fuel and boost your running, if your winding 50 and + Psi into it, you may want to have a hemi design, lots of 26 heads have the squish pads removed on big power builds for a reason.

[menthol369]

yeah true^

im just going to bite the bullet and run the 8.3:1 run as thin HG as possible and give it a shave on the head.

[Ben C34]

I'm interested in this too, was planning to take 2mm off the RB25DET head I have and bolt it down with a stock head gasket to get

close to a 9.5ish to 1 comp.

Does it really ruin the chamber shaving a couple of mm off or would it hardly be noticeable as spending $1300 on pistons is

expensive.. I prefer the stock pistons with a 2thou clearance for quietness.

Does anyone know how much clearance there is generally in this situation ?

Seems like I'm pushing the neo head on everyone, but you can get them for around 600, you will get the comp yiu want and due to solid lifters and differnt designt vct less oil has to go into the head.

[AngryRB]

I have roughly calculated that the head would need 2mm shaved off to bring it up to 9.5ish to 1, less with decking the block and a thin head gasket.

I would go the neo head but I have newish tomei 256 cams in my head and it seems to be in great condition, would a neo head be comparable to mine seeing as I

already have the tomei's?

on a side note, I recently asked an engine rebuilder about a stock rebuild and was told the cast pistons are not available for a rb25/30 build and I should go forged, is there any truth to that or were they trying to make more money from me?

[KiwiRS4T]

I have roughly calculated that the head would need 2mm shaved off to bring it up to 9.5ish to 1, less with decking the block and a thin head gasket.

I would go the neo head but I have newish tomei 256 cams in my head and it seems to be in great condition, would a neo head be comparable to mine seeing as I

already have the tomei's?

on a side note, I recently asked an engine rebuilder about a stock rebuild and was told the cast pistons are not available for a rb25/30 build and I should go forged, is there any truth to that or were they trying to make more money from me?

There seems to be plenty of non- forged pistons around e.g. here's a set but you probably want non turbo for the right CR:

http://www.ebay.com.au/itm/HYPATEC-PISTON-SET-OF-6-HOLDEN-COMMODORE-VL-RB30-TURBO-RB30ET-/180869720350

Personally if I were going to rebore I would get forged - rods and crank should be OK.

[Dale FZ1]

There's a stack of options on the market for RB30E cast pistons, stock bore and oversize. Look at ACL or Precision for starters.

There is nothing inherently wrong with quality cast pistons, but they won't tolerate knock. There are a lot of 30DET builds out there reliably making pretty stout numbers on good tunes ie. sensible AFR and ignition maps.

Opinions abound on what is the best target static CR. Safe bet is the 8.3 - 9.0 range, with a number of brands supplying the market for forged pistons. Higher end of that scale will give a little more off boost efficiency but tolerate less overall ignition advance under load.

If running it exclusively on petrol and regular track, go for the lower comp. If you're going E85 and/or dedicated road car, go for the higher comp.

[AngryRB]

is there a certain pressure/torque that will break the rings/ring landings on a stock piston or does the piston fatigue over time?

just wondering because my rb25 dropped cyl 5 and wasn't knocking at all, I am thinking it was a sudden boost spike to 25psi from the usual 20psi, so in the case of

new pistons/rings in an rb30 can I expect a ring landing to break or should it be fine at the same power level 320-340kw (gt3076).

KiwiRS4T- only problem with forged is the cold start slap, apart from that id not think twice.

Edited March 15, 2014 by AngryRB

[KiwiRS4T]

Broke ring lands on my Rb25DET as well. Currently running old pistons with new rings and bearings (and conrod bolts) on my RB30 seems plenty of people manage 340kw with stock pistons. Still I would go for forged pistons if you can afford them - how is "cold start slap " a problem? Put it together right, fully balanced and don't thrash it when it is cold.

[Ben C34]

I have roughly calculated that the head would need 2mm shaved off to bring it up to 9.5ish to 1, less with decking the block and a thin head gasket.

I would go the neo head but I have newish tomei 256 cams in my head and it seems to be in great condition, would a neo head be comparable to mine seeing as I

already have the tomei's?

on a side note, I recently asked an engine rebuilder about a stock rebuild and was told the cast pistons are not available for a rb25/30 build and I should go forged, is there any truth to that or were they trying to make more money from me?

What would that do to the piston to valve clearance? I would be very hesitant to take that much the head.

If you are going to bore a block, I think it is pretty silly to buy new cast pistons, i would go for forged.

If your engine needs more than a hone, you might aswell just get another engine that would get away with just a hone, and keep using the stock pistons. The engines are everywhere and are cheap as chips.

Or preferably find a good nick engine and just use it untouched.

[AngryRB]

ok so sounds like my easiest cheapest option is to sell my non neo head for a neo head and bolt it down on a decent stock 3L

or

aim for 350+ and get some forgies, seems overkill to get forgies for the low 300's, especially being a streeter.

how would a stock neo head compare to my non neo head with 256 tomei poncams for flow etc, will there be any difference/restriction upto the 340kw?

KiwiRS4T- Im probably just a bit OCD, I like to start the engine stone cold and hear it quiet apart from the injectors ticking, plus the forgies

mean a rebore which ill might be stuffed up by machine shop and leave me with mega piston slap,

superben- I don't know what the clearance would be, just planning stage at the moment

Edited March 16, 2014 by AngryRB

[KiwiRS4T]

If you are going for a Neo head look for a DET.

Neo heads have 236deg inlet and 232 exhaust but the DE has 6.9mm lift and the DET 8.4mm

Neo head has solid lifters.

My engine with low compression and stock bottom end apart from rings bearings and conrod bolts has being going for about three years and makes 314AWKW limited to 7000 rev/min although you wouldn't lose anything on the street by limiting it to 6,500 (the extra 500 revs is sometimes handy for delaying gear changes on the track).

[Scando]

I have roughly calculated that the head would need 2mm shaved off to bring it up to 9.5ish to 1, less with decking the block and a thin head gasket.

I would go the neo head but I have newish tomei 256 cams in my head and it seems to be in great condition, would a neo head be comparable to mine seeing as I

already have the tomei's?

on a side note, I recently asked an engine rebuilder about a stock rebuild and was told the cast pistons are not available for a rb25/30 build and I should go forged, is there any truth to that or were they trying to make more money from me?

One option if you want to keep your current head, you can use standard RB25 pistons and deck the block 30 though to get them flush with the block (RB25 pistons have a 31.3mm pin to deck height vs 32mm for the RB30 pistons which is why you have to deck the block). This will give you around 9.5:1 with a RB25 non-neo or RB26 head.

[AngryRB]

One option if you want to keep your current head, you can use standard RB25 pistons and deck the block 30 though to get them flush with the block (RB25 pistons have a 31.3mm pin to deck height vs 32mm for the RB30 pistons which is why you have to deck the block). This will give you around 9.5:1 with a RB25 non-neo or RB26 head.

Sounds like it could work, but ive decided to keep things simple and get a stock rb30 and marry it to a neo head, its a long term project and failure will be more

likely if i start pulling things apart or trusting machine shops to do stuff right. Stocker RB30 and conservative boost should see happy days.

If there's anyone interested in a good non neo head with fresh tomei 256cams let me know.

Edited March 27, 2014 by AngryRB