Jump to content
SAU Community

Recommended Posts

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 :P )

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.

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.?

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.

  • Like 1

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 by AngryRB

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.

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.

  • 3 weeks later...

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?

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.

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.

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 by AngryRB

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.

  • Like 1

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.

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 by AngryRB

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).

  • 2 weeks later...

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.

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 by AngryRB

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Similar Content

  • Latest Posts

    • I know why it happened and I’m embarrassed to say but I was testing the polarity of one of the led bulb to see which side was positive with a 12v battery and that’s when it decided to fry hoping I didn’t damage anything else
    • I came here to note that is a zener diode too base on the info there. Based on that, I'd also be suspicious that replacing it, and it's likely to do the same. A lot of use cases will see it used as either voltage protection, or to create a cheap but relatively stable fixed voltage supply. That would mean it has seen more voltage than it should, and has gone into voltage melt down. If there is something else in the circuit dumping out higher than it should voltages, that needs to be found too. It's quite likely they're trying to use the Zener to limit the voltage that is hitting through to the transistor beside it, so what ever goes to the zener is likely a signal, and they're using the transistor in that circuit to amplify it. Especially as it seems they've also got a capacitor across the zener. Looks like there is meant to be something "noisy" to that zener, and what ever it was, had a melt down. Looking at that picture, it also looks like there's some solder joints that really need redoing, and it might be worth having the whole board properly inspected.  Unfortunately, without being able to stick a multimeter on it, and start tracing it all out, I'm pretty much at a loss now to help. I don't even believe I have a climate control board from an R33 around here to pull apart and see if any of the circuit appears similar to give some ideas.
    • Nah - but you won't find anything on dismantling the seats in any such thing anyway.
    • Could be. Could also be that they sit around broken more. To be fair, you almost never see one driving around. I see more R chassis GTRs than the Renault ones.
    • Yeah. Nah. This is why I said My bold for my double emphasis. We're not talking about cars tuned to the edge of det here. We're talking about normal cars. Flame propagation speed and the amount of energy required to ignite the fuel are not significant factors when running at 1500-4000 rpm, and medium to light loads, like nearly every car on the road (except twin cab utes which are driven at 6k and 100% load all the time). There is no shortage of ignition energy available in any petrol engine. If there was, we'd all be in deep shit. The calorific value, on a volume basis, is significantly different, between 98 and 91, and that turns up immediately in consumption numbers. You can see the signal easily if you control for the other variables well enough, and/or collect enough stats. As to not seeing any benefit - we had a couple of EF and EL Falcons in the company fleet back in the late 90s and early 2000s. The EEC IV ECU in those things was particularly good at adding in timing as soon as knock headroom improved, which typically came from putting in some 95 or 98. The responsiveness and power improved noticeably, and the fuel consumption dropped considerably, just from going to 95. Less delta from there to 98 - almost not noticeable, compared to the big differences seen between 91 and 95. Way back in the day, when supermarkets first started selling fuel from their own stations, I did thousands of km in FNQ in a small Toyota. I can't remember if it was a Starlet or an early Yaris. Anyway - the supermarket servos were bringing in cheap fuel from Indonesia, and the other servos were still using locally refined gear. The fuel consumption was typically at least 5%, often as much as 8% worse on the Indo shit, presumably because they had a lot more oxygenated component in the brew, and were probably barely meeting the octane spec. Around the same time or maybe a bit later (like 25 years ago), I could tell the difference between Shell 98 and BP 98, and typically preferred to only use Shell then because the Skyline ran so much better on it. Years later I found the realtionship between them had swapped, as a consequence of yet more refinery closures. So I've only used BP 98 since. Although, I must say that I could not fault the odd tank of United 98 that I've run. It's probably the same stuff. It is also very important to remember that these findings are often dependent on region. With most of the refineries in Oz now dead, there's less variability in local stuff, and he majority of our fuels are not even refined here any more anyway. It probably depends more on which SE Asian refinery is currently cheapest to operate.
×
×
  • Create New...