R33 Rb30 Conversion

[GTR-Ben]

Ok my compression testing is all done with the Rb26 head on the stock RB30 block.

Came up around 7.5 : 1 which is far to low obviously.

I am going to try and put some RB26 pistons or maybe RB25, and see what that gives me. Anyone that actually knows??

Other wise I have to take 60thou off the head.. which I don't really want to do.

Ben

With the GTR head though you get a CR of around 7.5 : 1,

A 2.5 head will give you the 8.3:1

So I need to know figures using the GTR head, which is quite differen't.

I would love to go and buy a set of forgies.. but now I am pushing close to my budget on this whole project car, and another $2000 or so would throw it over

Really need someone who has used the GTR head and has some numbers..

[Cubes]

Ben,

Have you thought about using a set of RB25DE pistons, then fiddle with the head gasket to suit the comp you want?!?

I used a Cometic metal head gasket that has several layers.

It also has a O-ring type setup around the bore.

Apparently they use them on a lot of the Group-A's, Nascars & Hi-comp diesels.

Mine set me back $150 from the US.

Basically the same as one of those HKS type headgaskets.

Steve is still running the stocker pistons in his 25t with the CR dropped to around 8.6:1. He's running around 1.3bar (20psi) with no problems YET.

[Steve]

Joel, I actually run around 1.6bar, it has spiked to 1.9bar though.

and what is this YET stuff? everyone seems to think my car will blow up - except the Jap guys that I looked to for guidance, they seem to think it will be no probs. Standard RB bottom end is good for 600bhp according to them, with everything else sorted, and I reckon they should know:p

[Cubes]

hehe Sorry Steve.

I'm sure it will be fine. The tuning & CR is sorted thats the main thing.

I had to throw in the YET as I'm a sceptical person. Hell I'm even worried about pushing over 300rwkw on the stock RB30 rods.

I've heard of them turning in to banana's at around 380rwkw. But thats only hearsay.

[GTR-Ben]

Sounds interesting Joel. Can you tell me more about the head gasket etc??

I am going to take in some GTR pistons and try and find some RB25 pistons to see how these change the CR and see if it helps me. If I get a CR of around 8.3 - 9.0 then I will be happy with it..

Steve you are using a RB25 head correct?

It is really fiddly work with the CR.. I can't just put any head gasket on to fix the CR.. there is only so much you can do.

And as for the forgies costing $2000. By the time I buy them, ship them and install them etc. it adds up.

Seems like no one actually knows what I can do with a GTR head.. will find out soon enough I suppose.

[GiJOr33]

well you can buy the pistons locally from many performance places, and you'll have to get the block rehoned and pistons fitted regardless of whether you use stock ones or forgies.

But from the 3 posts you've made i understand you don't want to go down the forged pistons road so i'll stop harping on the subject... just though i'd bring up that as an option.

[Sydneykid]

Hi GTR Ben, I have read the last few pages of this thread a couple of times and they make no sense. It must be me, maybe a few facts might help;

There is no difference in the combustion chamber size between an RB26 head and an RB25 head. They are both between 62 cc's and 64 cc's depending on casting differences.

RB30E and RB30ET (SOHC 2 valve cylinder) heads have around 56 cc's to 58 cc's combustion chambers.

If you use RB30E pistons with an RB25/26 head you end up at 8.3 to 1. This was posted a number of times in the 42 pages. If you use RB30ET pistons you end up at 7.5 to 1, this was also posted.

RB25/26 pistons have the gudgeon pins located around 2 mm lower than RB30 pistons. So if you use RB25DET or RB26DETT pistons in an RB30 with standard RB30 conrods, you end up with the pistons 2 mm further down the bore than RB30 pistons.

The answer is to take 1 mm off the block and 1 mm off the head and if you use RB25DET pistons you end up at 8.3 to 1. Same as if you used RB30E pistons. But the RB25 pistons certainly look tougher, so I would prefer to use them.

All of these dimensions are approximate, every engine is unique and you should do precise actual measurements of you own personal components.

Bottom line, I personally would not recommend taking 2 mm off the head alone, you would end up with too little metal at the water jacket to head gasket face. Plus it would remove too much squish zone which is a very effective feature of all RB DOHC, 4 valve cylinder heads.

Hope that clarifies.

[GTR-Ben]

Thanks for the info.. I don't really know what is going on but I have had numerous engine rebuilt and work done from this shop and I am 100% they know what they are doing..

Unless my GTR head is aftermarket?? as it is ported and had aftermarket valve guides etc, and the head is definetly 67cc's.. you can see a larger lip between the contact point with the block and the part where the valve seat.. about 2mm more than a RB25DE head??

These guys have gone over all the squish zones etc with me, (which I don't 100% understand) and they have set the engine up to be good in all these areas..

I understand you know what you are talking about Sydneykid... and I was sure that the RB26 head had about 63cc..

Only conclusion is that it is an aftermarket head..

It has already had the head shaved and setup. Compression ratio is now 8.45 : 1 with a stock RB30E gasket.

I think it was slightly more than 7.5 : 1 to start with as I didn't give them a gasket and they just estimated the amount.

Well it's all done now.. just have to put it back togethor. Got my GT3040 turbo and 42mm Turbonetics wastegate on its way.

And the Silvia is almost ready to drop it all in.

Thanks for all your help guys, wait for photos and vids

[Sydneykid]

Hi GTR-Ben, in an R25/26 cylinder head there is plenty of metal to increase the size of the combustion chamber. This is actually technically a better solution than using a thick headgasket. You may find that your head has been worked on to lower the compression ratio of the previous engine it was fitted to.

By going to 67 cc's for the combustion chamber, this would lower a standard 2.6 litre RB26 to 8.2 to 1. Or it would give a 2.7 litre an 8.5 to compression ratio. These are all common numbers for medium tune from Jap workshops.

I have 4 X RB26 cylinder heads in the flow bench room right now and the largest is 64.4 cc's. We have probably done 20 or so to date and none are over 65 cc's standard.

Hope that clarifies further

[GTR-Ben]

Yep not sure exactly what was going on but now it is all back to 8.45:1

Thanks for your help mate.

[dondesoto]

ive just finished putting on a rb25de n/a head on an rb30e block .

i used the rb25de n/a pistons on the rb30e conrods

pistons sat approx 1 mm lower in the bore

rb25de n/a pistons have a much larger dome than the rb30e pistons .

by my calculations ( no not guessing ) the compression works out at 10.5 - 1

[Cubes]

The bloke who built up mine & Darrens RB30 said after he's finished the 400chev sprintex supercharged motor he's building, he's going to have a go at a RB30DE(S) using a sprintex supercharger. He's asked around and researched in to it a bit but no one is really telling him of any results. He's dropping the motor in to a Commodore, VL I assume.

If he goes through with it i'll post up results and how he goes with it. Should be rather interesting. I also assume it will be up around the 400-500hp mark as that is what his current 400chev ute is making.

Whoohoo, Get to pick up the motor up tomorrow.

Its all bubble wrapped strapped on a pallet. Problem is I havn't got a fork to pull it off with. I can see my self stressing out tomorrow.

I can't remember if I posted or not. I ended up using a Series I RB25DET oil pump, Andrew checked the clearances etc. & lots of lock tight on the bolts.

Its been a long 44 page & 10 month journey since I was asking if the RB25 & rb30 pistons were the same dia.

:headspin: :headspin:

[Cubes]

Just to scare a few of you off.

I did the sums of how much it has cost me.

All up it came to around $6500-$6600.

Thats including extra's like head reco port/polish, head purchase, clutch, getting the head fitted, cam belt, oil pump, waterpump, flywheel, harmonic balancer etc.

Now add the turbo, injectors & ecu on top of that. Then the gearbox will die then my diff will definately be open wheeling.. lol

Oh well another 2 years of Uni left to throw money at it.

EDIT: I forgot the VRS kit.. thats an extra $275. Make it $6775-$6875. ouch.

Gary, I remember you saying a long long time ago. Performance isn't cheap.

Would have wasted that money through out the year anyway, at least its gone to something. Thats the way I look at it.

[Cubes]

It would have worked out basically the same for me to build up a R32 RB25DE engine.

If you are re-building an engine it is always going to cost around the 5k mark for some thing decent with forgy's etc.

My origional plan was to freshen up a RB30 bottom end (1100) then slap the $750 RB25DE head straight on top. Then I had the idea.. ahh I'll spend another 1k on the head to reco port/polish it etc. Then I had another idea.. ahh I'll spend another $1300 or so on forgy's... and another I really should get it all crack tested, blueprinted and well yer..

[Sydneykid]

HiGuys, I don't think DOHC, 4 valve engines go together very well with superchargers. They have such a wide RPM band that it is very difficult to match the supercharger sizing. You either end up with a torque monster, with nothing over 5,500 rpm. Or a down low unresponsive slug, that leaps into action for 1,500 rpm or so at the very top of the rpm range.

Being positive displacement and permanently running at crankshaft rpm X gearing, they make sizing a turbo charger seem like child’s play. Fantastic on a big block Chevy (we have a 1,500 bhp one) but anything over 6,000 rpm is flat line time.

I won't be putting one on my RB engine anytime soon.

[Cubes]

Gary,

How do the R32RB25DE cams work with cam gears?

Does it help peak power to be made higher in the rpm range?

ok I'll reword it.. Would u use a set of Adj. cam gears on a RB25DE RB30DET which will be running a HKS3037 to crack the 300rwkw?

[Sydneykid]

Hi Joel, yes.

[GiJOr33]

so you get rid of VCT after around 280rwkw and go with adjustable cam gears?

[Cubes]

Gary,

I've done a bit of research and calculations in to this Squish Band.

Well ... Sh*t.

If only I knew what I know now when I selected pistons/comp ratios.

(Piston sitting 2.5mm down the bore with a 1.14mm (.045") head gasket, 14cc dome & 62.2cc Head = 8.2:1)

That’s providing my calcs are correct which I am now starting to have a pretty good understanding of all the calcs. I am 100% sure its right.

The apparent perfect squish band tolerance is .76mm (.030") – 1.52mm (.060") (deck height + head gasket thickness). It isn't that simple though.

You also should check the maximum squish velocity (MVS) to ensure it isn't extreme (its not linear either) and well there is more but it is damn complicated.

I think sticking to the Nissan .053" (my calcs can any 1 check for sure?) squish spec should be about right as they have obviously done the calcs with regards to the size of the squish band.

So yer as you said previously.

Flat top pistons with a nice deck height and head gasket to suit is best.

Basically exactly the same as the way it rolled out from the factory.

BUT..

These apparent negatives.

I've searched hi and low but haven’t actually found any hard proof how much it actually does affect an engine, its mainly hearsay.

Economy? Emissions? Performance? Response?

Have two exact same spec RB30DET's, same boost comp ratio etc would it be noticible by the seat of the pants feel? One 3.5mm Squish one 1.35mm squish. hehe Sorry for the flicking between imperial/metric.

Surely some one has done a test in the US of A with their Hemi's.

One thing good that has came out of all this is I am feeling content that I have learnt a lot.

[slow_r31]

so more heresay joel, but i thought squish zone effected how prone an engine was to detonation. hence why running decomp plates is abit of a catch 22, in that you decrease compression, but change the squish zones.

this is only my ignorant heresay however