Pistons for RB25 / 30 head conversions, you can now buy them

[twinvl]

An r/c cars engine is slightly different to a car engine... Generly 2 stroke and compression ignition arn't they, or do they have 4 strokes and spark plugs these days??

But yeah your right, higher comp is better simply from a drivability POV, but low comp has it's advantage. u can make alot more power out of a low comp engine for the simple fact u can put ALOT more boost into it.
And it's more to do with better combustion chamber design as to why the comp ratio is going up and up with newer engines.

[Krieger]

yeh, true about the better chamber design. but i still disagree about more boost is better, depends on your compressor efficiency though. higher boost doesnt always net you bigger power, and it lessens reliability.

RB's dont have a prob at all handling high cylinder pressures, the low compression thinking stems from the old days when engines had crap head clamping and cylinder sealing.

btw, rc cars are four strokes nowdays, still use glow plugs though.

-jimmy

[B15HOP]

So to get a high compression of 9:1CR you would need dome pistons instead of dish pistons. With a negative displacement of 7cc each... Wouldn't it be easier to shave the block? And what is the volume when using n/a RB25 pistons?

[blownvn]

Didn't bother to read the whole thread so I appoligize if this was mentioned before but ACL is about the make low price forgies for the VL turbo, thought you guys might be interested.

[LukeVL]

Just CC'd my motor. Using stock, original bottom end form an NA and a VVT RB25 head, it came out to 8.8:1. Bummer. Now I have to buy forgies.
Be careful what you do with your motors, cause I believed what everyone else said and the comp. ratio was too high for a turbo motor. It just won't have enough timing in it to be a decent motor.

[twinvl]

Hey luke, what figures did u get when u CC'd it??? I did the same and got 8.06:1 not 8.8?

[roamin]

hi there i have a friend who works at sps pistons and there are a few ppl on this site which are personal friends who use them. one which ran a 10.5 at 125 mph last weekend at calder.


HSVSUX: i beleive your engine builder didnt tell sps exactly what he required obviously as your the only person i have seen with bad reports about them. oh and for wanting to throw them in the bin, thats silly ill have em

[LukeVL]

Cylinder head with spark plug - 61cc
1cc dome on the NA pistons

I got around 8.5 with this method, but when I torqued the head down and cc'd it as a unit, it came out to 8.8:1.

[LukeVL]

My final figure is 8.48:1. Sorry about the confusion. Thats with an Rb25 head (VVT if it makes any difference), nissan gasket and NA psitons.

[B15HOP]

RB30 pistons I guess?

[LukeVL]

yes mate, na RB30 stockers

[Jaws]

in the skylines australia forums several ppl have done the conversion using the rb25de head and rb25det head.

there has been alot of discussion into the finer details in the thread.

http://www.skylinesaustralia.com/for...ad.php?t=15420

[brett_32i]

is the rb20 head the same as rb25.
'cause you can pick up 20's cheap as.

[twinvl]

No they are very different, rb20's have smaller bores.

[brett_32i]

and bore is about 10mm smaller. back to the rb25.....

[LukeVL]

What? RB25,26,30 all have 86mm bore. Rb20s would be about 78mm or so.

[brett_32i]

so i've been told......rb20's have smaller bore than rb25, 26, 30.

[R32-Joel]

One important factor to consider when building up a performance motor is to setup the squish properly..

You want around 40thou.

[1RB.30T]

Originally Posted by 87calais

yeah but while 10.5:1 is ok for an N/A engine , its way too high for a turbo application. Special Piston Services in Melbourne (i think) have pistons for RB30/25 head conversions or they will make a custom piston for you. I am using the RB26 head and i just need to supply them with the cc of the combustion chamber and the compression ratio i want to use and they will make a set of forgies for me. Most quotes i got for forgies were around $1800. SPS sell 6cyl piston sets, custom made for $1500. cant beat that for value for money. :mrgreen:

how are u mate i thinking about putting a rb26 head on my rb30 block is it match modifactions to fit it i heard it goes on with no probs thanks

[FLWLSS]

non trade price for Forged ARIAS pistons to suite RB26/30 in .020” overbore with 8.5:1.
are $1320 with rings.

I will be using the Arias pistons but will be aiming for around 9:1 comp.

So machining will required.

PS: Arias are $1200 from modyourcar

[pigsy]

abit late but quantum racing sell cp forgies for around $1300with gudgens
but just remember when your rebuilding you can raise the comp quicker buy skimming the deck

[R32-Joel]

9:1 comp is fine with big boost (1.2+bar) providing the combustion chamber has been polished up nicely and you are running a nice set of cams 260degree+.

[nixcars]

Just about to build one using CP's, but u have to buy the rings sepearte, couple hundred is about the going price. These give between 8.3 & 8.5:1 depending how far u deck the block & head. I've also been told by my machineist that CP's are slightly better than Arias.

[R32-Joel]

What deck clearance are you looking at?

Having built an engine once I would always set the quench/squish gap to around 40-50thou. Hence have a zero deck clearance and use a 40thou headgasket to give you your 40thou clearance.

If you go too large you will loose power, economy and the benifits regarding the 'free' octane boost of a well setup quench/squish.