I was reading an artical the other day and Arias pistons now makes a piston to suit the RB30 block to RB25 Head conversion, this is good news in regards to compression ratio problems with this swap, if your interested go to http://www.rocket.au.com for info, they are the distributors. Carcrazy.

Wiseco has had these pistons on the market for over 6 months now. I thought everybody knew that. Oh well. 8)

If you buy the whole RB25 engine and use the pistons out of that motor on the RB30 conrods then there's the compression problem solved. Me,RB30-ZED and a few others around the place have fitted them to maintain a high compression ratio at 10.5:1,that ratio is not a problem to me,would probably be to someone who wants to use the crappy standard fuel but,even Shell Optimax may be marginal in my application as the R33 computer is programmed to run a far higher ignition advance than the cars here in Australia,I have heard of a 110 octane rating used over in Japan.

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:

What about running VL turbo pistons in the conversion if going for high boost & big turbo,etc? I suppose the only problem is running too lower compression as it will be at 7:1 thereabouts. Another option I suppose would be to use the N/A RB30 pistons which work out to be 8.3:1,but I am told that anything over 15 PSI cannot be run with those pistons.

Yeah VL turbo pistons make the compression ratio too low. It would be ok when on boost but very doughy off boost. Some people use the VL NA pistons and machine part of the crown off the piston to bring the compression ratio back to around 8:1. Has anyone here used Special Piston Services forgies in an engine???. I was quite impressed with the price and service, just after peoples opinions on them. Cheers

I rang Allan Engineering in Mallala a while back... Can get Wiseco forgies machined to whatever comp ratio u like, they are a RB26 piston with a blank top so they machine it down to suit! $1600 with rings and gudgons. I would have them already but i keep finding other stuff to spend all my $$$ on so can never afford them! 1 day! BTW, this is a great thread!

1500 to 1800 is a good price for forgies, depending on what you get for it. Friend recently got some rb25 pistons, standard forgies for 1500, might be SPS ones im not sure. Truly an art work, i think that was wtih rings and gudgons, but they were albit incomplete.... needed to cross drill the gudgion pin housing for an oil squirter, but oitherwise great.

I have sps forgies, ringlands were machined 3 thou too wide, my engine builder wanted to throw them in the bin. :cry: I would have ordered CP's from the US if I had have known. 10:1 is sweet for aftermarket ecu and race type use, all the Honda turbo boys are 10:1 sometimes higher and with big boost.

rb30 N/A pistons with RB25 Head = 8.3:1 compression ratio by memory, not 100% thou, i would of thought that to be a perfect balance for off-boost drivability

Quote: Originally Posted by HARRIS rb30 N/A pistons with RB25 Head = 8.3:1 compression ratio by memory, not 100% thou, i would of thought that to be a perfect balance for off-boost drivability Yep, it's 8.3:1 according to an article in a recent Zoom special.

8.3 would be ideal wouldnt it? I have been driving around a standard rb25det (comp ratio is 9 i believe) and i managed 18psi with good cooler and constant fine tuning to keep it on the boil and had no problems (except the occasional pinging to let me know i had gone too close to the edge!) I am considering the RB30DET motor soonish so i guess i might go that way...forgies of course.

Arias are expensive and hard to get. Go Wisecos

Uhm...if you look at the standard vlt pistons you will see that they are multivalve pistons....

There are 4 Valve reliefs in the RB30E pistons due to the valve arrangement in the single cam head. ie, cyc 1,2 and 3 inlet valves are on the front rh side and exhaust valves on the rear lh side. Then they swap fro cyc 4, 5 and 6. So instead of making 2 different pistons they made 1 with 4 reliefs! Just saying they are not a multivalve piston but they are....... It just so happends that the reliefs suit the twin cammer perfectly, no piston/valve contact possible! Atleast with a RB25DE VVT head anyway...

i think HSVSUX is onto something higher compression + lots of boost + good tunning = lots and lots of power but this is very hard to accomplish and needs to be very fine tuned and a strong motor but can be done

Just wanted to clear something up. I did some chamber measuring and came up with a figure of 8.06:1 using the twincam head on a RB30E block, not 8.3:1... The piston measured to be 1cc dished, turbo pistons 11cc dished. For the compression ratio i want, 8.5:1, i need a 3cc domed piston! I guess we shouldn't take whats in a mag as gospel! But then there can be alot of variation that throw the figure off too.

How do you measure the compression ratio anyway? Depends on the turbo but I was considering a smallish turbo 5 - 10psi with about 9:1 ratio?

Quote: Originally Posted by B15HOP How do you measure the compression ratio anyway? Depends on the turbo but I was considering a smallish turbo 5 - 10psi with about 9:1 ratio? U measure the volume of the combustion chamber in the head, and cyclinder with the piston down a little, by sealing around the piston/valves ect with grease, then place a piece of glass over the opening and fill with oil using a syringe to measure the volume! Measurements i got were 63cc for twin cam head 55cc for single cam head 21.3cc for 86mm bore, piston 3.5mm from TDC, cyc vol, and head gasket 87mm bore, 1mm thick. With these figures i got 20.33cc displacment of piston from 3.5mm down to TDC with measured at 21.3cc the piston is 0.97cc dished. Head gasket makes up 5.94cc. So add the head volume, 63cc with the piston @TDC volume and gasket volume together to get a total combustion chamber of 69.91cc. Then the compression ratio is the cyclinder volume at BDC compared to volume at TDC, BDCcc/TDCcc. BDC volume is for 86mm bore and 85mm stroke is 493.7cc piston displacment + CC of 69.91cc = 563.61cc. and 563.61cc / 69.91cc = 8.06 8.06:1 comp ratio. If you do all the same calculations with a 55cc head volume (SOHC head) you get 8.98:1, which verifies the results/measurements!

Hey I found the equation: ((boost psi / 14.7) + 1) x motor compression = effective compression That would explain why a BMW is soo @#$kn powerfull while still being n/a. The M3 has something like 12:1 compression ratio which equivelent of running 6psi of boost with a 9:1 compression ratio. This is why before it made me wonder why the BMW had such high compretion ratio and produced soo much power only n/a. The M3 with 3.3L vvti and high compression produces almost 250kW. Racing cars have an "effective compression ratio" of 22:1, thats a crap load. Due to laws of compression though that kind of ratio would kinda fry your standard engine. Cooling becomes a problem and day to day driving becomes unreliable. Forged pistons are a must in these situations because of heat and pressure differences. Changing to dohc should be worth it in the long run due to better cooling the head provides as well as the increased flow at higher rpm. With forced induction like turbo or supercharger and 15 psi, your effective compression would be more like 18:1 which is over double the ammount of air traveling through the engine.