wow this thread has been goign for two years now Some original photos have not been recovered yet.

So the bottom line is: Rb25t pistons will fit straight into a rb30 and run the same compression (with rb25det head) Rb30 na pistons will provide sufficient low compression for an rb25det head Rb30t pistons will make to lower compresion ratio. Is this correct?

RB25 pistons have a different pin height (the block needs to be decked) RB30E pistons give a C/R of 8.3 RB30ET pistons 7:1 or something way too low. Most people use N/A RB30E pistons or the forged oned to suit the conversion are usually around 8.5. Im going to use some high comp RB30E pistons and machine the tops so its around 9:1, as most new BB turbos these days dont need to boost more than 15psi.

Quote: wow this thread has been goign for two years now Yeah, and its the first time ive actually read it since i made it, man there is some interesting stuff said in here..... Some original photos have not been recovered yet.

Quote: Originally Posted by carcrazy Yeah, and its the first time ive actually read it since i made it, man there is some interesting stuff said in here..... Just out of curiousity does any one knnow maximum diameter for a rb25 piston and if any one knows off the top how much these wiseco pistons are worth and there biggest size?

hey im plannin a build with the 26 head on the 30 block and wondering wot pistons ud recommend for it. it should be seeing round 1.5bar tuned

CP's are definitely the most suited.

im buying a big dollar rb30 bottom end thats been built with rb30 forgies and are converting to a rb25 head. as all parts are new and block is already oringed, balanced and decked. is it goin to be a big fuss or do i have to change to rb30/25 forgies and is it possible to trade in or have to start gain

hi i have done the rb30-rb25det head conversion cr was 8.3:1 and instead of using a turbo i have used a procharger supercharger c2 rated at 725hp. max boost 24psi. intercooled at 15 psi using the LinkPlus G2 plugin ecu for vls i have tuned it with my computer it realy needs to go on the dyno couse its a litle rough still but it eats my mates rb26dett with out trying. i will be getting it on the dyno hopefully soon and will post the figures for you to have a look at should be interesting

The compresssion of those forgies may be a bit low maybe 8:1 or there abouts, all depending on what power output you want though, if you want redicuousboost youll need it but if you want stockish boost try opt for a 9:1 compression

9:1 is fine for up to ~18psi using the twin cam head and pump fuel providing you don't have excessive turbine back pressure. i.e trying to push 18psi through a highflowed rb20/25det turbo or something silly.

just wondering about doing this combo. at current i am running a standard rb30et with high mount t04e, fmic, 2 extra injectors, 19psi. If i convert over to the rb25det head the comp ratio will drop down to around 7.1:1, would this compression ratio still be to low if i up the boost to around 22psi. Not sure if it matters or not but the car is manual not auto.

Apparently CP now do a piston that achieves a 9:1 comp ratio. Some original photos have not been recovered yet.

Quote: Originally Posted by VL_30_LT If i convert over to the rb25det head the comp ratio will drop down to around 7.1:1, would this compression ratio still be to low if i up the boost to around 22psi. still want to use the original rb30et pistons, not all of us can afford cp pistons and all the rest the would go with doing a rebuild of the bottom end.

Im looking at purchasing a RB25DE head, and im going to also be performing a RB30E->RB30ET, So the RB25DE head can bolt straight onto the RB30 block without to much **** around? a big dirty RB30DET would chop! thanks for your help.

check out the 100+ page thread on sau they have all the info your after

Quote: Originally Posted by 650hp 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 Arais Rb30/25 pistons $1130 Click me (http://midnightmods.com.au/index.php?page=17)

CP All compression ratios are based on zero deck height 1.2mm head gasket (7.13cc) RB25 @ 62.5cc RB26 @ 64.5cc They assure me they have done alot of work to make sure the facts are correct. p/n= CP-RB30/26-.20 Flat top with no valve pockets 8.2:1 COMPRESSION RATIO with RB25 head and bit less with RB26 head 21mm pin 1.280" compression height Only stocked in 0.20 thou oversize but can be custom ordered in any size. p/n= CP-RB30/26-.20-9.0 6.0cc dome top, 9.0:1 COMPRESSION RATIO with RB25 head and bit less with RB26 head 21mm pin 1.280" compression height Only stocked in 0.20 thou oversize but can be custom ordered in any size. *Care must be taken when using on RB26 with more than 11.5mm valve lift* ARIAS Suit RB30/26 p/n= AP332103 flat top 21mm pin 1.280" compression height Only stocked in 0.20 thou oversize (86.5mm). COMPRESSION RATIO 8.4-8.6 Suit RB30/26 p/n= AP332104 10.6cc dome top 21mm pin 1.280" compression height Only stocked in 0.20 thou oversize (86.5mm). COMPRESSION RATIO 9.2-9.4 Suit RB30/25 p/n= AP332105 13cc dome top 21mm pin 1.280" compression height Only stocked in 0.20 thou oversize (86.5mm). COMPRESSION RATIO 9.5-9.8 WISECO Skyline RB30 w/ DOHC RB25det cylinder head 1.260" compresion height, 66.5cc head chamber, Dish top -11.3cc, 7.0:1 compression ratio, 21mm pin, 1mm gasket thickness, 1mm deck clearance. K577M865 86.5mm K577M87 87.00mm ACL 6MKRY9608 (VL turbo) 87mm, -4.5cc dished top, 1.261" compression height, 21mm pin, Standard RB30ET pistons are 7.8:1, with RB25 head it is 7.1:1 but these ACL pistons are bumped up to 8.5:1 with SOHC head, so with the RB25 head it may give about 7.5:1??

so if i buy cp 9.1 ratio forgies, will it drop to the ideal ratio once the 26 head goes on? also do i have to use the oil squirters?

hello all. First post =) (had to register, been trolling the boards for too long now) can we add some high performance N/A applications here? I'm looking at getting VG30DE Pistons from JE Pistons coupled with Spool Rods. They make a 10.5:1 compression piston which should fit spool rods quite nicely. they come in 87.5, so an overbore is required (the block needs it anyways). does anyone know if the valve reliefs on the pistons are gonna be an issue, or Are they most likely the same degree/angle? I've been advised by a reputable RB30 builder on this, but I'd like some added assurance, because these parts aren't cheap. RB26 R33 head is going on there too. EDIT: After cubes and I conversed, and after I doubled his calculations, we arrived at 9.11 to 1 with VG30DE pistons and some decking here and there. Add to the fact that I've come to realize these 87.5 mm head gaskets are harder to come by, it sorta makes it hard to justify going with the VG30DE pistons. I think I should probably get some custom CP Pistons made or something. I know this is a black-magic type art in picking pistons, but what's everyones general opinion about pistons? Ideally I'm looking for 10.5 but I'll settle for 10.1 compression ratio. Are there any 87 or 87.5 mm forged pistons for RB's that can achieve that with an RB26 head, or will i have to customize them? Even a bit more than 10.5 is cool. EDIT: what's the chance of me getting arias to make these in 87 mm? Suit RB30/26 p/n= AP332104 10.6cc dome top 21mm pin 1.280" compression height Only stocked in 0.20 thou oversize (86.5mm). COMPRESSION RATIO 9.2-9.4 assurance, because these parts aren't cheap. RB26 R33 head is going on there too. EDIT: After cubes and I conversed, and after I doubled his calculations, we arrived at 9.11 to 1 with VG30DE pistons and some decking here and there. Add to the fact that I've come to realize these 87.5 mm head gaskets are harder to come by, it sorta makes it hard to justify going with the VG30DE pistons. I think I should probably get some custom CP Pistons made or something. I know this is a black-magic type art in picking pistons, but what's everyones general opinion about pistons? Ideally I'm looking for 10.5 but I'll settle for 10.1 compression ratio. Are there any 87 or 87.5 mm forged pistons for RB's that can achieve that with an RB26 head, or will i have to customize them? Even a bit more than 10.5 is cool. EDIT: what's the chance of me getting arias to make these in 87 mm? Suit RB30/26 p/n= AP332104 10.6cc dome top 21mm pin 1.280" compression height Only stocked in 0.20 thou oversize (86.5mm). COMPRESSION RATIO 9.2-9.4 EDIT 2: hmmm. seems that even these won't give me anything over 9.4 with a 0.040 quench and a 1.2 mm HG. any help out there?