Rb25De Pistons In Rb30 Block For Rb25/30 N/a

[walky01]

hey guys im new to this forum and have done some research and this seems like the best one to go to, im currently in the mindset of building a n/a rb25/30 for my vl commodore. At this stage i want it pureley for n/a not turbo setup later etc... ive gone to get some quotes and it just seems more expensive for what it is inexcess of 7k. so im going to attempt it myself with the help of a few people. anyway what i was wondering is im using rb25de pistons on the 30rods. the block is standard hasnt been touched before so i can track down some 25 pistons no problems. ill be taking it to a machine shop to get the work done but what needs to be done to acheive a comp ratio of 11:1? ill be running on 98 and have an aftermarket haltek system once its up and running. do you reccommend strengthening the rods? or should be fine for n/a. im assuming i just need to get the block decked by 0.8m but just want some more opinions. thanks guys all help will be appreciated !

[SKiT_R31]

The stock rods will be fine, deck the block 1mm and your onto a winner, though expect the compression ratio to be around 12:1. That should be fine on 98ron with management (esp running MAP), but if you intend to turbo it in the future I'd be using factory turbo pistons (with the block decked) should still have good N/A power but still capable of N/A + T.

[walky01]

I thought if i decked the block 1mm it would drop compression to 10.5-1? to around 11:1? also with the head if it a r32 rb25de non vvt head it bolts straight up right? no mods required.

[TwoCents]

Download the PDF from the first thread in this post. Good guide on what heads can be used, pistons etc etc http://www.skylinesaustralia.com/forums/topic/15420-r33-rb30-conversion/

[StevenCJR31]

Download the PDF from the first thread in this post. Good guide on what heads can be used, pistons etc etc http://www.skylinesa...b30-conversion/

i have had a look at this but would like specific information regarding compression ratio, am looking at using low k 25DE pistons and DE 25 head and making up a 25/30, can anyone tell me what the CR would be using this combo please?

[walky01]

Got told once block is decked 1m its about 12:1

[StevenCJR31]

thats high!

[zebra]

Not really, sounds ideal to me.

I would however blend the chambers to make sure there is no sharp edges etc

[walky01]

For an n/a engine its perfect comrpession ratio. thanks to the help of a member on here i have most info now in my head on what to do and how to go about prepping the bottom end to wack a 25 head on.

[zebra]

Spend some time on the head, thats where the power is made. Bottom end just supports it.

If you dont have the cash for cams thats ok, but have the ports cleaned up as a minimum.

[StevenCJR31]

what would be the approx CR without decking the block?

[zebra]

without working it out properly i would say mid 10s

[StevenCJR31]

without working it out properly i would say mid 10s

thanks zeb!

[walky01]

Not only that you would be wasting your time and money, my bloke said it costs 99 bucks to get the block decked, and why would you put rb25de pistons in the first place? its worth the compression made