Stealth-Z Posted November 18, 2008 Share Posted November 18, 2008 After years of effort, I finally have the right combination for my never ending RB30DETT build. This may get long winded. Several years ago Wiseco custom made a set of pistons for myself and AKWIKZ. This is what they send us. These flat top pistons had 5cc valve relief cuts. They were supposed to yield a compression ratio of around 9:1. After installing and measuring them in the block I discovered the compression ratio would be around 7:1. After a discussion with Wiseco it was discovered there was an error in the order. So after some more time they ended up sending us these. The new pistons have 16cc domes. Initial measurements indicated these would give me a 9:1 compression ratio. Now me being the perfectionist wanted to have a closer quench between the piston and head chamber. By the time I got the pistons to a zero deck height, the compression ratio numbers became to high for my idea of a turbo motor. There is a lot of information on Skylines Australia about the "Ideal" compression ratio of the RB30DET conversion being around 9:1. I am building a street car that has to run on 91 octane California fuel. So instead of having new pistons made, I decided to have the 16cc dome area cut down. Fortunately I found a machinist local to me who was competent enough to keep a uniform thickness. He removed about half the original dome area. This new configuration will give me an 8.6:1 compression ratio. The machinist also made this torque plate for doing the final hone and piston clearance. I kept getting conflicting information about the benefit or need of a torque plate when boring a motor. I can say I did see deformation around the head stud areas. Quote Link to comment Share on other sites More sharing options...
Careless Posted November 18, 2008 Share Posted November 18, 2008 those look amazing. what was the final compression ratio you got with those 9:1 pistons when you decked the block/head, since I have the same pistons. Also, would your machinist be willing to rent out the torque plate? Quote Link to comment Share on other sites More sharing options...
Stealth-Z Posted November 18, 2008 Author Share Posted November 18, 2008 It figured out to be around 9.75:1 with the 66cc chamber volume on the one head. The torque plate is mine. I am eventually going to trade it for tuning work once the motor is built and ready to run. Until then, we may be able to figure something out. Quote Link to comment Share on other sites More sharing options...
Z24O Posted November 18, 2008 Share Posted November 18, 2008 CP make an ideal piston for the rb30det comes in at 8.2:1,they also make a 9.0:1 they are very good quality Quote Link to comment Share on other sites More sharing options...
RIPSNZ Posted November 18, 2008 Share Posted November 18, 2008 mmmmm, it seems you have over complicated the whole thing, its very easy to get the correct flat top pistons off the shelf that give low to mid 8 C/R that have been well proven in a huge range of power output RB30s. My 2c is, don't get too worried about squish, keep it simple and you'll be surprised how good they can be. Rob Quote Link to comment Share on other sites More sharing options...
Z-Monster Posted November 19, 2008 Share Posted November 19, 2008 guys.. i'm going to use vg30dett pistons... i've decked the block, taken about 2mm off the top of it... i wonder if anyone else here has used these pistons and what will my compression ratio be........ i wonder why one one uses these pistons??? i also had to port the cylinder 1.5mm to make these pistons fit... i'm affraid that after i put this pistons in there it would'nt work and i would get an engine that has such low compression that it drive like crap. i doubt that the compression ratio would be higher then 8.5:1 Quote Link to comment Share on other sites More sharing options...
Careless Posted November 19, 2008 Share Posted November 19, 2008 guys.. i'm going to use vg30dett pistons... i've decked the block, taken about 2mm off the top of it... i wonder if anyone else here has used these pistons and what will my compression ratio be........ i wonder why one one uses these pistons??? i also had to port the cylinder 1.5mm to make these pistons fit... i'm affraid that after i put this pistons in there it would'nt work and i would get an engine that has such low compression that it drive like crap. i doubt that the compression ratio would be higher then 8.5:1 it will work. if I remember correctly the only difference in stock piston to vg30dett pistons being the compression height @ 1.280 for the RB30 pistons vs. 1.260 for the VG30DETT pistons, right? I'm going off the top of my head. infact, the pistons I have now, and the pistons that net the high compression ratio that were cut down by stealth-z's machinist are near identical to the vg30de non turbo pistons. same compression height, same piston pin offset, same gudgeon pin size, same dome volume (16cc), but only 0.5 larger in diameter. I too wondered about the combustion chamber size being that the pistons are positive deck clearance. usually this is taken up by the gasket. if you really wanna get down to it, the big end bearings for the RB30 rods will be the same as the VG30 too. Quote Link to comment Share on other sites More sharing options...
Careless Posted November 19, 2008 Share Posted November 19, 2008 It figured out to be around 9.75:1 with the 66cc chamber volume on the one head. The torque plate is mine. I am eventually going to trade it for tuning work once the motor is built and ready to run. Until then, we may be able to figure something out. interesting. 66cc. now I've heard it all! LOL. 66, 64, 63, and 62! and would you believe it was all from different piston manufacturers. I guess the only way would be to bust out the turkey baster or the graduated cylinder, and measure my real piston volume after boring. Quote Link to comment Share on other sites More sharing options...
kiwi303 Posted November 19, 2008 Share Posted November 19, 2008 I've a pair of landrover Rover V8 heads arriving to replace the P6B rover V8 heads on my block that I'll need to size the chambers on, to find the compression chamber size is pretty simple if you have builders putty. The stuff sold in DIY hardware stores that dries without shrinking in any way. just spray the chamber with a light oil, squish in the putty and use a straight edge to level flat with the head face. once dry push a valve to pop it loose and submerge in either a graduated chemistry water volume measure, or glass cup with water on it and a line where the water level is. Now either just read off the increase on the graduated marks, or use a syringe to draw out water until the water level is back to where it was before you dropped in the mould of the chamber. it pays to fill in th tip of the spark plug or the cured putty will bind on the squiggly shapes and you'll have a devil of a time getting it loose. Quote Link to comment Share on other sites More sharing options...
Z-Monster Posted November 19, 2008 Share Posted November 19, 2008 it will work. if I remember correctly the only difference in stock piston to vg30dett pistons being the compression height @ 1.280 for the RB30 pistons vs. 1.260 for the VG30DETT pistons, right? I'm going off the top of my head. infact, the pistons I have now, and the pistons that net the high compression ratio that were cut down by stealth-z's machinist are near identical to the vg30de non turbo pistons. same compression height, same piston pin offset, same gudgeon pin size, same dome volume (16cc), but only 0.5 larger in diameter. I too wondered about the combustion chamber size being that the pistons are positive deck clearance. usually this is taken up by the gasket. if you really wanna get down to it, the big end bearings for the RB30 rods will be the same as the VG30 too. Careless, you are right about the rods.. the vg rods are exectly the same length as well just a tad bit thinner. going back to the piston, does this mean that i do not need to deck my block? should i have just left the block without skimming the top and dump the piston in as it is? i should end up with more or less 8.5:1 compression ratio? i am not sure about the compression height being so close between the stock rb30 vs the vg30dett pistons.... the vg pistons look much lower in height... only problem is that i have not put them in the engine and brought it uo TDC yet. pls advise. Quote Link to comment Share on other sites More sharing options...
Stealth-Z Posted November 19, 2008 Author Share Posted November 19, 2008 I have two RB26 heads. One measured 64cc's and the other 66cc's. I measured multiple times. Z-Monster, I never looked at VG pistons. What are the specs on them? I know the heads have a smaller chamber volume than the RB's. If they have a dished like appearance, you will not like the CR numbers. Using flat top pistons in my current configuration would give me 7.8:1. Quote Link to comment Share on other sites More sharing options...
stony Posted November 19, 2008 Share Posted November 19, 2008 mmmmm, it seems you have over complicated the whole thing, its very easy to get the correct flat top pistons off the shelf that give low to mid 8 C/R that have been well proven in a huge range of power output RB30s. My 2c is, don't get too worried about squish, keep it simple and you'll be surprised how good they can be. Rob Remember guys this is coming from a guy that is running +1000HP on his RB30 and has built many of these motors. I have seen many including myself reinvent the wheel when it comes to performance rather that enlist the help of the people that have allready gone done that particular road. Quote Link to comment Share on other sites More sharing options...
Z-Monster Posted November 20, 2008 Share Posted November 20, 2008 stealth-z and stony, yes the vg30dett pistons are dish like and are relatively flat. i'm hoping for a 8.5:1 CR but how does the 7.8:1 cr engine drive??? i hear its sluggish and would not be an ideal daily driver. stony you quoted RIPZ and he said to just use whatever we had. i dont think a 7.8:1 CR would be good right. is the only way to solve this by taking at least 2mm off the top of my block and closing the gap between head and piston @tdc? or is there another way of doing this?? bear in mind i'm also using a cometic metal head gasket at 1.2mm. dont think i can get a thinner head gasket. Quote Link to comment Share on other sites More sharing options...
stony Posted November 20, 2008 Share Posted November 20, 2008 I think what RIPS was saying was that there are off the shelf pistons available to get the correct or desired C/R. no need to have pistons custom made or modified.... I low compression engine will be more sluggish and require more boost. Quote Link to comment Share on other sites More sharing options...
Careless Posted November 20, 2008 Share Posted November 20, 2008 stealth-z and stony, yes the vg30dett pistons are dish like and are relatively flat. i'm hoping for a 8.5:1 CR but how does the 7.8:1 cr engine drive??? i hear its sluggish and would not be an ideal daily driver. stony you quoted RIPZ and he said to just use whatever we had. i dont think a 7.8:1 CR would be good right. is the only way to solve this by taking at least 2mm off the top of my block and closing the gap between head and piston @tdc? or is there another way of doing this?? bear in mind i'm also using a cometic metal head gasket at 1.2mm. dont think i can get a thinner head gasket. nissan's gasket is 0.041 inches = roughly 1mm. there are some people seeing 500 -600 bhp on standard gaskets. And there is a "Nismo" gasket kit too. Quote Link to comment Share on other sites More sharing options...
Careless Posted November 20, 2008 Share Posted November 20, 2008 mmmmm, it seems you have over complicated the whole thing, its very easy to get the correct flat top pistons off the shelf that give low to mid 8 C/R that have been well proven in a huge range of power output RB30s. My 2c is, don't get too worried about squish, keep it simple and you'll be surprised how good they can be. Rob Forgive me for saying this, but Eric has been posting updates and progress and his trials and tribulations over finding correct pistons to work for about a year and a half... and I know he's waited that long because his block has been from machine shop to incompetent machine shop throughout this whole ordeal... I have bought a set exactly like his that will work well for me because of the result of manufacturer specification errors that prevented AKWIKZ from using them as well (cancellation of project/compression too high), and that was over a year ago... so I know how long he's been at this. It's kinda too late to even bother making a comment like that, considering he asked for help a while ago and got no response. He (as well as I) have even posted on Skylines Australia, and the feedback is poor in respect to sharing info to help someone out. I mean... pistons, unless designed in-house like Engine Dynamics Roller Wave pistons are not the "underlying secret" of any build in my eyes. I just think it hurts the community, especially RB goers who are trying to get expensive parts and info from overseas without error when someone who could have saved "over complication" could have also spoke up earlier or perhaps gave a car/model/year that an off the shelf piston could come from. That's how I see it. And that's why I choose to post parts numbers when someone asks for specific parts. I've been through that, and I know how much of a bitch it is when you get the wrong parts. Quote Link to comment Share on other sites More sharing options...
Careless Posted November 20, 2008 Share Posted November 20, 2008 I've a pair of landrover Rover V8 heads arriving to replace the P6B rover V8 heads on my block that I'll need to size the chambers on, to find the compression chamber size is pretty simple if you have builders putty. The stuff sold in DIY hardware stores that dries without shrinking in any way. just spray the chamber with a light oil, squish in the putty and use a straight edge to level flat with the head face. once dry push a valve to pop it loose and submerge in either a graduated chemistry water volume measure, or glass cup with water on it and a line where the water level is. Now either just read off the increase on the graduated marks, or use a syringe to draw out water until the water level is back to where it was before you dropped in the mould of the chamber. it pays to fill in th tip of the spark plug or the cured putty will bind on the squiggly shapes and you'll have a devil of a time getting it loose. i like this idea. Quote Link to comment Share on other sites More sharing options...
Stealth-Z Posted November 20, 2008 Author Share Posted November 20, 2008 This piston thing started for me July 2006! Here is the threat for the first set of pistons from Wieseco. http://forums.hybridz.org/showthread.php?t=112975&highlight=wiseco After installing and measuring the above fore mentioned pistons, February 2007. http://forums.hybridz.org/showthread.php?t=135307&highlight=rb30+measure Asking what compression calculators people are using, June 2008 http://forums.hybridz.org/showthread.php?t=135307&highlight=rb30+measure I have asked for technical assistance. Not trying to reinvent the wheel here. Hopefully the above threads are clear on that. Cutting the pistons down, yes, it added a bit of complexity to the whole thing. My choice was get new pistons or work with what I have. The dome looks that way because of the way the pistons were forged. Wiseco pushed up the dome from the inside. I could not simply shave off the top of the dome and retain adequate thickness. Yes, CP now makes off the shelf pistons. Put a set of flat tops in and what do you get for a CR? You still have to measure all the parameters. Quote Link to comment Share on other sites More sharing options...
Z-Monster Posted November 25, 2008 Share Posted November 25, 2008 hi guys.. i have finally fitted the vg30dett pistons into my block and at TDC it seems that the pistons line up right at the edge of the block to make both surfaces level with the buldge in the center of the piston protruding a little. The pistons come out way father then Stealth-Z's picture in the first post@TDC. i do not think i will be skimming the block as i will try to see if the compression ratio on this current setup will work nicely as a daily driver or not. i am guess for now that if the stock rb30 pistons were giving me 8.5:1 compression ratio, the vg30dett pistons are going to be a tad bit lower maybe 8-8.3:1 compression ratio and thats should be bearable and responsive enough i hope. guys pls correct me if i'm wrong. i had compared my new rb30 to my stock rb25 which i replaced and the piston height at TDC are very similar withthe exception of the extra dome on the rb25 piston protruding over the block deck much more then the vg30 piston. if the rb25 compression ratio is 8.7:1 then my current set up on the rb30 should not fall far from this right guys... as you all can see i'm not as scientific as you guys and i work alot on theory and pictures... hahahaha Quote Link to comment Share on other sites More sharing options...
Careless Posted November 25, 2008 Share Posted November 25, 2008 hi guys.. i have finally fitted the vg30dett pistons into my block and at TDC it seems that the pistons line up right at the edge of the block to make both surfaces level with the buldge in the center of the piston protruding a little. The pistons come out way father then Stealth-Z's picture in the first post@TDC. just for technical terminologies sake, the bulge in the middle of the piston is referred to as the "dome", and the edge of the pistons that come out passed the block are referred to (and someone correct me on this) the "crown" of the piston, and not the ring-lands like some people incorrectly say (as there is no ring on it, persay). Now, as I had mentioned earlier, the RB30 pistons are 1.280 compression height pistons, and RB26 pistons stick out of the block by 0.020 inches, or 0.5 mm. This is referred to as "positive deck" clearance. The VG30 pistons, being 1.260 compression height would give you a "zero deck" clearance. This isn't too bad, because you will not have to worry about the piston crown hitting the head if you incorrectly select your gasket, or the rods start to stretch while they're hot @ high RPM. i do not think i will be skimming the block as i will try to see if the compression ratio on this current setup will work nicely as a daily driver or not. i am guess for now that if the stock rb30 pistons were giving me 8.5:1 compression ratio, the vg30dett pistons are going to be a tad bit lower maybe 8-8.3:1 compression ratio and thats should be bearable and responsive enough i hope. guys pls correct me if i'm wrong. You could get anywhere from 8.2 - 8.9:1 compression ratio without skimming the block by some rough calculations I did. I'm at work so I can't really look into it. Quote Link to comment Share on other sites More sharing options...
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