sickness14 Posted January 30, 2010 Share Posted January 30, 2010 hello guys, first post here. Im a member of the RB community on Nicoclub, and really needed some widespread opinions on this dilemma so here i am. -John When using RB26 crank, rods, and cp 87mm pistons (8.5:1) on an RB25neo will this setup work? or are you supposed to use the RB25 pistons, because my pistons are above the block at TDC. My piston to deck height is twenty two and a half thousandths out of the hole. im getting a crazy compression calculation of 10.7:1 using a 2mm gasket! wtf. help me please. i dont know if my head has been shaved too much, or the block deck shaved too much, or i have the wrong pistons and should be using rb25 pistons. The rb26 87mm pistons i have are domed. using this site: http://www.csgnetwork.com/compcalc.html I came up with the compression ratio. Im not happy right now... here's what i've come up with... Enter Bore/Stroke Designation Type 1 = Inches 2 = Millimeters 2 Enter Cylinder Bore Size 87 Enter Piston Stroke Length 73.7 Enter Head Gasket Bore Diameter 88 Enter Compressed Head Gasket Thickness 2.0 Enter Combustion Chamber Volume In CCs 51.5 Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-') 15.5 Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') : -0.5715 Calculated Engine Compression Ratio 10.78 to 1. Total Displacement Volume 44.766 cc Im pretty much screwed . i think its more the head than the block's fault. I found that the neo head is supposed to be around 56cc from skylines australia (which i dont know if its accurate) and my head is 51.5 ccs. this change alone results in almost a whole point increase in compression. The stock piston is dished... The RB25 CP piston i think is dished based on google image search Yet, the piston that was used by previous owner was a custom cut CP RB26 piston stock bore and at 9.1:1 comp. and it was domed... you can see where the valves were hitting when used with RB26 crank and RB26 rods. and my piston now... RB26 87mm 8.5:1 off the shelf piston, domed. Im going to cry. help. original owner of car's original build... "well, i thought long and hard about it over the last few days and decided to get a set of cp pistons at 9.5:1, pretty much an identical (forged) replacement. so i called up CP Pistons for a custom set because they don't offer it off the shelf. Since they don't deal with the public they referred me to JGY Customs because that's their closest vendor to me. so i'll be sending #1 and #6 piston to JGY tomorrow, then they send the specs to CP and CP sends me my pistons." crap, so i guess he did have custom pistons ordered to use the rb26 pistons in an rb25. he also said: "i went with the rb26 crank with the good oil drive and the rb25 pistons wouldn't work because of the wrist pin." which explains that i guess an rb26 piston has to be used with the rb26 crank. I talked to the guys at CP already today. they said that these pistons could not be modified, because i initially thought i could have the dome cut down. they said theres not enough material on there underneath to cut them down. they said they could make me a custom set of flat top RB26 pistons in 87mm for around $1200. I just dont know that even then it would drop the compression enough. The stroke of the crank and the deck height also play a part in this. The block has been decked twice, and this head surfaced 3 times now. stuff is just getting to thin. CJMartz2k can hook me up with another RB25 block, unmolested, but i still dont know if that'll work with the piston / head combo. Quote Link to comment Share on other sites More sharing options...
kiwi303 Posted January 30, 2010 Share Posted January 30, 2010 IIRC without going to look at the saved links I have, RB26DETT wrist pins are further up the piston, so distance between wrist pin centre and piston deck is shorter, hence why RB25 pistons don't work on the 26 crank/rods, they'd be way out of the bore trying to occupy the spark plugs space. The RB25 and RB26 rod length is IIRC the same, so the higher pin is used instead of a shorter rod to allow for the longer stroke. So yah, you're out of luck with pistons, can't use 25's on the 26 crank. Quote Link to comment Share on other sites More sharing options...
sickness14 Posted January 30, 2010 Author Share Posted January 30, 2010 IIRC without going to look at the saved links I have, RB26DETT wrist pins are further up the piston, so distance between wrist pin centre and piston deck is shorter, hence why RB25 pistons don't work on the 26 crank/rods, they'd be way out of the bore trying to occupy the spark plugs space. The RB25 and RB26 rod length is IIRC the same, so the higher pin is used instead of a shorter rod to allow for the longer stroke. So yah, you're out of luck with pistons, can't use 25's on the 26 crank. So in theory, my RB26 pistons should work fine, but i still dont get why my compression is way off. Maybe the Neo head has something to do with it. Quote Link to comment Share on other sites More sharing options...
RB26powered74zcar Posted January 30, 2010 Share Posted January 30, 2010 Yet, the piston that was used by previous owner was a CP RB25 piston stock bore and it was domed... you can see where the valves were hitting when used with RB26 crank and RB26 rods. Im going to cry. help. I don't see where any valve was hitting... If you are referring to all that damage and specs of missing piston along the edge, that's detonation damage if you ask me... Quote Link to comment Share on other sites More sharing options...
sickness14 Posted January 30, 2010 Author Share Posted January 30, 2010 The line on the left side of the dome, semi-circle is where the valve was touching. of course the piston has detonation on it! lol. it seized up a ring, and spun a bearing. had the block bored .40 over, new set of pistons, bearings, etc. Quote Link to comment Share on other sites More sharing options...
RB26powered74zcar Posted January 30, 2010 Share Posted January 30, 2010 The line on the left side of the dome, semi-circle is where the valve was touching. of course the piston has detonation on it! lol. it seized up a ring, and spun a bearing. had the block bored .40 over, new set of pistons, bearings, etc. Had to put on the glasses... lol. I see the mark now Quote Link to comment Share on other sites More sharing options...
sickness14 Posted January 31, 2010 Author Share Posted January 31, 2010 figured it out my head being a neo, has a smaller CC than the regular rb25 head. thus, those pistons will not work. the previous owner of the car had built the motor with the same wrong pistons. and when i bought it , had a compression ratio of 11:1, thus it blew up under base map tune The R34 NEO head has a smaller combustion chamber than any of the earlier R33 or R32 RB25 or RB26 engines. RB25 combustion chamber = 63cc RB25 Neo combustion chamber = 51.5cc I need a custom set of pistons made, which gives me 0 deck height, and / or flat top RB30 combustion chamber = 55cc Quote Link to comment Share on other sites More sharing options...
kiwi303 Posted January 31, 2010 Share Posted January 31, 2010 Don't the NEOs also have a mm or so smaller valve? why not take the piston and head to a machinist and have the head milled out to have chambers accommodating the piston? Quote Link to comment Share on other sites More sharing options...
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