RB26 Crank/rods/pistons In RB25 Neo block... crazy compression ratio?! - Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum

whatbrokethistime · 01-30-2010, 11:02 AM

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.

Head CC's: 51.5cc
Piston dome CC's: 15.5cc
Piston-to-deck height: 22.5 thou above deck
Bore on piston: 87mm
Stroke: 73.7mm

using this site:

Engine Compression Ratio (CR) Calculator

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 CP RB26 piston stock bore and it was domed... you can see where the valves were hitting even back theb, when used with RB26 crank and RB26 rods.

and my piston now... RB26 87mm 8.5:1 off the shelf piston, domed.

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.

Found this on wiseco's site.

the block deck height is listed here too. if i pull the rotating assembly out i can find out how much has been decked. It shows the deck clearances being different on both motors.

the RB25: 0.33+
the RB26: 0.15-

hmm.

http://www.wiseco.com/PDFs/Cat...a.pdf

i found that using that compression ratio calculator, if my pistons were flat, and still come out of the hole the same amount, my CR would be 8.7:1 which should be fine for a turbo car. now thats if using a 1.5mm HG and if it there wasnt piston to valve contact given that size head gasket and the piston being that far out of the hole.

Anyone have any suggestions, or know how this would work?

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01-30-2010, 11:02 AM	#1
whatbrokethistime Zilvia Member Join Date: Dec 2008 Location: Raleigh, NC Age: 32 Posts: 166 Trader Rating: (1) Feedback Score: 1 reviews	RB26 Crank/rods/pistons In RB25 Neo block... crazy compression ratio?! 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. Head CC's: 51.5cc Piston dome CC's: 15.5cc Piston-to-deck height: 22.5 thou above deck Bore on piston: 87mm Stroke: 73.7mm using this site: Engine Compression Ratio (CR) Calculator 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 CP RB26 piston stock bore and it was domed... you can see where the valves were hitting even back theb, when used with RB26 crank and RB26 rods. and my piston now... RB26 87mm 8.5:1 off the shelf piston, domed. 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. Found this on wiseco's site. the block deck height is listed here too. if i pull the rotating assembly out i can find out how much has been decked. It shows the deck clearances being different on both motors. the RB25: 0.33+ the RB26: 0.15- hmm. http://www.wiseco.com/PDFs/Cat...a.pdf i found that using that compression ratio calculator, if my pistons were flat, and still come out of the hole the same amount, my CR would be 8.7:1 which should be fine for a turbo car. now thats if using a 1.5mm HG and if it there wasnt piston to valve contact given that size head gasket and the piston being that far out of the hole. Anyone have any suggestions, or know how this would work?