RB26 Crank/rods/pistons In RB25 Neo block... crazy compression ratio?!

[sickness14]

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: 51.5ccPiston dome CC's: 15.5ccPiston-to-deck height: 22.5 thou above deckBore on piston: 87mmStroke: 73.7mm

using this site:

http://www.csgnetwork.com/compcalc.html

I came up with the compression ratio. Im not happy right now...

[Joe]

are you actually running a dome top piston?

[sickness14]

are you actually running a dome top piston?

yup. thats how CP makes their off the shelf 87mm RB26 piston.

here's what i've come up with...

Enter Bore/Stroke Designation Type

1 = Inches 2 = Millimeters 2Enter Cylinder Bore Size 87Enter Piston Stroke Length 73.7Enter Head Gasket Bore Diameter 88Enter Compressed Head Gasket Thickness 2.0Enter Combustion Chamber Volume In CCs 51.5Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-') 15.5Enter 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.

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 RB25 piston stock bore 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.

Modified by sickness14 at 4:31 PM 1/29/2010

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.

Modified by sickness14 at 4:54 PM 1/29/2010
Modified by sickness14 at 7:52 AM 2/1/2010

[jdmser]

You could go with a custom 3mm Cometic head gasket. It would bring you down to 9:1 and put an extra mm between the pistons and valves. You could also check with CP to see if they or another machinist can cut valve reliefs in them. It would help with clearance and help lower the cr.

[sickness14]

You could go with a custom 3mm Cometic head gasket. It would bring you down to 9:1 and put an extra mm between the pistons and valves. You could also check with CP to see if they or another machinist can cut valve reliefs in them. It would help with clearance and help lower the cr.

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.
Modified by sickness14 at 5:20 PM 1/29/2010

[jdmser]

I'm guessing you haven't checked the piston to valve clearance yet. That's what's really going to tell you if you can use the pistons with that head. You could also look at having the combustion chamber reshape.
Modified by jdmser at 1:38 AM 1/30/2010

[RustspecS13]

I thought the neos had a smaller combustion chamber then the regualr rb25, thus your compression ratio problem.

~Alex

[sickness14]

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.


Modified by sickness14 at 7:38 PM 1/29/2010

[Bluefire]

I am not aware of any differences in deck height between rb25's and rb26's. You should be able to drop in RB26 crank, rods, and pistons into a Rb25 block. You would not be able to mix and match any of the parts, so either all rb25 or all rb26. Also if there is piston sticking out above the deck with all rb26 stuff, I would find out how much has been decked out of the surface...

-Bluefire

[sickness14]

I am not aware of any differences in deck height between rb25's and rb26's. You should be able to drop in RB26 crank, rods, and pistons into a Rb25 block. You would not be able to mix and match any of the parts, so either all rb25 or all rb26. Also if there is piston sticking out above the deck with all rb26 stuff, I would find out how much has been decked out of the surface...

-Bluefire

Ive got no way of knowing how much the block has been decked before, the previous owner had it decked, and then i had it decked again. I can get another neo block and swap everything over into it if i have to.

so is the piston not supposed to be out of the hole at all on an all RB26 setup in 25 block?-John

[sickness14]

I thought the neos had a smaller combustion chamber then the regualr rb25, thus your compression ratio problem.

~Alex

can anyone verify this with factual information such as CC's from both heads?i need to have proof from now on because "i think" doesnt work for me when im spending thousands lol

[eh?]

can anyone verify this with factual information such as CC's from both heads?i need to have proof from now on because "i think" doesnt work for me when im spending thousands lol

Yes they are different. The RB25 has a cc volume of around 62-64cc. The neo smaller cc combined with it's longer valve lift creates a problem. What I would do is find a company that makes Neo specific pistons and tell them you are using a stroked crank. See if they can make you a custom neo piston, not a custom RB26 piston.

[sickness14]

Yes they are different. The RB25 has a cc volume of around 62-64cc. The neo smaller cc combined with it's longer valve lift creates a problem. What I would do is find a company that makes Neo specific pistons and tell them you are using a stroked crank. See if they can make you a custom neo piston, not a custom RB26 piston.

thank you, i just now stumbled across that same conclusion. crap. this is going to be fun. yeah the neo head is coming up around 51.5 cc's. Ive got to get a flat top piston made or something. im going to call CP on monday.

i found this on skylines australia...

"RB25 combustion chamber = 63ccRB30 combustion chamber = 55cc

I’m going to use a NEO head and as most of you know it only has a capacity of 50-51cc compared to the normal rb25 head of 60-64cc, obviously this will raise compression.

Please Note : The R34 NEO head has a smaller combustion chamber than any of the earlier R33 or R32 RB25 or RB26 engines.

The best way to measure a non flat top piston is, after zeroing your indicator, to set your indicator on the flat or quench part of the piston, rock it all the way to one skirt or side, record the measurement. Rock the piston against the other skirt & record the measurement. Divide the total of these numbers by two & that is as close to what your piston to deck height will be.Do it a few times & you will get a "feel" for when the piston is "rocked" over all the way If you have a flat top piston then you can use the center of the piston."

[sickness14]

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 = 63ccRB25 Neo combustion chamber = 51.5ccRB30 combustion chamber = 55cc

I need a custom set of pistons made, which gives me 0 deck height, and flat top which would give me a compression ratio of 8.45:1

if i wanted 9:1 id have to calculate to keep some of the dome. I talked to brad at spool imports in australia who specializes in Neo RB30 builds and got some great info:

first email:

"hello,

Do you happen to know what kind of piston i need to use with an RB25DET Neo head, using the original block bored .40 over 87mm with RB26 crank, RB26 rods?

I just bought an off the shelf 87mm CP piston for an rb26 and did the calculation for compression, and the piston is going to make the compression too high. its domed as well. any input on this?

-John"

Response:

"Hi John

As the RB25 NEO has a smaller combustion chamber, and the RB26 has a longerstroke. You are going to have to order a custom piston to suit.

Cheers"

Second email

"Thank you. How do i go about determining the proper piston specifications in order to have a custom one made? Can i just use a custom flat top CP RB26 piston, vs the dome type i have? this will save the 15.5 cc's of the dome, bringing my compression down to roughly 8.7:1. the piston to deck height is still 0.5715 above the deck.

Ive been using an on-line compression ratio calculator.

i really appreciate the help. i cant seem to find much information on this dilemma anywhere, and im in the U.S.! -John"

Response

"Hi John You cant have any positive deck height on the piston, due to the squish area on the head.As a zero deck height gives you about 40thou ( 1mm ) gap which is just the right amount allowing for conrod stretch etc. What you need to do is just calculate the compression height ( centre of gudgeon pin to the top of the piston ) that you require to get a zero deck height.Then if a flat top with zero deck height works out to be the right comp – woo hoo

Otherwise you need to calculate the dish or dome required. Hope that helps Cheers"

AWESOME.




Modified by sickness14 at 1:08 AM 1/31/2010