Piston Comp Ratio & Deck Clearance 26/30

[Fastrotor]

Hi Guys,

I've just taken delivery of my RB30 bottom end. Now it was going to be 25 head, but i changed to 26 head. The motor came with ACL RB25DET pistons 6MKRY9603 's.

Now i just spent abit of time on this site, http://www.csgnetwork.com/compcalc.html

All the values are easy to get, except the deck height . So i actually measured it myself at .254mm (below)

So these are the values i used below.

6MKRY9603

Bore 86.5mm

Stroke 85mm

Gasket Bore 87mm

Gasket Thickness 1.2mm

Combustion Chamber CC's 64cc (average of what i have read on here)

Piston Dome CC's 11.5cc (off ACL website)

Piston Deck Clearance .254mm (Measured)

Gives a comp ratio of 9.17 : 1

If using the RB26 ACL Piston

6MKRY9604

Bore 86.5mm

Stroke 85mm

Gasket Bore 87mm

Gasket Thickness 1.2mm

Combustion Chamber CC's 64cc (average of what i have read on here)

Piston Dome CC's 16.3cc (off ACL website)

Piston Deck Clearance 1.254mm (Measured + 1mm as ACL lists the RB26 1mm lower COMPRESSION HEIGHT)

Gives a comp ratio of 9.03 : 1

NOW ..... what i wanted to know is..... the comp ratios are very close, so either would suit, however the 1mm lower deck clearance on the rb26 piston, is this for extra valve clearance of high lift cams in the 26 ??

Will the 25 piston be ok in my motor with the 26 head and 260-280 10.8mm lift cams ?

Hopefully someones been down this road.

Thanks

Gary

[Fastrotor]

Does anyone know for sure the stock RB30 DECK HEIGHT ??? Only reference i found on here was 230.5mm

Gary

[T04GTR]

zero.

your calc is wrong. with them sitting down in the bores will be lower. ie 7.7:1 for 25 pistons 31.3mm pin height

and 7.6:1 for 26 pistons. ie:lower pin height of 30.3mm

i suspect you enterd a minus in your deck height box. (only if its poking above the bore.)

[T04GTR]

26 has a longer stroke. with the same length rod. so the lower pin height.

[Fastrotor]

Dave i wanted to know the stock rb30's Deck Height, not deck clearance. So i can work out how much has been shaved off my block. (ie deck to centerline of crank)

No i used a positive value in the calc (.245mm) just as i measured it. The piston flats are just below the deck (.254mm) and the dome is above it.

How did you get those values from the calc ??

[T04GTR]

stock rb30 block height or piston pin height or compresson height is 32mm.

zero deck clearance. ie flush.

i used the same calc as you?.. like 5 times as my results were so differant to yours.

i dont know how high the block is. break out the ruler i spose

[T04GTR]

if your piston was a flat top, you would have 15cc below the top of the deck.

your 25 pistons have a + 11.5cc. so... 15-11.5=3.5cc

so efectivly you have a piston with a 3.5cc dish in it. and since flat tops yield about 8:1 i figure my calculations arnt wrong.

which says 7.7:1 with rb25 piston.

if you were to deck the block your 2.54mm (way to much) it would be 9.38:1 evan decking 1mm its 8.28:1,1.5mm is 8.6:1 , 2mm its 8.96:1

great calc but.

[Fastrotor]

Something is wrong for you to enter the same numbers and get a different result. Here are the exact numbers in order of putting them in.

2

86.5

85

87

1.2

64

11.5

0.254

= 9.17:1

[Fastrotor]

I just worked it all out by hand, and i get 7.7:1 same as you .! Now im worried. Engine builder says it will be 8.7:1

My results by manual were..

3.426cc Effective piston to deck clearance

7.051cc Headgasket volume

64cc assumed head volume

=74.47cc at TDC

Piston bore volume at BDC (using 86.5 bore and 85 stroke) is 499.5cc

Add the above value and get 573.97cc and divide by the 74.47cc = 7.7:1

Why do i get the feeling something is wrong......hopefully with the calcs !!

Gary

[T04GTR]

now im confused, i get the 9.17:1 too......

[T04GTR]

ah haaa, you deck clearance. is 0.254, not 2.54.... it is 2.54mm isnt it, not 0.254mm

so unless you want to deck 2.5mm off the block.. id be making some queries.

[Fastrotor]

My Deck clearance IS 10 thou, (0.254mm) BELOW the DECK , i measured it from the deck to the piston flats at tdc.

Seems pretty suspect if we enter 2.54mm in the calculator, that it shows the exact result we both manually calculated from the values.

Makes me think the calc has a decimal place bug perhaps.

More worrying is that it seems the motor is 7.7:1 ........

Im going to cc the heads tonight, then whack it on with gasket and tighten it all down an cc the whole lot to be totally sure.

As for the deck height , there is no point measuring this one as it has been shaved down.... i need to know the original deck height to work out how much has been removed.

I would have though with all the guru's on here someone would know......(and confirm our manual comp calcs) !

Gary

[Fastrotor]

Hmm seems like somehow our manual calcs are wrong.. i've used quite a few different calcs , and finally the standalone calc for comp ratio and Dynamic comp ratio from Pat Kelly , what a great little bit of software that is. They all gave over 9:1 so i guess its fine. Anyway i cc'd one cyl on the head and got 65.5cc so the actual comp works out to 8.9:1 ... which id still like alot higher , due to wanting to run 280 degree cams. Either that or get VCT on the inlet cam to raise the Dynamic comp ratio drastically down low.

Gary

[T04GTR]

well its its zero point two five four millimeters (0.254mm) thats not all that bad. i was under the impression it was 2.54mm. which would seem corect to me if they are rb25 pistons (2mm comp height differance on the piston)

0.7mm lower down than rb30 wich has a zero deck height. but rb25 has a +.5mm deck height stock. so that would be your .2mm so all good. dam it im half tempted to get a stock 25 piston a throw it im my rb30 just to see lol..

so after all that, it seems ok.

[Fastrotor]

well its its zero point two five four millimeters (0.254mm) thats not all that bad. i was under the impression it was 2.54mm. which would seem corect to me if they are rb25 pistons (2mm comp height differance on the piston)

0.7mm lower down than rb30 wich has a zero deck height. but rb25 has a +.5mm deck height stock. so that would be your .2mm so all good. dam it im half tempted to get a stock 25 piston a throw it im my rb30 just to see lol..

so after all that, it seems ok.

I knew the calcs must be right, but i wondered what we both did wrong in the manual calcs..... and i just worked it out.

When we did the volume of the cylinder for the gap below the deck, we used the 'diameter' in the "Pi*R2*H" equation ...stupid mistake but easy to do.

So its definately 8.9:1 ..... cool.... now i can calc in confidence what i need to do to get it higher.

Gary

[dreamers]

Hi All,

Sorry for hijacking

Please mind my english. i am new to RB26/30 and i am no engineer or good in maths.

i have been browsing all the posts and topic but i still can't get an exact comfirmation of using RB26 piston in RB30 block.

i have actually ordered a RB30 block and it is on the way now and my planning is this

RB26 standard head (not deck)

RB30block (NA)

Cometic 1.2mm

The question is i have a set of RB26 HKS 87mm forged piston around, can i put it in and what would be the compression be?

i notice that someone actually said that RB26 piston is 0.5mm lower which means that compression would be very low. (please correct me if im wrong)

if base on standard RB26 head + RB26 HKS 87mm + cometic 1.2mm head gasket + standard RB30 NA block

Should i deck the block to get better comp eg 8.7:1? if yes how much should i deck the block?

If not what about if i would to use the standard RB26 86mm piston?

i am so sorry to ask such lame question even after reading all the treads in the forum but still confuse.

Thanks

DC

Edited February 24, 2010 by dreamers

[T04GTR]

stock 26 pistons will get 7 ish to 1. way to low. you are better of putting rb25det pistons in. 26 pistons sit 2mm lower. 25 pistons sit down about .5mm but stock 25/26 deck height is +0.5mm above the deck.

[XKLABA]

if your piston was a flat top, you would have 15cc below the top of the deck.

your 25 pistons have a + 11.5cc. so... 15-11.5=3.5cc

so efectivly you have a piston with a 3.5cc dish in it. and since flat tops yield about 8:1 i figure my calculations arnt wrong.

which says 7.7:1 with rb25 piston.

if you were to deck the block your 2.54mm (way to much) it would be 9.38:1 evan decking 1mm its 8.28:1,1.5mm is 8.6:1 , 2mm its 8.96:1

great calc but.

sorry for digging this up from the dead, I have been searching for different pistons and came across this and have seen what went wrong.

flat top pistons with a 0.254 deck height give a capacity of 1.49cc, not 15cc , so 1.49cc - 11.5cc ( for dome tops) = -10.01

-10.01cc effective piston to deck clearance

7.051cc headgasket volume

64 cc assumed combustion camber

= 61.041cc

61.041 + 499.5 = 560.541 then divide by 61.041= 9.18

[KiwiRS4T]

Sorry just noticed the dates

[Gerg_R31]

Yeah more detailed on how to work out static comp ratio;

http://www.skylinesaustralia.com/forums/topic/378823-engine-internals-comp-ratios-general-info/page__pid__6104330#entry6104330