Skimming Rb26 - Combustion Chamber Volume?

[Uncustom]

Hi guys,

I have a few questions regarding bringing down the combustion chamber volume in an RB26 head. Reason for this is out of interest in increasing the compression ratio of an RB26/30 build (trying to acheive 9.0:1 or above). The RB30 bottom end is using flat top pistons, I know most people would say to get dome top pistons or have custom ones made up, but I'm curious as to what can be done to the head without touching the bottom end to reduce the combustion chamber volume.

Firstly, how much is a safe amount to skim off a the cylinder head? I know machining a head too much can introduce external problems, but out of curiosity, I'd like to know how much they can be machined without causing any other major problems.

Secondly, assuming that a stock RB26 has a combustion chamber volume of 64cc, how much would this amount of skimming bring down the volume? This is probably a question that'll need to be answered by someone who's CC'd a head after machining it, as it would be difficult to actually calculate the volume given the geometry of the chamber.

The piston to deck clearance is zero on the block.

Any answers are appreciated,

Cheers!

[Blackkers]

with the factory RB30e pistons you need a combustion chamber of around 53ccs to get a CR of around 9.0:1 with a factory size head gasket. I'm not sure how correct that statement is but I think that's what the RB30 boys seem to agree on

in order to reduce the combustion chamber by 10cc's you need to remove exactly 1.72mm from the head. (assuming the combustion chamber is perfectly cylindrical for that 1.7mm)

Not sure if that's do-able but you're welcome to try

[iruvyouskyrine]

Blackkers are you using some sort of formula to get these measurements? If so, share the love!

[Blackkers]

haha its simpler than you think. I figured that if the combustion chamber is close enough to 86mm in diameter at the base of the head (which it isn't exactly) then the height of a cylinder with volume 10cc's (10,000mm^3) and diameter of 86mm is 1.72mm

V=pi x r^2 x height

if the lengths are in mm then volume has to be mm^3 obviously

[GTSBoy]

Yuh, and while that formula will be wrong given the combustion chamber is not cylindrical, it will most certainly be right enough that the error will be small in comparison to the size of the answer.

[Blackkers]

yeah I'd be more worried about the actual diameter of the chamber rather than where it goes flat on the sides.

but ballpark seems to be good enough anyway.

I don't know how much you could get away with taking out of the head. A combination of thin HG around 1mm and head + deck skimming might get you there

Edited June 1, 2014 by Blackkers

[Uncustom]

Haha so you guys are treating the combustion chamber (a certain height close to the deck) as a cylinder then, I guess it may be valid in order to obtain an approximation of the volume. As for skimming 1.72mm off the head - no thanks haha, that's a fair chunk to take out. I'll use a 1mm head gasket, but if I can somehow get the head chamber to 56cc or so I'd be happy. I'll speak to some engine builders and machinists and see what they think. For now it'd be great to hear from more people. Thanks for the replies guys

Edited June 1, 2014 by Uncustom

[GTRPSI]

First calculate your valve to piston clearance before removing anything from the head, you may end up needing a thicker headgasket when you realize the error.

Id love 9.1 compression, however in my case ill be unshrowding the valves around the combustion chamber and actually dropping compression because of it.....then if room, going for a thinner headgasket after checking valve to piston clearance.

[KiwiRS4T]

Sell the RB26 head and bolt a Neo head on.