Rb26 Compression Ratio - Cp Pistons

[drew6]

Hey guys, quick question:

I am putting together an rb26 using CP pistons and an 87mm bore and .010" shaved off the block. Everything else is std sizes. I am looking for help getting the right head gasket thickness - I would like to end up with around 8.5:1. However, I can not find out what cc the dome of the pison is so I can not calculate it myself.

I tried calling CP to get help on this but have a several attempts I couldn't wait any longer.

Thanks

[N1GTR]

Bore (in) 3.425 in.

Bore (mm) 87.000mm

Piston Style Dome, with no valve reliefs

Piston Material Forged aluminum

Compression Distance (in) 1.193 in.

Piston Head Volume (cc) -15.50cc

Wrist Pin Style Floating

Pin Diameter (in) 0.827 in.

Gapless No

File Fit No

Top Ring Thickness 1.0mm

Top Ring Material Steel

Second Ring Thickness 1.2mm

Second Ring Material Cast iron

Oil Ring Thickness 2.8mm

Thats from CP for part number SC7311

[The Dan]

You are thinking too much about it.

Just put on a 1.3-1.5mm and drive it.

If it's too laggy.....add Nitrous

[R32 TT]

Depends on which CP piston you're talking about. If its the so-called 9.0:1 CP's, then the dome is 6cc.

Here's a piccy of one - is this what you have?

Edited June 15, 2011 by R32 TT

[zebra]

Piston Style Dome, with no valve reliefs

Piston Head Volume (cc) -15.50cc

How can you have a dome top slug with a -15.5cc volume

doesnt make sense to me lol

Silly Silly CP

[DVS JEZ]

How can you have a dome top slug with a -15.5cc volume

doesnt make sense to me lol

Silly Silly CP

NOOB

[zebra]

how so?

[DVS JEZ]

Have a good hard think about how to measure cc volume of the combustion chamber.

Then think about if a piston is protruding into that chamber...

[DVS JEZ]

Just like sex. The further u stick it in the less volume there is.

Some people use up more volume than others lol. Just like pistons

[zebra]

Talking about pistons though, not chambers.

- vol means the piston is dished

+ vol means the top of the piston has a dome top.

[DVS JEZ]

Its reffering to chamber cc

[zebra]

no it isnt

are you saying the piston has a 40 something cc dome? or that the 26 chamber vol is 15.5cc?

edit - just worked it out and using a 0 deck height piston at 40cc dome the comp ratio is 25:1

[Duncan]

26 chamber volume is about 66cc

So I would expect a -15cc piston to give me about 51cc

[DVS JEZ]

Correctamundow

[ahh 33 s2]

i have these pistons, are they ment to stick above the deck hieght, and by how much, mine look stupidly high??

btw running tomie 1.2mm HG

[zebra]

You guys are missing the point.

Combustion chambers have NOTHING to do with piston crown volume

Compression Distance (in) 1.193 in - So from the centreline of the gudgeon pin to the edge of the piston crown is 1.193". If the piston was dead level along the crown aka a flat top piston the crown volume would be classed as 0cc, if it is raised aka dome top than it would be + 15.5cc. A dished piston (or inverted dome as some call it) will have a negative volume.

As an example the first one in this list (SRP 138093) has a -5cc volume due to the valve reliefs

The one underneath (SRP 140674) has a +11cc dome due to the raised crown.

http://www.jegs.com/webapp/wcs/stores/servlet/KeywordSearchCmd?Ne=1%2b2%2b3%2b13%2b1147708%2b1147708&Ntt=350&langId=-1&Ntk=all&storeId=10001&catalogId=10002&N=4294964093&Nty=0

[zebra]

So really I was just having a dig at the CP website.

For eg, im building a V8 up.

Now I have heads with a 58cc chamber volume and I need pistons with a -6cc volume at 0 deck height to get the comp I want (11.5:1). If I was building it to take a turdbro or a supercharger I would need something like a -15cc dish to have a lowish comp ratio.

Now if CP listed there Ford Cleveland pistons like that, I would see the -15.5cc volume and order those because they will give me low comp....So imagine my suprise when I open the box and...."Oh look they are a dome top, I wonder how a blown v8 will like 16:1 comp?"

See my point?

[Beer Baron]

I dunno? it made perfect sense to me. -15cc dome top piston means take 15cc off your chamber volume...

[Beer Baron]

oh and to the bloke that asked yes they can and do sit slightly proud. up to you to set the right deck height and chose the right thickness gasket to get the right CR and also to ensure they interact properly with the squish pads but don't hit valves or anything else. not always just bolt them in and whack it together. you do need to do some measuring.

[zebra]

I dunno? it made perfect sense to me. -15cc dome top piston means take 15cc off your chamber volume...

Ye. It works that way as well. I'm not denying that. Just simpler to do work it how all the other piston maker dudes do it.

RBs are easy because they are application specific. But when you start doing funky stuff like 6" chev rods and pistons designed for a 347 stroker windsor v8 in a Cleveland v8 it becomes dicky when some people say -ve and others say +ve lol