Jump to content
SAU Community

Recommended Posts

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

Link to comment
https://www.sau.com.au/forums/topic/367818-rb26-compression-ratio-cp-pistons/
Share on other sites

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

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

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?

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.

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

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Similar Content

  • Latest Posts

    • Ok i will get those 310mm. I found one but on a different site. This is the description on those...is it ok? Technical parameters: - Axle: front. - Disc type: ventilated. - Number of holes: 5. - Disc diameter: 310mm. - Total height with center: 54mm. - Thickness (new/min.): 30/28mm. - Designed for brake calipers manufacturer: Sumitomo.
    • You Gregged a whole racetrack!?
    • Look for broken wire or bad connector at the motor. Might not be it, but is worth starting there, as it is easy.
    • Hi everyone, I’m having an issue with my R32 GT-R. Sometimes, when the car goes over a bump or experiences some vibration, the 4WD warning light comes on the dashboard. When I check the code from the control unit in the trunk, it shows Code 19 – ETS Motor. However, everything seems to be working fine — if I turn off the engine and restart the car, the light goes away and everything functions normally. Has anyone experienced this before? Where should I start troubleshooting this issue? Thanks in advance!
    • I'm back from the dyno - again! I went looking for someone who knew LS's and had a roller dyno, to see how it shaped up compared to everything else and confirm the powerband really is peaking where Mr Mamo says it should. TLDR: The dyno result I got this time definitely had the shape of how it feels on the road and finally 'makes sense'. Also we had a bit more time to play with timing on the dyno, it turns out the common practice in LS is to lower the timing around peak torque and restore it to max after. So given a car was on the dyno and mostly dialled in already, it was time for tweaking. Luis at APS is definitely knowledgable when it came to this and had overlays ready to go and was happy to share. If you map out your cylinder airmass you start seeing graphs that look a LOT like the engine's torque curve. The good thing also is if you map out your timing curve when you're avoiding knock... this curve very much looks like the inverse of the airmass curve. The result? Well it's another 10.7kw/14hp kw from where I drove it in at. Pretty much everywhere, too. As to how much this car actually makes in Hub Dyno numbers, American Dyno numbers, or Mainline dyno numbers, I say I don't know and it's gone up ~25kw since I started tinkering lol. It IS interesting how the shorter ratio gears I have aren't scaled right on this dyno - 6840RPM is 199KMH, not 175KMH. I have also seen other printouts here with cars with less mods at much higher "kmh" for their RPM due Commodores having 3.45's or longer (!) rear diff ratios maxing out 4th gear which is the 1:1 gear on the T56. Does this matter? No, not really. The real answer is go to the strip and see what it traps, but: I guess I should have gone last Sunday...
×
×
  • Create New...