bruce Posted July 6, 2012 Report Share Posted July 6, 2012 (edited) Would need to know the head volume to work it out. Stock is around 63cc. Did your machinist measure it? You could measure it with a clear sheet of plastic (like a CD cover with a hole in it) to cover over the chamber, and a pipette or syringe to introduce metho or turps etc., like here: http://www.rollaclub...attach_id=87204 Then whatever milliliters fits into the chamber gives the cubic centimeters. There is a program that takes the piston ring height into account, but you could do a quick calculation with a pen and paper and it should be fine. Or here are some online calculators http://www.csgnetwor...m/compcalc.html http://www.rbracing-rsr.com/compstaticcalc.html Edited July 6, 2012 by bruce Quote Link to comment Share on other sites More sharing options...
agius Posted July 6, 2012 Author Report Share Posted July 6, 2012 yea the head people said they measured it and got 63cc after they shaved it Quote Link to comment Share on other sites More sharing options...
bruce Posted July 6, 2012 Report Share Posted July 6, 2012 OK, even shaved down close to the erg holes. Mine was shaved 1mm and it still measured 63cc as well. Measured it a few times. Also measured an unshaven head as well and it was the same. That was without the clear plastic sheet though, so thought I couldn't measure the difference. Maybe because the chamber is a spherical shape it needs alot shaved off to notice the reduction in volume. So perhaps the head gasket, piston dome and oversized pistons would be more effective in increasing the compression ratio. Using this calculator: http://www.csgnetwork.com/compcalc.html with these values: Bore: 85mm Stroke: 78mm gasket thickness:1.5mm dome: 20.6cc (from Wiseco catalog) gives a compression of 9.69 with 1mm gives a compression of 10.2 with a 30 thou gasket (0.762mm) gives a compression of 10.47 What piston dome volume did you use in the calculations? That needs to be known accurately. Could imprint the piston dome on some clay or plasticine and measure the volume, similar to the cylinder head chamber volume measurement. Quote Link to comment Share on other sites More sharing options...
agius Posted July 6, 2012 Author Report Share Posted July 6, 2012 Its making alot more sence now, i never made any calculations till now, but i belive when the head place made the calculations, piston dome value was worked out as 20.0cc Head gasket was worked out to 1.0mm and dome as 63.0cc Thus giving me the rough number as 10.1 static compression Guess they got lazy and decided to work it out in whole numbers Unluss there programs do so.. 4 posts ago if i read what I'm writing now i wouldnt know what I'm on about, ive learnt so much it just came all together The edges of the cumbustion chambers of the head must be almost dead streight for atleast over 40thou to shave 1mm off and have the exact same cc volume as completly stock Of corse the cc is going to decrease, but not in way if the edges where curved dramaticly to the sphere right from the very start.. Ive honed my bores coz they were still in great condition and ive already purchased standard size rings Thats why ive never botherd about high comp pistons... I think the best option is just chuk the motor all together put up with the low compression meanwhile shave a 2t head to the right spec.. but hopefully the motor runs in correctly with the low comp ratio hrmm.. Quote Link to comment Share on other sites More sharing options...
agius Posted July 6, 2012 Author Report Share Posted July 6, 2012 i don't want to bother about the thin head gasket with the 3t head i rather save it for the 2t head so i can save that extra 1mm on the 2t head just incase god forbid it ever overheats or i ever need to reco it or something ya know Quote Link to comment Share on other sites More sharing options...
philbey Posted July 7, 2012 Report Share Posted July 7, 2012 http://cochise.uia.net/pkelley2/DynamicCR.html This is the best dynamic comp ratio calculator I found. It's very handy. I ended up with nearly 12:1 by accident on my 5k and I got away with it running a big ass cam, premium only. If I run stock fuel, as I had to once, it's not good at all. Quote Link to comment Share on other sites More sharing options...
agius Posted July 10, 2012 Author Report Share Posted July 10, 2012 http://cochise.uia.n.../DynamicCR.html This is the best dynamic comp ratio calculator I found. It's very handy. I ended up with nearly 12:1 by accident on my 5k and I got away with it running a big ass cam, premium only. If I run stock fuel, as I had to once, it's not good at all. thanks for that philbey Quote Link to comment Share on other sites More sharing options...
agius Posted July 10, 2012 Author Report Share Posted July 10, 2012 so i got the head back, ive taken a few photos there is hardly any meat left between the valve and the edge of cumbustion chamber EGR holes i was talking about shaved almost to the thread Quote Link to comment Share on other sites More sharing options...
bruce Posted July 11, 2012 Report Share Posted July 11, 2012 (edited) Looks like the compressed head gasket thickness is 1.3mm, probably a permaseal one? Then the effective dome volume: dome volume - piston reliefs = 30-8 = 22cc Head has been shaved 30 thou (0.762mm). Looks like they've taken the piston ring height into account in the compression calculation. Edited July 11, 2012 by bruce Quote Link to comment Share on other sites More sharing options...
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