piston identification and compression ration maths ??
Turbo Mini Forums

Help !!!

new to this lot. got a 72 mini fitted with a 12hd26 turbo lump. pulled the lump out 'cos it smokes. wipped the head off and found ...... nice clean bores, new ish looking dished pistons with numbers stamped on them and a cylinder head with combustion chambers that look a bit larger than normal ??? what does it all mean??

The pistons have 20831 stamped on one side and idi .020 on the other BUT the .020 has x's stamped on top of them ??

thought i would check the compression ratio so i used a syringe filled with light oil and filled up one combustion chamber

i then turned the crank to tdc and did the same

i measured the head gasket and worked that out ..

i got the following

c chamber ........25 cm3
pstn dish...........11 cm3
h gskt.............4.25 cm3

giving a clearance volume of 40.25 cm3
i measured the bore and stroke

bore ........70.6 mm
stroke ......81.5 mm

pi r 2 h gave me a swept volume of 319.04 cm3

swept v + clearance v / swept v gave me a cr of 8.9:1

lower than standard ???

what i did notice was that the bore seemed standard ??

if you can make head or tale from this please help

try it on here just fill the blanks in

http://www.maverickracing.co.uk/compratio.exe

Metro turbo weekend driver,Mini turbo in the making again!

don't forget the pistons don't come to the top of the deck, so you need to account for this,

there's also the ring land volume but i think this is so small that it tends to be neglected

www.afracing.com/cr.xls

designed for minis!!

Alex

AlexF