just a quicky
Turbo Mini Forums

can i use a modifyed normal 1275 cylinder head for a turbo?
(if i put sodium filled valves in)

probably not, it is likely if the head has been modified, it has been skimmed, which will result in too high a compression.... get the head burreted, (or do it yourself) and work out what the cr will be..... alexf has a cr calcualtor on his site iirc.....

oh, and dont worrk too much about the sodium filled exhaust valves... a lot of people are no longer using these......

Pete

And on the 7th day........... God created turbochargers!

if its a "standard" 12g940 head off a Normall aspirated engine you could run a sane boost levell of 7psi without detonation. no worries !

like nutter driver said dont worry about the ex valves

surely all heads that are removed need skiming

ive done some research and found its the pistons that change the compression, using 21251 +060 pistons so will jus remove some material
from the top (1mm or there abouts) to make them the same as turbo pistons. should do the trick i hope!

"surely all heads that are removed need skiming"

i have neder had a head skimmed that i have changed gasket on and never had a problem and i have done thousands of miles on them. i agree if you want to be safe then do so but if the head was jusst removed out of cureosity (sp?) or like me the gasket had gone and i drove no more than 30 miles at low revs i took the risk and was fine:) sorry its off topic but just a thought to share:)

Dan

why would all heads that are remover need skimming? only ever skim ahead when they warp, or want to increase compression (as is usual when modifying a head).....

regarding the pistons changing the compression, whilst this is true, it is only one factor..... the easiest way to lower compression is to cut the head (assuming the pistons you want to use make it too high).

before you do ANY machine work (either cutting head, or chopping pistons) you MUST calulate the compression ratio. like i said, use to calculator on alex f's site and see what it is.... you may need to make big changes to your spec, or you may need to change nothing.

You must actually measure the volume of the head...... i do this as follows.... take one old cd case (i actually have a thicker piece of perspex, but a cd case will work fine) and drill two small holes in it. take one syringe (or any other easily graduated container, must be acurate to a fair degree) place the head on a flat surface with valves etc all fitted, place a smear of grease around the chamber and press the perspex into postion, making sure the holes are both over it. then using water (i have heard of people using parafin, i use water because i never lose the tap, its always where it was last time so its easy to find.... NOTE, do NOT use petrol, it evapourates too fast, and will fubar the perspex as well!) fill the chamber completely untill the last of the air goes from under the perspex, and note the amount of liquid the chamber has taken.

you should really also measure the other factors accurately, but as you do not have the pistons etc fitted yet, you can make a accurate guess, there are figures around (in dv's yellow book iirc) that will make this easier.....

a side note.... i personally would be seriously thinking about weather cutting as much as 1mm from a piston is a good idea in terms of hte strength of the piston.... i have no idea what the 21251 pistons are, (i recognise the number but thats about it!! lol!!) so i would be looking into wheather or not you will be ok doing this....

again i really cannot emphaisise enough, please do not just cut down the pistons untill you know all of the other factors involved!!

Apoligies if any of the above sounds at all patronising, its not supposed to, but i know things can come across differently on the net!!!

Hope that helps,

Pete

And on the 7th day........... God created turbochargers!

im cuttin the top of the pistons off to make them the same height as the 21254's and lower the compression ratio. it will not affect the strength as its just off the crown. im very lightly skimming the head as im not sure of its history (its already been crack tested), and jus touching the block face.
I will use a thicker gasket than usual if i can get hold of one.

21251's are econoglide squeeze cast pistons made by hepolite go up to +060"

different manufactures change the compression in different ways, changing the height of the piston is one, the other is changing the depth of the dish.

i cant see the point in checking the compression ratio if im replicating other ways of doing it.

the theory checks out will let you know if it works
no reason why not

by the way i work at an engine remanufactures and ive managed to blag this as a college project. im takin photos of everything so the head and block need to be skimmed ju

just to look good really

mate, you really need to work out what the current compression ratio is as i said, before you do any machine work! imagine if you already had a compression ratio of 7:1, then you mill 1mm of the top of the pistons..... you should be working out what you have at the mo, then working out how much extra volume you need to get what you want, then working out where to get it from.....

Pete

And on the 7th day........... God created turbochargers!

the ratio shouldnt be any different to a standard 1275. niether the block or head have been skimmed previously the only variable is the pistons. very slightly skimming (im talkin max 002" on the head and less than that on the block, if thay clean up ok) the head and block will make almost no difference to compression ratio, specially with a thicker gasket.

this aint a quicky at all is it lol
stu

also a dish in the piston is better for combustion efficiency (in theory) than reducing the piston height

Chopping the tops of the pistons would be a bad move as the rings will be nearer the top.
If you use low hieght pistons in a std block that will gain you 4cc

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