27 replies to this topic

[oltonlad]

why not give it a compression test with my standard mg metro head on was

around 195, my mates with the lower compression was about 165


the mg metro head and cam or 266 will make it go nice

i thought that all of the 940 heads were the same "cc".......around 22-ish i think, thats why i assumed that the compression ratio was governred by the pistons....or am i wrong?

[racingbob]

why not give it a compression test with my standard mg metro head on was

around 195, my mates with the lower compression was about 165


the mg metro head and cam or 266 will make it go nice

i thought that all of the 940 heads were the same "cc".......around 22-ish i think, thats why i assumed that the compression ratio was governred by the pistons....or am i wrong?

the higher compression mg metro ones as in mine 195 comp

i think my mates had the lower comp pistons 165 comp

all heads i believe the same

[oltonlad]

i've been looking this up and the total combustion volume in my engine is 41cc, if i had the head skimmed by 40 thou then that would reduce the combustion volume down to 38cc which would result in a compression ratio of around 10.2:1.

[ACDodd]

No the later factory unleaded head are nominal 23cc chambers the MG metro big valve factory heads and all other 940 head were nominal 21cc. Compression ratio chamges were made by varying the piston deck height. MG metro nominal 0.010". For lower compression, 0.045" nominal deck height was used.

AC

Edited by ACDodd, 24 January 2013 - 11:07 PM.

[Vipernoir]

How do the obsolete 21962s compare to 21251s and 21253 in terms of compression/deck height ?

[ACDodd]

For an A+ block (1275) 21962 are nominally 0.045" down the bore. The 21963s are nominally 0.015". the 21251's are nominally 0.065", the 21253 are nominally 0.005". All are nominally 8cc dish.

AC

[Cooperman]

Quite right AC, I just stripped an engine which had 21251 and they were 0.067" down the bores as an average. After skimming about 0.004" off the deck the 21253's are virtually flush. Dish is quoted as 8.4 cc.

[blacktulip]

what cc engines are the 21253 pistons used on?

[Cooperman]

High compression 1275 to 1330 units as after-market parts. The 21253-00 is standard and the 21253-60 gives 1330 cc.

[Cask_D]

I am overhauling a 1275 A+ metro turbo engine for use without a turbo, looking at using a MED stage 3 head, unsure on what cam yet although will be a fast road/rally,

I have been informed due to the use of a turbo the engine will run with a very low CR, would this effect the power being delivered ? even with the change of the cylinder head ?

If so how can i go about trying to reduce the losses in performance ? a change in piston ??

[oltonlad]

I am overhauling a 1275 A+ metro turbo engine for use without a turbo, looking at using a MED stage 3 head, unsure on what cam yet although will be a fast road/rally,

I have been informed due to the use of a turbo the engine will run with a very low CR, would this effect the power being delivered ? even with the change of the cylinder head ?

If so how can i go about trying to reduce the losses in performance ? a change in piston ??

i think that you will have to change the pistons as the metro turbo ones will have a very low compression rating

[Cask_D]

After a closer look the pistons are 20831's, after a bit of research i beleive they have a 12cc dish,

i've been lloking at some pistons a set with a flat head, another with a 6cc dish,

im fairly new to all the technical properties of engines but will halfing the dish size have a linear effect on CR, or is it all exponential ?

if its the latter i think i will go for the 6cc dish

[oltonlad]

After a closer look the pistons are 20831's, after a bit of research i beleive they have a 12cc dish,

i've been lloking at some pistons a set with a flat head, another with a 6cc dish,

im fairly new to all the technical properties of engines but will halfing the dish size have a linear effect on CR, or is it all exponential ?

if its the latter i think i will go for the 6cc dish

as far as i'm aware the compression in the A series engines are determined more by the crown height rather than the dish of the piston.