AE 21253 pistons +40 EAN
Turbo Mini Forums

i recently bought a engine with AE 21253 pistons +40 already fitted its done 0 miles and still has the honing marks

its got a 266 cam shaft
turbo oil pump
uprated clutch and lightened flywheel

now i want to fit a Supercharger to it when it comes to getting the engine up and running in my mini but are the pistons a complete no no for Forced induction ?

i currently do not have a cylinder head for it but i intend on fitting a mg metro head to the engine and having the chambers enlarged but before i do that i have to work out by how much

searched for a while prior to starting this thread just done another search and found yes they are ok for turbo use as long as the compression ratio is lowered!

worked out my Compression ratio to be 10.1:1 not sure if the details i have entered are correct but

bore size 71.61
piston stroke length 81.28
headgasket bore diameter 73.5
compressed headgasket 0.8
cc in head 21.4
dish in piston -9.5
deck clearance 0

if anything looks out of the ordinary could someone point me in the right direction

Edited by blister on 15th Jan, 2013.

Bk450 headgasket is 3.8cc compressed.

21253's are being used in a few boosted engines on here, one of them was pushing 200bhp or there abouts

Edited by Brett on 15th Jan, 2013.

Yes i moved to the darkside

Instagram @jdm_brett

Right and going by having +40 pistons and standard stroke length my compression ratio is high am i right in thinking i need to find 9cc somewhere to bring my ratio down to the 9s?

if you have callipers and the piston to hand ill work it out perfectly for you

if not i can do you a best guess?

bear in mind though some pistons dont come up to the top of the block as standard, this gives you the option of running them slightly down the bore and trade squish for CR ( mine are 0.5mm down)

Edited by Brett on 15th Jan, 2013.

Yes i moved to the darkside

Instagram @jdm_brett

Yes i moved to the darkside

Instagram @jdm_brett

will measure everything tomorow but 10.29 sounds bang on however the pistons do come flush with the blocks face i think but again i will investigate this tomorow,

going by a bodge job of 9cc decompression plate and 2 headgasket it would take me in to the 8-9cr area where its just right i believe when running a eaton suckthrough

If you have it in pieces why not get the head sent off to one of the head wizards around?

That way you can get the chambers enlarged, and any maintenance (guides etc) done, plus any porting work you want, in one hit. Believe me you dont want to have to do it after you've got the car running...

http://www.turbominis.co.uk/forums/index.p...tid=469104&fr=0

Edited by Brett on 15th Jan, 2013.

Yes i moved to the darkside

Instagram @jdm_brett

oh the reason i suggested the decom plate was because i didnt think the head could be taken out to 29cc or even 30cc wouldnt it effect the performance?

so waht would my CR be if i was to say 9cc in mean of mods to the head

28cc in head = 8.83
30cc in head = 8.47

power wise, it will make less power off boost but allow alot more safe boost to be run resulting in an overall power gain when on boost
but since the supercharge makes boost from the get-go it should be win-win imo

these are based on a few guesses though CR may shift about by 0.5cc either way, the spread sheet i made up takes into account piston measurements

Yes i moved to the darkside

Instagram @jdm_brett

brilliant thank you so i have to really add 9cc into the head which is 21cc not to bad really will investigate further

im not sure what the max is i seen as much as 32cc but imo you want some meat left in there otherwise the combustion chamber may well endup blowing through into the waterways
imo you want to keep some sort of beak too, i had a head that was 28cc no beak and it was a little det monster, changed that for another head 28cc but with a beak and much better shape chamber and i can run 35 degrees advance on 12psi boost... not that i do

Yes i moved to the darkside

Instagram @jdm_brett

You can also hack a lump off the top of the 21253s and remachine the dish - I did this on the wife's engine (due to inherited issues) to drop the CR down to 10.2ish.

Metric is for people who can't do fractions.

yup, i had the dishes enlarged on my 21253's to 11cc but i've seen a set out at 14.7cc too.

they took 8 years to kill but i finally did it last year at around 25/30 psi at a guess after taking 20psi quite happily for years.

sounds like youve got a good set-up for a low pressure supercharge set-up

28cc head, doublevalve springs
and possibly bigger valves

dont forget electric fuelpump

and as its out i'd get venier duplex adjustable timing chain

its the AE 21251's that are chocolate

Reading up on RTS Clutches