ok, i have brand new standard spec cast mahle pistons ready for my engine build

im looking into lowering the compression slightly, and was wondering how much is to be taken off the face of the piston to achieve this?

if i get this done via a specialist tuners, they merely send the pistons to a machinist and charge me extra for the priviledge, whereas if i know the amount to be removed, i can go direct to the machinist and get it done at half the bloody price!! lol

so, anybody know the typical amount removed for a typical lowered compression ratio on a s2?

 

60 thou max.

 

if thats the max value to aim for, whats the best amount to remove? i realise its a broad question, but hoping sum1 wil have an answer lol

i dnt really kno how far to go, and dnt wana go too far, as dnt really know the exacts of what removing certain amounts wil result in

 

Why do you want to lower the compression, what sort of boost are you trying to run?
High 20's? 30's? (psi)

 

RSandy- i'd go with the 60 thou.

 

All depends how thick the piston is to start with.

 

all i know is its a brand new cast item, so i guess as thick as any other standard compression standard piston, within its set casting tolerances lol

as for why i want to lower it, i kinda got advised to go down this route as i may choose to run 20/21psi high boost, and for reliability and the life of the engine, got told in the long run it would be beneficial to go low comp now whilst the pistons are out, save a strip down if i decide to go down thsi route at a later date

 

Why not ask one of the long term experts in this field......

 

iv foned a couple of ford tuners, and they dont seem to give much away, other than they will send them off and get it done for 50quid+vat. now im not stingy, but i foned an engine machinist, and was advised that if i got the measurements required, it could easily be done for less than half that. not really feeling paying a company 25 to 30quid to hold my pistons for me and look at them lol.

so i figure maybe 1 of the less secretive tuners or the like who use this forum might b able to shed some light on the answers im after

 

Best of buying some proper forged items

 

perfect answer my boy !!

 

I agree completely, it is BEST.. but it may not be BEST for him, or his budget!

 

You'd of been even cheeper buyin XR3i pistons and machining them seeing as your going to machine the RST ones

 

Are they the same piston

If they are then go for it

 

totally a diffrent piston

 

in what respects?

I've been told be someone I trust that it is the way to do it if your buying new RST pistons...







Both MAHLE pistons, dont see how they are 'totally' different, the crown is different, but err your going to machine that anyway? same block they go into, same rods?.....

 

i bought the stuff for my engine build 3 years ago, with the intention of building it up instantly, but the shell underwent a massive resto, so its only now being sorted

at the time, forged pistons cost wedge, most of my budget!!! even cast items were like 350quid, so i had to go with these. in retrospect i did the wrong thing, but hindsight is a wonderful thing

so, iv got these now, not forged, not xr3i, not anythin special, just these lol

 

sum xr3i pistons are not flat top pistons.although i think efi xr3i pistons are flat top

 

efi 3i pistons arnt flat top . i have a set

can they be machined to run in turbo lump only reason i ask is i have a set and a efi turbo block if it can be done i could build up a tubby lump that way on small budget.


tony

 

I'd be comparing the crown thickness between the XR3 and RST pistons before acting on some of the advice in this thread

 

ill get them out tomorrow i think and compair them i think

 

going forged pistons is the budget way
skimming ur pistons will lead to a more expensive route when needin a rebuild
am not sayin its a bad idea just 21 psi is a lot of boost for skimmed pistons,playin russian roulette on ur engine
may get lucky and they last well,then again may not and thats the budget engine out the window

 

crown thickness,a bit bodgey way of doing thingsyou can see in the pics that the rst pistons have thicker crown,you machineing 3i pistons and they will proberly break

 

agreeddo it once,and not have all the trobles and probs!

 

When built and mapped correctly there is very little risk - I know of many engines running at this spec, some with small amount of nitrous, with a perfect reliability record (going back almost 20 years!).

 

sum r flat top.my old fiesta xr2i was flat top. my old mfi wernt flat top but my mates mfi was flat top.so bit of mix and match really

 

when you say some nitrous how much is some and is it used bottom end or top of rev range and what type of spec are these engines as been toying with building a low comp engine with t34.48/.55 25 psi and a touch of nos to help spool turbo up

 

you make me laugh
u use words like correctly and perfect yet still say that in the same sentence as skimmin pistons,if u want to be perfect and correct then do it right
its a budget build,perfect and correctly very rarely are used in budget builds
perfect and correct would be too get decent management and do it right but is that called budget,he's want more power at little cost as possible,skimmin pistons would be budget but risky where forged pistons could still be budget comparin to what he may spend in future
also takinn into considration prices of mappers time,u dnt really grasp the meanin of budget do u

so u know of cars that have thousands spent on manangement runnin nitrous and runnin skimmed psitons,why,does that not sound madness to spend that money doin that and skimpin on one of the important parts

 

Sorry but it's not my fault you are unable to understand the words and the context they are said in.

Again, i've never seen a problem but I am interested to hear your views on what crown thickness is acceptable at what power levels on the OE Mahle RST pistons? With regards to budget, I am very aware of what is budget and what is not. Surely if I wasn't, I woudn't have assisted the thread originator with his quest to do it on the cheap?

I don't believe I mentioned management, however the point of my mentioning the nitrous was to demonstrate that it's not as risky as some/you may believe.

But i'm glad I make you laugh, everyone needs to smile every now and again

 

I wouldn't run skimmed OE pistons at the power level you're talking about!

 

u never mentioned management,but u mentioned mapping,therefore i presume diff management as mfi cant be mapped
maybe ur choice of words could be better
i ran skimmed pistons on ma rst,made 214@wheels on 17psi of boost,car runnin sweet as ava set it up as always,but yet pistons just couldnt take that power at that boost,option was forged pistons or keep gettin rebuilds,decided to throw in the towel in the end
so ma argument isnt skimmed pistons dont work or are shite,just could save this chap a lot of hastle in the long run if he goes the forged route the now while the engines out
could be difference of a budget relaible engine to a cheap unreliable one

 

sorry i didn't mention it'll be forged pistons with steel rods with flowed efi head on custom inlet and exhaust maniflods on megasquirt

 

wow, certainly opened up a can of worms with this thread!! lol

im reluctant to buy forged items, as im coming to the end of the build, and my remaining funds are for gearbox and other random parts.

if however i could find a cheap but yet perfectly useable and sound forged pistons, then maybe it would be an option

if not, and the apparent skimming of my cast pistons is going to cause major grief, then i will have to maybe consider keeping them standard compression and running less boost, keeping it around a bar maybe

advice is appreciated, but please bear in mind that help finding said budget forged pistons is even more appreciated

 

also, could i even sensibly go down the forged piston route now, as iv already had my block rebored to suit the 1 oversize pistons, +.27 or whatever it is..

if it would mean yet another rebore, then thats also extra cost i didnt fancy incurring

 

Forgive me, but I can't see the thread originator mention mfi anywhere. Best not to assume, eh? Oh and 'mfi' can be mapped, well at least the spark ecu can.

You say your car ran sweet as ever but the pistons couldn't take it? Isn't that a contradiction? Do you mean it DID run okay and then melted? If so, refer back to my comment about it being well mapped/setup


This is all getting very distracted now; the TO wanted to know about machining the standard pistons. He's received various answers now. He can make his own judgements.

Good job.

 

appreciate the help, sorry it kicked off into a bit of a war lol

the car is a s2, running mfi still

im still non the wiser as to exactly how much i should remove if i choose to go ahead, but thanks for the input everybody lol

 

mfi can be mapped???this gets better
i wont even go into that,mfi cant be mapped,end of
your clutchin at straws,wotever u think u can do its not called mappin

is wasnt a melt,it was cracked pistons 3 times,and ava dnt need to be judged on there mappin/setup skills,i know alan well and he's up there with the best in the business

basically the skimmed pistons couldnt take the power or boost that was asked of it
on another note same thing happened to another ava car that did get forged pistons and is goin strong still,coincidence????i think not,problem solved????i think so

whatever he decides i hope it works out for him but i suspect we may get a my engines fucked topic soon

 

Nooooooooo, Don't skim the pistons, I've been down that road twice now,
Each time ended with trail of smoke, and cracked u know whats....
Either use a decompression plate - ie. 2mm copper head gasket ?,
or get proper forged jobbies with 10cc - 15cc bowls,

 

If your going for anything above 200bhp then it has to be forged if you want it to last

 

Bet it goes bang

Champagne idea's lemonade pockets