whats the max you can over bore a 2.5

[gorgeee]

is it possible to overbore lots to make alot higher cc
i read somewhere does a 2.9 for this engine and wonder with a eta crank can this be over bored?

[harry]

Hi.

I have a magazine article on an MK motorsport e30. Its bored to exactly 2700cc. Its quoted as 210 bhp, but they wouldnt give a torque figure so its probably lower than the stroked 2.7.

Any help?

[gorgeee]

thanks for that Harry
do you know if its the same block they used?

[E30Adam]

The M20 can be bored to 3 litres but it needs to be stroked as well.

[Boots_Walker]

i've heard 86mm is about the max you would want to go

[Ade]

whats the standard bore size ?

[gorgeee]

standards 84mm
i dont understand why no 1 has bored and stroked this engine
by boring it out to 86mm what piston would be used?
and would this take it to 3litres?

[jamesroools]

http://www.bmw2002.com/

if you look under e30 performance they have a kit that has everythin you need to take the m20 to a 3.0 litre, i read on the e30 tech forum that it uses a modified m50 crank, but thats sounds a bit impossible as the m50 is a twin cam and the m20 isnt, and btw its about Ԛ£1400 in our money

[Karan]

m50 crank can fit...

but the front needs to be machined where the crank pulley fits...

ask e30 adam he has a 328 crank in his car i think

Karan

[gorgeee]

thanks for that!
i thought i could do it with oversize pistons and a 324d crank
so if i can stroke the engine to 2.7 with this crank would it be worth it to try and get it rebored to 86mm
does any1 know what size the engine would be come?
thanks alot for your help on this
im just being forced to rebuild my tired high mileage engine and want to do it once and for all

[Ade]

going from 84 to 86 pistons would make a 4.8% difference in displacement, which may be worth it to some?

erm i think im correct...

[Turbo-Brown]

Yeah think that's about right

So what would that give, 2500x1.048=2620cc

[M5pilot]

With an 81mm crank and 86mm Bore you would get 2823cc


M52b28 crank with 2mm over bore would give 2927cc.

Considering Adam's car revs as well as any other m20 (using M52 crank), doing a 2mm overbore aswell would be even better!

PIstons would cost around Ԛ£600 for nice high compression 10.5:1 lightweights.

Would make a nice engine! Imagine the torque

[Turbo-Brown]

I'm probably totally wrong here, but I was finishing off my M20 bottom end yesterday and noticed that the skirt of the piston comes down into the block at BDC which presumably means that you'd need different rods if the stroke was drastically longer?

Obviously the piston's have been covered.

Does anyone know that the rod/stroke ratio with the uber crank is?

[Ade]

if youre stroking a motor i believe youre supposed to increase the rod length o keep the rod-length/stroke ratio similar?

so unless you use aftermarket pistons with a reduced deck height then wont you need to put a spacer under the block ??

How do the stroker kits do it?

anyone?

[Turbo-Brown]

Might have difficulty putting a spacer under the block on these as it's integral to the crank case

[Ade]

aha, so what piston/con rod/crank combinations are known to work and allow you to slip in a stroker crank with the least hassle?

Also can a standard 320 block fit a 325 crank...?

hehehe

[gorgeee]

does anyone know where you can buy pistons in england
not the megga expensive ones just the same quality as standard and how much they cost

[Turbo-Brown]

Should be able to get them through the dealers or any good machine shop.

I was quoted about Ԛ£90 per piston for the standard Mahle ones for the 325i but thankfully mine were OK and the block didn't need a re-bore

[Ade]

KIN HELL!

a set of 4 Forged Mahle pistons (and 4 barrels) for a VW Type one engine costs Ԛ£170...

I guess thats due to demand then?

[Jhonno]

ahh now i see why the m20 revs so well... even with the 2.7 crank its still nicely oversquare Excellent

[gorgeee]

i thought the pistons were alot too but then i've seen tuners charging about Ԛ£800 for a set of six
so didnt think any thing of it
but still the price of a piston is way too much prabably why people dont over bore much now and rather buy a different 2nd hand engine

[Turbo-Brown]

ahh now i see why the m20 revs so well... even with the 2.7 crank its still nicely oversquare Excellent

An engine being oversquare doesn't affect it's ability to rev, if you had a one mile stroke and a 1.1 mile bore, it'd be oversquare but not very rev happy.

A longer stroke crank will decrease an engine's ability to rev as the inertial loads in the reciprocating components will increase.

As a general rule, oversquare engines can make more power (as long as they were designed over-sq) as you can have a larger valve area per unit displacement with an over-sq engine than an under-square.