'unique' 2.8 build, need a bit of help with c/r

[Beavy_OZ]

Hi guys,

Well I am nearly at the end of ~8 months of research, $3k saved up, and I just need to clarify what I intend to do will work. I am hoping to build a 2.8L stroker using:

m20b25 head (EARLY high comp), 288 schrick cam + springs, ported(?)
m20b23 block, bored to 84mm
m52b28 crank, 84mm
m20b23 rods, 130mm
m20b25 piston (EARLY high comp)

At the moment I have my 325i and a 323i engine, so all I am missing is a crank! I just need to know this is possible, and whether I will need to shave the block given I have the higher comp pistons/head? What would be the expected C/R, power etc. I am hoping the cam will retain the 'peaky' nature of the 325i motor (it bogs down with it at the mo, a little more torque would be nice!)

Also, can someone tell me why I need the non-TU crank, and how to tell the difference when I go to pick one up from the wreckers?

How much machine work will be needed to make this combo work?

Thanks guys, can't wait!!

ps. it's only 'unique' here in OZ cos we don't have the high comp motor so it's been difficult asking around!
It's out of an imported rust bucket

[Andyboy]

The TU crank (double Vanos) has a TDC trigger ring on the rear crank web. It can be unbolted but some users have reported that the crank webs foul the bottom of the pistons and the rear block webs. Just use an early non TU crank - cheaper and they drop straight in.
If you are using early high comp pistons they will need to have 12mm taken off the bottom anyway. Your compression ratio will be 10.8:1 assuming a standard head and stock BMW head gasket. Possibly a bit too high, and you will need to pun it on high opctane fuel to avoid detonation.

March 109 has the 325i with the 10.9:1 CR 2.8 I built some years ago so ask him how he gets on. Using later 8.8 pistons will give a compression ratio of 9.73:1 which will be fine on 95 fuel and iirc they don't need shortening for the 2.8 crank webs. Horses for courses!

[Beavy_OZ]

The wheel you speak of is quite large isn't it? About the size of the water pump pulley, with notches for the hall sensor? Should be easy to spot the difference ;) Cheers

I always run 98RON, would this be ok for the high comp pistons? If the skirt is shorter on the later pistons, then it is indeed much easier either way I guess. Thanks very much!

I'm about 30whp shy of a mate with an old 5L V8, should be interesting to see how close we are in about 6 months.

[march109]

My M20B28 Andyboy built goes very well

But it helps that the 'sport' 284/272 cam installed lowers the dynamic compression though, by all accounts on a standard cam shes not happy on anything less than quality 98RON. As it is shes mapped for 98RON now anyway with the schrick cam she makes good power with pleny of potential for more when its all up and running right.

[Beavy_OZ]

Thanks for the info, I'm all ready to go, except my mate with a 2.7 stroker reckons there's no way I could bore an m20b23 block to 84mm, the wall would be too thin?

[Ant]

M20B20 and M20B23 will bore to 85mm with plenty to spare so the stock 84mm is not an issue

indeed, the castings are all from the same basic patten, only the final machining stage dictates the capacity.

I use B20 blocks +4mm for my 2.7's to ensure the bores are perfect.

HTH

[tomson]

Thanks for the info, I'm all ready to go, except my mate with a 2.7 stroker reckons there's no way I could bore an m20b23 block to 84mm, the wall would be too thin?

Thats bollox as Ant has pointed out above