M47TU 90mm Crank

[stuartgallafant]

Hi there, im looking to do a stroker conversion on my spare m42 lump and i was wondering if the 90mm crank can be used instead of the 88mm crank?

is there any difference in design and what will have to be done to make it fit?

thanks

[christophbmw]

Hi there, im looking to do a stroker conversion on my spare m42 lump and i was wondering if the 90mm crank can be used instead of the 88mm crank?

is there any difference in design and what will have to be done to make it fit?

thanks

 i think it is 89.6mm and it has the OBDII crank sensor wheel attached to it, ive heard it will fit there is just ALOT of modifying involved. (10hp or so more power potential that the 88mm).

[Boyracer]

Lookie over here: http://www.m42club.com/forums/showpost.php?p=31900&postcount=97

Based on that I did bit of research.

Crank nose must be lenghtened by custom made part, will cost something and durability is a question, especially if you want to rev shite out of the engine.

Biggest question mark I have right now is that M47N crankshaft does have less counterweights and it is lighter. Instead the engine has what appears to be balancer shafts.

What happens if you use this crankshaft without balancer shafts? :confused:

[Boyracer]

Few thoughts after well (not really) slept night.

While almost 90 mm stroke sounds easy way to achieve almost 2 liter capacity (198? cm3) there are few things that speak against using this crankshaft:

1. Rod/stroke ratio. With stock 140 mm rod and 90 mm stroke it is around 1.55, if you use 135 mm rod then it is 1.5. Stock engine has 1.72 if I recall correctly and 1.75 is consired ideal for some reason.

I checked that many Honda Type-R engines (Civic, Integra and S2000) has ratios of 1.57. So engines are clearly able to sustain very high revs with that kind of ratio.

2. Durability. You have to manufacture extension to crankshaft nose to mount chain sprocket and vibration damper. I think that due vibration damper the extension joint goes under quite significant stress especially in high revs small engine requires to produce any significant amount of power.

3. Crankshaft is designed to work with balancer shafts. Not sure how this affects the suitability for M42 use which does not have balancer shafts but I'm quite sure it is not a positive

So, in my eyes this is not best option to increase power... The crank nose extension would make me hesitant to use high revs in fear of it breaking. I think better way is to build a engine that can sustain higher revs from smaller capacity then increasing capacity in expense of revvability (is that a word?).

88 mm crank from M47 seems to be far better option in my opinion, bit less capacity, lot less work.

[stuartgallafant]

good points made. Im just having such a difficulty locating a bloody 88mm crank!!

BMW use balance shafts in all the 4 cylinder engines after the year '99 due to 4 cylinders not being as smooth as their inline 6 engines. It wouldnt be a problem using the crank without balance shafts, as you could get the whole crank, flywheel, and vibration damper all balanced up together.

Im pretty sure a decent engineering/macj=hine shop could suitably extend the crank nose, maybe by interference fir or such like? maybe i will look into as a friend of mine works in a machine shop in central london. he's gonna be getting a call!!

thanks for your help. i'll let you know what i find

[2002maniac]

Im pretty sure a decent engineering/macj=hine shop could suitably extend the crank nose, maybe by interference fir or such like? maybe i will look into as a friend of mine works in a machine shop in central london. he's gonna be getting a call!!

It wouldn't be too difficult.  One method I can think of would be to use dowel pins to locate the extension on the snout.