Stroking/boring M20B25/B27: Sectioned blocks show critical dimensions. PICS!

[pantelones]

Definitely looks like a hard weld cam lobe.

Ehh, no... This is a heat treating process called induction hardening...

[digger]

hiop used to do a 91mm and 95.8mm for the M50/M52 stokers
BMW made a 93.8mm stroke for the S52 block that went into an ALpina B3 3.3 and B3S
MM do an offset ground S54 crank (s54 has bigger rod bearing so can be offset down the M50 size while increasing the stroke) for M50 based engines with 93mm stroke but adapting the nose is an issue

also VAC are taking preorders for a 97mm crank for M50 based engines but this wont fit i would say. you also be looking at $5500 to get your hands on one yikes....

[whodwho]

Getting closer to building mine out and wanted to know for sure so picked up a pair of 300mm calipers and gave it check, this is an 87 b25i block not cleaned.

[Madhatter]

unless its something machined to give a smooth surface for a weld. Looks like lots of penetration on either side of the "lobe", a lot of heat has been transferred here.

Edit: Looking at the other pics, it's certainly eccentric in shape. Very weird.

[LJ851]

dumb question what is the thing between the bearings. it looks like it has been added and looks like a cam lobe of sorts

http://www.ecstuning.com/BMW-E30-325...w_All/ES23062/

Bizarre.

It looks like a fuel pump lobe maybe but M20 never had a mech one....

[tinkerputzer]

Wow that's odd. Definitely looks like a hard weld cam lobe. For what though. That's no good for bigger cranks.

[digger]

dumb question what is the thing between the bearings. it looks like it has been added and looks like a cam lobe of sorts

http://www.ecstuning.com/BMW-E30-325...w_All/ES23062/

[Ray Smoodiver]

Stickied.

[tinkerputzer]

I get what you're saying. Smooth transitions are always best. On mine, with the 89.6 mm crank and s52 rods, it was only the raised text on the shaft that was making contact every couple revolutions. It was barely anything. All that was really needed was to knock down the text on a lathe. For an even bigger crank do like you said and remove the required material in a localized area then blend it all out to have nice smooth transitions.

[digger]

Nope. Maybe give mm a call. I would think the oil pump drive shaft would give out before the intemediate shaft.

i thought i had heard but could not find anything.

my thinking is the shaft is stressed in bending due to the belt tension. the bending moment is highest at the front bearing of the block and where you notch it out is midway between the two bearings so bending moment is only half the maximum. so it makes sense to locally notch it with a nice transition shape not like the sharp corner the MM put really close to the high bending moment areas. the one i have from MM is 17.75mm diameter ending in a sharp notch. i bet you could go alot thinner midway between the bearings with better shape and not impact fatigue life. perhaps you could polish or peen it aswell

the torsion loads are small enough to be carried by the skinny oil pump hex drive so wont affect the main shaft at all

[tinkerputzer]

Nope. Maybe give mm a call. I would think the oil pump drive shaft would give out before the intemediate shaft.

[digger]

has anyone ever heard of a broken intermediate shaft? i am referring to the shaft not the gear that used to be pressed

[digger]

The above is only true if the flow capacity of the head remains the same though. I wasn't suggesting that increasing the capacity will develop more power if you are already at max VE.

things are not very far from the maximum flow capacity at 220CFM for the 2.8L

[Madhatter]

there is a limit to how much power you can make with a given CFM and head characteristics and fuel, changing the stroke will for the most part only affect the rpm where it happens. there is more tunability in higher rpm so a smaller higher rpm engine has a bit of an advantage with wave tuning etc

if i run a sim on my engine with more cubic inches all i do is get more torque no more peak power but it peaks a few hundred rpm earlier.....its not until i start influencing the head flow and cam profile do i start getting much higher peak hp

The above is only true if the flow capacity of the head remains the same though. I wasn't suggesting that increasing the capacity will develop more power if you are already at max VE.

11.5mm increase in throw is a big difference and you have a small margin as it is, think it's going to be closer than you think. I wouldn't be surprised if its one of the reasons why companies like MM have gone with a much longer rod on purpose, even having some made which will reduce the angle. Now we are reducing the length of the rod and increasing the stroke over those combinations which could be a problem. They note in their own info that when going to 89.6 mm stroke of the M3, the rod can already strike the block in their sport engines with a 135mm rod. So I think you can be pretty certain the further increase in stroke is going to be a problem at some point. Interesting to note they went to 138mm rods on the rally 3.2L engines, 206 - (138 + 44.8 ) = 23.2mm. Wouldn't be surprised if they reduced the pin diameter to due to the piston/rod combination too, but there is obviously a point where things physically won't fit or can't be made reliable otherwise they could have just run a longer 140mm rod they already had to reduce the angle further. All conjecture though of course until someone want's to give it a try though. I've seen them use doweled sandwich plates between the block and the head, then sleeved the block to increase deck height before, kind of old school I guess and you don't see it a lot anymore. I guess if you were hell bent on creating the biggest capacity M20, it's not that big a deal to give everything a try.

[digger]

Yeah, I wasn't talking about the combination fouling on the crank counterweights, more that increased stroke changes the rod angle in relation to the block and the bottom of the bore and side of the block. What did you end up with before to the block and rod? about 5mm? Was Wondering if there would be enough room for the rod to clear as the added stroke is going to push the rod closer to the block.
You can see why bmw made the M50 block taller when they replaced the M20. Even just a few mm is enough to make a difference with much longer strokes.

I think it would be ok, its manly the counterweight if too big would cause problems with the rotating assembly and piston design. Id be confident that the rod near the cap would not hit the side of the block and that clearance to the bottom of bore for the rod shoulder would be touch and go with as big of a chamfer as possible. You would probably need to get a few sample rods or get dimensions and make a prototype to choose the most compact brand.