M52TU stroker ideas

[ChaseN]

So I'm going to pick up a M52TUB25 next weekend, and just thinking through some ideas about what to do with it. Here are my thoughts:

M52B28 84mm crank
M52B28 135mm rods
Stock 84mm pistons

Ends up at 2.8l (obviously) and 10.2:1 compression

OR:

S50 85.8mm crank
M52B28 rods
stock pistons

Bumps me to 2.9ish, appx. 10.4:1 compression

I'd like to keep the stock pistons just because of cost. Here's the rub - M52TUB25 & M52TUB28 pistons are physically identical, except that the B28 pistons have an anti-friction graphite coating to withstand higher power/worse rod-to-stroke ratios. Should I be worried about running the B25 pistons without this coating, in a 2.8ish street engine?

What do you all think, anything I'm missing? Probably cheaper to find an M52 crank, and I doubt I'll miss that .1 liter of displacement...As an aside, boring isn't really an option as this is a sleeved aluminum block that can only be bored once - and as it only has 23k on it, I'd like to save my one chance to bore for someday in the future when I potentially really need to.

I have my reasons for using this motor as a base, and am committed to it.

[Leo_328i]

Cool idea - good on you for going for an alloy block motor over a cast iron one!

I think you may be mistaken on the B25 and B28 pistons being identical besides the graphite coating. The B28s have a shorter piston skirt I believe.

I have an official BMW pdf that documents this.

I think for simplicity of ECU management you may be best going with the 2.8l unless you can find someone that does a 2.9l software (maybe TRM does it?).

I certainly hope that you are prepared to setup the Double VANOS!

[ChaseN]

Thanks for the advice Leo. I have the same pdf I believe, and the way I read it led me to believe that the piston were the same - but they very well may not be. Based on simple math, I think the dome profile at least is nearly identical. Assuming I machine some skirt material off the b25 piston, will I miss the coating?

Question - if I used a s52 or m52b28 crank, and kept the 140mm rods, (which would take me up to almost 11.9:1) do you think there is enough meat on the top of the stock m52 piston to machine back down to around 11.0ish?

And fortunately, hoveringuy on this board makes a solution for dual VANOS that is easy peasy.

[Leo_328i]

Hmmmm, never investigated getting the pistons getting machined down for less compression. I would assume that there would be at least a couple of mm to play with that should be enough to reduce your compression down to 11:1.

My reason for this assumption is that the stock B28 pistons have small valve indentations for clearance which look around 3mm deep approx.

A cheaper alternative would be to run a thicker head gasket to reduce compression.

[dave.a.cote]

I put a S50 crank, and M52B28 pistons and rods in my M50 for 2.9l. I machined 0.015" off the top of the pistons to get my CR down to 11.0:1.

Overall, I am quite happy with it. With I had an allow block though!

[dave.a.cote]

I don't know for certain, but according to my numbers a S50 crank with 135mm rods and B25 pistons that were designed for 140mm rods give a compression ratio of 12.4

[dirtysix]

Use a M54B30 rotating assembly?

[ChaseN]

I don't know for certain, but according to my numbers a S50 crank with 135mm rods and B25 pistons that were designed for 140mm rods give a compression ratio of 12.4

75/2 = 37.5 + 140 = 177.5mm wrist pin height (stock)
85.8/2 = 42.9 +135 = 177.9 wrist pin height (s50 crank/135mm rods)

Unless my math is screwed up in a way I don't understand (likely) I don't think the compression ratio could change more than a tenth or two?

Use a M54B30 rotating assembly?

Logical, easy, and somewhat expensive. This would be my preferred scenario if I can snag a complete assembly on the cheap.

[dirtysix]

Well, if it helps, the M54 cranks the same as your s52.

[ChaseN]

I don't have an s52...???

[dirtysix]

But the cranks might be cheaper than M54?.....
Comon, work with me here.

[ChaseN]

Haha, the way it was written I thought you meant I had a S52 I could cannibalize for parts. But anyway, pick up a s52 crank, M54 pistons and a cheap set of 135mm rods (out of like 80% of m5x motors it seems) and I've got a M54 rotating assembly(-ish). Good thinking.

[dirtysix]

That would be the general idea.
You'd have to confirm cause I could be talkin' out me arse.

[dave.a.cote]

I still think your CR will be high with the S50 crank, 135mm rods and M52TUB25 pistons. As per your calcs - your writs pin goes up slightly for less compressed volume but the game changer is that your volume when the piston is at the bottom of it's stroke is increased significantly because of the longer stroke crank.

Even if your compressed volume was exactly the same, your stroke increases by 14.4% so your CR will increase the same - turning 10.5 into 12.0.

I thought about going M54 rotating assembly but found these parts first.

[Stu]

I still think your CR will be high with the S50 crank, 135mm rods and M52TUB25 pistons. As per your calcs - your writs pin goes up slightly for less compressed volume but the game changer is that your volume when the piston is at the bottom of it's stroke is increased significantly because of the longer stroke crank.

Even if your compressed volume was exactly the same, your stroke increases by 14.4% so your CR will increase the same - turning 10.5 into 12.0.

I thought about going M54 rotating assembly but found these parts first.

With the 3.0 crank, 135's and 84 bore pistons that came with the aluminum block, you'll have a 2.9 and cr set closer to 11:1 (~11.2) after piston machining. Unfortunately I don't remember off hand how much material needs to come off the pistons.